increased tooth crown size in females from …...increased tooth crown size in females from...

Increased tooth crown size in females from opposite-sex

dizygotic twins: a possible intrauterine hormonal

influence on dental development

School of Dentistry

The University of Adelaide

Daniela Cisoto Ribeiro

Submitted for the degree of Doctor of Philosophy in Dentistry

April 2012

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Table of Contents

List of Tables ........................................................................................................................ iv

List of Figures ........................................................................................................................ x

List of Abbreviations ........................................................................................................... xii

Abstract ............................................................................................................................... xiii

Thesis Declaration ............................................................................................................... xv

Acknowledgements ............................................................................................................. xvi

1. Introduction ........................................................................................................................ 1

2. Literature review ................................................................................................................ 4

2.1. The importance of studying the human dentition ....................................................... 4

2.2. Genetic, epigenetic and environmental influences on dental variation ...................... 5

2.3. The value of twins in dental research ......................................................................... 7

2.3.1. The twinning process ........................................................................................... 8

2.3.2. Special features of the twinning process ............................................................ 10

2.3.3. Importance of zygosity determination ............................................................... 12

2.3.4. Different twin models ........................................................................................ 13

2.4. Studies of tooth size variation ................................................................................... 14

2.4.1. Sexual dimorphism in tooth size ........................................................................ 15

2.4.2. Hormonal influences on tooth size .................................................................... 18

2.4.3. Dental asymmetry .............................................................................................. 21

2.5. Odontogenetic processes ........................................................................................... 24

2.6. Methods of obtaining dental measurements ............................................................. 27

2.6.1. Sliding calipers .................................................................................................. 28

2.6.2. New techniques for measuring dental crowns ................................................... 29

2.7. Statistical approaches for describing tooth size variation ......................................... 30

3. Aims of this research ....................................................................................................... 33

3.1. Format of the thesis ................................................................................................... 34

4. Materials and methods ..................................................................................................... 35

4.1. Sample size and zygosity determination ................................................................... 35

4.1.1 Inclusion and exclusion criteria .......................................................................... 36

4.2. Defining the landmarks ............................................................................................. 37

4.3. The 2D Image Analysis System ................................................................................ 42

4.3.1. Image analysis .................................................................................................... 43

4.4. Power analysis .......................................................................................................... 44

4.5. Statistical analysis ..................................................................................................... 44

5. Errors of measurements ................................................................................................... 46

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5.1. Introduction ............................................................................................................... 46

5.2. Methods .................................................................................................................... 48

5.3. Results ....................................................................................................................... 49

5.3.1. Repeatability test: phase 1 ................................................................................. 49

5.3.2. Image Pro Plus 5.1 software and ImageJ software: phase 2 .............................. 50

5.3.3. Repeatability test for all dimensions using ImageJ software: phase 3 .............. 51

5.4. Discussion ................................................................................................................. 61

6. Results of descriptive analysis of tooth size data in monozygotic (MZ) and dizygotic

same-sex (DZSS) twins ....................................................................................................... 62

6.1. Introduction ............................................................................................................... 62

6.2. Results ....................................................................................................................... 64

6.2.1. Sexual dimorphism ............................................................................................ 64

6.2.2. Coefficients of variation (CV) ........................................................................... 80

6.2.3 Association between variables ............................................................................ 81

6.2.4. Comparison between different populations ....................................................... 86

6.3. Discussion ................................................................................................................. 91

7. Hormonal effects on tooth development in dizygotic opposite-sex (DZOS) twin pairs . 95

7.1. Introduction ............................................................................................................... 95

7.2. Methods .................................................................................................................... 97

7.3. Results ....................................................................................................................... 98

7.3.1. Univariate analysis ............................................................................................. 98

7.3.2. Multivariate analysis ........................................................................................ 136

7.4. Discussion ............................................................................................................... 139

8. Dental asymmetries ........................................................................................................ 142

8.1. Introduction ............................................................................................................. 142

8.2. Methods .................................................................................................................. 145

8.3. Results ..................................................................................................................... 146

8.2.1. Directional asymmetry ..................................................................................... 146

8.2.2. Fluctuating asymmetry .................................................................................... 152

8.3. Discussion ............................................................................................................... 162

9. General discussion ......................................................................................................... 166

10. Conclusions .................................................................................................................. 176

11. References .................................................................................................................... 177

12. Appendices ................................................................................................................... 200

Appendix 1 – List of achievements and professional development activities of Daniela

Ribeiro during PhD candidature 2008-2012 .................................................................. 200

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Appendix 2 – Bland and Altman graphs of intra- and inter-operator measurements

obtained in the School of Dental Sciences, the University of Liverpool, Liverpool, UK.

....................................................................................................................................... 203

Appendix 3 - Ethical approval ....................................................................................... 211

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List of Tables

Table 5.1: Measures of repeatability error.................................................................. 50

Table 5.2: Measures of repeatability errors between Image Pro Plus 5.1 and ImageJ

software..........................................................................................................

51

Table 5.3: Mean difference in millimeters between first and second measurements

for mesiodistal (MD), buccolingual (BL), crown height (CH) and

intercuspal dimensions (IC) of primary and permanent dentitions of a

sample of monozygotic (MZ), dizygotic same-sex (DZSS) and dizygotic

opposite-sex (DZOS) male and female twins – right side............................

54

Table 5.4: Mean difference in millimeters between first and second measurements

for mesiodistal (MD), buccolingual (BL), crown height (CH) and

intercuspal dimensions (IC) of primary and permanent dentitions of a

sample of monozygotic (MZ), dizygotic same-sex (DZSS) and dizygotic

opposite-sex (DZOS) male and female twins – left side................................

55

Table 5.5: Dahlberg statistics (in mm) for mesiodistal (MD), buccolingual (BL),

crown height (CH) and intercuspal dimensions (IC) of primary and

permanent dentitions of a sample of monozygotic (MZ), dizygotic same-

sex (DZSS) and dizygotic opposite-sex (DZOS) male and female twins......

58

Table 6.1: Descriptive statistics for mesiodistal (MD) dimensions of primary and

permanent dentitions in monozygotic (MZ) male and female twins.............

70

Table 6.2: Descriptive statistics for buccolingual (BL) dimensions of primary and


71

Table 6.3: Descriptive statistics for crown height (CH) dimensions of primary and


72

Table 6.4: Descriptive statistics for intercuspal (IC) dimensions of primary and


73

Table 6.5: Descriptive statistics for mesiodistal (MD) dimensions of primary and

permanent dentitions in dizygotic same-sex (DZSS) male and female

twins...............................................................................................................

74

Table 6.6: Descriptive statistics for buccolingual (BL) dimensions of primary and


twins...............................................................................................................

75

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Table 6.7: Descriptive statistics for crown height (CH) dimensions of primary and


twins...............................................................................................................

76

Table 6.8: Descriptive statistics for intercuspal (IC) dimensions of primary and


twins...............................................................................................................

77

Table 6.9: Percentages of sexual dimorphism for mesiodistal (MD), buccolingual

(BL), crown height (CH) and intercuspal (ICP, IC1, IC2, IC3 and IC4)

dimensions of primary and permanent dentitions in MZ males and

females...........................................................................................................

78

Table 6.10: Percentages of sexual dimorphism for mesiodistal (MD), buccolingual

(BL), crown height (CH) and intercuspal (ICP, IC1, IC2, IC3 and IC4)

dimensions of primary and permanent dentitions in DZSS males and

females...........................................................................................................

79

Table 6.11: Correlation coefficients for mesiodistal (MD), buccolingual (BL),

crown height (CH) and intercuspal (IC) crown dimensions between

antimeric pairs of teeth in males from MZ and DZSS twin pairs..................

82


crown height (CH), and intercuspal (IC) crown dimensions between

antimeric pairs of teeth in females from MZ and DZSS twin pairs...............

83



isomeric pairs of teeth in males from MZ and DZSS twin pairs...................

83



isomeric pairs of teeth in females from MZ and DZSS twin pairs................

84

Table 6.15: Correlation coefficients for mesiodistal (MD), buccolingual (BL) and

crown height (CH) dimensions of primary and corresponding sucessional

permanent teeth in males and females from MZ and DZSS twin pairs.........

84

Table 6.16: Correlation coefficients between all variables in the same tooth in MZ

and DZSS female twins: primary upper second molar (MZ: n=47-52;

DZSS: n=35-39).............................................................................................

85

Table 6.17: Mesiodistal (MD) dimensions of primary and permanent teeth in

different populations measured in mm......................................................

87

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Table 6.18: Buccolingual (BL) dimensions of primary and permanent teeth in

different populations measured in mm......................................................

88

Table 6.19: Crown height (CH) dimensions of primary and permanent teeth in

different populations measured in mm...........................................................

89

Table 6.20: Intercuspal (IC) dimensions of primary and permanent teeth in

different populations measured in mm...........................................................

90

Table 7.1: Comparison of mesiodistal (MD) dimensions in the primary and

permanent dentitions of males from dizygotic opposite-sex (DZOS) and

monozygotic (MZ) twin pairs........................................................................

101

Table 7.2: Comparison of buccolingual (BL) dimension in the primary and



102

Table 7.3: Comparison of crown height (CH) dimensions in the primary and



103

Table 7.4: Comparison of intercuspal (IC) dimensions in the primary and



104



dizygotic same-sex (DZSS) twin pairs...........................................................

105

Table 7.6: Comparison of buccolingual (BL) dimensions in the primary and


diygotic same-sex (DZSS) twin pairs............................................................

106




107




108


permanent dentitions of females from dizygotic opposite-sex (DZOS) and


111


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112




113




114




117




118




119




120

Table 7.17: Percentage increase in size for mesiodistal (MD) dimensions in the

primary and permanent dentitions between dizygotic opposite-sex (DZOS)

and monozygotic (MZ) female twins.............................................................

122

Table 7.18: Percentage increase in size for buccolingual (BL) dimensions in the



123

Table 7.19: Percentage increase in size for crown height (CH) dimensions in the


and monoygotic (MZ) female twins...............................................................

124

Table 7.20: Percentage increase in size for intercuspal (IC) dimensions in the



125

Table 7.21: Percentage increase in size for mesiodistal (MD) dimensions in the


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and dizygotic same-sex (DZSS) female twins............................................... 126

Table 7.22: Percentage increase in size for buccolingual (BL) dimensions in the


and dizygotic same-sex (DZSS) female twins...............................................

127

Table 7.23: Percentage increase in size for crown height (CH) dimensions in the



128

Table 7.24: Percentage increase in size for intercuspal (IC) dimensions in the



129

Table 7.25: Percentage of sexual dimorphism for mesiodistal (MD) dimension

between males and females from dizygotic opposite-sex (DZOS) co-twins,

monozygotic (MZ), and dizygotic same-sex (DZSS) twins..........................

132

Table 7.26: Percentage of sexual dimorphism for buccolingual (BL) dimension



133

Table 7.27: Percentage of sexual dimorphism for crown height (CH) dimension



134

Table 7.28: Percentage of sexual dimorphism for intercuspal (IC) dimension


monozygotic (MZ), and dizygotic same-sex (DZSS) twins............................

135

Table 8.1: Summary statistics of directional asymmetry (DA) for mesiodistal (MD)

dimensions in the primary and permanent dentitions of males and females

from monozygotic (MZ), dizygotic same-sex (DZSS) and dizygotic

opposite-sex (DZOS) twin pairs....................................................................

148

Table 8.2: Summary statistics of directional asymmetry (DA) of buccolingual (BL)




149

Table 8.3: Summary statistics of directional asymmetry (DA) of crown height

(CH) dimensions in the primary and permanent dentitions of males and

females from monozygotic (MZ), dizygotic same-sex (DZSS) and

dizygotic opposite-sex (DZOS) twin pairs.....................................................

150

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Table 8.4: Summary statistics of directional asymmetry (DA) of intercuspal (IC)


from monozygotic (MZ), dizygotic same-sex (DZSS), and dizygotic


151

Table 8.5: Estimates of fluctuating asymmetry (FA) for mesiodistal (MD)

dimension in the primary and permanent dentitions of males and females



155

Table 8.6: Estimates of fluctuating asymmetry (FA) for buccolingual (BL)




156

Table 8.7: Estimates of fluctuating asymmetry (FA) for crown height (CH)




157

Table 8.8: Estimates of fluctuating asymmetry (FA) for intercuspal (IC) dimension

in the primary and permanent dentitions of males and females from

monozygotic (MZ), dizygotic same-sex (DZSS) and dizygotic opposite-

sex (DZOS) twin pairs...................................................................................

158

Table 9.1: Table of predictions of sexual dimorphism from two models presented

by Guatelli-Steinberg and co-workers (2008)................................................

170

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List of Figures

Figure 4.1: Mesiodistal (MD) crown dimension measured on a permanent

upper central incisor from the labial view..............................................

38

Figure 4.2: Mesiodistal (MD) crown dimension measured on a permanent

upper first molar from the occlusal view................................................

38

Figure 4.3: Buccolingual/labiolingual (BL) crown dimension measured on a

permanent upper central incisor from the labiolingual view..................

39

Figure 4.4: Buccolingual/labiolingual (BL) crown dimension measured on a

permanent upper first molar from the buccolingual view......................

39

Figure 4.5: Crown height (CH) dimension measured on a permanent upper

central incisor from the labial view........................................................

40

Figure 4.6: Intercuspal (IC) dimensions shown on a permanent upper first

molar and measured from the occlusal view..........................................

41

Figure 4.7: 2D image analysis system............................................................... 43

Figure 5.1: Values of mean differences in millimeters between first and

second determinations (intra-operator systematic errors) for crown

dimensions in primary and permanent dentitions – right side................

56

Figure 5.2: Values of mean differences in millimeters between first and

second determinations (intra-operator systematic errors) for crown

dimensions in primary and permanent dentitions – right side................

57

Figure 5.3: Values of Dahlberg statistics in millimeters (intra-operator

random errors) for crown dimensions in primary and permanent

dentitions – right side.............................................................................

59

Figure 5.4: Values of Dahlberg statistics in millimeters (intra-operator

random errors) for crown dimensions in primary and permanent

dentitions – left side................................................................................

60

Figure 7.1: Percentage difference of mesiodistal (MD), buccolingual (BL),

crown height (CH), and intercuspal (IC) dimensions in the primary

and permanent dentitions between females from dizygotic opposite-

sex (DZOS) and monozygotic (MZ) twin pairs. (positive

values=DZOS larger)..............................................................................

130

Figure 7.2: Percentage difference of mesiodistal (MD), buccolingual (BL),

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crown height (CH), and intercuspal (IC) dimensions in the primary

and permanent dentitions between females from dizygotic opposite-

sex (DZOS) and dizygotic same-sex (DZSS) twin pairs. (positive

values=DZOS larger)..............................................................................

130

Figure 7.3: Least squares mean values (in mm) and standart errors (SE) for

mesiodistal (MD), buccolingual (BL) and crown height (CH)

dimensions in the primary and permanent dentitions of monozygotic

(MZ), dizygotic same-sex (DZSS) and dizygotic opposite-sex

(DZOS) female twins.............................................................................

137

Figure 8.1: Estimates of fluctuating asymmetry (FA) and standard errors

(SE) for mesiodistal (MD) dimensions in the primary and permanent

dentitions for males and females from all zygosities.............................

159


(SE) for buccolingual (BL) dimensions in the primary and permanent


159


(SE) for crown height (CH) dimensions in the primary and permanent


160


(SE) for intercuspal (IC) dimensions in the primary and permanent


160

Figure 8.5: Comparisons of fluctuating asymmetry (FA) estimates and

standard errors (SE) for mesiodistal (MD), buccolingual (BL), crown

height (CH), and intercuspal (IC) dimensions in the primary and

permanent dentitions of males and females from all zygosities. ...........

161

Figure 9.1: Representation of dental crown measurements: A) intercuspal

dimensions (IC); B) mesiodistal dimension (MD); C) buccolingual

dimension (BL); D) crown height dimension (CH)...............................

173

Figure 9.2: Schematic representation of the stages of formation of the upper

central incisor and surges in testosterone production.............................

174

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List of Abbreviations

MZ Monozygotic (twin pairs)

DZSS Dizygotic same-sex twin pairs

DZOS Dizygotic opposite-sex (twin pairs)

DA Directional asymmetry

FA Fluctuating asymmetry

MD Mesiodistal (crown dimension)

BL Buccolingual (crown dimension)

CH Crown height (dimension)

IC Intercuspal (dimensions)

LL Labiolingual (crown dimension)

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Abstract

Studies in rodents and humans have suggested that masculinisation of females and

feminisation of males can occur between fetuses in utero due to hormonal diffusion.

Opposite-sex twin pairs provide a useful model to study the effects of prenatal hormone

diffusion on tooth size. This study aims to determine whether primary and permanent

tooth sizes are altered in females and/or males from opposite-sex dizygotic (DZOS) twins

compared with same-sex monozygotic (MZ) or dizygotic (DZSS) twins; to assess the

sexual dimorphism between males and females from all zygosities; and to quantify the

amount and magnitude of dental asymmetries, both directional and fluctuating

asymmetries, in the different twin groups.

Serial dental models of the primary, mixed and permanent dentitions of 122 males

and 135 females, aged from 4 to 16 years of age from DZOS, MZ and DZSS twin pairs,

were used. Mesiodistal (MD) and buccolingual (BL) crown dimensions, crown heights

(CH) and intercuspal (IC) dimensions of all primary teeth and the permanent central

incisors, lower lateral incisors, canines, second premolars, first and second molars were

measured to an accuracy of 0.1mm using a 2D image analysis system.

Dental crowns of DZOS females were consistently larger by approximately 1-3%

in MD and BL dimensions, by 2.7-4.7% in CH dimensions and by 0.5-0.8% in IC

dimensions of permanent teeth compared with other female groups. Although the

differences were smaller than in the permanent dentition, the primary dentition also

showed larger dental crown size in DZOS females by 0.5-2% in MD and BL dimensions,

3% in CH and by 1% in IC dimensions. No systematic trend for altered tooth size was

found in either dentition or for any of the crown dimensions between the male twin groups.

There was no evidence of systematic directional asymmetry in either dentition in males or

females from all the different zygosity groups, whereas fluctuating asymmetry was evident

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in all groups with the magnitude being smaller for MD and BL dimensions compared with

CH and IC dimensions. There was no evidence that the magnitude of fluctuating

asymmetry differed significantly between the different zygosity groups.

This analysis shows a trend for primary and permanent dental crown size to be

larger in DZOS females than in the other female twins. A multivariate analysis of variance

confirmed that there were statistically significant differences between the twin zygosity

groups, with the CH dimensions showing the greatest effect. No effects were found in

DZOS males. The effects on DZOS females may be related to circulating male hormones

from the co-twin in utero, supporting the view that variation in tooth size reflects an

interplay between genetic, epigenetic and environmental influences during development,

with hormonal effects playing a small but significant role.

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Thesis Declaration

Name: Daniela Cisoto Ribeiro Program: PhD in DENTISTRY

This work contains no material which has been accepted for the award of any other

degree or diploma in any university or other tertiary institution to Daniela Cisoto Ribeiro

and, to the best of my knowledge and belief, contains no material previously published or

written by another person, except where due reference has been made in the text.

I give consent to this copy of my thesis, when deposited in the University Library,

being made available for loan and photocopying, subject to the provisions of the Copyright

Act 1968.

I also give permission for the digital version of my thesis to be made available on

the web, via the University’s digital research repository, the Library catalogue, the

Australasian Digital Theses Program (ADTP) and also through web search engines, unless

permission has been granted by the University to restrict access for a period of time.

Signature: __________________________ Date: __________________

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Acknowledgements

First and foremost, I would like to acknowledge the immense debt that this thesis

owes to my principal supervisor Professor Grant Townsend for his unwavering trust in my

abilities to handle such a project and for introducing me to the world of twin studies.

During the extended early period of uncertainty, when I struggled with communicating the

unorthodox theoretical premise of this study, it was Prof. Townsend’s ability to look

beyond conventions and offer insightful comments that gave me the courage to continue

with the project. Therefore, in this instance it would not be overstating to claim that the

present thesis could not have been possible without his support and guidance. I would also

like to thank my co-supervisor Prof Wayne Sampson for his prompt and timely advice

which has kept this project on schedule and for helping me in the beginning with the first

contact with the Craniofacial Biology and Dental Education Group. I thank Dr Toby

Hughes for his patience in guiding me with the data analysis. Special thanks go with

Professor Alan Brook for his valuable comments that helped me build up my knowledge

and his wife Brenda Brook’s priceless kindness in receiving me in their home during my

trip to Liverpool, UK, for laboratory studies. I would like to thank all my supervisors and

express my deepest and enduring gratitude for taking on a student who knew nothing about

genetics and twin studies and generously providing me with time, assistance and resources

to learn. Thanks must go to all my supervisors for reading the drafts of this thesis and

patience with my spelling errors.

I also acknowledge Associate Professors John Kaidonis and Tracey Winning, Dr.

Susanna Mihailidis and Dr. Sarbin Ranjitkar for their weekly contribution to enrich the

Craniofacial Biology and Dental Education Group seminars sessions. Special thanks go to

Dr. Sarbin Ranjitkar for reading a draft of my thesis and providing comments.

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This project would not have been possible without the help of my many

collaborators. Special thanks go to Michelle Bockmann for always giving me support on

the most difficult steps over these years, and for the lovely cupcake recipes that burst my

sweet income helping me cope with the stress. Special thanks go to Karen Squires, Abbe

Harris, Corinna Bennett and Lucia Hatch for the immense support throughout this journey.

Thanks also go to Sandra Pinkerton, James Rogers and Graham Scriven, fellow researchers

in the Craniofacial Biology and Dental Education Group who always made my Tuesdays

in the M. J. Barrett Laboratory less heavy and lonely.

For Nor Atika Md Ashar, postgraduate student and highly regarded friend, thank

you for the countless times we escaped for a quick coffee at Aroma and for helping me

with any inopportune computer problems I had. For Ruba Odeh, postgraduate student,

thanks for making my life in the lab less lonely. We were together in this journey.

Special thanks go to Dr Richard Smith, James Hibbard and Tom Coxon from the

School of Dental Sciences, The University of Liverpool, UK, for their guidance throughout

my laboratory work using the 2D image analysis system in Liverpool. Special thanks go to

Tom Coxon for his help with the double determinations while in Liverpool and to James

Hibbard for his assistance with the 2D image analysis system and guidance in setting up a

similar image system in the M. J. Barrett Laboratory in the School of Dentistry, The

University of Adelaide. I also would like to thank Assistant Professor Raija Lahdesmaki

from the University of Oulu, Oulu, Finland, for her countless support while she was in

Adelaide and for guiding me during my visit to Oulu.

The project has also benefitted from the inputs of various other members of staff at

the School of Dentistry over the course of its development, and I am grateful for their

comments. The funding for pursuing this research was afforded by the International

Postgraduate Research Scholarship (IPRS) and I am thankful to the University of Adelaide

for giving me this opportunity to pursue my research in Australia. I am also thankful for

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the Travel Scholarships provided by the School of Dentistry, The Faculty of Health

Sciences and the Australian Twin Registry.

I would like to thank my husband Gilberto, who knew how to be an anchor during

the storm, keeping me firm and in a safe port; who knew how to be the mainsail during this

trip, transmitting me the necessary strength for this journey; who knew how to be the

ballast when I needed stability and knew how to be my wing when I needed to fly. For his

constant love and support throughout this journey.

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1. Introduction

A complete understanding of human dental development is yet to be reached, although

considerable research has been performed recently to elucidate the mechanisms of

odontogenesis and the factors that can lead to dental anomalies (Sharpe, 2001; Matalova et

al., 2008). Dental crown size has been studied extensively in human populations with the

aim of determining the extent of genetic contribution to observed variation, as well as the

influence that the environment exerts over specific dental phenotypes (Garn et al., 1965a;

Townsend, 1978; Townsend and Brown, 1978a; Brook, 1984). Teeth are useful for

morphometric studies in living and ancient populations as their external morphology

remains generally unchanged throughout life and they are also often the last parts of the

body to remain in fossil discoveries (Townsend, 1978; Townsend and Richards, 1990;

Townsend et al., 1994a; Townsend et al., 2003b). Moreover, it is possible to study

variability of tooth size in both the deciduous and permanent dentitions of individuals,

giving insights into genetic, epigenetic and environmental influences on developmental

events over time (Townsend et al., 2005).

By studying twins and their relatives, researchers are able to determine whether variation

of dental features is due mainly to genetic factors or to environmental influences

(Lundström, 1948; Horowitz et al., 1958; Osborne and De George, 1959; Garn et al.,

1965a; Potter and Nance, 1976). Recently, epigenetic factors, which refer broadly to

changes in phenotypic expression without changes in the DNA sequence of genes, have

been proposed to account for specific phenotypic variations seen in monozygotic twin pairs

(Townsend et al., 2005). Moreover, studies of twins have improved our understanding of

early human development, including hormonal effects prenatally (Dempsey et al., 1999b;

Heikkinen et al., 2005) and the development of body symmetry (Townsend and Richards,

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55

1990; Townsend et al., 1992; Lauweryns et al., 1993; Townsend et al., 1999). Studies of

dentofacial development in twins have proved to be an important means of clarifying the

causes of many human malformations and diseases (Boklage, 1987a; Townsend et al.,

1992; Lauweryns et al., 1993; Townsend et al., 1998).

The determination of body symmetry is believed to occur after amniogenesis and to finish

at around the eighth day after conception (Boklage, 1981). Furthermore, it has been

suggested that events responsible for brain asymmetries may also appear as alterations or

variations within the dentition (Boklage, 1981). Thus, the use of dental measurements

(e.g., mesiodistal and buccolingual crown dimensions) may provide a more concrete and

reliable way to explore human asymmetries compared with other subjective features such

as measurements of brain function and behaviour (Boklage, 1987a). However, there is a

lack of information regarding the nature and extent of asymmetry in the deciduous and

permanent dentitions of individuals due to a paucity of studies using serial dental casts.

There is also a lack of studies on dental asymmetry in twins. Twins are particularly

suitable for studying asymmetry as it is possible to make comparisons both within and

between monozygotic and dizygotic twin pairs, providing insights into genetic, epigenetic

and environmental influences. Furthermore, earlier methods of measuring dental casts

using hand-held calipers are now being replaced by 2D and 3D image analysis systems that

preserve the integrity of the casts as well as allowing measurement of more variables

(Brook et al., 1999; Brook et al., 2005).

This proposed study has implications for clinical dentistry, as well as the fields of human

biology, physical anthropology and forensic odontology. Studies of tooth size variation,

including associations between primary and permanent dentitions, are directly relevant to

understanding the causes of dental crowding and spacing. It is also generally accepted that

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common dental anomalies, such as extra and missing teeth, are linked to tooth size and

influenced by a common set of genes (Brook, 1984). In the fields of human biology and

anthropology, there is a need to understand more about symmetry and asymmetry and the

contributions of genetic, epigenetic and environmental factors to observed variation. The

findings of this study should also contribute to the field of forensic odontology, by

explaining the causes of morphological variation observed within the dentition and thereby

facilitating the process of identification.

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2. Literature review

2.1. The importance of studying the human dentition

Although much valuable work has already been undertaken, further investigation is

needed to understand the complex mechanisms and interactions occurring during dental

development and how these result in the clinical phenotype. Studies of variation in tooth

size and shape within and between related individuals provide a particularly valuable

means of assessing genetic, epigenetic and environmental influences on dental variation

(Garn et al., 1965a; Townsend, 1976; 1978; Townsend and Brown, 1978a; Brook, 1984;

Townsend et al., 2005).

The human dentition provides a useful model system to explore the role of genetic,

epigenetic and environmental factors over time because teeth start to form around 4 - 6

weeks after conception and continue to develop until around 21 years after birth

(Townsend et al., 1994a; Hillson, 1996; Townsend et al., 2009c). Once a tooth crown has

formed it does not change in shape or size, except due to post-eruption alterations such as

processes of wear, caries, dental treatment or cultural issues (Townsend, 1976; Townsend

et al., 1994a). Moreover, tooth size and shape can be studied in living populations and in

fossil collections, and also directly in the mouth or by using dental models, making it

possible to have records of both the primary and permanent dentitions of the same

individual (Hillson, 1996).

Moreover, the measurement of mesiodistal crown diameters (MD), which refer to

the distance between the mesial and distal contact points of the tooth crown, and

buccolingual (BL) or labiolingual diameters (LL), which refer to the breadth or distance

between the buccal/labial and lingual surfaces of the crown has been shown to be reliable

and accurate, when derived using calipers and dental models (Moorrees et al., 1957;

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Hunter and Priest, 1960). However, there is still a need to study dental crown morphology

more comprehensively by defining new dental phenotypes and by using different methods

of measurement.

2.2. Genetic, epigenetic and environmental influences on dental variation

It has been shown that variation in tooth number, size, and shape reflects a

multifactorial mode of inheritance, with complex interactions between genetic and

environmental factors during tooth formation (Bailit, 1975; Kabban et al., 2001). More

recently, researchers have suggested that epigenetic factors may be important during dental

development, perhaps explaining why phenotypic differences can be noted between

monozygotic co-twins (Townsend et al., 2005; Brook, 2009; Townsend et al., 2009b).

Although the term “epigenetic” can be defined in a narrow sense as “the addition or

removal of methyl groups to DNA or the attachment of acetyl groups of histones”, a

broader definition will be used in this thesis, namely “an alteration of gene expression

without changing the DNA sequence” (Townsend and Brook, 2008; Barros and

Offenbacher, 2009; Brook, 2009; Townsend et al., 2009b; Bell and Spector, 2011).

Tooth dimensions, tooth position, the sequence of tooth eruption, and palatal

dimensions have been examined to elucidate the influence of genetic, epigenetic and

environmental factors (Potter and Nance, 1976; Townsend et al., 1994b; Townsend et al.,

1995; Brook et al., 2009a). Studies investigating tooth anomalies indicate that there is an

association between variations of tooth number and size, as well as between the sexes

(Brook, 1984; Brook et al., 2002; Brook, 2009). Hypodontia, which is the congenital

absence of one or more teeth, is associated with microdontia (smaller teeth) and is more

common in females, while hyperdontia or supernumerary teeth, which is the presence of

one or more extra teeth apart from the normal dentition, is linked to megadontia (larger

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teeth) and is more common in males (Brook, 1984; Brook et al., 2009a). Moreover,

familial studies correlating tooth size with the remaining dentition have indicated that an

association between anomalies of tooth number and size exists, as reduced tooth

dimensions with variable degrees of severity were found in the relatives of patients with

hypodontia (McKeown et al., 2002; Brook et al., 2009a; Parkin et al., 2009). Furthermore,

teeth adjacent to hypodontia/hyperdontia sites presented more abnormal development and

morphology compared to patients with normal dentition, suggesting a combination of

genetic, epigenetic and environmental factors are likely to be important in determining

tooth number, size and morphology (Brook et al., 2002; Brook et al., 2009a).

Studies in South Australian twins have shown that the prevalence of agenesis of

upper lateral incisors does not differ between twins and singletons but that there is a higher

prevalence in females than in males with a ratio of 2:1, corroborating Brook’s findings

(Townsend et al., 1995; Brook et al., 2002; Brook et al., 2009a). Brook (1984) proposed a

multifactorial model incorporating a polygenic mode of inheritance with environmental

influences to explain the aetiology of tooth anomalies such as hypodontia, microdontia,

megadontia and hyperdontia. He proposed an underlying normal curve of tooth size for

each gender with superimposed thresholds. Individuals located outside the normal range of

variation will be more likely to display anomalies of tooth size, number and position, all of

which seem to be linked. This model emphasises the fact that we can learn much about the

aetiology of dental anomalies by studying so-called normal variation (Brook, 1984).

The relative contributions of genetic, epigenetic and environmental influences to

variation in human dental features differ depending on the phenotypic feature under

investigation. For example, tooth emergence has been studied in the primary dentition of

South Australian twins and it has been shown that variation in eruption timing is controlled

mainly by genes but there is also some influence from the environment (Hughes et al.,

2007; Woodroffe et al., 2010). Interdental spacing has also been studied in South

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Australian twins and it has been shown that monozygotic (MZ) twin pairs are more

concordant for interdental spacing than dizygotic (DZ) twin pairs, suggesting that genetic

factors play a role in the determination of interdental spacing in the maxilla but

environmental factors may be more important in the mandible (Thomas and Townsend,

1999). Studies of occlusal features in MZ and DZ twins have confirmed that

environmental factors exert an important role in the determination of some features such as

overbite, overjet and crossbite (Harris and Smith, 1980; Townsend et al., 1988). The

morphology of the dental arches has been assessed to estimate the contribution of genetic

and environmental influences in twins and it has been suggested that asymmetry in dental

arch shape is influenced mainly by the environment (Richards et al., 1990). Palatal

features have also been studied in twins and it has been verified that palatal width and

height are strongly influenced by genetic factors (Townsend et al., 1990).

2.3. The value of twins in dental research

Twin studies came to the fore when Galton (1875) suggested that it should be

possible to assess the interactions between “nature” and “nurture” of many body features

by comparing identical or monozygotic (MZ) twin pairs, who share the same genes, with

non-identical or dizygotic (DZ) twin pairs, who share half of their genes on average

(Galton, 1875; Townsend et al., 2009b; Townsend et al., 2009c). Since then, many studies

using twins and their families have been carried out to improve our understanding of

genetic, epigenetic and environment influences on dental and facial phenotypic variation

(Lundström, 1948; Hatton, 1955; Horowitz et al., 1958; Osborne and De George, 1959;

Lundström, 1963; Garn et al., 1965a; Townsend, 1978; Townsend and Brook, 2008).

Heritability estimates have also been calculated in different populations and in twins to

quantify the genetic contribution to variation in many dental traits (Biggerstaff, 1975;

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1976; Townsend and Brown, 1978b; Harris and Smith, 1980; Townsend, 1980; Townsend

et al., 1998) and genetic modelling has been applied to calculate the genetic contributions

of additive (A) and non-additive (D) genetic variance along with the effects of the common

environment (C) and factors exclusive to each individual (E) (Dempsey et al., 1995;

Dempsey and Townsend, 2001; Hughes et al., 2001; Hughes et al., 2007).

A link between specific genes and many dental anomalies and disorders has now

been established (Thesleff, 2006; Matalova et al., 2008), but common dental anomalies

such as variations in tooth number, tooth size and shape seem to be associated with each

other, suggesting a multi-factorial aetiology. A good example is the discrepancy found in

tooth size and number between MZ co-twins, suggesting that groups of genes are

responsible for the establishment of certain dental features and that there may be a

combined pleiotropic effect linked with time of development and spatial variation

(Townsend et al., 1995; Townsend et al., 2003b; 2005; Townsend and Brook, 2008;

Townsend et al., 2009a; Townsend et al., 2009b).

Studying twins and their families allows us to assess the genetic, epigenetic and

environmental basis of many common dental disorders and diseases, such as risk of caries,

periodontal disease and malocclusion (Townsend et al., 1998; Townsend et al., 2003b).

However, to better understand the applications of twin research it is necessary to further

our knowledge of the twinning process, the importance of zygosity determination, the

different types of twin models that can be used for scientific studies, and the special

features of the twinning process.

2.3.1. The twinning process

Twin pregnancies involve the development of two or more individuals who share

the same intrauterine space and resources (Townsend et al., 2009b). There are some

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developmental events that are specific for each type of twin and this deserves some

comment. Monozygotic (MZ) or “identical twins” are formed when the zygote cleaves

soon after conception producing two individuals who have the same sex and who generally

share the same genes (Townsend and Richards, 1990). However, the moment at which this

cleavage occurs seems to be of great importance in the determination of variability in MZ

twins, as it is associated with the number of placentas, chorions and amnions (Townsend et

al., 1992; Townsend et al., 1999; Townsend et al., 2009c; Weber and Sebire, 2010). If

cleavage has occurred between the first and third day post-fertilization and prior to

implantation, MZ twins will present with separate placentas, chorions and amnions and

they are referred to as dichorionic - this situation occurs in 20-30% of MZ twin pairs. If

cleavage has occurred between the fourth and ninth day after conception and after

implantation inside the uterus, MZ twins will present a single placenta and chorion but

double amnion. Most MZ twin pairs (around 60%) belong to this monochorionic

placentation category. If cleavage has occurred around the ninth or tenth day after

conception and after implantation, the MZ twin pair will present a single placenta, chorion

and amnion - around 3% of twins fall in this category. Later cleavage can lead to

conjoined or Siamese twins (Boklage, 1980; 1981; Townsend and Richards, 1990;

Townsend et al., 1992; Race et al., 2006; Weber and Sebire, 2010).

The term “identical twins” tends to be misused as both genetic and phenotypic

differences can exist between MZ co-twins. During the process of meiosis, mis-

segregation of genetic and/or cytoplasmatic material can occur probably due to post-

mitotic crossing over, or non-disjunction can cause chromosomal aneuploidy (Bohec et al.,

2010), imprinting (Prins, 2008), inactivation or altered expression of genes, X-inactivation,

altered cytoplasmatic segregation, and differences in telomere size (Machin, 2009; Weber

and Sebire, 2010).

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Fraternal or dizygotic (DZ) twins are formed from two different zygotes each one

having their own placenta, chorion and amnion. Because of this, they can be of the same

sex or opposite sex, and they only share half of their genes on average (Townsend and

Richards, 1990). Dizygotic twins have been genetically considered as full siblings who

share the same intra-uterine environment and studying these twins may help to clarify the

influence of pre-natal environmental factors on the determination of dental features

(Lauweryns et al., 1993; Townsend et al., 2003b).

2.3.2. Special features of the twinning process

Apart from the special meiotic and mitotic events that occur during the twinning

process, the peri-natal developmental events of twins, including birth itself, are also special

as they represent important environmental factors affecting phenotype expression.

Twinning is associated with higher mortality and morbidity rates and premature births

compared with single births (Boklage, 1987a; Townsend et al., 2009c). Many twin

pregnancies may not progress above 16 weeks of gestation due to the “vanishing” of one or

both fetuses in utero in the first or second trimester of pregnancy (Townsend et al., 1992;

Townsend et al., 2009c). In fact, there is no evidence of how many single births actually

began as twin pregnancies, as in a high proportion of twin pregnancies it is possible that

one of the co-twins dies in utero resulting in a single birth (Boklage, 1980; Townsend et

al., 1992). The twinning process is also associated with a high rate of congenital

anomalies, such as congenital heart defects and neural tube defects, which occur due to

failure in the closure of bilateral structures during development (Townsend et al., 1992;

Machin, 2009; Weber and Sebire, 2010).

Apart from increased intrauterine mortality and morbidity rates and congenital

anomalies in twins, chorion type seems to be an important environmental factor that can

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influence MZ twins during their development in utero. Monochorionic MZ twin pairs do

not equally share the intrauterine space and resources, and around 30% develop vascular

anastomoses between arteries and veins causing an imbalance in the blood flow between

fetuses (Townsend et al., 2009c; Weber and Sebire, 2010). This can cause differences in

physical development in both fetuses, such as hypovolaemia and hypoxia in the donor fetus

and polycythaemia, polyuria and polyhydramnios in the recipient fetus, leading to

discordant weight at birth. This twin-twin transfusion syndrome (TTTS) is a serious

condition that can cause death of one or both twins (Burris and Harris, 2002; Race et al.,

2006; Machin, 2009; Townsend et al., 2009c; Weber and Sebire, 2010).

Chorion type seems to be an important intrauterine environmental stressor that

could contribute to differences between MZ twins in many different phenotypes (Derom et

al., 1996; Prescott et al., 1999; Loos et al., 2001a; Machado et al., 2010). Studies using

chorion type and dental dimensions have shown that chorionicity can affect permanent

tooth crown diameters during odontogenesis (Burris and Harris, 2002; Race et al., 2006).

They also suggest that heritability estimates of tooth size in previous MZ twin studies

might be underestimated by not accounting for differences in chorion type (Burris and

Harris, 2002). An Australian study, using MZ twin pairs, also found greater intrapair

variance for permanent tooth crown dimension in monochorionic MZ twins compared with

dichorionic MZ twins, suggesting that twins who share the same chorion are under more

environmental stress than twins who develop inside separate chorions (Race et al., 2006).

Furthermore, female twins from both zygosities and with low birthweight display

significantly reduced tooth size for both the deciduous and permanent dentitions (Apps et

al., 2004). Although there have been several studies of dental features in twins, there are

relatively few studies in which dental asymmetry has been quantified, particularly in both

the deciduous and permanent dentitions of the same group of individuals.

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2.3.3. Importance of zygosity determination

The accurate determination of zygosity in twins is important as twin studies are

often based on comparisons within and between MZ and DZ pairs, as well as with other

siblings and relatives. Correct determination of zygosity is of great importance as it helps

to establish the validity and reliability of the twin model proposed – any errors in

determination can influence the results of twin studies (Townsend and Richards, 1990;

Townsend et al., 2009c). Moreover, accurate zygosity determination will help to provide

appropriate genetic counselling to families, to offer correct preventive treatment outcomes

in the case of diseases and to provide the best matches in organ transplantation (Machin,

2009).

Early studies used facial resemblance, such as colour of the eyes and hair, ear form

and taste sensitivity, to establish the zygosity of twins. However, these methods were not

reliable enough to determine zygosity and errors could influence twin analyses. The

analysis of blood groups such as ABO, MN, Rh, Kell, Duffy and also secretor status has

also been used to assess zygosity in twins and has been shown to be reliable when

associated with other features such as physical appearance, eye colour and finger prints

(Sutton et al., 1962; Lundström, 1963; Lauweryns et al., 1993). Tooth morphology

assessed on dental casts of like-sexed twin pairs has been used to determine the zygosity of

twins but the author stressed that the examiner needs to have a very good knowledge of

dental morphology (Lundström, 1963). Placental morphology, fetal membranes and also

serum proteins and enzymes have also been used in establishing the zygosity of twins

(Derom et al., 1985). However, the analysis of highly polymorphic DNA markers, such as

D3S1358, vWA, FGA, AMEL, D81179, D21S11, D18S51, D5S818, D13S317, D7S820,

extracted from the cells of the cheek has been shown to be a very effective, efficient and

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precise way to determine the zygosity of twin pairs (Derom et al., 1985; Townsend and

Richards, 1990; Townsend et al., 2003b; Hughes et al., 2007).

2.3.4. Different twin models

There are many different types of twin models that can be used to assess genetic,

epigenetic and environmental influences on specific phenotypic variations. The classical

twin model used by many researchers involves searching for similarities between identical

or MZ twin pairs and making comparisons with fraternal or DZ twin pairs. By using this

model it is possible to calculate heritability estimates (Townsend et al., 2003a). The MZ

co-twin model allows researchers to study the influence of non-genetic factors on specific

phenotypes that differ in expression between MZ co-twins, as MZ twin pairs are matched

by sex, age and their genes (Townsend et al., 2003b). Epigenetic influences are believed to

be a possible explanation for some of these types of phenotypic variation (Townsend et al.,

2005). For example, discordances between MZ co-twins for missing or supernumerary

teeth were recorded in a large sample of MZ twins, reflecting an effect of environment

and/or possibly due to epigenetic influences during odontogenesis (Townsend et al., 2005).

The MZ co-twin reared-apart model attempts to overcome the confounding effects

of common family environment by studying MZ twins who were separated soon after birth

and then raised in different family environments. This model allows researchers to

overcome the influence of common environment, highlighting that any similarities

between the co-twins must be due to shared genes (Townsend et al., 2003b; Townsend et

al., 2009b; Townsend et al., 2009c). Incisor tooth crown size, occlusion, and caries

predisposition have been assessed using this MZ co-twin reared apart model, indicating

that there is strong genetic control for these dental traits (Boraas et al., 1988).

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The twin half-sib model is another model that involves comparisons between the

offspring of MZ twin pairs with unrelated partners. The basis for this twin model is that the

offspring of MZ twin pairs, born to different mothers or fathers, are genetically half-

siblings even though they are socially first cousins. This twin model can help to unravel

the familial contributions to disease by addressing the influence of maternal effects and

assortative mating (Potter, 1990).

The opposite-sex DZ twin pair model makes comparisons between opposite-sexed

DZ twin pairs and other zygosity groups. By using this particular twin model it is possible

to assess the influence of the pre-natal environment over phenotypic variation. For

example, the larger buccolingual dental crown size of females with co-twin brothers

compared to females from same-sex DZ or MZ pairs has been suggested to be due to

diffusion of male sex hormones in utero (Dempsey et al., 1999b; Townsend et al., 2003b).

Other body features such as finger-length ratios (van Anders et al., 2006), length of

gestation (Loos et al., 2001b; Goldman et al., 2003), behaviour and attitudes (Miller and

Martin, 1995) and birthweight (Goldman et al., 2003) have also been studied in opposite-

sexed DZ twin pairs, providing some additional support for an effect of male sex hormone

diffusion on female co-twins in utero.

2.4. Studies of tooth size variation

It is well established that tooth size dimensions vary between populations. Studies

based on a worldwide distribution of populations have reported that differences in tooth

size dimensions between populations may reflect improvements in food cultivation and

preparation over time (Bailit, 1975; Hillson, 1996; Hanihara and Ishida, 2005). However,

a study comparing tooth crown size between Southern Chinese, North Americans, British

populations and skeletal remains of a Romano-British population showed that differences

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exist between them not only due to genetic factors but possibly also due to environmental

factors such as diseases, poor nourishment and poisoning that could account for the smaller

tooth size in the ancient Romano-British (Brook et al., 2009b). Complete understanding of

the evolutionary basis of tooth size variation between populations across the centuries is

still a challenge to many anthropologists and geneticists.

2.4.1. Sexual dimorphism in tooth size

By studying tooth crown size it is possible to quantify the amount of sexual

dimorphism within and between populations (Moorrees et al., 1957; Garn et al., 1965c;

Kieser, 1990). Overall, males have larger tooth dimensions than females and this is

evident for both mesiodistal (Garn et al., 1965c; Garn et al., 1967b; Harris and Lease,

2005) and buccolingual (Garn et al., 1966b) diameters and for both deciduous (Black,

1978; Harris and Lease, 2005; Adler and Donlon, 2010) and permanent (Garn et al., 1966b;

Schwartz and Dean, 2005) dentitions in humans. The pattern of sexual dimorphism seems

to vary between ethnic groups. Sexual dimorphism has been reported for deciduous tooth

size among siblings and twins of two ethnic groups from the United States and significant

differences between males and females have been reported in Caucasians and Afro-

American children, with Afro-Americans being more dimorphic than Caucasians

(Heikkinen et al., 2005). The amount of sexual dimorphism also varies between groups of

teeth, with canines showing most dimorphism and premolars the least (Kieser, 1990;

Hillson, 1996). Deciduous and permanent molar crown components of a sample of

Australian Aborigines have also been assessed to quantify the amount of sexual

dimorphism and it has been shown that females tend to have smaller dimensions for crown

components than males (Kondo and Townsend, 2004). These authors also stated that

mesiodistal and buccolingual diameters, as well as intercuspal distances, seem to be

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affected depending on the order of cusp calcification and the timing of tooth formation

(Kondo and Townsend, 2004; Takahashi et al., 2007).

2.4.1.1. Morphogenetic field theory

By studying the evolutionary basis of dental development, Butler (1939) observed

in the mammalian dentition that teeth tended to be organized in an anterior-posterior axis

into three morphogenetic fields (incisor, canine and molar) where each field had a more

stable polar tooth and other teeth followed in a graded pattern of variability around the

polar tooth (Butler, 1939; Lombardi, 1975; Hillson, 1996; Townsend et al., 2009a).

Dahlberg (1945), building on Butler’s theory, proposed a revised morphogenetic field

model for the human dentition by including the premolars as a new morphogenetic field

(Dahlberg, 1945). He suggested that the most stable tooth in shape, morphology and

timing of emergence would be the most mesial tooth of each group of teeth, with the

exception of the mandibular incisors, where the most stable tooth would be the lateral

incisor, and the most variable tooth would be towards of the extremities of each field

(Dahlberg, 1945; Lombardi, 1975). Based on this concept, the most posterior teeth of each

class that develop later and over a longer period of time seem to present more phenotypic

variation than the more mesial teeth, with exception of the lower central incisors

(Townsend and Brown, 1981; Townsend et al., 2009a).

2.4.1.2. Associations between teeth and dentitions

Tooth size variation can also be studied by exploring associations between the

primary and permanent dentitions. Mesiodistal and buccolingual diameters have been used

to assess correlations between deciduous and permanent dentitions (Moorrees et al., 1957;

Townsend and Brown, 1978b; Brown et al., 1980; Kieser, 1990). Correlations in tooth size

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between deciduous and permanent dentitions have been calculated in a sample of North

American children and it has been shown that there are lower correlations between

dentitions than within dentitions, suggesting that each dentition is affected differently by

genetic and environmental factors (Arya et al., 1974). More recent studies have shown that

both primary and permanent dentitions present the same genetic interactions but they can

be affected differently by environmental factors (Thesleff, 2006). A study using an

Australian Aboriginal population also found a moderate correlation between dentitions

when considering single teeth, but a higher correlation was found when considering groups

of teeth (Townsend and Brown, 1980). Common environmental effects also appeared to

influence buccolingual diameters more than mesiodistal diameters (Townsend and Brown,

1980).

2.4.1.3. Sex chromosomes

Sexual dimorphism in the dentition seems to be linked with the sex chromosomes

as the growth of tooth crowns is affected differently by the sex chromosomes during dental

development (Garn et al., 1965b; Alvesalo, 2009). Alvesalo and his colleagues have

studied tooth crown size extensively by measuring dental casts from individuals with

various chromosomes abnormalities, such as males with an extra Y-chromosome

(47,XYY) and males with Klinefelter syndrome (47,XXY) and found that, overall, these

males present larger tooth crown dimensions than unaffected males (Townsend and

Alvesalo, 1985a; b; Alvesalo, 1997; Townsend and Alvesalo, 1999). They have also

studied the influence of the X-chromosome by comparing tooth crown dimensions of

primary and permanent dentitions from individuals with Turner syndrome (45,X) and

females with the 45,X/46,XX chromosome mosaic syndrome and found smaller tooth

crown dimensions compared with normal females (Varrela et al., 1988; Alvesalo, 1997;

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2009). Comparisons of the thickness of dentine and enamel measured on radiographs in

normal males and females with males and females with chromosomal abnormalities have

also been used to evaluate the influence of sex chromosomes and it has been shown that

both Y- and X-chromosomes affect these dental tissues differently. The Y-chromosome

seems to affect tooth growth by promoting the formation of enamel and dentine, possibly

through increased mitotic activity at various stages of development during tooth formation,

while the X–chromosome seems to influence mainly crown enamel deposition (Alvesalo et

al., 1987; Alvesalo et al., 1991; Alvesalo, 2009).

More recently, researchers reported that female twins with low birthweight have

smaller tooth size in both permanent and deciduous dentitions compared with twins with

normal birthweight, suggesting that females are more susceptible to environment

disturbances than males (Apps et al., 2004). Further information is required on the effects

of gender, the X- and Y-chromosomes and intrauterine environmental disturbances that

could lead to differences in tooth crown sizes in both primary and permanent dentitions.

2.4.2. Hormonal influences on tooth size

Many researchers have proposed that sex hormones might be an important factor in

explaining the variations observed between males and females in various features (Cohen-

Bendahan et al., 2005a; Hines, 2006; Knickmeyer and Baron-Cohen, 2006; Vuoksimaa et

al., 2010b; Tapp et al., 2011), even though some studies have indicated that the

development of sexual dimorphism in the human dentition occurs mainly due to the effects

of the sex chromosomes (Guatelli-Steinberg et al., 2008; Alvesalo, 2009). Knowledge of

the effects of sex hormones on many body traits remains incomplete. Several studies have

been carried out recently to gain further understanding of the effects of male hormones on

body traits since there is some evidence from animal studies (Ryan and Vandenbergh,

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2002) and in humans (Resnick et al., 1993; Miller, 1994) that females of opposite-sex DZ

pairs may be influenced by male hormones in utero.

Studies in animals have demonstrated that male hormones can diffuse across the

amniotic membranes between fetuses. Another possible mechanism of male hormone

diffusion in utero is the intrauterine position of litters (Ryan and Vandenbergh, 2002).

Female rats which are positioned between males in utero showed more masculine

characteristics, such as longer anogenital distance, increase aggressiveness and increased

weight, than females positioned between females in utero (Ryan and Vandenbergh, 2002).

Furthermore, researchers have also raised questions about whether hormone diffusion in

utero could influence tooth crown size differently in males and females, as they observed

that androgenised female Rhesus monkeys showed increased values for tooth crown

dimensions compared to normal female Rhesus monkeys (Zingeser and Phoenix, 1978).

In humans, studies have demonstrated that intrauterine maternal levels of male

hormone vary with the sex of the fetus carried, suggesting that male hormone can pass

through the placenta from the male fetus into the maternal circulation (Meulenberg and

Hofman, 1991; Miller and Martin, 1995). It is possible that a similar mechanism can occur

between twin fetuses of opposite sex pairs (Miller and Martin, 1995). However, some

studies questioned the viability of this maternal-fetal route since they found no difference

in hormone concentration in women pregnant with male or female babies (Hines et al.,

2002; van de Beek et al., 2004). Studies have demonstrated that masculinisation of some

physiological, behavioural, and morphological traits can occur in females from opposite-

sex twins compared with females from same-sex twins or female singletons. Females with

a co-twin brother seem to present reduced spontaneous otoacoustic emissions (McFadden,

1993), enhanced mental rotation performance (Vuoksimaa et al., 2010b; Heil et al., 2011),

decreased visual acuity (Miller, 1995), reduced fecundity rates (Lummaa et al., 2007),

reduced length of gestation (Loos et al., 2001b), and increased growth in utero (Glinianaia

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et al., 1998). Behaviour and attitudes have also been studied in opposite-sex twins and it

has been shown that females with a co-twin brother seem to be more aggressive, have

higher scores of thrill and adventure seeking, as well as disinhibition and experience

seeking than females from same sex twins and female singletons (Resnick et al., 1993;

Miller and Martin, 1995; Cohen-Bendahan et al., 2005a). Females who have a male co-

twin also display lower finger-length ratios (Putz et al., 2004; van Anders et al., 2006),

increased total brain volume (Peper et al., 2009), altered craniofacial growth and dental

asymmetries (Boklage, 1985), and increased tooth crown size (Dempsey et al., 1999b).

Dempsey and colleagues found larger permanent tooth crown size in females with co-twin

brothers in comparison to females from same-sex DZ or MZ pairs, suggesting that

diffusion of male sex hormones occurred in utero and could influence tooth development

(Dempsey et al., 1999b). Furthermore, a study using female-to-male transsexuals (FtM)

and control males and females found increased mesiodistal and buccolingual tooth crown

dimensions in the permanent dentition of FtM transsexuals compared with control females,

with values tending or shifted towards control males. This study placed FtM transsexuals

in an intermediary position between males and females, suggesting that another

genetic/environmental mechanism other than behaviour is involved in the determination of

transsexualism in humans (Antoszewski et al., 2009).

In contrast to effects of male hormones in utero, feminization effects of some

behavioural traits, such as eating disorders affecting males with a co-twin sister, have also

been described in the literature (Procopio and Marriott, 2007; Culbert et al., 2008; Baker et

al., 2009), suggesting that both masculinization and feminization pathways should be

considered when analysing the effects of sex hormones in opposite-sex dizygotic twins.

Studies on levels of steroid hormones in normal males have shown that there is a

surge in testosterone levels soon after testicular differentiation in males which occurs

around 7-9 weeks of gestation. The levels of testosterone are higher between week 10 and

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week 20 of gestation, peaking around week 14, and the values are comparable to the levels

of normal adult males (Reyes et al., 1974; Knickmeyer and Baron-Cohen, 2006; Tapp et

al., 2011). Another surge of testosterone hormone occurs soon after birth due to the

inhibition of estrogen levels produced by the placenta and the levels remain high during the

first year of life peaking between the 3rd

and 4th

months after birth. These levels are

similar to those in normal adult males (Reyes et al., 1974; Knickmeyer and Baron-Cohen,

2006). A third testosterone surge occurs in males during the adolescent growth spurt. This

pubertal surge is necessary for sexual maturation and fertility in males. It is during this

period that male secondary sexual characteristics develop, such as growth of the testes and

penis, as well as development of axillary and pubic hair (Larsen et al., 2003).

Knowledge of the first two surges is important as teeth from primary and

permanent dentitions start to form before or during this period of time in utero and

continue development until early childhood. Male hormones could be an important

environmental factor influencing tooth crown formation. However, further information is

required on the extent and nature of sexual dimorphism in tooth crown size, including the

influence of sex chromosomes and the influence of sex hormones in utero on dental

development.

2.4.3. Dental asymmetry

Bilateral structures, which supposedly should develop as mirror images, are rarely

symmetrical even though they are often assumed to have the same genetic input (Potter et

al., 1976). Early studies suggested that differences in bilateral structures or asymmetries in

living organisms are a sign of genetic or environmental stress during their formation (Bailit

et al., 1970; Potter and Nance, 1976; Kieser, 1990). These differences between bilateral

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structures are described as reflecting an inability of the organism to “buffer” against

developmental accidents or noise during their formation (Van Valen, 1962).

Asymmetries refer to discrepancies between right and left sides of antimeric traits

and they can be divided into directional asymmetry (DA), fluctuating asymmetry (FA) and

antisymmetry. Hillson (1996) defined DA as ”the tendency for one side to be consistently

larger than the other”, whereas FA refers to small random variations in phenotype

expression between sides believed to occur as a result of developmental instability and/or

failure of individual to buffer against developmental disturbances (Van Valen, 1962;

Townsend and Brown, 1980; Hillson, 1996; Woodroffe et al., 2010). Antisymmetry refers

to a less common condition where asymmetry is normally present but with a variable

predominance between right and left sides. Handedness is a good example of

antisymmetry in humans where right- and left-handed individuals are standard in the

population while ambidextrous individuals are less common (Van Valen, 1962).

Bilateral asymmetries have a genetic component and seem to increase with

inbreeding, genetic disorders and syndromes, as well as in unfavourable environmental

situations, such as prenatal/maternal conditions, socio-economic status, malnourishment,

limited physical habitat, extreme weather conditions, and diseases (Bailit et al., 1970;

Townsend and Brown, 1980; Townsend, 1983; Kieser et al., 1997).

The dentition is particularly suitable for studying asymmetries, as tooth crown size

is determined before eruption into the oral cavity. Moreover, right and left sides of the

dental arches are assumed to be formed at the same time and under the same genetic

influences (Perzigian, 1977). Studies in rats have emphasized the importance of prenatal

and neonatal environment in the development of asymmetries as well as the influence of

some stressors such as cold, noise and lack of food on the determination of dental

fluctuating asymmetry in rats (Bailit and Sung, 1968; Sciulli et al., 1979). Many

populations have been assessed for dental asymmetries and researchers have shown that

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the more demanding the social, economical and health conditions operating on a

population are, the higher the levels of asymmetry (Bailit et al., 1970; Perzigian, 1977;

Townsend and Brown, 1980).

Some authors have found evidence of DA in tooth crown size, dental occlusion and

tooth emergence (Sharma et al., 1986; Townsend et al., 1999; Corruccini et al., 2005;

Harris and Bodford, 2007; Harris and Smith, 2009; Mihailidis et al., 2009) possibly due to

right hemisphere dominance of the brain over the left hemisphere, but FA is common and

thought to be caused by environmental disturbances (Boklage, 1987b; Townsend et al.,

1992; Townsend et al., 1994a; Townsend et al., 1999). Studies using monozygotic and

dizygotic twins have shown that asymmetry has little or no genetic origin but FA was

evident in both monozygotic and dizygotic twins (Potter and Nance, 1976).

Studies have showed that FA is non-directional and randomly distributed across

right and left sides and it affects both MD and BL dimensions (Garn et al., 1966a).

Moreover, increased FA values were found in individuals with hypodontia and in

individuals with larger teeth (Garn et al., 1966a). Males have been reported to display

increased FA compared with females, suggesting that paired X-chromosomes might be an

important buffering factor against environmental disturbances in the dentition (Garn et al.,

1965b), but other studies have reported more FA in females than in males (Harris and

Nweeia, 1980; Guatelli-Steinberg et al., 2006). Townsend et al. (1999b) have also found

that, in the deciduous dentition, those teeth which remain longer in the pre-calcified stage

show greater fluctuating asymmetry in crown dimensions than those that develop more

rapidly.

FA in the human dentition seems also to respect the morphogenetic field pattern,

with the later forming tooth in each class of teeth presenting greater FA than the key tooth

or more stable tooth (Garn et al., 1966a; Townsend and Brown, 1981). However,

researchers found less FA in primary second molars in comparison to other primary teeth,

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suggesting that the primary second molars might be the more stable molar in the primary

dentition (Guatelli-Steinberg et al., 2006).

A critical point in the study of FA is the sample size and measurement errors.

Because FA reflects small, random, non-directional differences between antimeres, studies

with small sample sizes are unlikely to detect the true extent of fluctuating asymmetry in

relation to the measurement error. Many different mathematical and statistical formulae

have been developed to quantify the amount of FA in many different traits (Palmer and

Strobeck, 1986; Palmer 1994). However, the lack of understanding of the developmental

stability mechanisms, allied to the different statistical approaches used to compare FA

results between groups and populations, makes the study of FA challenging.

In summary, there is a lack of information on the nature and extent of dental

asymmetries and the patterns of tooth-size correlations in the primary and permanent

dentitions of the same individuals. Furthermore, the effects of possible prenatal hormone

diffusion on dental asymmetries in opposite-sex dizygotic twins are still to be established.

2.5. Odontogenetic processes

Many molecular studies have been carried out in the past decade or so to improve

our understanding of odontogenetic processes (Sharpe, 2001; Matalova et al., 2008; Lesot

and Brook, 2009; Townsend et al., 2009a; Ishida et al., 2011). During odontogenesis, the

developing tooth germ passes through different stages of formation, such as the thickening

and specialization of dental lamina, bud, cap and bell stages (Nanci, 2003). In each of

these stages, mutual interactions between the epithelium and mesenchyme take place

leading to the processes of morphogenesis, epithelial histogenesis, and cell differentiation

(Lesot and Brook, 2009; Ishida et al., 2011). Cells derived from the ectoderm and

ectomesenchyme are engaged in these processes and tooth shape is determined by a

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combination of signalling molecules (Shh, Fgf8, Bmp4 and Wnt) with both positive and

negative coding effects (Sharpe, 2001). Complex processes involving signalling

molecules, transcription factors and genes are responsible for controlling tooth number,

shape and position. For example, failure in expression of signalling molecules, such as

Wnt/ β-catenin, can affect tooth formation at the early bud stage while its increase in

function can lead to abnormal tooth shape (Liu et al., 2008).

Signalling centres, called primary enamel knots, appear in the cap stage of tooth

formation as a result of interactions between signalling molecules and transcription factors

in the mesenchyme (Thesleff et al., 2001; Lesot and Brook, 2009). These enamel knots

appear at the site of the future cusp tips of the teeth and their positions seem to be

influenced by epithelial signalling molecules, as these molecules regulate the spatial

expression of homeobox genes in the ectomesenchyme (Thesleff et al., 2001;

Fleischmannova et al., 2008; Lesot and Brook, 2009). The enamel knots are also

responsible for surrounding cell proliferation and induction of dental papilla formation, and

they seem to stimulate the terminal differentiation of odontoblasts which then start the

deposition of dentine matrix (Thesleff et al., 2001; Fleischmannova et al., 2008; Brook,

2009). In multicusped teeth, the death of primary enamel knots seems to occur by

apoptosis and secondary enamel knots are then formed at the sites of future cusp tips

(Thesleff et al., 2001; Fleischmannova et al., 2008; Brook, 2009; Lesot and Brook, 2009).

Apart from the complex odontogenetic processes which involve molecules, cells,

and tissue interactions, time and space also contribute to variation in the clinical

phenotype. Disturbances in the temporo-spatial coordination of odontogenesis may lead to

dental abnormalities of number, size, form and structure (Brook, 2009). Simple

hypodontia, oligodontia which refers to the absence of more than six teeth, and anodontia

which is the complete absence of all teeth, seem to be associated with mutations in several

genes, such as MSX1, PAX9, AXIN2and EDA (Fleischmannova et al., 2008; Matalova et

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al., 2008; Brook, 2009). Hypodontia patients display smaller tooth size (microdontia) and

altered tooth morphology (peg-shaped crowns) (Parkin et al., 2009), and it seems that the

pattern follows the morphogenetic fields, with the latest-formed tooth in each field being

more affected (Brook, 2009; Brook et al., 2009a). Hyperdontia is also associated with

gene mutations and is linked with increased tooth size (megadontia) (Khalaf et al., 2005b;

Brook et al., 2009a) and altered tooth shape (Brook, 2009; Brook et al., 2009a). Other

genes, such as p63 and Pitx2, associated with syndromes or craniofacial defects can also be

associated with hypodontia and hyperdontia (Thesleff, 2006; Fleischmannova et al., 2008;

Matalova et al., 2008). Later disturbances in tooth development will produce teeth with

abnormal dentine and enamel structure, such as amelogenesis imperfecta and

dentinogenesis imperfecta respectively (Fleischmannova et al., 2008; Brook, 2009).

Studies of mesiodistal crown diameters in different human populations have

showed that the pattern of variability across the dentition follows the same morphogenetic

fields proposed by Butler (1939). This holds even though the overall crown sizes may vary

according to the population studied (Brook et al., 2009b).

Another theory to explain patterning in the dentition is the clone theory (Osborn,

1978). This theory suggests that the development of teeth in each class is determined by a

clone of mesenchymal cells which induce the dental lamina to initiate tooth development

(Hillson, 1996; Townsend et al., 2009a). However, the clone theory does not explain the

development of the whole dentition.

With increasing understanding of the molecular basis of odontogenesis, a

homeobox code model has been proposed (Sharpe, 1995; Townsend and Brook, 2008;

Townsend et al., 2009a). The homeobox code model is based on expression of different

homeobox genes in the ectomesenchyme cells during odontogenesis. By overlapping or

mixing these homeobox genes, it is suggested that the different tooth types are established

(Nanci, 2003; Townsend et al., 2009a).

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Studies of crown components, such as mesiodistal, buccolingual (or labiolingual),

intercuspal distances and cusp areas, have been carried out to elucidate the association

between the position of the enamel knots, the sequence of tooth development, and

morphogenetic field patterning (Townsend et al., 2009a). Crown components of the upper

molar teeth such as the cusp areas of the paracone (mesiobuccal cusp), protocone

(mesiolingual cusp), metacone (distobuccal cusp) and hypocone (distolingual cusp) have

been studied in a sample of Australian Aborigines and it has been shown that the latest-

forming cusp of the tooth, the hypocone, is also the most variable cusp in dimension and

expression (Takahashi et al., 2007). Intercuspal dimensions have been studied in

Australian twins and it has been shown that the intercuspal distances present high values of

fluctuating asymmetry compared with mesiodistal and buccolingual dimensions,

suggesting that the position of cusp tips is influenced more by the environment or

epigenetic effects during their development (Townsend et al., 2003a). Although there have

been several studies of molecular and cell interactions during the development of the

dentition, a complete understanding of how genetic, epigenetic and environmental factors

influence tooth morphogenesis is still to be elucidated.

2.6. Methods of obtaining dental measurements

Dental measurements have been used largely in areas such as clinical and forensic

dentistry, physical anthropology, and to understand genetic and environmental effects

during tooth formation. Measurements of the tooth crown and roots can be obtained by

using different techniques, such as digital hand-held sliding calipers and dental casts,

standardized radiographs, computer tomography, standardized photography, and laser

scanning (Hillson, 1996; Brook et al., 1999; Lahdesmaki and Alvesalo, 2004; Brook et al.,

2005; Lahdesmaki and Alvesalo, 2005; Smith et al., 2009; El-Zanaty et al., 2010).

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2.6.1. Sliding calipers

The traditional method of obtaining dental measurements is by using sliding

calipers with beaks sharpened to fit in between the interdental spaces (Moorrees et al.,

1957; Hunter and Priest, 1960). This manual method can be used directly inside the mouth

or indirectly by using dental models. The direct method offers potentially a high level of

accuracy but it is difficult to perform in comparison with indirect methods as there are

problems with access and in establishing the correct mesiodistal crown diameters of

posterior teeth, especially in the maxilla (Hunter and Priest, 1960). Moreover, this method

is patient-dependent, as the patient needs to be present every time a measurement is

performed.

The most commonly used indirect method consists of making measurements from

dental models or casts. This method has some advantages over the direct method such as

easy visualization of both upper and lower dental arches, easy access once the dental

models are stored in a laboratory, and the ability to obtain records at different

developmental stages of both dentitions (Moorrees et al., 1957). However, there are

disadvantages in using hand-held calipers because they can damage the models with their

sharp beaks, thereby altering measurements. This method is limited as it only allows a few

linear measurements of the dental crown, such as MD, BL and IC dimensions, to be

obtained. Crowded and rotated teeth cannot be appropriately measured by using calipers

(Moorrees et al., 1957; Brook et al., 1999; Brook et al., 2005; Smith et al., 2009).

Moreover, many factors can affect the measurement accuracy of this method, such as the

technique used, the conditions of the tooth and surrounding tissues, impression and casting

procedures, inaccuracy in locating the correct landmarks and operator skills (Hunter and

Priest, 1960; Brook et al., 1999; Brook et al., 2005; Harris and Smith, 2009).

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2.6.2. New techniques for measuring dental crowns

More recently, new techniques have been developed to measure dental crowns in

order to improve the accuracy and reliability of measurements as well as allow the study of

new dental phenotypes such as areas, perimeters, volumes, angles, and other subdivisions

of the tooth crown (Brook et al., 1999; Brook et al., 2005; Harris and Smith, 2009). The

2D image analysis system is a standardized photographic system which uses a digital

camera to obtain images with high resolution from both occlusal and buccal views of

dental casts. It allows the acquisition of more data in comparison to the manual technique

and it has been shown to be accurate and reliable for some tooth dimensions, such as MD

and BL, while other dimensions such as tooth crown areas and perimeters present an

acceptable level of measurement error. However, this system presents some disadvantages

such as image orientation and calibration procedures as well as subjectivity in the

identification of the landmark such as contact points and cervical areas, which is operator

dependent and this might incur measurement errors (Smith et al., 2009).

The 3D image analysis system consists of obtaining images of dental casts using

laser scanning. The dental models are scanned in three different planes or axes and the

images are superimposed to create a virtual 3D dental model. The advantage of this

system over calipers and 2D systems is that it allows the measurement of angles, volumes

and other subdivisions of the tooth crown in 3D. 3D systems also enable surface curvature

of the tooth to be recorded (Smith et al., 2009). This system allows the storage of dental

casts in an electronic form and they can be easily manipulated using the software (Smith et

al., 2009).

Both 2D and 3D image analysis systems have been shown to be reliable and

accurate as well as having the advantage of not damaging models and enabling

measurement of rotated and crowded teeth (Brook et al., 1999; Brook et al., 2005; Smith et

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al., 2009; Ashar et al., 2012). At present, there is limited information of new dental

phenotypes such as perimeters, areas, angles, and volumes, but it is anticipated that these

new imaging technologies will help to establish a bridge between descriptions of the

variation of dental phenotypes and actually identifying the genes responsible for observed

variation. Moreover, these systems will be more efficient as images will only need to be

obtained once and then stored for future use.

2.7. Statistical approaches for describing tooth size variation

Descriptive statistics such as mean values, standard deviations and coefficients of

variation are most commonly used to describe variation in tooth size. Differences between

mean values for males and females can be expressed as percentages of sexual dimorphism

and the formula most used to quantify the percentage of sexual dimorphism between males

and females in the same population is , where M is the mean value of

males and F is the mean value of females (Garn et al., 1967b). Another way used to

measure sex dimorphism between males and females is by computing the “D score” or the

area underneath the curves defined by males and females (Chakraborty and Majumder,

1982). Unpaired Student’s t-tests are also used to assess statistical significance between

tooth dimensions in males and females and statistical significance is normally set at

p<0.05. Coefficients of correlation are another method to describe associations between

dental crown variables, and values can be calculated between different dentitions, arches,

right and left sides of the same arch, teeth and tooth dimensions (Moorrees and Reed,

1964; Edgar and Lease, 2007). Dental asymmetries can also be explored within and

between the dentitions, and various approaches have been described for computing

directional and fluctuating asymmetry. Directional asymmetry of tooth dimensions, or the

systematic difference between right and left sides, is generally assessed using paired t-tests

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or modified/nested paired t-tests (Sharma et al., 1986). Fluctuating asymmetry is often

quantified by calculating the absolute difference between right and left sides as follows:

, where L is the mean value for the left side and R is the mean

values for the right side of the arch (Harris and Nweeia, 1980).

Accuracy in dental measurements increases with an increase in the recorder’s

experience in performing the measurement (Hunter and Priest, 1960). Repeated

measurements are the best approach to obtain measurements as close as possible to the

object’s true size. Systematic and random errors of the method can arise when dental

measurements are performed and they can be generated by many different sources, such as

impression and casting procedures, calibration of the measurement equipment, landmarks

identification and operator’s skills. The first statistical formula used to calculate the

difference between double measurements was published by Dalhberg (1940). Another

statistical approach to quantify the errors of the method is by using the paired t-test, which

compares measurements of the same sample obtained either by two examiners or by

comparing two measurements recorded by the same examiner (Harris and Smith, 2009).

The intraclass correlation coefficient can also be used to calculate the reliability of the

method by quantifying the concordance within and between examiners and expressing the

results using a percentage base (Kieser et al., 1990; Khalaf et al., 2009; Smith et al., 2009).

Univariate analyses have been largely used to explore differences in tooth size

within and between populations, with each dental variable analysed as a single independent

variable. However, due to a relatively high degree of correlation found between different

tooth dimensions, more sophisticated analyses, such as principal component analysis

(PCA) and multivariate analysis (MANOVA), have been used to calculate associations

between different variables in a multidimensional space (Potter, 1972; Potter et al., 1981).

Descriptive statistics such as means, standard deviations and coefficients of

variation will be used initially in this thesis to describe each variable and they will be

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reported according to dentition, sex and zygosity. Sexual dimorphism will be assessed

using the formula proposed by Garn et al. (1967): , where M is the

mean value of males and F is the mean value of females. Unpaired t-tests will be used to

compare male and female mean values. Comparisons of variances and mean values

between the primary and permanent dentitions and zygosities will be made using F-tests

and Student’s t-tests respectively, with significance set at p<0.05. Dental asymmetries will

be assessed by using paired t-tests to describe directional asymmetry, while fluctuating

asymmetries will be assessed by the formula , where L refers to

mean values on the left side of the arch and R refers to mean values on the right side of the

arch. Mixed linear model analysis will also be used to compare the amount of fluctuating

asymmetry between all groups and multivariate analysis of variance (MANOVA) will be

used to compare differences between males and females from the different zygosity

groups, taking account of the correlations between variables.

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3. Aims of this research

The general aim of this research is to measure different dental crown dimensions

(mesiodistal, buccolingual, crown height, and intercuspal distances) in the primary and

permanent dentitions of males and females from MZ, DZ same-sex (DZSS) and DZ

opposite-sex (DZOS) twin pairs to determine whether tooth size is altered in females

and/or males from DZOS twin pairs compared with same-sex MZ or DZSS twins.

The main hypothesis is that females from DZOS twin pairs will have larger

tooth crown dimensions than females from either MZ or DZSS pairs, possibly

reflecting intrauterine male hormone diffusion.

More specifically, the objectives are:

- to describe tooth crown dimensions in males and females from MZ, DZ same-

sex and DZ opposite-sex twins and to assess the extent and patterns of sexual

dimorphism in the primary and permanent dentitions in all twin groups.

- to analyse possible hormonal influence in utero for each dental crown

dimension by making associations with the time that each tooth starts to form

and the time that its crown formation is completed.

- to assess dental asymmetry, both directional asymmetry (DA) or fluctuating

asymmetry (FA), in all twin groups to determine whether there are any trends or

patterns within or between groups, including any evidence of females being

better buffered than males against developmental disturbances.

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3.1. Format of the thesis

This thesis is presented as ten main chapters. The first two chapters provide an

overall introduction and literature review, focusing on setting the scene of this research and

identifying gaps in our current knowledge, and Chapter 3 presents the aims. The fourth

chapter outlines the methods used in this project, while the fifth chapter focuses on the

systematic and random errors of the methods used in this research. Chapters 6, 7 and 8

present results of this study. Specifically, Chapter 6 presents descriptive statistics and the

result of comparisons between monozygotic and dizygotic same-sex twins. Chapter 7

focuses on univariate and multivariate comparisons between DZOS twins and twins from

other zygosity groups. Chapter 8 presents data on directional and fluctuating dental

asymmetries in the twin samples. Chapter 9 presents a general discussion of this research

with key findings and suggestions for further research, while Chapter 10 provides general

conclusions. A list of references is provided at the end of this thesis, as well as a list of

achievements during the PhD candidature in an Appendix.

The thesis has been set up to facilitate future publication of results, so there is some

repetition between chapters.

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4. Materials and methods

4.1. Sample size and zygosity determination

The study sample consisted of monozygotic (MZ) and dizygotic (DZ) twin pairs

enrolled in an ongoing study of dentofacial development of Australian twins and their

families being undertaken in the School of Dentistry at the University of Adelaide

(Townsend et al., 2006; Townsend et al., 2012b). A cohort of twins comprising

approximately 300 twin pairs with serial models of primary, mixed and permanent

dentitions, oral examinations, intra-oral photographs, mono- and stereo-photographs, palm-

and finger-prints, blood and cheek cells for zygosity determination, medical history and

laterality tests as well as other information such as questionnaires from families and family

environment were used in this study. The sample was divided by zygosity and gender into

six groups as follow: 52 MZ same-sex female (MZF), 46 MZ same-sex male (MZM), 39

DZ same-sex female (DZSSF), 42 DZ same-sex males (DZSSM), and 44 DZ opposite-sex

females (DZOSF), and 44 DZ opposite-sex males (DZOSM) twin pairs.

Zygosity has been determined by comparison of a number of genetic markers in the

blood (ABO, Rh, Fy, Jk, MNS), serum enzyme polymorphisms (GLO, ESD, PGM1, PGD,

ACP, GPT, PGP, AK1) and protein polymorphisms (HP, C3, PI, GC), as well as by

analysis of up to six highly variable genetic loci (FES, vWA31, F13A1, THO1, D21S11,

FGA) on six different chromosomes by using DNA extracted from buccal cells initially

and subsequently from highly polymorphic genetic loci (D3S1358, vWA, FGA, AMEL,

D8S1179, D21S11, D18S51, D5S818, D13S317, D7S820) on ten chromosomes. The

probability of dizygosity, given concordance for all systems, is less than 1 per cent

(Townsend et al., 1995; Townsend et al., 2005; Hughes et al., 2007). All participants are

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of European ancestry with no relevant medical and dental history that could influence the

study (Townsend et al., 2005; Hughes et al., 2007).

4.1.1 Inclusion and exclusion criteria

Only twins with primary, mixed and permanent dental casts available for the same

individual were selected in this study. Ages ranged between 4.1 – 16.5 years of age and

only fully erupted teeth were selected and measured.

One co-twin from each pair of same-sex MZ and DZ twins was randomly selected

for inclusion in this study to avoid bias that would be introduced by inclusion of tooth size

data from both co-twins who share, on average, all (MZ co-twins) or half (DZSS co-twins)

of their genes. In contrast, both twins from opposite-sexed DZ twin pairs were used. The

dental models were obtained by making impressions of the upper and lower arches in

alginate and then pouring up the impressions with yellow stone.

Because of the time-consuming nature of the data acquisition process, only some

permanent teeth were measured. Measurements of maxillary central incisors, canines,

second premolars, and first and second molars, as well as mandibular central and lateral

incisors, canines, second premolars, first and second molars, were obtained in the

permanent dentition. Maxillary central incisors, canines and first molars and mandibular

lateral incisors, canines and first molar were selected because they represent the most

stable or “key” tooth in each morphogenetic field class described by Butler (1939) and

Dahlberg (1945) in the permanent dentition. Maxillary second premolars and second

molars and mandibular central incisors, second premolars and second molars were selected

to confirm the concept of morphogenetic field theory in all twins studied as well as to

evaluate the level of asymmetries present in each tooth class. All teeth of upper and lower

arches of the primary dentition were used in this study since calcification of primary teeth

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starts in utero and any intrauterine hormonal influence on dental crowns should be evident.

Both dentitions were selected for all individuals to evaluate the entire process from a

longitudinal perspective.

Measurements of maximum mesiodistal (MD) crown dimensions, buccolingual

(BL) crown dimensions, crown height (CH) dimensions and intercuspal (IC) dimensions of

both MZ and DZ twin pairs from both sexes were assessed by using a standardized

photographic system, the 2D image analysis system described by Brook et al. (1999).

Each dimension is defined in detail in the next section. All phenotypes were assessed for

measurement accuracy and reliability before being included in the analysis (Brook et al.,

1999).

Permanent upper lateral incisors were excluded due to the fact that these teeth vary

greatly in shape and size, and the upper and lower first premolars were also excluded

because previous studies using the same sample had reported that first premolars had not

presented differences in MD and BL diameters greater than 0.1mm for both males and

females (Dempsey et al., 1999b; Townsend et al., 2009a). Third molars were excluded

because the impressions were made before eruption of these teeth. Teeth that were not

fully erupted, had carious lesions or restorations, or were crowded and/or exhibited any

evidence of tooth wear or model damage were excluded from this study.

4.2. Defining the landmarks

The maximum mesiodistal (MD) crown dimension was defined as the maximum

distance between the mesial and distal proximal surfaces of the tooth crown and, in the

case of minimal dental crowding or if adjacent teeth were missing, the measurements were

taken from the anatomical positions where the contact occurred (Moorrees et al., 1957;

Brook et al., 1999; Brook et al., 2005). Maximum mesiodistal dimensions of permanent

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upper central incisors and lower central and lateral incisors were assessed from the labial

view while the occlusal view was used to assess the mesiodistal dimensions of permanent

upper and lower canines, second premolars, and permanent first and second molars.

Maximum mesiodistal dimensions of upper and lower primary central incisors and lateral

incisors were assessed from the labial view, whereas upper and lower primary canines and

first and second molars were assessed from the occlusal view.

Figure 4.1: Mesiodistal (MD) crown dimension measured on a permanent upper central incisor from the labial view.

Figure 4.2: Mesiodistal (MD) crown dimension measured on a permanent upper first molar from the occlusal view.

The maximum labiolingual or buccolingual (BL) crown dimension was defined

as the greatest distance between buccal and lingual surfaces of the crown perpendicular to

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and bisecting the line defining the mesiodistal dimension (Tobias, 1967; Kieser, 1990;

Brook et al., 1999; Brook et al., 2005). In this study we defined the buccolingual

dimension as the maximum distance between the buccal and lingual surfaces, using as

landmarks the most cervical point in the buccal groove of buccal surface of tooth crown

and the most cervical point in the lingual groove of the lingual surface of the tooth crown.

Figure 4.3: Buccolingual/labiolingual (BL) crown dimension measured on a permanent upper central incisor from the

labiolingual view.

Figure 4.4: Buccolingual/labiolingual (BL) crown dimension measured on a permanent upper first molar from the

buccolingual view.

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Crown height (CH) varies between groups of teeth and can be affected by the

position of the gingiva in relation to the tooth, by the labiolingual or buccolingual

inclination of the tooth, by tooth wear and also when the tooth is not fully erupted. This

dimension was defined as follows:

for the upper central incisor and lower central and lateral incisor: the maximum

distance between the middle point in the incisal portion of the tooth crown and

the middle point in the cervical line of the tooth crown in a labial view;

for canines and premolars: the maximum distance between the cusp tip to the

cervical line of the tooth crown in a labial view;

for molars: the maximum distance between the mesio-buccal cusp tip to the

cervical line of the tooth crown in a buccal view.

Figure 4.5: Crown height (CH) dimension measured on a permanent upper central incisor from the labial view.

The intercuspal distance (IC) was defined as the distance between the cusp tips of

first molars and second molars in the primary dentition and premolars and molars in the

permanent dentition. In the primary dentition, it comprised one measurement in the first

molars (ic1) and four measurements in the second molar, while in the permanent dentition

it comprised one measurement in premolars and four measurements in molars as follows:

A) Primary dentition:

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first molars (ic1): the maximum distance between the buccal and lingual cusp tips

from an occlusal view.

second molars:

1) the maximum distance between the mesio-buccal and the mesio-lingual cusp tips (ic1);

2) the maximum distance between the mesio-buccal and the disto-buccal cusp tips (ic2);

3) the maximum distance between the disto-buccal and the disto-lingual cusp tips (ic3);

4) the maximum distance between the mesio-lingual and the disto-lingual cusp tips (ic4).

B) Permanent dentition

premolars (ICP): the maximum distance between the buccal and lingual cusp tips from

an occlusal view.

first and second molars

1) the maximum distance between the mesio-buccal and mesio-lingual cusp (IC1),

2) the maximum distance between the mesio-buccal and the disto-buccal cusp (IC2),

3) the maximum distance between the disto-buccal and the disto-lingual cusp (IC3),

4) the maximum distance between the mesio-lingual and the disto-lingual cusp (IC4).

The cusp tips were marked with small dots in pencil before taking each photograph

in order to make the cusp tips observable on the photographs (Harris and Dinh, 2006).

Figure 4.6: Intercuspal (IC) dimensions shown on a permanent upper

first molar and measured from the occlusal view.

IC 2

IC 1

IC 4

IC 3

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4.3. The 2D Image Analysis System

The dental models were placed on a platform under standardised illumination. The

platform was adjustable and could be moved in three planes allowing images to be

obtained from different orientations. Each tooth was imaged separately from both the

labial/buccal and occlusal views using a digital camera (Canon EOS 50D digital SLR

camera, Cannon Australia) with a resolution of 15.1 megapixels and the images were

displayed in an array of 4752 x 3168 pixels for analysis. A 100mm lens (Elicar macro

lens) was used to capture the images. The digital camera was mounted horizontally above

the study model on an adjustable rod. The study casts were illuminated with four

multidirectional spot lights surrounding the cast. A length of steel rule with a scale in

millimetres was placed adjacent to the tooth surface being imaged and at the same plane of

the tooth. The camera was connected to a computer (Intel Pentium 4 CPU 3.20GHz, 3192

MHz, 1 core, 2 logical processors, Australia) and EOS Digital Software (Cannon Australia

PTY. LTD) was used to acquire the images from the camera. The camera settings were

adjusted to the following: aperture (f) f16, ISO speed 160 and shutter speed 0.3 seconds

and the images obtained were than saved as a JPEG into the designated directory for later

calibration and measurement using the ImageJ software (National Institute of Health,

USA).

The labial and buccal images of each cast were captured first and then followed by

the occlusal images. Measurements were obtained following a specific sequence: firstly

from the upper left quadrant, followed by the upper right quadrant, lower left quadrant and

lastly for the lower right quadrant. The lens was focused parallel to the tooth surface

imaged and also parallel to the long axis of the clinical crown for buccal view images or at

right angles to the long axis of the clinical crown for occlusal views. Each image received

an appropriate file name and was saved and backed-up (Brook et al., 1999; Brook et al.,

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2005). Teeth from right and left quadrants, as well as teeth from the upper and lower

arches from both dentitions, were identified and recorded in the data files in order to

evaluate the patterns of dental asymmetry.

Figure 4.7: 2D image analysis system.

4.3.1. Image analysis

The images were analysed using ImageJ software (National Institute of Health,

USA). Every image was first calibrated and then measured and the data obtained were

stored in an Excel file for later statistical analysis.

The labial view was used to capture images to assess MD crown dimensions and

CH dimensions of upper central incisors and lower central and lateral incisors. The buccal

view was used to capture images to assess CH of upper and lower canines, second

premolars, first molars and second molars.

The occlusal view was used to capture images to assess MD crown dimensions of

upper and lower canines, second premolars, first and second molars; BL dimensions of

upper central incisors, lower central and lateral incisors, and upper and lower canines,

second premolars, and first and second molars; and IC dimensions of upper and lower

second premolars, first and second molars. Data acquisition, including obtaining over

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46,000 measurements from approximately 23,700 images, extended over a 12 month

period.

4.4. Power analysis

A power analysis was performed to estimate approximate sample sizes to test the

main hypotheses. The power analysis was based on the following hypotheses: H0:

Zygosity has no effect on measures of tooth size and Ha: Female opposite-sex twins have

larger tooth sizes than females from MZ or same-sex DZ pairs. Calculations were based

on unpaired t-tests of group differences in the tooth exhibiting the greatest estimate of the

required sample size to analyse all (partially-correlated) teeth. The unit was deemed to be

one female individual ascertained from either DZ female-female or DZ opposite-sex twin

pairs. The dependent variable was tooth size in millimetres. The smallest meaningful

group difference (Δ) was set at 0.2 mm. A number of assumptions were made in these

calculations including: equal numbers in each zygosity group; equal variances between

groups, with SD-0.5; Type I error rate (α) = 0.05; desired power (β) = 0.8. It was

determined that a total of approximately 50 individuals per group would be needed to

provide the desired power and these were randomly selected from the total sample of twins

in the collection.

4.5. Statistical analysis

All twin data were assessed for conformity to a normal distribution. Descriptive

statistics of mean values, standard deviations (SD) and coefficients of variation (CV) were

obtained for all tooth size variables and data were computed by tooth dimension, dentition,

gender and zygosities. Unpaired Student’s t-tests were used to make comparisons between

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males and females and between zygosities and statistical significance were set at p<0.05.

Variances were compared using F tests. Statistical significance was set at p<0.05.

Sexual dimorphism was assessed for all variables by calculating percentages of

sexual differences between males and females. These sex differences were defined by Garn

et al. (1967) as , where M is the mean value of males and F is the mean

values of females.

Associations between variables were assessed by calculating correlation

coefficients (r) for both dentitions and all zygosities studied. Correlation coefficients were

assessed for antimeric pairs of teeth, isomeric pairs of teeth, primary teeth and their

successional permanent teeth, as well as between all dimensions in the same tooth.

Dental asymmetries were assessed as follows:

a) Directional asymmetry (DA) was assessed by comparing tooth dimensions for

antimeric teeth using Student’s paired t-tests, with significance set at p<0.05.

b) Fluctuating asymmetry (FA) was assessed by calculating the absolute difference

between antimeric pairs according to the formula , where

R=right side values and L=left side values (Harris and Nweeia, 1980). A mixed linear

model analysis, with statistical significance set at p<0.05, was used to identify patterns of

FA between zygosity groups and dentitions.

A multivariate analysis of variance (MANOVA) was also performed to evaluate

sets of data between males and females from all zygosities taking account of the

intercorrelations between variables.

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5. Errors of measurements

5.1. Introduction

Traditionally, the most commonly-used method to measure dental crown

dimensions is by using hand-held calipers. Measurements can be made directly inside the

mouth or indirectly by measuring dental models, the latter being the most common

approach (Hunter and Priest, 1960; Kieser, 1990). Although the direct method has a high

level of validity, it is difficult to perform in more posterior teeth such as upper and lower

molars. Furthermore, this method is patient-dependent. Obtaining measurements from

dental models allows easy access and visualization of both upper and lower arches as well

as providing the opportunity to examine longitudinal records of the different

developmental stages of the dentition within individuals (Kieser, 1990). The disadvantage

of using sliding calipers is that their sharp beaks can damage stone models, therefore

altering the measurements obtained (Brook et al., 1999; Brook et al., 2005; Smith et al.,

2009). Moreover, measuring crown dimensions using calipers only allows an accuracy of

at best 0.1mm. Other issues with sliding calipers are that linear crown dimensions, such as

crown heights (CH) and intercuspal distances (IC), are difficult to obtain, and crowded and

rotated teeth cannot be measured appropriately (Brook et al., 1999; Smith et al., 2009).

An operator’s experience in recording dental measurements leads to an increase in

accuracy and repeated measurements are the best way to obtain measurements as close as

possible to the object’s true size (Hunter and Priest, 1960). It is possible to measure

several different dimensions of the tooth crown but all measurements will involve errors of

measurements. The fidelity of the measurements will depend on three main points: 1)

locating the landmarks, 2) the precision of the measuring equipment, and 3) how the

operator uses the equipment. Errors of measurement can be classified as systematic errors

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or random errors (Moorrees et al., 1957; Hunter and Priest, 1960; Houston, 1983). In this

study, systematic errors could occur during the impression and casting procedures, imaging

procedures and at the time of making the measurements. Random errors could arise from

variations in positioning the dental models in relation to the camera, by imaging

procedures such as camera set up, by light intensity and shadows that could interferes with

the measurements, from difficulties in identifying the landmarks, and due to limitations in

the skills of the operator.

Impression and casting procedures by themselves can lead to systematic errors as

they can lead to distortion of the impressions and consequently distortions of the dental

models. Alginate impressions shrink as they lose water by syneresis and should be poured

immediately after the impressions are obtained. The type of stone used is also of

importance as different types of stone/gypsum expand linearly in their outer dimensions at

different rates (Powers and Sakaguchi, 2006). Because of this, all the impressions used in

this study were obtained in alginate and poured immediately after the impressions were

made using type III yellow stone.

The aims of this chapter are to quantify the errors of the method. In this study,

intra- and inter-operator repeatability was assessed to evaluate both systematic and random

errors. To quantify intra- and inter-operator errors of the methods, the Fleiss intra-class

correlation coefficient (ICCC) was used and results were classified according to the

Donner and Eliasziw scale (Donner and Eliasziw, 1987). Bland and Altman graphs were

also used to display intra- and inter-operator repeatability (Appendix 2) (Bland and

Altman, 1986). Paired t-tests were used to compare the different types of software used,

Image Pro Plus 5.1 and ImageJ. Paired t-tests were used to assess systematic errors of the

method, while Dahlberg statistics were used to assess random errors of measurement

(Dahlberg, 1940).

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5.2. Methods

To avoid systematic errors of measurement due to the setting up of the camera, the

camera was set up to work with the manual exposure, with ISO (image sensor’s sensitivity

to light) set to 160, with shutter speed of 0.3 seconds and aperture-priority (f) of 16.

During all shots, the lens was adjusted to the same position (number 0.31 in the lens

display) to allow the same distance between the camera and the object as well as the same

depth of field for all images.

To minimize random errors, the camera was positioned and levelled using a spirit

level device positioned on both the equipment and the camera. The lights were dimmed to

graduation 4 of the lighting equipment and all lamps were angled at 45 degrees to allow the

same radiant light throughout the object. External light sources were turned off when

taking shots to avoid any unwanted source of lighting that could interfere with the image

quality. Altered positioning of the models on the holder could also lead to random errors

and depends on the skills of the operator. The surfaces to be imaged were visually

positioned and carefully checked before the images were recorded in order to ensure that

the tooth crown surface to be imaged was parallel to the lens of the camera. To minimize

these errors, all dimensions were assessed by performing repeated measurements and

accuracy was optimised by obtaining measurements as close as possible to the true

dimension (Houston, 1983).

In this study, the errors of the method were assessed in three distinct phases. In the

first phase, intra- and inter-operator repeatability tests were performed by two operators

(operator 1: Daniela Ribeiro [DR]; operator 2: Tom Coxon [TC]) with the objective of

calibrating both operators to correctly localize the landmarks and also to enhance accuracy

and precision of operator 1 by reducing intra-operator systematic and random errors. This

phase concentrated on obtaining measurements of MD and BL dimensions of permanent

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upper central incisors and first molars of 20 study models stored in the Imaging Laboratory

of the School of Dental Sciences, University of Liverpool, UK. The second phase

consisted of comparing the first measurement obtaining by the operator 1 (DR) in

Liverpool using the Image Pro Plus 5.1 software and digitizing the same images using the

ImageJ software. The third phase involved obtaining measurements of MD, BL, CH and

IC dimensions in a sample of 26 twins from MZ, DZSS and DZOS twin pairs.

5.3. Results

5.3.1. Repeatability test: phase 1

Consistency between two or more measurements of the same object under the same

experimental conditions can be enhanced by making repeated measurements of this object

(Harris and Smith, 2009). Repeatability was assessed in this study and performed initially

in the Imaging Laboratory of the School of Dental Sciences, University of Liverpool, UK.

Intra-operator repeatability was determined by imaging the permanent right and left upper

central incisor, and also the permanent right and left upper first molars twice on 20 study

models using the 2D image analysis method. All images were captured using the same

camera (Kodak/Nikkon DCS 410), measured using the Image Pro Plus 5.1 software (Media

Cybernetics, Buckinghamshire, UK) and stored in a Windows 2003 Excel file.

Measurements of the MD and BL dimensions were obtained. Intra-operator repeatability

was assessed by comparing the first and second measurements obtained by operator 1

(DR). Inter-operator repeatability was assessed by using the first of the repeated

measurements of the operator 1 (DR) and the measurements of operator 2 (TC) under the

same experimental environment.

The intra- and inter-operator repeatability data were analysed using the Fleiss intra-

class correlation coefficient (ICCC) and the results were then classified using the Donner

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and Eliasziw scale (Donner and Eliasziw, 1987) as being slight (0.00-0.20), fair (0.21-

0.40), moderate (0.41-0.60), substantial (0.61-0.80) and excellent (0.81-1.00). High

reproducibility results were found for intra-operator (0.96 – 0.99) and for inter-operator

(0.86 – 0.97) measures and both were scored as excellent according to the Donner and

Eliasziw scale (Table 5.1). Errors of the method were small and therefore considered to be

unlikely to bias the results of this study.

Table 5.1: Measures of repeatability error 1

Intraclass

correlation SE

Mean

Difference

(in mm)

SD

Difference

(in mm)

Intra-operator

right I1 MD 0.993 0.013 0.007 0.059

BL 0.988 0.188 0.029 0.084

M1 MD 0.961 0.041 0.008 0.187

BL 0.975 0.035 0.026 0.158

left I1 MD 0.990 0.014 0.005 0.064

BL 0.957 0.028 0.077 0.124

M1 MD 0.986 0.017 0.052 0.074

BL 0.983 0.032 0.043 0.144

Inter-operator

right I1 MD 0.973 0.028 0.034 0.124

BL 0.911 0.031 0.230 0.138

M1 MD 0.935 0.051 0.087 0.227

BL 0.914 0.062 0.087 0.277

left I1 MD 0.937 0.030 0.101 0.133

BL 0.862 0.042 0.204 0.190

M1 MD 0.903 0.050 0.022 0.222

BL 0.947 0.054 0.066 0.241 1 Data were based on repeated measures of 20 individuals taken by operator 1 (DR) and by operator 2 (TC). I1=permanent upper central incisor; M1=permanent upper first molars. MD=mesiodistal dimension; BL=buccolingual

dimension.

5.3.2. Image Pro Plus 5.1 software and ImageJ software: phase 2

The first set of images obtained in Liverpool by operator 1 (DR) was measured

again using the ImageJ software (Image Processing and Analysis in Java, National Institute

of Health, USA) in Adelaide and the results were compared to assess possible systematic

errors. Comparisons between the same measurements obtained by two different software

packages, Image Pro Pus 5.1 and ImageJ, were performed using the Fleiss intra-class

correlation coefficient (ICCC) and the results were classified using the Donner and

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Eliasziw scale as being excellent, ranging between 0.86 and 0.97 for all measures. This

comparison was necessary because it was decided to use this ImageJ software in Adelaide

to measure the data instead of the Image Pro Plus 5.1 used initially in Liverpool, UK. The

results showed that it was reliable to use the ImageJ software to obtain all the

measurements in this study (Table 5.2).

Table 5.2: Measures of repeatability errors between Image Pro Plus 5.1 and ImageJ software.

Intraclass

correlation SE

Mean

Difference

(in mm)

SD

Difference

(in mm)

Intra-operator

right I1 MD 0.973 0.028 0.017 0.124

BL 0.911 0.031 0.000 0.138

M1 MD 0.935 0.051 0.018 0.227

BL 0.914 0.062 0.019 0.277

left I1 MD 0.937 0.030 0.008 0.133

BL 0.862 0.042 0.012 0.190

M1 MD 0.903 0.050 0.118 0.222

BL 0.947 0.054 0.014 0.241 Data were based on repeated measures of 20 individuals taken by operator 1 (DR). I1=permanent upper central incisor; M1=permanent upper first molars. MD=mesiodistal dimension; BL=buccolingual dimension.

5.3.3. Repeatability test for all dimensions using ImageJ software: phase 3

Repeatability tests were performed for all tooth crown dimensions (MD, BL, CH

and IC) of primary and permanent dentitions in a randomly selected sub-group of 26

individuals from MZ, DZSS and DZOS male and female twin pairs. Duplicate

measurements were obtained by one operator (DR) using the same standardized 2D image

analysis photography system and each tooth was imaged and the image digitized and

measured. Paired t-tests, with significance set at p<0.05, were also used to determine

whether there were any systematic differences in data generated by the first and second

determination and results are presented for the right (Table 5.3) and left sides (Table 5.4).

Values of mean differences between first and second determinations (intra-operator

systematic errors) for dental crown dimensions in primary and permanent dentitions are

described for the right side (Figure 5.1) and for the left side (Figure 5.2). The Dahlberg

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statistics were also calculated using the formula: , where X1 is the

measurement of first determination, X2 is the measurement of second determination, and n

is the sample size and comparisons made between the two samples (Table 5.4) (Dahlberg,

1940).

Overall, double determination showed small differences between the right and left

sides (Tables 5.3 and 5.4) with few significant values (right: 12 / left: 7) for both sides.

The total number of significant values was small compared with the amount of data

collected and may have occurred due to chance. The most posterior teeth in the primary

dentition showed the largest differences between the first and second determinations in all

crown dimensions studied, while in the permanent dentition the largest differences between

two measurement occasions varied according to tooth and dimension. Permanent upper

premolars and lower central incisors displayed the largest differences for MD dimensions,

while upper and lower first molars displayed the larger differences between both

determinations for the BL dimensions, with upper second molars displaying the largest

differences between the first and second determination (Tables 5.3 and 5.4). BL

dimensions were associated with high errors for the primary lower lateral incisors on both

sides (Figures 5.1 and 5.2) while CH dimensions were associated with high errors for the

primary upper second molars and permanent upper and lower second molars (Figures 5.1

and 5.2). Crown heights (CH) were also associated with larger systematic errors of the

method compared with the other crown dimensions measured (Figure 5.1 and 5.2).

The values of the Dahlberg statistics (intra-operator random errors) for crown

dimensions in the primary and permanent dentitions are reported in Table 5.5, as well as

for the right side (Figure 5.3) and left side (Figure 5.4). Random errors were smaller in the

primary dentition compared with the permanent dentition (Table 5.5). Posterior teeth

displayed greater random errors for all crown dimensions studied than the more anterior

teeth in the primary dentition, while the permanent dentition did not display the same

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pattern (Table 5.5). Primary lower central incisors were associated with small random

errors for MD, BL and CH dimensions, while larger random errors were found for CH

dimensions in the permanent upper left second molars and for BL dimensions in the

permanent lower second premolars.

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Table 5.3: Mean difference in millimeters between first and second measurements for mesiodistal (MD), buccolingual (BL), crown height (CH) and intercuspal dimensions (IC)

of primary and permanent dentitions of a sample of monozygotic (MZ), dizygotic same-sex (DZSS) and dizygotic opposite-sex (DZOS) male and female twins – right side.

MD BL CH ICP IC1/ic1 IC2 IC3 IC4

n

Mean

Diff

(mm)

SE

Diff n

Mean

Diff

(mm)

SE

Diff n

Mean

Diff

(mm)

SE

Diff n

Mean

Diff

(mm)

SE

Diff n

Mean

Diff

(mm)

SE

Diff n

Mean

Diff

(mm)

SE

Diff n

Mean

Diff

(mm)

SE

Diff n

Mean

Diff

(mm)

SE

Diff

Primary

Upper

i1 24 -0.003* 0.013 23 -0.008* 0.022 12 -0.056* 0.729

i2 24 -0.001* 0.165 25 -0.017* 0.014 15 0.018* 0.041

c 26 -0.004* 0.013 25 -0.004* 0.017 3 0.043* 0.130

m1 26 0.006* 0.023 26 0.036* 0.035 6 -0.072* 0.101

5 -1.000 0.175

m2 24 -0.025* 0.025 25 -0.025* 0.022 15 0.103* 0.573

13 -0.025 0.035 13 -0.015 0.077 13 0.109* 0.034 13 -0.052 0.052

Lower

i1 13 0.029* 0.010 13 -0.005* 0.025 9 -0.002* 0.024

i2 21 -0.008* 0.010 21 0.032* 0.020 15 -0.017* 0.035

c 25 0.040* 0.057 25 -0.004* 0.015 7 0.020* 0.128

m1 26 0.040* 0.026 26 0.008* 0.034 5 0.238* 0.086

4 -0.088 0.068

m2 26 0.010* 0.018 26 -0.173* 0.039 6 0.115* 0.079

8 -0.141 0.089 7 -0.074 0.058 9 -0.063 0.060 9 0.073 0.059

Permanent

Upper

I1 26 -0.021* 0.014 24 -0.045* 0.017 22 -0.056* 0.026

C 13 0.000* 0.022 13 0.017* 0.161 11 0.121* 0.045

PM2 15 -0.029* 0.024 15 0.016* 0.024 15 0.016* 0.032 15 -0.057 0.032

M1 24 -0.018* 0.038 24 -0.041* 0.020 20 0.024* 0.057

21 -0.060 0.057 21 0.009 0.061 21 0.044 0.514 21 0.043 0.046

M2 5 -0.008* 0.033 5 -0.024* 0.045 4 0.200* 0.078

5 0.122 0.051 5 0.024 0.106 4 -0.013 0.034 5 -0.078 0.084

Lower

I1 26 -0.074* 0.062 25 0.028* 0.072 18 -0.041* 0.028

I2 21 -0.043* 0.040 21 -0.023* 0.024 15 0.000* 0.031

C 18 0.070* 0.031 15 -0.019* 0.029 11 0.025* 0.059

PM2 16 0.001* 0.025 16 -0.003* 0.025 13 0.091* 0.071 13 0.040 0.044

M1 22 -0.031* 0.045 24 -0.193* 0.044 11 0.076* 0.080

15 -0.019 0.041 13 -0.134* 0.041 13 -0.026 0.052 14 0.048 0.045

M2 4 0.008* 0.110 6 -0.178* 0.070 4 0.350* 0.079 5 0.028 0.031 6 -0.003 0.082 5 -0.032 0.046 6 -0.013 0.045

i1=primary central incisor; i2=primary lateral incisor; c=primary canine; m1=primary first molar; m2=primary second molar; I1=permanent central incisor; I2=permanent lateral incisor; C=permanent canine;

PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; MD=mesiodistal dimension; BL=buccolingual dimension; CH=crown height dimension; ICP=intercuspal dimension in permanent second premolars; ic1=intercuspal dimension in primary first molars; IC1= mesio-buccal/mesio-lingual intercuspal dimensions; IC2=mesio-buccal/disto-buccal intercuspal dimensions; IC3=disto-buccal/disto-

lingual intercuspal dimensions; IC4=mesio-lingual/disto-lingual intercuspal dimensions. *Significant difference between first and second determination: p<0.05.

55

Table 5.4: Mean difference in millimeters between first and second measurements for mesiodistal (MD), buccolingual (BL), crown height (CH) and intercuspal dimensions (IC)

of primary and permanent dentitions of a sample of monozygotic (MZ), dizygotic same-sex (DZSS) and dizygotic opposite-sex (DZOS) male and female twins – left side.


n

Mean

Diff

(mm)

SE

Diff n

Mean

Diff

(mm)

SE

Diff n

Mean

Diff

(mm)

SE

Diff n

Mean

Diff

(mm)

SE

Diff n

Mean

Diff

(mm)

SE

Diff n

Mean

Diff

(mm)

SE

Diff n

Mean

Diff

(mm)

SE

Diff n

Mean

Diff

(mm)

SE

Diff

Primary

Upper

i1 23 0.000 0.009 22 0.011* 0.226 12 -0.023 0.024

i2 25 0.016 0.021 25 -0.032* 0.019 16 -0.029 0.050

c 26 -0.123 0.026 26 0.012* 0.019 4 -0.025 0.159

m1 26 -0.008 0.015 26 0.072* 0.052 6 -0.675 0.746

7 -0.801 0.666

m2 24 -0.038 0.040 24 0.008* 0.025 16 -0.303 0.336

15 -0.032 0.057 15 0.057* 0.025 14 -0.037 0.033 14 -0.053 0.070

Lower

i1 12 0.006 0.018 13 0.025* 0.028 10 -0.016 0.033

i2 20 0.016 0.017 19 -0.158* 0.218 16 0.021 0.021

c 26 0.037 0.038 26 -0.063* 0.045 7 -0.070 0.137

m1 26 0.046 0.023 26 -0.062* 0.044 6 0.177 0.089

4 -0.180* 0.033

m2 26 0.048 0.044 26 -0.128* 0.028 4 0.235 0.088

7 -0.107* 0.036 6 0.113 0.100 6 0.010 0.038 7 -0.046 0.064

Permanent

Upper

I1 26 -0.065 0.044 23 -0.027* 0.028 23 -0.126 0.061

C 15 -0.005 0.020 14 0.050* 0.034 10 0.071 0.048

PM2 17 -0.168 0.181 17 -0.030* 0.018 17 0.018 0.070 16 -0.098* 0.035

M1 25 -0.592 0.041 25 -0.057* 0.021 22 0.023 0.037

20 -0.086 0.059 21 -0.046 0.031 20 0.002 0.043 20 0.072 0.040

M2 4 -0.068 0.084 5 -0.074* 0.057 5 -0.146 0.221

5 0.106 0.094 5 -0.018 0.100 5 -0.020 0.057 5 0.066 0.151

Lower

I1 24 -0.129 0.011 25 -0.017* 0.038 21 -0.003 0.021

I2 23 -0.004 0.028 20 -0.012* 0.029 18 -0.029 0.032

C 18 0.018 0.053 17 -0.015* 0.030 12 -0.063 0.086

PM2 16 -0.062 0.055 17 -0.149* 0.099 15 -0.075 0.065 15 -0.031 0.075

M1 24 -0.010 0.035 25 -0.160* 0.034 13 0.097 0.097

17 -0.055 0.043 13 0.012 0.056 12 -0.007 0.068 15 -0.085 0.082

M2 9 -0.039 0.065 9 -0.032* 0.033 2 0.005 0.325 4 0.020 0.055 4 -0.018 0.010 2 0.035 0.145 4 -0.023 0.042


PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; MD=mesiodistal dimension; BL=buccolingual dimension; CH=crown height dimension; ICP=intercuspal dimension in permanent second premolars; ic1=intercuspal dimension in primary first molars; IC1= mesio-buccal/mesio-lingual intercuspal dimensions; IC2=mesio-buccal/disto-buccal intercuspal dimensions; IC3=disto-buccal/disto-

lingual intercuspal dimensions; IC4=mesio-lingual/disto-lingual intercuspal dimensions. *Significant difference between first and second determination: p<0.05.

56

Figure 5.1: Values of mean differences in millimeters between first and second determinations (intra-operator systematic errors) for crown dimensions in primary and permanent

dentitions – right side.

i1=primary central incisor; i2=primary lateral incisor; c=primary canine; m1=primary first molar; m2=primary second molar; I1=permanent central incisor; I2=permanent lateral incisor,

C=permanent canine; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; MD=mesiodistal dimension; BL=buccolingual dimension; CH=crown height

dimension; ICP=intercuspal dimension in permanent second premolars; ic1=intercuspal dimension in primary first molars; IC1= mesio-buccal/mesio-lingual intercuspal dimensions; IC2=mesio-

buccal/disto-buccal intercuspal dimensions; IC3=disto-buccal/disto-lingual intercuspal dimensions; IC4=mesio-lingual/disto-lingual intercuspal dimensions.

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

0.400

0.450

i1 i2 c m1 m2 i1 i2 c m1 m2 I1 C PM2 M1 M2 I1 I2 C PM2 M1 M2

Upper Lower Upper Lower

MD

BL

CH

ICP

IC1

IC2

IC3

IC4

57

Figure 5.2: Values of mean differences in millimeters between first and second determinations (intra-operator systematic errors) for crown dimensions in primary and permanent

dentitions – left side.


PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; MD=mesiodistal dimension; BL=buccolingual dimension; CH=crown height dimension; ICP=intercuspal dimension in

permanent second premolars; ic1=intercuspal dimension in primary first molars; IC1= mesio-buccal/mesio-lingual intercuspal dimensions; IC2=mesio-buccal/disto-buccal intercuspal dimensions; IC3=disto-buccal/disto-lingual intercuspal dimensions; IC4=mesio-lingual/disto-lingual intercuspal dimensions.

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

0.400

0.450



Primary Permanent

MD

BL

CH

ICP

IC1

IC2

IC3

IC4

58

Table 5.5: Dahlberg statistics (in mm) for mesiodistal (MD), buccolingual (BL), crown height (CH) and intercuspal dimensions (IC) of primary and permanent dentitions of a

sample of monozygotic (MZ), dizygotic same-sex (DZSS) and dizygotic opposite-sex (DZOS) male and female twins.


n Right n Left n Right n Left Right Left Right Left Right Left Right Left Right Left Right Left

Primary

Upper

i1 24 0.044 23 0.029 23 0.072 23 0.072 16 0.152 14 0.054

i2 24 0.056 25 0.074 25 0.049 25 0.069 16 0.105 18 0.132

c 26 0.047 26 0.092 26 0.058 26 0.066 3 0.133 4 0.091

m1 26 0.080 26 0.052 26 0.127 26 0.189 7 0.155 5 0.160

5 0.258 7 0.118

m2 24 0.087 24 0.137 25 0.078 24 0.083 16 0.163 16 0.159

14 0.084 15 0.153 14 0.208 15 0.077 14 0.110 14 0.088 14 0.127 14 0.157

Lower

i1 13 0.031 12 0.042 13 0.062 13 0.070 9 0.048 10 0.072

i2 21 0.032 20 0.054 21 0.067 19 0.066 15 0.093 16 0.060

c 25 0.086 26 0.094 25 0.053 26 0.164 7 0.126 7 0.073

m1 26 0.095 26 0.086 26 0.120 26 0.160 5 0.207 6 0.188

4 0.104 4 0.134

m2 26 0.065 26 0.160 26 0.184 26 0.135 6 0.149 4 0.198

9 0.183 8 0.092 8 0.105 5 0.064 9 0.128 6 0.060 9 0.128 7 0.115

Permanent

Upper

I1 26 0.053 26 0.162 24 0.067 23 0.094 22 0.093 23 0.220

C 13 0.055 15 0.053 13 0.110 14 0.093 11 0.132 10 0.113

PM2 15 0.067 17 0.057 15 0.064 17 0.055 15 0.132 17 0.197 15 0.095 16 0.117

M1 24 0.130 25 0.149 24 0.073 25 0.082 20 0.177 22 0.122

21 0.184 20 0.192 21 0.110 21 0.105 21 0.165 20 0.133 21 0.150 20 0.132

M2 5 0.046 4 0.113 5 0.066 5 0.096 4 0.171 5 0.329

5 0.113 5 0.153 5 0.150 5 0.142 4 0.043 5 0.082 5 0.131 5 0.176

Lower

I1 26 0.225 24 0.038 25 0.249 25 0.131 18 0.088 21 0.067

I2 21 0.129 23 0.092 21 0.077 20 0.088 15 0.082 18 0.094

C 18 0.103 18 0.155 15 0.077 17 0.084 11 0.132 12 0.207

PM2 16 0.068 16 0.157 16 0.068 17 0.300 13 0.184 15 0.180 13 0.112 15 0.200

M1 22 0.147 24 0.118 24 0.201 25 0.165 11 0.187 13 0.248

15 0.110 17 0.126 13 0.138 13 0.136 13 0.129 12 0.159 14 0.119 15 0.224

M2 4 0.134 9 0.132 6 0.168 9 0.069 4 0.266 0 - 5 0.048 4 0.069 6 0.130 4 0.018 5 0.069 2 0.105 6 0.072 4 0.053


PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; MD=mesiodistal dimension; BL=buccolingual dimension; CH=crown height dimension; ICP=intercuspal dimension in

permanent second premolars; ic1=intercuspal dimension in primary first molars; IC1= mesio-buccal/mesio-lingual intercuspal dimensions; IC2=mesio-buccal/disto-buccal intercuspal dimensions; IC3=disto-buccal/disto-lingual intercuspal dimensions; IC4=mesio-lingual/disto-lingual intercuspal dimensions.

59

Figure 5.3: Values of Dahlberg statistics in millimeters (intra-operator random errors) for crown dimensions in primary and permanent dentitions – right side.

i1=primary central incisor; i2=primary lateral incisor; c=primary canine; m1=primary first molar; m2=primary second molar; I1=permanent central incisor; I2=permanent lateral incisor;

C=permanent canine; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; MD=mesiodistal dimension; BL=buccolingual dimension; CH=crown

height dimension; ICP=intercuspal dimension in permanent second premolars; ic1=intercuspal dimension in primary first molars; IC1= mesio-buccal/mesio-lingual intercuspal

dimensions; IC2=mesio-buccal/disto-buccal cusp dimensions; IC3=disto-buccal/disto-lingual intercuspal dimensions; IC4=mesio-lingual/disto-lingual intercuspal dimensions.

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350



Primary Permanent

MD

BL

CH

ICP

IC1

IC2

IC3

IC4

60

Figure 5.4: Values of Dahlberg statistics in millimeters (intra-operator random errors) for crown dimensions in primary and permanent dentitions – left side.


C=permanent canine; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; MD=mesiodistal dimension, BL=buccolingual dimension, CH=crown height

dimension, ICP=intercuspal dimension in permanent second premolars, ic1=intercuspal dimension in primary first molars, IC1= mesio-buccal/mesio-lingual intercuspal dimensions, IC2=mesio-

buccal/disto-buccal intercuspal dimensions, IC3=disto-buccal/disto-lingual intercuspal dimensions, IC4=mesio-lingual/disto-lingual intercuspal dimensions.

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350



Primary Permanent

MD

BL

CH

ICP

IC1

IC2

IC3

IC4

61

60

5.4. Discussion

Double determination analysis was performed for all variables, both dentitions and

for all zygosities studied. Systematic and random errors of the method were assessed

within and between operators.

According to the methodology used to quantify the errors of the method in this

study, variation in the measurements obtained by the same operator or between two

operators was minimal. Inter-operator errors exceeded intra-observer errors and this is

consistent with the findings of Kieser et al. (1990) (Table 5.1). Moreover, high reliability

was found between both software systems used to obtain the measurements (Image Pro

Plus 5.1 and ImageJ software), enabling the use of ImageJ software in Adelaide to obtain

the measurements in this study (Table 5.2). Different crown dimensions displayed

different amounts of systematic and random errors of measurements, with MD dimensions

and BL dimensions displaying almost the same systematic errors, less then errors for CH

and IC dimensions. Random errors were small and seemed to affect each tooth differently

(Figures 5.3 and 5.4).

On the basis of this double determination analysis, there were no obvious trends in

systematic errors of the method, even though some results reached statistical significance

(Tables 5.3 and 5.4). Some variables, such as mesiodistal (MD) and buccolingual (BL)

dimensions, were easier to measure than others, e.g., crown heights (CH) and intercuspal

(IC) dimensions. There certainly were random errors associated with all variables but

these errors were relatively small compared with the differences found between males and

females and between zygosity groups and, therefore, unlikely to bias the results of this

study.

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6. Results of descriptive analysis of tooth size data in

monozygotic (MZ) and dizygotic same-sex (DZSS) twins

6.1. Introduction

Comparisons of tooth size and shape within and between related individuals

provide a particularly valuable means of assessing genetic, epigenetic and environmental

influences on dental variation (Horowitz et al., 1958; Garn et al., 1965a; Townsend, 1976;

1978; Townsend and Brown, 1978a; Brook, 1984; Townsend et al., 2005; Townsend et al.,

2009c). The human dentition is especially suitable for genetic studies because teeth start to

form around 4 – 6 weeks after conception and continue to develop until around 21 years

after birth (Townsend et al., 1994a; Hillson, 1996; Townsend et al., 2009c; AlQahtani et

al., 2010).

Sexual dimorphism is evident in human tooth crown size (Garn et al., 1965c; Garn

et al., 1967b) and its magnitude and patterning varies within and between populations

(Moorrees et al., 1957; Garn et al., 1965c). On average, males present larger tooth crown

dimensions than females and this is evident for both primary (Black, 1978; Harris and

Lease, 2005; Adler and Donlon, 2010) and permanent dentitions (Garn et al., 1966b;

Schwartz and Dean, 2005), with the latter being more dimorphic than the former. Sexual

dimorphism also differs according to the tooth and dimension studied but is present in both

mesiodistal (Garn et al., 1965c; Garn et al., 1967b; Kieser et al., 1990; Hanihara and

Ishida, 2005; Harris and Lease, 2005) and buccolingual (Garn et al., 1966b) crown

dimensions, although little or no evidence of sexual dimorphism has been found for crown

components such as intercuspal distances (Biggerstaff, 1975; Townsend, 1985).

Given the advantages of using dental dimensions to make inferences about the

magnitude and patterning of sexual dimorphism in MZ and DZSS twins, this chapter aims

63

57

to describe tooth crown size in the primary and permanent dentitions of monozygotic (MZ)

and dizygotic same-sex (DZSS) males and females twins of Caucasian ancestry enrolled in

an ongoing study of dentofacial development of Australian twins and their families in the

School of Dentistry at The University of Adelaide. The aim is to describe the different

tooth crown dimensions (mesiodistal, MD; buccolingual, BL; crown height, CH;

intercuspal, IC) in terms of means, standard deviations (SD) and coefficients of variation

(CV) in both dentitions and zygosities, and to explore associations with the timing of tooth

formation in both dentitions and the morphogenetic field theory of tooth development

described by Butler (1939) and Dahlberg (1945). It also aims to determine the magnitude

and pattern of the percentages of sexual dimorphism for each variable between primary

and permanent dentitions of both MZ and DZSS twins, as well as quantify the amount of

sexual dimorphism in the primary and permanent dentitions of the same individuals of both

MZ and DZSS twins.

Associations were also calculated between the different tooth crown dimensions

using Pearson’s coefficients of correlation (r). Correlation coefficients between teeth from

right and left sides (antimeric pairs) as well as correlations between teeth from the same

side in the upper and lower arches (isomeric pairs) were calculated for all variables and for

both dentitions and zygosities. Other correlations calculated included correlations between

primary and successional permanent teeth and correlations between all variables in the

same tooth, also making associations with the timing of formation of each dimension

studied. Mean values and standard deviations calculated for the twin samples were also

compared with published data for other Caucasian groups.

64

57

6.2. Results

Sample sizes differed for some variables due to the fact that some participants did

not present with full primary or permanent dentitions when impressions were obtained.

Tooth wear and teeth that were not fully erupted also contributed to smaller sample sizes

for some teeth in both dentitions. Analysis of histograms showed that all variables were

normally distributed and descriptive statistics including means, standard deviations (SD)

and coefficients of variation (CV) were calculated.

Tables 6.1 – 6.4 present descriptive statistics for mesiodistal (MD), buccolingual

(BL), crown height (CH) and intercuspal (IC) dimensions in monozygotic (MZ) males and

females. Tables 6.5 – 6.8 present descriptive statistics for the same dimensions in

dizygotic same-sex (DZSS) males and females. Tables 6.9 and 6.10 present the differences

between male and female mean values as well as the percentages of sexual dimorphism for

all variables in the primary and permanent dentitions of MZ and DZSS twins.

Associations between variables are presented in Tables 6.11 - 6.16, while comparisons of

MD, BL, CH and IC dimensions between MZ and DZSS males and females with published

data are reported in Tables 6.17 – 6.20.

6.2.1. Sexual dimorphism

Overall, mean values of MD, BL, CH, and IC dimensions were generally greater in

males compared with females for both MZ and DZSS twins, for most of the teeth measured

in both dentitions. The sex difference was significant (p<0.05) for all teeth in MZ twins

for MD and BL dimensions, except for the MD dimension of the primary upper lateral

incisors and permanent lower central incisors (Table 6.1) and for the BL dimension of the

primary upper left lateral incisor (Table 6.2), which showed similar values between males

and females. Sex differences were also significant (p<0.05) in DZSS twins for both MD

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57

and BL dimensions, but more evident in the later-forming teeth in both primary and

permanent dentitions (Tables 6.5 and 6.6). However, the magnitude and pattern of the

differences varied across zygosities, dentitions, teeth studied and all dimensions.

Differences between males and females were also evident between dentitions and

teeth and varied according to the dimension studied because different tooth dimensions are

formed at different times during tooth development. The first dimensions to be determined

during odontogenesis are the intercuspal (IC) dimensions and they form during the cap

stage of tooth formation after the positioning of the secondary enamel knots and prior to

the beginning of calcification in multicuspid teeth. The second dimensions to be

established during tooth formation are the MD dimensions and they are formed after the

beginning of calcification and before calcification reaches approximately half of the

occluso-cervical crown distance, although this varies depending on the teeth type. This

dimension develops later than the IC dimension but before the establishment of the BL and

CH dimensions. The other two dimensions, BL and CH, are established when tooth crown

calcification is almost complete and the root is starting to form, therefore they develop at a

later time compared with IC and MD dimensions. Because each tooth dimension measured

in this study (MD, BL, CH and IC) is formed at different times of crown formation it is

expected that each dimension might display different percentages of sexual dimorphism.

Calculations of the percentage of sexual dimorphism were performed for all IC, MD, BL

and CH dimensions for both MZ and DZSS twins (Tables 6.9 and 6.10). Overall, the

primary dentition presented smaller percentages of sexual dimorphism on average than the

permanent dentition for all variables studied in both MZ (Table 6.9) and DZSS (Table

6.10) twins.

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57

6.2.1.1. Mesiodistal (MD) crown dimensions

Overall, mean values of MD crown dimensions were greater in males compared to

females for all teeth and zygosities studied (Table 5.1 and Table 5.5). The primary

dentition displayed smaller differences in MD crown dimensions between males and

females compared with the permanent dentition, showing a smaller percentage of sexual

dimorphism in both MZ and DZSS twins. Primary upper lateral incisors showed no

difference in MD crown dimensions in MZ twins while primary upper right canines and

lower left central incisors presented the highest percentage of sexual dimorphism (6.1%

and 7.9%, respectively) in the same group (Table 6.1 and Table 6.9). No differences were

found in mean MD crown dimensions between DZSS males and females for primary

central incisors, upper right canines, upper second molars and lower right central incisors,

while lower right lateral incisors and lower left canines showed the highest percentages of

sex dimorphism (2.2% and 3.5%, respectively) in the primary dentition of DZSS twins

(Tables 6.5 and 6.10). For this group, upper lateral incisors presented a negative value for

sexual dimorphism (-1.9%), with females being larger than males for the MD dimension

(Table 6.10). For the permanent dentition, lower central incisors showed the smallest

percentage of sexual dimorphism (1.9%) for MD dimensions for both MZ and DZSS twin

pairs, while lower right and left canines showed the highest values of sexual dimorphism in

both MZ and DZSS twins (9.1% and 6.0%, respectively) (Table 6.9 and Table 6.10). The

high values of sexual dimorphism found for upper left (9.4%) and lower left (9.8%) second

molars in MD dimensions in MZ twins are likely to reflect the small sample sizes.

Permanent lower second molars also showed no sexual dimorphism in DZSS twins

possibly due to small sample sizes.

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6.2.1.2. Buccolingual (BL) crown dimensions

Overall, buccolingual (BL) crown dimensions were larger in males compared with

females for all teeth in both MZ and DZSS twins (Tables 6.2 and 6.6). The primary

dentition displayed smaller percentages of sexual dimorphism for BL crown dimensions

compared with the permanent dentition in both MZ and DZSS twins. Similar to MD

dimensions in MZ twins, the primary upper left lateral incisor displayed no sexual

dimorphism for the BL dimension in MZ twins, while lower right central incisors and

lower left second molar presented the highest percentage of sexual dimorphism (8.3% and

6.0%, respectively) in the primary dentition (Tables 6.2 and 6.9). No sexual dimorphism

in BL dimensions was found for upper left canines, upper right first molars and lower right

central incisors, whereas upper right second molar and lower left first molar presented the

highest percentages of sexual dimorphism in the primary dentition of DZSS twins (3.1%

and 4.4%, respectively) (Tables 6.6 and 6.10). For the permanent dentition, upper central

incisors showed the smallest percentage of sexual dimorphism in MZ twins (2.8%), while

upper left canines and lower right canines displayed the highest percentage of sexual

dimorphism (10.3% and 8.3%, respectively) in MZ twins (Table 6.9). Lower second

premolars (right: 6.1%; left: 4.9%) and lower right first molars (6.1%) also showed high

values of percentage of sexual dimorphism in DZSS twins (Table 6.10). The high

percentages of sexual dimorphism found for upper second molars (right: 9.1%; left: 11.0%)

and lower second molars (right: 10.2%; left: 9.1%) in MZ twins and for upper second

molars (right: 7.2%; left 6.4%) are likely to reflect small sample sizes.

6.2.1.3. Crown height (CH) dimensions

Overall, crown height (CH) dimensions displayed larger mean values for males

compared with females for both MZ and DZSS twins and for all teeth studied (Tables 6.3

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57

and 6.7). The primary dentition displayed smaller differences in CH dimensions between

males and females compared with the permanent dentition, therefore presenting smaller

percentages of sexual dimorphism for both MZ and DZSS twins (Tables 6.9 and 6.10).

The smaller mean values found in males than females for CH dimensions in the permanent

upper second molars (right and left sides) and permanent lower second molars (right side

only) in DZSS twins, might have been an effect of small sample sizes. Another factor that

could have influenced the reversed sexual dimorphism in the permanent upper and lower

second molars is the degree of eruption and gingival maturation of these teeth at the

moment the impressions were taken.

6.2.1.4. Intercuspal (IC) dimensions

Overall, males displayed larger mean values for IC dimensions than females in both

primary and permanent dentitions and for both MZ and DZSS twins (Tables 6.4 and 6.8).

However, IC dimensions displayed smaller percentages of sexual dimorphism compared

with MD, BL and CH dimensions in MZ twins (Table 6.9) but a different pattern was

found for DZSS twins where IC dimensions displayed larger percentages of sexual

dimorphism compared with MD, BL and CH dimensions, possibly because of small

sample size in DZSS twins (Table 6.10). Percentages of sexual dimorphism differed

between zygosities, with MZ twins displaying, on average, less percentage of sexual

dimorphism for IC dimensions compared with DZSS twins, except for IC2 which

displayed larger values in MZ twins (MZ: right=8.3%, left=5.0%; DZSS: right=6.4%,

left=3.5%). Negative percentages of sexual dimorphism found for IC may have been

related to the relatively small sample sizes.

Overall, males from both zygosities displayed larger mean values for all

dimensions studied compared with females from the same zygosities. However, the

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percentages of sexual dimorphism varied across dentitions, teeth and dimensions studied,

with primary dentition displaying smaller percentages of sexual dimorphism than the

permanent dentition. Lower canines were the most dimorphic teeth in the permanent

dentition while the lower central incisors were the least dimorphic. Intercuspal dimensions

displayed the smallest amount of sexual dimorphism compared with the other dimensions

studied, while CH dimensions displayed the highest amount in both dentitions and for both

zygosities studied.

70

Table 6.1: Descriptive statistics for mesiodistal (MD) dimensions of primary and permanent dentitions in

monozygotic (MZ) male and female twins.

MZ Males MZ Females

Right (MD)

Left (MD)

Right (MD) Left (MD)

n

mea

n SD CV

n

mea

n SD CV

n

mea

n SD CV

n

mea

n SD CV

Primary

Upper

i1 29 6.4* 0.40 6.2

33 6.4* 0.40 6.2

40 6.2 0.39 6.3

41 6.2 0.37 6.0

i2 34 5.1* 0.30 5.8

33 5.1* 0.33 6.4

44 5.1 0.33 6.5

45 5.1 0.31 6.2

c 45 7.0* 0.35 5.0

44 6.9* 0.35 5.0

51 6.6 0.44 6.6

52 6.6 0.52 7.9

m1 44 7.2* 0.39 5.5

45 7.2* 0.43 6.0

51 6.8 0.36 5.3

52 6.9 0.39 5.7

m2 45 9.0* 0.42 4.7

42 8.9* 0.41 4.6

52 8.5 0.36 4.3

52 8.6 0.36 4.2

Lower

i1 22 4.1* 0.32 7.8

22 4.1* 0.32 7.7

27 3.9 0.29 7.4

24 3.8 0.26 6.9

i2 33 4.6* 0.32 6.9

34 4.7* 0.32 7.0

41 4.4 0.33 7.5

38 4.4 0.33 7.5

c 46 6.0* 0.35 5.9

46 6.0* 0.29 4.9

51 5.8 0.27 4.7

51 5.7 0.30 5.3

m1 43 8.0* 0.39 4.9

45 8.0* 0.41 5.1

51 7.6 0.35 4.7

49 7.6 0.38 5.0

m2 45 10.1* 0.44 4.3

45 10.1* 0.44 4.4

51 9.8 0.37 3.8

51 9.7 0.37 3.8

Permanent

Upper

I1 45 8.7* 0.56 6.4

45 8.6* 0.48 5.6

52 8.4 0.53 6.2

51 8.4 0.50 5.9

C 29 8.1* 0.41 5.1

30 8.2* 0.38 4.7

33 7.6 0.34 4.5

33 7.6 0.40 5.3

PM2 30 6.9* 0.23 3.4

31 7.0* 0.35 4.9

38 6.7 0.32 4.8

36 6.7 0.36 5.3

M1 43 10.5* 0.47 4.4

44 10.5* 0.53 5.0

50 10.0 0.47 4.6

51 10.1 0.42 4.2

M2 19 10.5* 0.57 5.4

15 10.5* 0.51 4.8

9 9.7 0.54 5.3

9 9.6 0.40 4.1

Lower

I1 43 5.4* 0.39 5.2

42 5.5* 0.34 6.2

49 5.3 0.29 5.5

48 5.3 0.29 5.4

I2 39 6.0* 0.37 6.2

41 6.0* 0.35 5.9

48 5.8 0.37 6.3

49 5.8 0.34 5.9

C 33 7.2* 0.47 6.6

35 7.2* 0.46 6.4

38 6.6 0.37 5.7

39 6.6 0.35 5.4

PM2 32 7.5* 0.40 5.3

31 7.5* 0.37 4.9

36 7.1 0.40 5.6

35 7.2 0.40 5.5

M1 39 11.4* 0.66 5.8

43 11.4* 0.62 5.4

45 10.7 0.58 5.4

50 10.8 0.45 4.5

M2 16 11.0* 0.45 4.1 16 11.2* 0.57 5.1 15 10.2 0.37 3.7 16 10.2 0.48 4.7

i1=primary central incisor; i2=primary lateral incisor; c=primary canine; m1=primary first molar; m2=primary second molar; I1=permanent central incisor; I2=permanent lateral incisor; C=permanent canine; PM2=permanent second premolar; M1=permanent

first molar; M2=permanent second molar; n=sample size; mean=mean values; SD=standard deviation; CV=coefficient of variation.

(*mean values differ between the sexes at p<0.05) (yellow=larger mean values in males compared with females, blue=equal mean values).

71

57

Table 6.2: Descriptive statistics for buccolingual (BL) dimensions of primary and permanent dentitions in


MZ Males MZ Females

Right (BL)

Left (BL)

Right (BL) Left (BL)

n mean SD CV

n

mea

n SD CV

n

mea

n SD CV

n

mea

n SD CV

Primary

Upper

i1 31 5.2* 0.28 5.5

33 5.2* 0.28 5.3

43 4.9 0.33 6.8

41 5.0 0.31 6.3

i2 36 4.9* 0.40 8.1

35 4.8* 0.41 8.6

45 4.7 0.33 7.1

46 4.8 0.34 7.2

c 44 6.3* 0.34 5.4

44 6.3* 0.33 5.3

52 6.1 0.41 6.8

52 6.1 0.40 6.7

m1 44 9.0* 0.38 4.2

45 8.9* 0.33 3.7

52 8.5 0.35 4.1

52 8.5 0.32 3.7

m2 46 10.1* 0.40 3.9

45 10.1* 0.41 4.1

52 9.7 0.38 3.9

52 9.6 0.39 4.0

Lower

i1 22 3.9* 0.27 7.0

21 3.9* 0.22 5.5

26 3.6 0.33 9.0

25 3.7 0.27 7.4

i2 33 4.4* 0.28 6.4

34 4.4* 0.27 6.1

40 4.3 0.28 6.6

39 4.3 0.28 6.6

c 46 5.7* 0.33 5.8

46 5.7* 0.30 5.3

51 5.6 0.38 6.8

51 5.6 0.39 7.0

m1 43 7.2* 0.37 5.2

45 7.3* 0.44 6.1

51 6.8 0.41 6.0

50 7.0 0.37 5.4

m2 46 8.8* 0.37 4.2

45 8.9* 0.38 4.3

51 8.2 0.42 5.1

51 8.4 0.39 4.6

Permanent

Upper

I1 45 7.3* 0.58 8.0

44 7.3* 0.60 8.2

48 7.1 0.46 6.4

47 7.1 0.52 7.4

C 26 8.4* 0.61 7.3

28 8.6* 0.49 5.8

31 7.8 0.40 5.2

33 7.8 0.45 5.8

PM2 30 9.8* 0.51 5.2

31 9.8* 0.49 5.0

38 9.2 0.34 3.7

36 9.3 0.33 3.6

M1 44 11.9* 0.54 4.5

44 11.8* 0.51 4.4

50 11.1 0.48 4.3

51 11.1 0.47 4.3

M2 19 12.0* 0.63 5.3

15 12.1* 0.74 6.1

11 11.0 0.41 3.8

12 10.9 0.39 3.6

Lower

I1 41 6.3* 0.45 7.2

43 6.2* 0.47 7.6

48 5.9 0.38 6.4

45 6.0 0.38 6.3

I2 37 6.6* 0.42 6.4

38 6.5* 0.40 6.1

49 6.2 0.45 7.3

47 6.2 0.44 7.0

C 29 7.8* 0.64 8.3

34 7.8* 0.64 8.2

37 7.2 0.48 6.7

38 7.3 0.49 6.7

PM2 32 8.7* 0.44 5.0

31 8.7* 0.47 5.4

36 8.3 0.52 6.3

35 8.3 0.53 6.4

M1 42 10.6* 0.51 4.8

44 10.7* 0.53 5.0

49 9.9 0.48 4.8

50 9.9 0.45 4.5

M2 17 10.8* 0.54 5.0 16 10.8* 0.47 4.4 22 9.8 0.54 5.6 23 9.9 0.54 5.4


first molar; M2=permanent second molar; n=sample size; mean=mean values; SD=standard deviation; CV=coefficient of variation. .

(*mean values differ between the sexes at p<0.05) (yellow=larger mean values in males compared with females, blue=equal mean values)

.

72

57

Table 6.3: Descriptive statistics for crown height (CH) dimensions of primary and permanent dentitions in


MZ Males MZ Females

Right (CH)

Left (CH)

Right (CH) Left (CH)

n mean SD CV n mean SD CV n mean SD CV n mean SD CV

Primary

Upper

i1 18 5.6* 0.52 9.3

17 5.7* 0.51 8.9

35 5.3 0.66 12.5

36 5.3 0.68 12.9

i2 16 5.1* 0.41 8.0

17 5.0* 0.40 8.1

43 4.7 0.57 12.2

44 4.7 0.60 12.8

c 3 6.3* 0.96 14.9

3 6.0* 0.19 3.2

42 5.4 0.62 11.6

39 5.4 0.60 11.1

m1 10 4.7* 0.33 7.0

6 4.8* 0.33 6.9

47 4.2 0.40 9.5

49 4.3 0.48 11.2

m2 19 4.2* 0.38 9.0

15 4.3* 0.44 10.3

48 3.8 0.38 10.0

49 3.9 0.36 9.3

Lower

i1 11 5.0* 0.42 8.4

11 5.0* 0.54 10.7

24 4.8 0.64 13.3

25 4.8 0.63 13.2

i2 20 5.3* 0.43 8.1

22 5.3* 0.53 10.2

40 5.1 0.58 11.3

37 5.1 0.53 10.6

c 10 6.2* 0.51 8.2

9 6.3* 0.60 9.5

45 5.8 0.49 8.4

43 5.9 0.55 9.3

m1 4 5.1* 0.35 6.8

5 5.3* 0.54 10.1

49 4.7 0.43 9.2

46 4.7 0.43 9.1

m2 5 4.3* 0.47 11.1

6 3.7* 0.48 13.0

48 3.6 0.47 12.8

48 3.6 0.52 14.2

Permanent

Upper

I1 40 10.0* 0.79 7.9

41 10.0* 0.85 8.5

48 9.1 0.74 8.1

51 9.2 0.72 7.8

C 18 9.1* 0.85 9.3

20 9.1* 1.19 13.1

25 8.1 0.82 10.1

28 8.4 0.76 9.0

PM2 24 6.3* 0.76 12.0

28 6.1* 0.69 11.3

31 5.7 0.67 11.7

35 5.6 0.65 11.7

M1 35 5.3* 0.75 14.3

30 5.4* 0.70 12.9

46 4.7 0.65 13.9

49 4.7 0.60 12.7

M2 13 5.1* 0.66 13.0

11 5.5* 0.55 10.1

9 4.7 0.62 13.1

11 4.7 0.56 11.9

Lower

I1 36 8.2* 0.68 8.3

35 8.2* 0.83 10.1

41 7.8 0.75 9.6

42 7.9 0.71 9.0

I2 27 8.1* 0.82 10.1

32 8.1* 0.71 8.7

42 7.5 0.70 9.4

42 7.4 0.77 10.3

C 17 9.4* 1.17 12.4

22 9.2* 1.13 12.3

37 8.3 0.77 9.3

36 8.3 0.86 10.3

PM2 23 6.4* 0.78 12.3

19 6.4* 0.87 13.6

35 5.7 0.67 11.8

31 5.7 0.73 12.8

M1 14 5.1* 0.60 11.9

12 4.9* 0.67 13.9

43 4.7 0.65 13.9

45 4.6 0.70 15.3

M2 7 4.7* 0.59 12.6 6 4.8* 0.59 12.3 17 4.3 0.64 14.9 17 4.5 0.55 12.3




73

57

Table 6.4: Descriptive statistics for intercuspal (IC) dimensions of primary and permanent dentitions in


MZ Males MZ Females

Right (IC)

Left (IC)

Right (IC) Left (IC)

n mean SD CV n mean SD CV

n mean SD CV n mean SD CV

Primary

Upper

m1(ic1) 6 4.4* 0.35 8.0

6 4.4* 0.68 15.5

44 4.1 0.30 7.3

47 4.3 0.31 7.2

m2(ic1) 20 4.9* 0.41 8.4

18 5.0* 0.38 7.8

47 4.8 0.45 9.2

47 4.9 0.46 9.4

m2(ic2) 21 4.0* 0.46 11.7

18 4.0* 0.34 8.4

47 4.0 0.36 8.9

46 4.0 0.37 9.1

m2(ic3) 21 5.0* 0.45 8.9

18 5.1* 0.45 8.9

48 5.0 0.47 9.4

48 4.9 0.41 8.3

m2(ic4) 20 3.7* 0.36 9.8

18 3.6* 0.41 11.4

47 3.7 0.36 9.8

48 3.6 0.31 8.4

Lower

m1(ic1) 5 2.4* 0.25 10.7

5 2.4* 0.24 10.1

42 2.6 0.38 14.8

39 2.5 0.36 14.5

m2(ic1) 5 3.6* 0.54 15.1

7 3.5* 0.37 10.4

45 3.6 0.40 11.4

46 3.6 0.34 9.5

m2(ic2) 4 3.4* 0.48 14.1

6 3.4* 0.49 14.4

45 3.3 0.35 10.5

46 3.3 0.36 10.7

m2(ic3) 6 4.9* 0.38 7.9

6 4.9* 0.36 7.3

46 4.7 0.43 9.2

46 4.7 0.50 10.6

m2(ic4) 7 4.4* 0.44 10.1

7 4.2* 0.30 7.1

47 4.5 0.36 8.1

45 4.5 0.37 8.4

Permanent

Upper

PM2(ICP) 28 5.5* 0.47 8.5

29 5.4* 0.47 8.6

38 5.3 0.48 9.1

36 5.4 0.48 9.0

M1(IC1) 36 6.2* 0.39 6.3

31 6.2* 0.40 6.4

50 6.0 0.56 9.3

51 6.0 0.48 7.9

M1(IC2) 37 4.9* 0.48 9.7

36 5.0* 0.46 9.3

50 4.8 0.48 10.1

51 4.8 0.46 9.6

M1(IC3) 37 6.3* 0.50 7.9

32 6.2* 0.51 8.1

51 6.0 0.61 10.3

51 6.0 0.53 8.8

M1(IC4) 35 4.9* 0.53 11.0

29 4.8* 0.51 10.6

51 4.5 0.38 8.3

51 4.6 0.38 8.3

M2(IC1) 16 6.6* 0.46 6.9

13 6.5* 0.41 6.3

13 6.0 0.49 8.2

14 6.0 0.33 5.5

M2(IC2) 16 5.1* 0.47 9.2

12 4.9* 0.62 12.5

13 4.8 0.36 7.5

14 4.9 0.29 6.0

M2(IC3) 9 6.5* 0.80 12.4

8 6.3* 0.58 9.2

8 6.0 0.48 7.9

9 5.8 0.55 9.5

M2(IC4) 9 4.2* 0.56 13.3

8 4.0* 0.43 10.6

8 4.1 0.55 13.6

9 4.4 0.51 11.6

Lower

PM2(ICP) 27 4.4* 0.54 12.2

23 4.3* 0.47 10.9

36 4.2 0.51 12.2

35 4.2 0.52 12.4

M1(IC1) 15 5.2* 0.56 10.6

10 5.0* 0.49 9.8

45 5.0 0.47 9.4

45 5.0 0.55 11.0

M1(IC2) 12 4.9* 0.61 12.5

10 4.5* 0.45 10.0

44 4.3 0.45 10.6

44 4.2 0.45 10.6

M1(IC3) 13 5.9* 0.62 10.5

15 5.8* 0.41 7.0

43 5.6 0.63 11.2

46 5.7 0.59 10.4

M1(IC4) 17 5.7* 0.56 9.8

15 5.5* 0.50 9.0

45 5.7 0.37 6.6

49 5.6 0.44 7.8

M2(IC1) 8 4.8* 0.32 6.5

8 5.3* 0.35 6.6

21 4.8 0.47 9.9

22 4.9 0.71 14.5

M2(IC2) 7 5.2* 0.32 6.2

5 5.1* 0.19 3.7

21 4.7 0.50 10.7

23 4.7 0.38 8.2

M2(IC3) 7 5.2* 0.53 10.3

5 5.4* 0.67 12.4

21 5.0 0.58 11.6

22 4.9 0.56 11.4

M2(IC4) 7 5.9* 0.24 4.1 7 5.6* 0.47 8.4 21 5.2 0.60 11.5 21 5.1 0.61 11.9

m1=primary first molar; m2=primary second molar; PM2=permanent second premolar; M1=permanent first molar; M2=permanent

second molar; (ic1)=bucco-lingual intercuspal dimension in primary first molars; (ic1)=primary mesiobuccal-mesiolingual intercuspal dimension; (ic2)=primary mesiobuccal-distobuccal intercuspal dimension; (ic3)=primary mesiodistal-linguodistal intercuspal dimension;

(ic4)=primary mesiolingual-distolingual intercuspal dimension; (ICP)=permanent bucco-lingual intercuspal dimension;

(IC1)=permanent mesiobuccal-mesiolingual intercuspal dimension; (IC2)=permanent mesiobuccal-distobuccal intercuspal dimension; (IC3)=permanent distobuccal-distolingual intercuspal dimension; (IC4)=permanent mesiolingual-distolingual intercuspal dimension;

n=sample size; mean=mean values; SD=standard deviation; CV=coefficient of variation. (*mean values differ between the sexes at

p<0.05) (yellow=larger mean values in males compared with females, blue=equal mean values).

74

57

Table 6.5: Descriptive statistics for mesiodistal (MD) dimensions of primary and permanent dentitions in

dizygotic same-sex (DZSS) male and female twins.

DZSS Males DZSS Females

Right (MD)

Left (MD)



Primary

Upper

i1 32 6.3* 0.34 5.4

31 6.3* 0.40 6.3

30 6.3 0.45 7.2

31 6.3 0.46 7.4

i2 35 5.1* 0.38 7.4

35 5.1* 0.31 5.9

35 5.2 0.36 6.9

35 5.2 0.36 6.9

c 42 6.8* 0.42 6.1

42 6.8* 0.42 6.2

39 6.8 0.36 5.3

38 6.7 0.35 5.2

m1 42 7.0* 0.49 7.0

42 7.1* 0.43 6.1

39 6.9 0.49 7.1

38 7.0 0.46 6.6

m2 41 8.7* 0.54 6.2

40 8.7* 0.45 5.2

39 8.7 0.44 5.1

36 8.7 0.46 4.9

Lower

i1 23 4.0* 0.26 6.3

23 4.0* 0.25 6.1

25 4.0 0.30 7.6

24 3.9 0.30 7.7

i2 34 4.6* 0.33 7.0

31 4.6* 0.32 6.9

33 4.5 0.38 8.4

30 4.5 0.35 7.7

c 41 5.8* 0.40 6.9

42 5.9* 0.37 6.3

39 5.7 0.34 6.0

39 5.7 0.34 5.9

m1 39 7.8* 0.57 7.4

42 7.9* 0.43 5.5

38 7.7 0.38 5.0

39 7.7 0.41 5.4

m2 40 10.0* 0.49 4.9

41 10.0* 0.46 4.6

39 9.8 0.45 4.6

39 9.8 0.47 4.8

Permanent

Upper

I1 40 8.7* 0.47 5.4

38 8.6* 0.48 5.5

38 8.4 0.52 6.2

37 8.4 0.49 5.8

C 21 8.0* 0.32 4.0

21 8.0* 0.38 4.7

24 7.8 0.62 8.0

26 7.7 0.49 6.4

PM2 26 6.8* 0.36 5.3

25 6.9* 0.34 4.9

20 6.6 0.50 7.6

22 6.7 0.52 7.7

M1 37 10.3* 0.55 5.3

37 10.4* 0.61 5.9

36 10.0 0.48 4.8

37 10.0 0.48 4.8

M2 4 10.0* 0.53 5.3

6 10.0* 0.50 4.9

8 9.8 0.65 6.6

12 9.9 0.64 6.5

Lower

I1 38 5.4* 0.26 4.8

39 5.5* 0.32 5.8

37 5.3 0.37 6.9

34 5.2 0.34 6.5

I2 38 6.0* 0.32 5.4

38 5.9* 0.34 5.7

36 5.8 0.39 6.8

32 5.7 0.35 6.2

C 28 7.1* 0.34 4.8

29 7.1* 0.41 5.8

28 6.7 0.43 6.4

28 6.7 0.50 7.5

PM2 28 7.4* 0.39 5.3

28 7.4* 0.38 5.1

25 7.1 0.46 6.4

25 7.2 0.54 7.6

M1 38 11.2* 0.60 5.3

38 11.2* 0.61 5.5

34 10.8 0.66 6.1

35 10.8 0.63 5.8

M2 3 10.4* 0.81 7.8 5 10.6* 0.86 8.1 12 10.4 0.66 6.3 13 10.6 0.65 6.2




75

57

Table 6.6: Descriptive statistics for buccolingual (BL) dimensions of primary and permanent dentitions in



Right (BL)

Left (BL)



Primary

Upper

i1 35 5.0* 0.31 6.2

33 5.1* 0.34 6.7

30 4.9 0.40 8.2

31 5.0 0.41 8.2

i2 36 4.8* 0.38 7.9

36 4.8* 0.35 7.4

36 4.7 0.39 8.4

36 4.7 0.42 9.0

c 42 6.1* 0.49 8.0

42 6.0* 0.50 8.4

39 6.0 0.36 6.0

38 6.0 0.34 5.7

m1 42 8.6* 0.47 5.5

42 8.6* 0.42 4.8

38 8.6 0.35 4.1

39 8.5 0.36 4.2

m2 41 9.9* 0.45 4.6

42 9.9* 0.45 4.6

39 9.6 0.44 4.6

38 9.6 0.42 4.3

Lower

i1 24 3.8* 0.26 6.8

26 3.8* 0.23 6.2

25 3.8 0.25 6.7

25 3.7 0.24 6.3

i2 36 4.3* 0.33 7.6

37 4.3* 0.31 7.2

32 4.2 0.28 6.7

31 4.3 0.30 7.0

c 41 5.6* 0.40 7.1

41 5.6* 0.39 7.0

39 5.5 0.33 6.0

39 5.5 0.32 5.9

m1 40 7.0* 0.44 6.3

42 7.1* 0.41 5.8

38 6.8 0.33 4.8

39 6.8 0.34 4.9

m2 42 8.6* 0.41 4.8

41 8.6* 0.41 4.7

39 8.4 0.36 4.4

39 8.4 0.38 4.5

Permanent

Upper

I1 40 7.1* 0.63 8.9

40 7.1* 0.60 8.5

36 6.9 0.61 8.9

31 6.9 0.58 8.4

C 21 8.1* 0.55 6.7

21 8.1* 0.67 8.2

23 7.9 0.67 8.4

24 8.1 0.55 6.8

PM2 26 9.5* 0.55 5.8

25 9.5* 0.48 5.0

21 9.2 0.61 6.6

22 9.2 0.60 6.5

M1 37 11.6* 0.54 4.7

38 11.6* 0.51 4.4

35 11.2 0.59 5.3

38 11.2 0.50 4.5

M2 3 11.9* 0.56 4.7

6 11.7* 0.81 6.9

12 11.1 0.59 5.3

14 11.0 0.49 4.4

Lower

I1 40 6.0* 0.47 7.8

41 6.0* 0.52 8.7

36 5.9 0.49 8.3

34 5.9 0.47 8.0

I2 38 6.3* 0.53 8.4

38 6.3* 0.52 8.2

35 6.2 0.58 9.4

34 6.2 0.52 8.3

C 24 7.5* 0.74 10.0

26 7.5* 0.65 8.7

27 7.2 0.54 7.6

28 7.2 0.67 9.3

PM2 28 8.7* 0.56 6.5

28 8.6* 0.53 6.2

24 8.2 0.53 6.5

25 8.2 0.49 6.0

M1 39 10.5* 0.47 4.4

38 10.5* 0.43 4.1

36 9.9 0.50 5.0

38 10.1 0.50 4.9

M2 5 10.3* 0.68 6.6 7 10.3* 0.76 7.4 15 10.1 0.56 5.6 14 10.2 0.53 5.3




76

57

Table 6.7: Descriptive statistics for crown height (CH) dimensions of primary and permanent dentitions in



Right (CH)

Left (CH)



Primary

Upper

i1 12 6.0* 0.65 10.9

15 5.9* 0.53 9.0

31 5.3 0.58 11.0

28 5.3 0.52 9.8

i2 19 5.0* 0.64 12.8

17 5.0* 0.47 9.5

35 4.8 0.52 10.8

35 4.7 0.59 12.6

c 2 5.6* 0.47 8.4

3 6.0* 0.63 10.5

29 5.5 0.57 10.2

30 5.4 0.59 10.9

m1 11 4.6* 0.28 6.0

7 4.5* 0.47 10.3

34 4.3 0.48 11.0

33 4.4 0.39 8.9

m2 17 4.1* 0.44 10.9

16 4.0* 0.45 11.1

38 3.9 0.53 13.4

37 4.0 0.45 11.3

Lower

i1 7 4.7* 0.33 7.0

12 4.7* 0.51 10.9

24 4.5 0.43 9.6

23 4.6 0.45 9.9

i2 16 5.4* 0.40 7.5

25 5.2* 0.47 8.9

32 5.0 0.45 9.0

30 5.0 0.39 7.8

c 5 5.9* 0.38 6.3

8 6.2* 0.66 10.7

28 5.9 0.37 6.2

28 5.9 0.44 7.5

m1 5 5.5* 0.74 13.3

6 5.2* 0.50 9.6

36 4.8 0.38 7.9

32 4.7 0.52 10.9

m2 9 4.0* 0.28 7.0

5 4.0* 0.23 5.8

33 3.8 0.45 12.0

30 3.7 0.44 11.7

Permanent

Upper

I1 31 9.8* 1.00 10.2

29 9.9* 0.99 10.0

36 9.1 0.70 7.7

35 9.1 0.83 9.1

C 10 9.3* 0.85 9.1

10 9.4* 1.19 12.6

24 8.7 0.99 11.4

23 8.9 1.18 13.2

PM2 24 5.9* 0.70 11.9

24 5.9* 0.78 13.1

20 5.5 0.73 13.1

20 5.6 0.61 11.1

M1 27 5.1* 0.86 17.0

27 5.2* 0.65 12.5

34 4.9 0.65 13.4

34 5.1 0.78 15.4

M2 3 5.1* 0.61 12.0

3 5.2* 0.58 11.2

10 5.2 0.63 12.1

12 5.4 0.67 12.5

Lower

I1 13 8.5* 1.02 12.0

14 8.3* 0.98 11.8

36 7.7 0.95 12.3

33 7.9 0.76 9.6

I2 16 8.0* 0.73 9.2

14 8.1* 0.72 8.9

36 7.2 0.92 12.7

34 7.3 0.76 10.5

C 8 9.1* 0.74 8.1

14 8.7* 0.91 10.5

28 8.6 1.07 12.4

26 8.4 0.90 10.7

PM2 23 5.8* 0.64 11.0

24 6.1* 0.66 10.9

24 5.7 0.65 11.4

22 5.9 0.73 12.4

M1 8 5.1* 0.63 12.4

6 5.4* 0.47 8.6

33 4.8 0.60 12.5

30 4.8 0.62 12.9

M2 2 4.1* 0.08 1.9 2 4.7* 0.27 5.7 9 4.5 0.57 12.8 9 4.5 0.60 13.4




77

57

Table 6.8: Descriptive statistics for intercuspal (IC) dimensions of primary and permanent dentitions in



Right (IC)

Left (IC)



Primary

Upper

m1(ic1) 11 4.4* 0.27 6.1

7 4.5* 0.20 4.5

33 4.1 0.31 7.6

33 4.2 0.29 7.1

m2(ic1) 15 4.9* 0.37 7.5

16 5.0* 0.32 6.4

36 4.7 0.37 7.9

36 4.7 0.31 6.6

m2(ic2) 16 3.9* 0.30 7.8

17 4.0* 0.45 11.4

36 3.9 0.35 8.9

34 3.9 0.33 8.5

m2(ic3) 17 5.1* 0.33 6.5

16 5.0* 0.39 8.0

35 4.8 0.46 9.5

33 4.8 0.45 9.5

m2(ic4) 16 3.6* 0.29 8.0

15 3.7* 0.44 12.1

35 3.6 0.29 8.0

35 3.6 0.33 9.3

Lower

m1(ic1) 3 2.5* 0.43 17.3

3 2.8* 0.15 5.4

31 2.4 0.34 14.0

29 2.4 0.26 10.8

m2(ic1) 8 3.7* 0.24 6.4

4 3.9* 0.28 7.2

30 3.5 0.32 9.4

32 3.6 0.32 9.1

m2(ic2) 7 3.7* 0.41 11.1

4 3.4* 0.28 8.4

31 3.4 0.32 9.3

32 3.3 0.46 13.8

m2(ic3) 7 4.9* 0.39 8.0

5 5.0* 0.44 8.7

33 4.6 0.44 9.6

32 4.5 0.43 9.5

m2(ic4) 8 4.9* 0.27 5.5

5 4.7* 0.36 7.7

32 4.6 0.46 10.1

32 4.4 0.42 9.6

Permanent

Upper

PM2(ICP) 25 5.5* 0.36 6.6

24 5.4* 0.39 7.4

21 5.2 0.60 11.6

22 5.3 0.45 8.5

M1(IC1) 26 6.2* 0.49 7.9

28 6.2* 0.51 8.3

35 5.8 0.44 7.5

39 5.9 0.46 7.8

M1(IC2) 25 4.9* 0.54 11.0

28 4.8* 0.48 9.9

36 4.8 0.44 9.1

39 4.7 0.43 9.1

M1(IC3) 26 6.2* 0.58 9.4

27 6.1* 0.60 9.8

36 6.0 0.48 8.0

37 6.0 0.56 9.3

M1(IC4) 26 4.7* 0.53 11.2

27 4.7* 0.57 12.0

35 4.5 0.39 8.7

37 4.5 0.39 8.8

M2(IC1) 2 6.2* 0.25 4.0

3 6.3* 0.41 6.6

12 5.9 0.50 8.5

14 6.0 0.58 9.7

M2(IC2) 3 5.2* 0.65 12.4

4 5.0* 0.21 4.3

12 4.7 0.61 12.8

14 4.8 0.52 10.9

M2(IC3) 1 6.7* - -

3 6.5* 0.71 11.1

4 6.0 0.78 13.0

6 6.2 0.92 14.9

M2(IC4) 1 4.9* - -

2 4.8* 0.05 1.0

4 4.1 0.49 11.8

6 4.5 0.35 7.8

Lower

PM2(ICP) 24 4.5* 0.49 10.9

27 4.3* 0.46 10.7

24 4.0 0.52 12.9

23 4.0 0.53 13.4

M1(IC1) 7 5.2* 0.34 6.5

7 5.3* 0.18 3.3

37 4.8 0.46 9.5

37 4.8 0.36 7.5

M1(IC2) 6 4.4* 0.34 7.6

6 4.6* 0.29 6.3

36 4.3 0.30 7.0

38 4.2 0.49 11.7

M1(IC3) 7 5.9* 0.33 5.6

6 6.0* 0.45 7.6

36 5.4 0.56 10.3

37 5.6 0.59 10.6

M1(IC4) 8 5.9* 0.53 9.0

7 6.0* 0.42 7.0

37 5.7 0.54 9.4

36 5.6 0.51 9.1

M2(IC1) 3 5.0* 0.34 6.8

1 5.1* - -

16 4.9 0.49 10.0

15 4.7 0.60 12.9

M2(IC2) 3 5.3* 0.59 11.2

1 5.0* - -

15 4.8 0.41 8.5

15 5.1 0.47 9.2

M2(IC3) 3 5.6* 0.61 10.9

2 5.6* 0.54 9.7

15 5.0 0.74 15.0

15 4.9 0.77 15.7

M2(IC4) 3 6.0* 0.46 7.7 2 6.0* 0.21 3.4 15 5.7 0.46 8.0 15 5.6 0.75 13.6

m1=primary first molar; m2=primary second molar; PM2=permanent second premolar; M1=permanent first molar; M2=permanent

second molar; (ic1)=bucco-lingual intercuspal dimension in primary first molars; (ic1)=primary mesiobuccal-mesiolingual intercuspal dimension; (ic2)=primary mesiobuccal-distobuccal intercuspal dimension; (ic3)=primary mesiodistal-linguodistal intercuspal dimension;

(ic4)=primary mesiolingual-distolingual intercuspal dimension; (ICP)=permanent bucco-lingual intercuspal dimension;

(IC1)=permanent mesiobuccal-mesiolingual intercuspal dimension; (IC2)=permanent mesiobuccal-distobuccal intercuspal dimension; (IC3)=permanent distobuccal-distolingual intercuspal dimension; (IC4)=permanent mesiolingual-distolingual intercuspal dimension;

n=sample size; mean=mean values; SD=standard deviation; CV=coefficient of variation. (*mean values differ between the sexes at

p<0.05) (yellow=larger mean values in males compared with females, blue=equal mean values).

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Table 6.9: Percentages of sexual dimorphism for mesiodistal (MD), buccolingual (BL), crown height (CH) and intercuspal (ICP, IC1, IC2, IC3 and IC4) dimensions of primary

and permanent dentitions in MZ males and females.


Right Left Right Left Right Left Right Left Right Left Right Left Right Left Right Left

n % n % n % n % n % n % n % n % n % n % n % n % n % n % n % n %

Primary

Upper

i1 69 3.2 74 3.2 74 6.1 74 4.0 53 5.7 53 7.5

i2 78 0.0 78 0.0 81 4.3 81 0.0 59 8.5 61 6.4

c 96 6.1 96 4.5 96 3.3 96 3.3 45 16.7 42 11.1

m1 95 5.9 97 4.3 96 5.9 97 4.7 57 11.9 55 11.6

50 7.3 53 2.3

m2 97 5.9 94 3.5 98 4.1 97 5.2 67 10.5 64 9.2

67 2.1 65 2.0 68 0.0 64 0.0 69 0.0 66 4.1 67 0.0 66 0.0

Lower

i1 49 5.1 46 7.9 48 8.3 46 5.4 35 4.2 36 4.2

i2 74 4.5 72 6.8 73 2.3 73 2.3 60 3.9 59 3.9

c 97 3.4 97 5.3 97 1.8 97 1.8 55 6.9 52 6.8

m1 94 5.3 94 5.3 94 5.9 95 4.3 53 8.5 51 12.8

47 -7.7 44 -4.0

m2 96 3.1 96 4.1 97 7.3 96 6.0 55 8.3 54 2.8

50 0.0 53 -2.8 49 3.0 52 3.0 52 4.3 52 4.3 54 -2.2 52 -6.7

Average 4.2 4.5 4.9 3.7 8.5 7.6 0.4 -0.6 1.5 1.5 2.2 4.2 -1.1 -3.4

Permanent

Upper

I1 97 3.6 96 2.4 93 2.8 91 2.8 88 9.9 92 8.7

C 62 6.6 63 7.9 57 7.7 61 10.3 43 12.3 48 8.3

PM2 68 3.0 67 4.0 68 6.5 67 5.4 61 10.7 63 9.5 66 3.8 65 0.0

M1 93 5.0 95 4.0 94 7.2 95 6.3 87 13.0 79 15.4

86 3.3 82 3.3 87 2.1 87 4.2 88 5.0 83 3.3 86 8.9 80 4.3

M2 28 8.2 24 9.4 30 9.1 27 11.0 26 11.1 22 16.1

29 10.0 27 8.3 29 6.3 26 0.0 17 8.3 17 8.6 17 2.4 17 -9.1

Lower

I1 92 1.9 90 3.8 89 6.8 88 3.3 77 5.1 77 4.2

I2 87 3.4 90 3.4 86 6.5 85 4.8 72 6.7 74 9.2

C 71 9.1 74 9.1 66 8.3 72 6.8 58 7.2 58 10.4

PM2 68 5.6 66 4.2 68 4.8 66 4.8 63 7.0 50 12.4 63 4.8 5.8 2.4

M1 84 6.5 93 5.6 91 7.1 94 8.1 63 11.1 57 5.9

60 4.0 55 0.0 56 14.0 54 7.1 56 5.4 61 1.8 62 0.0 64 -1.8

M2 31 7.8 32 9.8 39 10.2 39 9.1 27 4.7 9 8.3 29 0.0 30 8.2 28 10.6 28 8.5 28 4.0 27 10.2 28 13.5 28 9.8

Average 5.5 5.8 7.0 6.6 9.0 9.9 4.3 1.2 4.3 5.0 8.3 5.0 5.7 6.0 6.2 0.8


PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; n=number of males and females; %=percentage of sexual dimorphism; MD=mesiodistal dimension; BL=buccolingual dimension; CH=crown height dimension; ICP=intercuspal dimension in permanent second premolars; ic1=intercuspal dimension in primary first molars; IC1= mesio-buccal/mesio-lingual intercuspal dimensions;

IC2=mesio-buccal/disto-buccal intercuspal dimensions; IC3=disto-buccal/disto-lingual intercuspal dimensions; IC4=mesio-lingual/disto-lingual intercuspal dimensions.

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Table 6.10: Percentages of sexual dimorphism for mesiodistal (MD), buccolingual (BL), crown height (CH) and intercuspal (ICP, IC1, IC2, IC3 and IC4) dimensions of primary

and permanent dentitions in DZSS males and females.


Right Left Right Left Right Left Right Left Right Left Right Left Right Left Right Left

n % n % n % n % n % n % n % n % n % n % n % n % n % n % n % n %

Primary

Upper

i1 62 0.0 62 0.0 65 2.0 64 2.0 43 13.2 43 11.3

i2 70 -1.9 70 -1.9 72 2.1 72 2.1 54 4.2 52 6.4

c 81 0.0 80 1.5 81 1.7 80 0.0 31 1.8 33 11.1

m1 81 1.4 80 1.4 80 0.0 81 1.2 45 7.0 40 2.3

44 6.1 40 7.1

m2 80 0.0 76 0.0 80 3.1 80 3.1 55 5.1 53 0.0

51 4.3 52 6.4 52 0.0 51 2.6 52 6.3 49 4.2 51 0.0 50 2.80

Lower

i1 48 0.0 47 2.6 49 0.0 51 2.7 31 4.4 35 2.2

i2 67 2.2 61 2.2 68 2.4 68 0.0 48 8.0 55 4.0

c 80 1.8 81 3.5 80 1.8 80 1.8 33 0.0 36 5.1

m1 77 1.3 81 2.6 78 2.9 81 4.4 41 15.2 38 10.6

34 4.2 32 16.7

m2 79 2.0 80 2.0 81 2.4 80 2.4 42 5.3 35 7.0

38 5.7 36 8.3 38 8.8 36 3.0 40 6.5 37 11.1 40 6.5 37 6.8

Average 0.7 1.4 1.8 2.0 6.4 6.0 5.1 9.6 4.4 2.8 6.4 7.7 3.3 4.8

Permanent

Upper

I1 78 3.6 75 2.4 76 2.9 71 2.9 67 7.7 64 8.8

C 45 2.6 47 3.9 44 2.5 45 0.0 34 6.9 33 5.6

PM2 46 3.0 47 3.0 47 3.3 47 3.3 44 7.3 44 5.4 46 5.8 46 1.9

M1 73 3.0 74 4.0 72 3.6 76 3.6 61 4.1 61 3.0

61 6.9 67 5.1 61 2.1 67 2.1 62 3.3 64 1.7 61 4.4 64 4.4

M2 12 2.0 18 1.0 15 7.2 20 6.4 13 -2.3 15 -3.5

14 5.1 17 5.0 15 10.6 18 4.2 5 11.7 9 4.8 5 19.5 8 6.7

Lower

I1 75 1.9 73 5.8 76 1.7 75 1.7 49 10.4 47 5.1

I2 74 3.4 70 3.5 73 1.6 72 1.6 52 11.1 48 11.0

C 56 6.0 57 6.0 51 4.2 54 4.2 36 5.8 40 3.6

PM2 53 4.2 53 2.8 52 6.1 53 4.9 47 1.8 46 3.6 48 12.5 50 7.5

M1 72 3.7 73 3.7 75 6.1 76 4.0 41 6.1 36 12.0

44 8.3 44 10.4 42 2.3 44 9.5 43 9.3 43 7.1 45 3.5 43 7.1

M2 15 0.0 18 0.0 20 2.0 21 1.0 11 -8.5 11 4.9 19 2.0 16 8.5 18 10.4 16 -2.0 18 12.0 17 14.3 18 5.3 17 7.1

Average 3.0 3.3 3.7 3.0 4.6 5.4 9.2 4.7 5.6 7.3 6.4 3.5 9.1 7.0 8.2 6.3


PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; n=number of males and females; %=percentage of sexual dimorphism; MD=mesiodistal dimension; BL=buccolingual dimension; CH=crown height dimension; ICP=intercuspal dimension in permanent second premolars; ic1=intercuspal dimension in primary first molars molars; IC1= mesio-buccal/mesio-lingual intercuspal dimensions;

IC2=mesio-buccal/disto-buccal intercuspal dimensions; IC3=disto-buccal/disto-lingual intercuspal dimensions; IC4=mesio-lingual/disto-lingual intercuspal dimensions.

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6.2.2. Coefficients of variation (CV)

Estimates of the relative variability of all dimensions in both dentitions of MZ and

DZSS males and females were assessed by calculating coefficients of variation (CV).

Overall, MZ males displayed larger CV values for MD dimensions compared with MZ

females in both dentitions, suggesting that males are more variable for this dimension than

females (Table 6.1). However, the same pattern was not evident for DZSS twins where

males displayed smaller CV values for MD dimension compared with DZSS females

(Table 6.5).

In the primary dentition, upper lateral incisors (i2) displayed larger CV values for

MD dimensions compared with upper central incisors (i1), suggesting that central incisors

are less variable, therefore the “key” tooth of the incisor field in the upper arch. This was

evident for both MZ and DZSS males and females. However, the same pattern was also

found for the lower incisors, suggesting that, for this study, the most stable tooth in the

mandibular incisor field in the primary dentition is the central incisor (i1). In the primary

dentition, first molars (m1) displayed larger CV values for MD dimensions compared with

the same dimension in the more posterior second molar (m2) in both upper and lower

arches, suggesting that m2 is the “key” tooth of the molar field in the primary dentition and

this pattern was evident in both MZ and DZSS males and females (Tables 6.1 and 6.5).

Coefficients of variation (CV) were also calculated in the permanent dentition. A

mandibular incisor field was evident in DZSS males and females, with lateral incisors

displaying smaller CV values for MD dimensions compared with the central incisors,

suggesting that the “key” tooth in the lower incisor field is the lateral incisor. However, a

similar lower incisor field was not evident in MZ males and females as this group

displayed larger CV values for the lateral incisor (I2) compared with the central incisor

(I1). For the molars in the permanent dentition, a maxillary molar field was evident in MZ

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79

males and females for the right side while the left side failed to demonstrate the same

pattern. For DZSS twins, a maxillary molar field was evident in females, while males

displayed equal values on the right side (CV=5.3) and a reversed pattern on the left side

(CV: M1=5.9; M2=4.9), with M2 showing smaller CV values compared with M1. The

lower arch failed to demonstrate evidence supporting the field theory in MZ males as CV

values in M1 were larger than in M2 on both right and left sides while MZ females

displayed a molar field effect only on the left side, with M1 being the “key” tooth. A small

sample size for M2 might have contributed to these differences found in both maxillary

and mandibular molar regions in MZ twins. For DZSS males and females, a mandibular

molar field was evident in both right and left sides, with M1 displaying smaller CV values

compared with M2, therefore, being the most stable tooth in the mandibular arch of DZSS

twins.

6.2.3 Association between variables

Coefficients of correlation (r) were calculated for all variables in both primary and

permanent dentitions and for both males and females from MZ and DZSS twins. Tables

6.11 and 6.12 show the values of correlation coefficients for MD, BL, CH and IC crown

dimensions between antimeric pairs of teeth in males and females from MZ and DZSS

twins. Overall, corresponding teeth from right and left sides of the same arch (antimeric

pairs) displayed high correlations for MD and BL dimensions in males and females from

MZ and DZSS twins, and this was evident for both upper and lower arches in the primary

and permanent dentitions. Crown height (CH) dimensions of antimeric pairs of teeth

displayed moderate to high correlations in the primary and permanent dentitions for both

males and females and both MZ and DZSS twins, while IC dimensions displayed low to

high correlations in the primary and permanent dentitions of males and females of both MZ

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and DZSS twins. The smaller sample sizes available for both CH and IC crown

dimensions might have contributed to variation in correlations found between antimeric

pairs of teeth.

Tables 6.13 and 6.14 show the values of correlation coefficients for MD, BL, CH

and IC dimensions between isomeric pairs of teeth in males and females from MZ and

DZSS twins. Overall, corresponding teeth from upper and lower quadrants of the same

side (isomeric pairs) displayed moderate to high correlations for MD and BL dimensions

of males and females from MZ and DZSS twins, and this was evident for both right and

left sides in the primary and permanent dentitions. Crown height (CH) and IC dimensions

displayed low to moderate correlations between isomeric pairs of teeth in both primary and

permanent dentitions of MZ and DZSS twins. The low values of correlation coefficients

found for CH and IC dimensions might have been due to the relatively small sample sizes

for both of these variables.

Table 6.11: Correlation coefficients for mesiodistal (MD), buccolingual (BL), crown height

(CH) and intercuspal (IC) crown dimensions between antimeric pairs of teeth in males from

MZ and DZSS twin pairs.

Males

MD BL CH IC

r n r n r n r n

MZ

Primary UR x UL

0.9* (29-45) 0.8-0.9 (31-46) 0.3-1.0 (3-19) 0.3-0.9 (6-21)

LR X LL

0.8-0.9 (22-46) 0.7-0.9 (21-46) 0.5-0.9 (4-22) 0.0-0.9 (6-18)

Permanent

UR x UL

0.9* (15-45) 0.9-1.0 (15-45) 0.8-0.9 (13-41) 0.0-0.8 (9-37)

LR x LL

0.4-0.9 (16-43) 0.2-1.0 (16-42) 0.5-0.9 (7-36) 0.0-0.9 (8-36)

DZSS

Primary

UR x UL

0.6-0.9 (31-42) 0.8-0.9 (33-42) 0.3-0.7 (2-19) 0.1-0.7 (11-17)

LR x LL

0.7-0.9 (23-42) 0.6-0.9 (24-42) 0.7-0.9 (5-25) 0.1-0.5 (7-17)

Permanent

UR x UL

0.9* (4-40) 0.8-1.0 (3-40) 0.7-1.0 (3-31) 0.4-0.8 (28-37)

LR x LL 0.4-1.0 (3-39) 0.4-1.0 (5-41) 0.0-1.0 (2-24) 0.3-0.7 (8-36)

UR=upper right quadrant, UL=upper left quadrant, LR=lower right quadrant, LL=lower left quadrant, MD=mesiodistal dimensions, BL=buccolingual dimensions, CH=crown height dimensions, IC=intercuspal

dimensions, n=sample size, r=Pearson’s coefficient of correlation, MZ=monozygotic twins, DZSS=dizygotic

same-sex twins. *In a few cases, rounding of minimum and maximum r values produced a single value.

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(CH), and intercuspal (IC) crown dimensions between antimeric pairs of teeth in females

from MZ and DZSS twin pairs.

Females

MD BL CH IC

r n r n r n r n

MZ

Primary

UR x UL

0.8-1.0 (40-52) 0.8-0.9 (41-52) 0.5-1.0 (35-49) 0.2-0.8 (44-48)

LR X LL

0.8-0.9 (24-51) 0.8-0.9 (25-51) 0.7-0.9 (24-49) 0.3-0.8 (39-47)

Permanent

UR x UL

0.8-0.9 (9-52) 0.8-0.9 (11-51) 0.8-0.9 (11-51) 0.1-0.8 (8-51)

LR x LL

0.4-0.9 (15-49) 0.4-0.9 (22-50) 0.7-0.9 (20-48) 0.3-0.8 (21-49)

DZSS Primary

UR x UL

0.8-0.9 (30-39) 0.8-1.0 (30-39) 0.6-0.8 (28-38) 0.6-0.8 (33-36)

LR x LL

0.7-0.9 (24-39) 0.8-0.9 (25-39) 0.6-0.8 (23-36) 0.4-0.7 (29-33)

Permanent UR x UL

0.5-0.9 (8-38) 0.6-1.0 (12-38) 0.7-0.9 (12-38) 0.3-1.0 (4-39)

LR x LL 0.3-0.9 (12-37) 0.4-0.9 (14-38) 0.3-0.8 (14-38) 0.2-0.9 (15-38)

UR=upper right quadrant, UL=upper left quadrant, LR=lower right quadrant, LL=lower left quadrant,

MD=mesiodistal dimensions, BL=buccolingual dimensions, CH=crown height dimensions, IC=intercuspal dimensions, n=sample size, r=Pearson’s coefficient of correlation, MZ=monozygotic twins, DZSS=dizygotic

same-sex twins.


(CH) and intercuspal (IC) crown dimensions between isomeric pairs of teeth in males from

MZ and DZSS twin pairs.

Males

MD BL CH IC

r n r n r n r n

MZ

Primary

UR x UL

0.5-0.8 (22-46) 0.3-0.7 (22-46) 0.1-0.7 (3-20) 0.1-1.0 (4-21)

LR X LL

0.5-0.8 (22-46) 0.5-0.7 (21-46) 0.7-0.9 (3-22) 0.2-0.8 (5-18)

Permanent

UR x UL

0.7-0.8 (16-45) 0.5-0.8 (17-45) 0.4-0.6 (4-36) 0.0-0.9 (7-37)

LR x LL

0.5-0.9 (15-45) 0.7-0.9 (15-44) 0.3-0.6 (7-41) 0.1-1.0 (5-36)

DZSS Primary

UR x UL

0.5-0.9 (23-42) 0.4-0.9 (24-42) 0.0-0.4 (2-19) 0.1-0.6 (3-17)

LR x LL

0.4-0.9 (23-42) 0.3-0.9 (26-42) 0.3-0.8 (3-25) 0.0-0.9 (3-17)

Permanent UR x UL

0.5-0.8 (3-40) 0.5-0.8 (3-40) 0.5-0.7 (2-31) 0.3-0.9 (7-37)

LR x LL 0.4-1.0 ((5-39) 0.6-1.0 (6-41) 0.6-0.9 (2-30) 0.1-1.0 (5-36)

UR=upper right quadrant, UL=upper left quadrant, LR=lower right quadrant, LL=lower left quadrant, MD=mesiodistal dimensions, BL=buccolingual dimensions, CH=crown height dimensions, IC=intercuspal

dimensions, n=sample size, r=Pearson’s coefficient of correlation, MZ=monozygotic twins, DZSS=dizygotic same-

sex twins.

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(CH) and intercuspal (IC) crown dimensions between isomeric pairs of teeth in females

from MZ and DZSS twin pairs.

Females

MD BL CH IC

r n r n r n r n

MZ

Primary

UR x UL

0.6-0.9 (27-51) 0.5-0.8 (26-52) 0.4-0.8 (24-49) 0.0-0.5 (42-48)

LR X LL

0.6-0.9 (24-52) 0.5-0.8 (25-52) 0.3-0.6 (25-49) 0.1-0.5 (39-48)

Permanent

UR x UL

0.6-0.8 (9-52) 0.6-0.8 (11-50) 0.2-0.6 (11-50) 0.2-0.7 (8-51)

LR x LL

0.4-0.7 (9-51) 0.6-0.8 (12-51) 0.2-0.6 (13-51) 0.1-0.7 (9-51)

DZSS Primary

UR x UL

0.6-0.9 (25-39) 0.5-0.8 (25-39) 0.4-0.6 (24-38) 0.1-0.5 (30-36)

LR x LL

0.7-0.8 (24-39) 0.7-0.9 (25-39) 0.3-0.6 (23-37) 0.0.-0.5 (29-36)

Permanent UR x UL

0.6-0.9 (8-38) 0.7-0.9 (12-38) 0.4-0.8 (12-38) 0.1-0.7 (4-37)

LR x LL 0.5-0.8 (12-37) 0.7-0.9 (14-38) 0.0-0.9 (14-38) 0.0-0.8 (6-39)

UR=upper right quadrant, UL=upper left quadrant, LR=lower right quadrant, LL=lower left quadrant,

MD=mesiodistal dimensions, BL=buccolingual dimensions, CH=crown height dimensions, IC=intercuspal dimensions, n=sample size, r=Pearson’s coefficient of correlation, MZ=monozygotic twins, DZSS=dizygotic same-

sex twins.

The values of correlation coefficients (r) between primary and corresponding

successional permanent teeth were also calculated for MD, BL and CH crown dimensions.

In general, primary and corresponding successional permanent teeth displayed low to

moderate correlations for most of the teeth studied in both males and females from MZ and

DZSS twins (Table 6.15).

Table 6.15: Correlation coefficients for mesiodistal (MD), buccolingual (BL) and crown height (CH)

dimensions of primary and corresponding sucessional permanent teeth in males and females from MZ and

DZSS twin pairs.

Males Females

MD BL CH

MD BL CH

r n r n r n r n r n r n

p x P

MZ

UR x UR 0.1-0.5 (29-45) 0.0-0.3 (26-46) 0.0-1.0 (3-40)

0.3-0.6 (33-52) 0.2-0.5 (31-52) 0.0-0.5 (25-48)

UL x UL 0.2* (30-45) 0.1-0.2 (28-45) 0.4-1.0 (3-41)

0.1-0.6 (33-52) 0.3-0.4 (33-52) 0.2-0.3 (28-51)

LL x LL 0.2-0.3 (22-46) 0.1-0.3 (21-46) 0.2-0.7 (6-36)

0.4-0.5 (24-51) 0.4-0.5 (51-25) 0.1-0.6 (25-48)

LR x LR 0.0-0.3 (22-46) 0.0-0.1 (22-46) 0.3-0.4 (7-36)

0.4-0.5 (27-51) 0.4-0.6 (26-51) 0.2-0.4 (24-48)

DZSS

UR x UR 0.3-0.7 (21-42) 0.2* (21-42) 0.1-0.6 (2-31)

0.2-0.6 (20-39) 0.4-0.7 (21-39) 0.0-0.3 (20-38)

UL x UL 0.2-0.7 (21-42) 0.1-0.4 (21-42) 0.4-0.5 (3-29)

0.2-0.5 (22-38) 0.3-0.8 (22-38) 0.1-0.3 (20-35)

LL x LL 0.3-0.5 (23-42) 0.2-0.5 (26-41) 0.1-1.0 (5-25)

0.2-0.5 (24-39) 0.4-0.5 (25-39) 0.3-0.4 (23-34)

LR x LR 0.3-0.4 (23-41) 0.1-0.5 (24-42) 0.2-1.0 (5-23) 0.4-0.5 (25-39) 0.4-0.5 (24-39) 0.3-0.4 (24-36)

p=primary dentition, P=permanent dentition, UR=upper right quadrant, UL=upper left quadrant, LR=lower right quadrant, LL=lower left quadrant, MD=mesiodistal dimensions, BL=buccolingual dimensions, CH=crown height dimensions, n=sample size, r=Pearson’s

coefficient of correlation, MZ=monozygotic twins, DZSS=dizygotic same-sex twins. *In a few cases, rounding of minimum and

maximum r values produced a single value.

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Pearson’s coefficients of correlation were also calculated for all size variables

within the same tooth. Overall, MD dimensions showed high correlations with BL

dimensions and moderate to low correlations with CH and IC in the same tooth, while

buccolingual (BL) dimension showed low to moderate correlations with CH and IC

dimensions in the same tooth. Overall, CH dimension showed low correlations with MD

and IC dimensions, but moderate correlations with BL dimensions. Although the

correlation coefficients varied between different dimensions, correlation coefficients

between different IC dimensions showed a moderate to high values. The primary upper

second molars of both MZ and DZSS twins were taken as examples to demonstrate

correlation coefficients between different dimensions within the same tooth (Table 6.16).

Table 6.16: Correlation coefficients between all variables in the same tooth in MZ

and DZSS female twins: primary upper second molar (MZ: n=47-52; DZSS: n=35-

39).

MZ

MD BL CH ic1 ic2 ic3 ic4

DZSS

MD - 0.75 0.20 0.53 0.23 0.62 0.41

BL 0.58 - 0.41 0.60 0.16 0.65 0.54

CH 0.13 0.22 - 0.07 -0.29 0.15 0.13

ic1 0.55 0.40 0.20 - 0.60 0.82 0.78

ic2 0.06 -0.27 0.07 0.09 - 0.44 0.71

ic3 0.57 0.47 -0.06 0.55 -0.29 - 0.73

ic4 0.35 0.26 -0.04 0.22 -0.09 0.60 - MD=mesiodistal dimensions, BL=buccolingual dimensions, CH=crown height dimensions, ic1= mesio-buccal/mesio-lingual intercuspal dimensions, ic2=mesio-buccal/disto-buccal intercuspal dimensions,

ic3=disto-buccal/disto-lingual intercuspal dimensions, ic4=mesio-lingual/disto-lingual intercuspal

dimensions, MZ=monozygotic twins, DZSS=dizygotic same-sex twins.

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6.2.4. Comparison between different populations

Mean values of MD, BL, and CH dimensions of males and females from MZ and

DZSS twins were compared with published data in Caucasians (Moorrees et al., 1957;

Black, 1978; Townsend et al., 1986; Bishara et al., 1989; Barberia et al., 2009) and are

shown in Tables 6.17 – 6.20. Overall, mean values for MD and BL dimensions in MZ and

DZSS male and female twins were consistent with mean values of the same dimensions

reported in Caucasians and North American whites. The larger mean values for CH

dimensions found for the primary molars of a Spanish sample compared with that for MZ

and DZSS twin pairs might have been due to different measurement techniques. In the

Spanish sample, CH dimensions were recorded directly on dental casts as the linear

distance between the anatomical tip of the cusp and anatomical gingival margin, while CH

dimensions in this thesis were measured as the linear distance between the same landmarks

using a 2D photographic approach.

There is a shortage of published data on IC dimensions in the primary and

permanent dentitions in other populations based on the same landmarks and measurements

used in this thesis. In fact, measurements of the distances between cusp tips can be made

in many different ways which make comparisons between studies difficult. However, IC

dimensions in MZ and DZSS males and female twins reported in this thesis were similar to

mean values of the same dimensions reported previously in the same Australian twin

sample (Townsend et al., 2003a).

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Table 6.17: Mesiodistal (MD) dimensions of primary and permanent teeth in different populations measured

in mm.

MZ

DZSS

Caucasians a

North American

Whites b

n mean SD n mean SD n mean SD n mean SD

Primary Upper

i1 M 29 6.4 0.40

32 6.3 0.34

27 6.5 0.44

64 6.6 0.36

F 40 6.2 0.39

30 6.3 0.45

35 6.4 0.33

69 6.4 0.43

i2 M 34 5.1 0.30

35 5.1 0.38

42 5.2 0.32

64 5.3 0.39

F 44 5.1 0.33

35 5.2 0.36

43 5.2 0.29

69 5.2 0.33

c M 45 7.0 0.35

42 6.8 0.42

54 6.9 0.32

65 6.9 0.36

F 51 6.6 0.44

39 6.8 0.36

74 6.7 0.34

69 6.7 0.35

m1 M 44 7.2 0.39

42 7.0 0.49

40 7.1 0.41

64 7.1 0.38

F 51 6.8 0.36

39 6.9 0.49

63 7.0 0.43

68 7.0 0.36

m2 M 45 9.0 0.42

41 8.7 0.54

42 8.9 0.40

63 9.1 0.46

F 52 8.5 0.36

39 8.7 0.44

76 8.8 0.47

68 8.8 0.55

Lower i1 M 22 4.1 0.32

23 4.0 0.26

17 4.0 0.31

64 4.1 0.30

F 27 3.9 0.29

25 4.0 0.30

24 4.0 0.30

68 4.0 0.30

i2 M 33 4.6 0.32

34 4.6 0.33

36 4.6 0.37

65 4.7 0.35

F 41 4.4 0.33

33 4.5 0.38

37 4.6 0.28

69 4.6 0.39

c M 46 6.0 0.35

41 5.8 0.40

53 5.9 0.32

65 5.9 0.32

F 51 5.8 0.27

39 5.7 0.34

66 5.7 0.26

68 5.7 0.35

m1 M 43 8.0 0.39

39 7.8 0.57

43 7.8 0.40

65 7.8 0.42

F 51 7.6 0.35

38 7.7 0.38

60 7.7 0.36

69 7.7 0.35

m2 M 45 10.1 0.44

40 10.1 0.49

43 10.0 0.47

63 9.8 0.52

F 51 9.8 0.37

39 9.8 0.45

65 9.8 0.50

69 9.6 0.49

Permanent

upper I1 M 45 8.7 0.56

40 8.7 0.47

104 8.8 0.53

87 8.8 0.46

F 52 8.4 0.53

38 8.4 0.52

125 8.6 0.55

87 8.4 0.53

C M 29 8.1 0.41

21 8.0 0.32

82 8.0 0.43

87 8.0 0.42

F 33 7.6 0.34

24 7.8 0.62

81 7.7 0.35

85 7.5 0.37

PM2 M 30 6.9 0.23

26 6.8 0.36

74 6.8 0.44

86 6.8 0.37

F 38 6.7 0.32

20 6.6 0.50

77 6.6 0.34

81 6.6 0.43

M1 M 43 10.5 0.47

37 10.3 0.55

81 10.7 0.50

83 10.8 0.56

F 50 10.0 0.47

36 10.0 0.48

107 10.4 0.47

85 10.5 0.51

M2 M 19 10.5 0.57

4 10.0 0.53

65 10.3 0.57

65 10.4 0.63

F 9 9.7 0.54

8 9.8 0.65

47 10.0 0.60

50 9.8 0.49

Lower

I1 M 43 5.4 0.39

38 5.4 0.26

100 5.5 0.32

85 5.4 0.31

F 49 5.3 0.29

37 5.3 0.37

123 5.4 0.31

87 5.3 0.36

I2 M 39 6.0 0.37

38 6.0 0.32

102 6.0 0.35

85 6.0 0.38

F 48 5.8 0.37

36 5.8 0.39

122 6.0 0.37

87 5.8 0.38

C M 33 7.2 0.47

28 7.1 0.34

87 7.0 0.40

84 7.0 0.36

F 38 6.6 0.37

28 6.7 0.43

87 6.7 0.33

87 6.5 0.32

PM2 M 32 7.5 0.40

28 7.4 0.39

79 7.3 0.46

82 7.3 0.52

F 36 7.1 0.40

25 7.1 0.46

78 7.0 0.38

83 7.0 0.40

M1 M 39 11.4 0.66

38 11.2 0.60

72 11.4 0.59

76 11.2 0.47

F 45 10.7 0.58

34 10.8 0.66

74 11.0 0.63

84 10.7 0.56

M2 M 16 11.0 0.45

3 10.4 0.81

54 10.9 0.65

53 10.8 0.71

F 15 10.2 0.37 12 10.4 0.66 50 10.5 0.63 53 10.3 0.62

i1=primary central incisor; i2=primary lateral incisor; c=primary canine; m1=primary first molar; m2=primary second molar;

I1=permanent central incisor; I2=permanent lateral incisor; C=permanent canine; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; n=sample size; mean=mean values; SD=standard deviation. a (Townsend et al., 1986), b (Moorrees et al., 1957).

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Table 6.18: Buccolingual (BL) dimensions of primary and permanent teeth in different populations measured

in mm.

MZ

DZSS

Caucasians a

North American

Whites b,c

n mean SD n mean SD n mean SD n mean SD

Primary Upper

i1 M 31 5.2 0.28

35 5.0 0.31

30 5.0 0.29

69 5.1 0.43

F 43 4.9 0.33

30 4.9 0.40

36 4.9 0.29

64 5.2 0.48

i2 M 36 4.9 0.40

36 4.8 0.38

42 4.8 0.39

69 4.7 0.40

F 4 4.7 0.33

36 4.7 0.39

41 4.7 0.36

64 4.6 0.39

c M 44 6.3 0.34

42 6.1 0.49

54 6.2 0.37

69 6.1 0.40

F 52 6.1 0.41

39 6.0 0.36

72 6.1 0.40

64 6.0 0.41

m1 M 44 9.0 0.38

42 8.6 0.47

48 8.8 0.42

69 8.8 0.50

F 52 8.5 0.35

38 8.6 0.35

69 8.5 0.42

64 8.6 0.55

m2 M 46 10.1 0.40

41 9.9 0.45

50 10.0 0.44

69 9.5 0.49

F 52 9.7 0.38

39 9.6 0.44

78 9.8 0.45

64 9.4 0.45

Lower i1 M 22 3.9 0.27

24 3.8 0.26

16 3.7 0.29

69 3.9 0.38

F 26 3.6 0.33

25 3.8 0.25

22 3.7 0.25

64 3.8 0.35

i2 M 33 4.4 0.28

36 4.3 0.33

34 4.3 0.22

69 4.4 0.38

F 40 4.3 0.28

32 4.2 0.28

35 4.3 0.23

64 4.4 0.28

c M 46 5.7 0.33

41 5.6 0.40

51 5.7 0.31

69 5.6 0.31

F 51 5.6 0.38

39 5.5 0.33

65 5.6 0.32

64 5.6 0.40

m1 M 43 7.2 0.37

40 7.0 0.44

49 7.4 0.43

69 7.4 0.48

F 51 6.8 0.41

38 6.8 0.33

65 7.2 0.41

64 7.3 0.44

m2 M 46 8.8 0.37

42 8.6 0.41

48 8.9 0.48

69 8.9 0.40

F 51 8.2 0.42

39 8.4 0.36

72 8.7 0.43

64 8.7 0.43

Permanent

upper I1 M 45 7.3 0.58

40 7.1 0.63

81 7.3 0.52

33 7.1 0.60

F 48 7.1 0.46

36 6.9 0.61

85 7.2 0.46

22 6.9 0.40

C M 26 8.4 0.61

21 8.1 0.55

81 8.6 0.55

33 8.1 0.70

F 31 7.8 0.40

23 7.9 0.67

78 8.2 0.45

22 7.9 0.50

PM2 M 30 9.8 0.51

26 9.5 0.55

83 9.5 0.60

33 9.3 0.60

F 38 9.2 0.34

21 9.2 0.61

82 9.2 0.54

22 9.0 0.60

M1 M 44 11.9 0.54

37 11.6 0.54

98 11.8 0.55

33 11.2 0.50

F 50 11.1 0.48

35 11.2 0.59

121 11.4 0.50

22 10.8 0.40

M2 M 19 12.0 0.63

3 11.9 0.56

69 11.8 0.72

33 - -

F 11 11.0 0.41

12 11.1 0.59

64 11.4 0.63

22 - -

Lower

I1 M 41 6.3 0.45

40 6.0 0.47

78 6.2 0.45

33 6.0 0.40

F 48 5.9 0.38

36 5.9 0.49

91 6.0 0.42

22 5.9 0.80

I2 M 37 6.6 0.42

38 6.3 0.53

78 6.4 0.42

33 6.2 0.40

F 49 6.2 0.45

35 6.2 0.58

89 6.3 0.41

22 6.0 0.30

C M 29 7.8 0.64

24 7.5 0.74

73 7.9 0.53

33 7.4 0.60

F 37 7.2 0.48

27 7.2 0.54

77 7.5 0.51

22 7.1 0.40

PM2 M 32 8.7 0.44

28 8.7 0.56

78 8.6 0.54

33 8.4 0.50

F 36 8.3 0.52

24 8.2 0.53

85 8.4 0.53

22 8.3 0.50

M1 M 42 10.6 0.51

39 10.5 0.47

92 10.8 0.49

33 10.7 0.40

F 49 9.9 0.48

36 9.9 0.50

116 10.5 0.47

22 10.3 0.40

M2 M 17 10.8 0.54

5 10.3 0.68

67 10.7 0.60

33 - -

F 22 9.8 0.54 15 10.1 0.56 58 10.3 0.55 22 - -

i1=primary central incisor; i2=primary lateral incisor; c=primary canine; m1=primary first molar; m2=primary second molar;

I1=permanent central incisor; I2=permanent lateral incisor; C=permanent canine; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; n=sample size; mean=mean values; SD=standard deviation. a (Townsend et al., 1986), b primary dentition (Black, 1978), c permanent dentition (Bishara et al., 1989).

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Table 6.19: Crown height (CH) dimensions of primary and permanent teeth in different

populations measured in mm.

MZ

DZSS

Spanish a

n mean SD n mean SD n mean SD

Primary Upper

i1 M 18 5.6 0.52

12 6.0 0.65

F 35 5.3 0.66

31 5.3 0.58

i2 M 16 5.1 0.41

19 5.0 0.64

F 43 4.7 0.57

35 4.8 0.52

c M 3 6.3 0.96

2 5.6 0.47

F 42 5.4 0.62

29 5.5 0.57

m1 M 10 4.7 0.33

11 4.6 0.28

75 4.9 0.51

F 47 4.2 0.40

34 4.3 0.48

76 4.7 0.39

m2 M 19 4.2 0.38

17 4.1 0.44

88 4.9 0.46

F 48 3.8 0.38

38 3.9 0.53

111 4.7 0.45

Lower i1 M 11 5.0 0.42

7 4.7 0.33

F 24 4.8 0.64

24 4.5 0.43

i2 M 20 5.3 0.43

16 5.4 0.40

F 40 5.1 0.58

32 5.0 0.45

c M 10 6.2 0.51

5 5.9 0.38

F 45 5.8 0.49

28 5.9 0.37

m1 M 4 5.1 0.35

5 5.5 0.74

60 5.2 0.56

F 49 4.7 0.43

36 4.8 0.38

65 5.2 0.44

m2 M 5 4.3 0.47

9 4.0 0.28

89 5.1 0.42

F 48 3.6 0.47

33 3.8 0.45

109 5.0 0.46

Permanent

upper I1 M 40 10.0 0.79

31 9.8 1.00

F 48 9.1 0.74

36 9.1 0.70

C M 18 9.1 0.85

10 9.3 0.85

F 25 8.1 0.82

24 8.7 0.99

PM2 M 24 6.3 0.76

24 5.9 0.70

F 31 5.7 0.67

20 5.5 0.73

M1 M 35 5.3 0.75

27 5.1 0.86

F 46 4.7 0.65

34 4.9 0.65

M2 M 13 5.1 0.66

3 5.1 0.61

F 9 4.7 0.62

10 5.2 0.63

Lower

I1 M 36 8.2 0.68

13 8.5 1.02

F 41 7.8 0.75

36 7.7 0.95

I2 M 27 8.1 0.82

16 8.0 0.73

F 42 7.5 0.70

36 7.2 0.92

C M 17 9.4 1.17

8 9.1 0.74

F 37 8.3 0.77

28 8.6 1.07

PM2 M 23 6.4 0.78

23 5.8 0.64

F 35 5.7 0.67

24 5.7 0.65

M1 M 14 5.1 0.60

8 5.1 0.63

F 43 4.7 0.65

33 4.8 0.60

M2 M 7 4.7 0.59

2 4.1 0.08 F 17 4.3 0.64 9 4.5 0.57

i1=primary central incisor; i2=primary lateral incisor; c=primary canine; m1=primary first molar; m2=primary

second molar; I1=permanent central incisor; I2=permanent lateral incisor; C=permanent canine; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; n=sample size;

mean=mean values; SD=standard deviation. a(Barberia et al., 2009).

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Table 6.20: Intercuspal (IC) dimensions of primary and permanent teeth in different populations measured in

mm.

MZ DZSS Australian Twins a

n mean SD n mean SD n mean SD

Primary Upper m1(ic1) M 6 4.4 0.35

11 4.4 0.27

F 44 4.1 0.30

33 4.1 0.31

m2(ic1) M 20 4.9 0.41

15 4.9 0.37

89 5.0 0.61

F 47 4.8 0.45

36 4.7 0.37

89 4.8 0.57

m2(ic2) M 21 4.0 0.46

16 3.9 0.30

90 4.4 0.46

F 47 4.0 0.36

36 3.9 0.35

90 4.3 0.55

m2(ic3) M 21 5.0 0.45

17 5.1 0.33

90 5.3 0.58

F 48 5.0 0.47

35 4.8 0.46

89 5.3 0.60

m2(ic4) M 20 3.7 0.36

16 3.6 0.29

89 4.2 0.44

F 47 3.7 0.36

35 3.6 0.29

89 4.2 0.50

Lower m1(ic1) M 5 2.4 0.25

3 2.5 0.43

F 42 2.6 0.38

31 2.4 0.34

m2(ic1) M 5 3.6 0.54

8 3.7 0.24

86 4.0 0.51

F 45 3.6 0.40

30 3.5 0.32

85 3.9 0.53

m2(ic2) M 4 3.4 0.48

7 3.7 0.41

84 3.7 0.36

F 45 3.3 0.35

31 3.4 0.32

84 3.6 0.38

m2(ic3) M 6 4.9 0.38

7 4.9 0.39

F 46 4.7 0.43

33 4.6 0.44

m2(ic4) M 7 4.4 0.44

8 4.9 0.27

86 5.1 0.47

F 47 4.5 0.36

32 4.6 0.46

87 5.1 0.53

Permanent Upper PM2(ICP) M 28 5.5 0.47

25 5.5 0.36

F 38 5.3 0.48

21 5.2 0.60

M1(IC1) M 36 6.2 0.39

26 6.2 0.49

91 6.0 0.65

F 50 6.0 0.56

35 5.8 0.44

91 5.7 0.64

M1(IC2) M 37 4.9 0.48

25 4.9 0.54

90 5.0 0.48

F 50 4.8 0.48

36 4.8 0.44

90 4.7 0.50

M1(IC3) M 37 6.3 0.50

26 6.2 0.58

F 51 6.0 0.61

36 6.0 0.48

M1(IC4) M 35 4.9 0.53

26 4.7 0.53

F 51 4.5 0.38

35 4.5 0.39

M2(IC1) M 16 6.6 0.46

2 6.2 0.25

F 13 6.0 0.49

12 5.9 0.50

M2(IC2) M 16 5.1 0.47

3 5.2 0.65

F 13 4.8 0.36

12 4.7 0.61

M2(IC3) M 9 6.5 0.80

1 6.7 -

F 8 6.0 0.48

4 6.0 0.78

M2(IC4) M 9 4.2 0.56

1 4.9 -

F 8 4.1 0.55

4 4.1 0.49

Lower PM2(ICP) M 27 4.4 0.54

24 4.5 0.49

F 36 4.2 0.51

24 4.0 0.52

M1(IC1) M 15 5.2 0.56

7 5.2 0.34

92 5.2 0.65

F 45 5.0 0.47

37 4.8 0.46

87 5.0 0.63

M1(IC2) M 12 4.9 0.61

6 4.4 0.34

90 4.5 0.53

F 44 4.3 0.45

36 4.3 0.30

88 4.4 0.55

M1(IC3) M 13 5.9 0.62

7 5.9 0.33

F 43 5.6 0.63

36 5.4 0.56

M1(IC4) M 17 5.7 0.56

8 5.9 0.53

89 6.1 0.68

F 45 5.7 0.37

37 5.7 0.54

85 6.1 0.58

M2(IC1) M 8 4.8 0.32

3 5.0 0.34

F 21 4.8 0.47

16 4.9 0.49

M2(IC2) M 7 5.2 0.32

3 5.3 0.59

F 21 4.7 0.50

15 4.8 0.41

M2(IC3) M 7 5.2 0.53

3 5.6 0.61

F 21 5.0 0.58

15 5.0 0.74

M2(IC4) M 7 5.9 0.24

3 6.0 0.46

F 21 5.2 0.60 15 5.7 0.46

m1=primary first molar; m2=primary second molar; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; (icp)=primary bucco-lingual intercuspal dimension; (ic1)=primary mesio-buccal/mesio-lingual intercuspal dimension;

(ic2)=primary mesio-buccal/disto-buccal intercuspal dimension; (ic3)=primary disto-buccal/disto-lingual intercuspal dimension;

(ic4)=primary mesio-lingual/disto-lingual intercuspal dimension; (ICP)=permanent bucco-lingual intercuspal dimension; (IC1)=permanent mesio-buccal/mesio-lingual intercuspal dimension; (IC2)=permanent mesio-buccal/disto-buccal intercuspal dimension;

(IC3)=permanent disto-uccal/disto/lingual intercuspal dimension; (IC4)=permanent mesio-lingual/disto-lingual intercuspal dimension;

n=sample size; mean=mean values; SD=standard deviation. a(Townsend et al., 2003).

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6.3. Discussion

Several studies have indicated that sex hormones might be an important

environmental factor in explaining the differences observed between males and females in

various physical features (Hines et al., 2002; Cohen-Bendahan et al., 2005a; Hines, 2006;

Knickmeyer and Baron-Cohen, 2006; Vuoksimaa et al., 2010b; Tapp et al., 2011),

although some researchers have claimed that the development of sexual dimorphism in the

human dentition occurs mainly due to the effects of the sex chromosomes (Guatelli-

Steinberg et al., 2008; Alvesalo, 2009).

Studies on levels of steroid hormones in normal males have shown that three surges

of testosterone occur during male development (for reference, see page 20). In this study,

the percentage of sexual dimorphism in the permanent dentition was found to be higher

than in the primary dentition for both the MZ and DZSS twins and this may be associated

with greater hormonal influence on permanent teeth that form over a longer period of time

than primary teeth.

The two human dentitions start to form at different times in utero. Primary teeth

start to develop around 4 - 6 weeks post-conception and continue their crown development

until around one year after birth, while permanent teeth start to form around 14 weeks post-

conception, and continue their formation until third molar crown calcification is

completed, at around 14 years of age (AlQahtani et al., 2010). During odontogenesis, the

developing tooth passes through different stages of formation, such as thickening and

specialization of the dental lamina, followed by bud, cap, and bell stages before crown

calcification starts (Nanci, 2003). The peak of the first testosterone surge occurs around 14

weeks post-conception, when primary teeth have already passed through the soft tissue

stages of tooth formation but before commencement of calcification. At this time, the

permanent teeth are just starting to develop. In addition, the primary dentition develops

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much faster, i.e. in a shorter period of time compared with the permanent dentition, and

consequently is less exposed to the effects of hormones either in utero or neonatally.

The magnitude and patterning of sexual dimorphism on tooth size varies according

to the dentition and tooth studied. In this study, the primary dentition displayed smaller

percentages of sexual dimorphism than the permanent dentition and this was evident for

both MZ and DZSS twin pairs and also for all variables studied, agreeing with a previous

study on sexual dimorphism in primary and permanent dentitions (Moorrees et al., 1957).

Permanent lower canines showed the highest percentage of sex dimorphism and lower

central incisors the lowest for MD dimensions in both MZ and DZSS twins, being

consistent with previous findings that permanent lower canines are the most dimorphic

teeth and lower central incisors the least for MD dimensions in the human dentition (Garn

et al., 1967b; Harris and Nweeia, 1980). It seems that the whole process of permanent

tooth development occurs under relatively high levels of testosterone influence in males

and this may be an explanation for the difference in sexual dimorphism found between

primary and permanent dentitions of the same individuals.

The tooth calcification process starts from the incisal/cuspal tip portion of the

crown and proceeds all the way to the cervical region and then to the root apex, so different

crown dimensions are defined at different times during tooth formation. For example, in

an upper central incisor, the MD dimension is determined soon after calcification starts,

while the BL dimension and CH are not determined until calcification of the entire tooth

crown is completed, some four years after the MD dimension is defined. In this study,

mesiodistal dimensions generally displayed smaller percentages of sexual dimorphism,

followed by BL dimensions and CH dimensions, and this pattern was evident for both twin

groups studied. This means that earlier-forming crown dimensions (MD) showed smaller

percentages of sexual dimorphism when compared to tooth dimensions that form later in

odontogenesis such as BL and CH. BL dimensions also tended to show less sexual

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dimorphism when compared to CH dimensions, possibly because the measurement of BL

dimensions does not always coincide with the most cervical aspect of the crown whereas

CH dimensions always extend to the cervical region. This indicates that dimensions that

form over a longer period of time tended to display more sexual dimorphism than earlier-

forming dimensions, possibly because later-forming dimensions are more likely to be

exposed to tooth-size increasing hormones than dimensions that form early on (Tables 6.9

and 6.10). Although previous reports showed no sex dimorphism for IC dimensions in a

study of Aboriginal Australians (Townsend, 1985), the present study found a small

percentage of sexual dimorphism for IC dimensions for both dentitions, with the primary

dentition showing smaller percentages than the permanent dentition. Differences in the

methodology used between these two studies, where IC dimensions in Aboriginal studies

were measured using calipers and in this study a 2D image analysis system was used,

might have contributed to this outcome. This suggests that even structures that form early

on during odontogenesis may be influenced by hormones, leading to small differences

between males and females.

Although crown dimensions are formed at different times, MD dimensions still

showed a relatively moderate to high correlation with BL dimensions in MZ and DZSS

twins, but only a moderate to low correlation with CH and IC dimensions (Table 6.16). A

possible explanation for this is that MD and BL crown dimensions reflect overall tooth

crown size in the same plane, thus being closely related to each other. In contrast, CH

dimensions are defined just before root formation commences and IC dimensions become

established very early on during the folding of the inner enamel epithelium before crown

calcification starts. They are, therefore, separated temporally in terms of crown formation.

Some researchers have stated that IC dimensions seem to be correlated according to

the order of cusp calcification (Kondo and Townsend, 2004; Takahashi et al., 2007). In

this study, correlations between IC dimensions in MZ twins were moderate to high, with

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IC1 presenting a high correlation with IC3, but a moderate correlation with IC2 and IC4

(Table 6.16). However, moderate to low correlations were found for all IC dimensions in

DZSS twins (Table 6.16). These findings are consistent with previous studies (Biggerstaff,

1975; 1976; Townsend, 1985; Harris and Dinh, 2006) and were evident in both the primary

and permanent dentitions, with primary dentition displaying smaller amounts of sexual

dimorphism than the permanent dentition for this trait.

Crown height dimensions (CH) showed the lowest correlations with MD and IC

dimensions but moderate correlations with BL dimensions. These low correlations may

have occurred due to the difficulty in locating the landmarks as this dimension is affected

by the position of the gingival tissues, by the inclination of the tooth, and by tooth wear.

Moreover, BL and CH are tooth crown dimensions that are established almost at the same

time during tooth development when crown calcification is almost completed.

The question of what produces sexual dimorphism in tooth size is not fully

answered. Whether the sex chromosomes themselves are responsible for producing sexual

dimorphism in tooth size or a combination of sex chromosomes and hormones produce the

variation within the dentition observed between males and females is uncertain. The

preliminary findings of our study of tooth size in twins are consistent with an influence of

male hormones on the human dentition, leading to greater sexual dimorphism in the

permanent dentition than the primary dentition and greater sexual dimorphism of certain

teeth and tooth dimensions. In the next chapter, this question will be addressed more

directly by exploring whether the surge in testosterone in utero has an effect on the females

from opposite-sex dizygotic twin pairs.

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7. Hormonal effects on tooth development in dizygotic

opposite-sex (DZOS) twin pairs

7.1. Introduction

A study using Rhesus monkeys found that prenatally androgenised females

displayed larger upper and lower permanent canines when compared with normal females,

suggesting that high levels of male hormone available in utero before tooth bud

development may play a role in masculinising the dentition (Zingeser and Phoenix, 1978).

Primary and permanent dentitions start to form at different times in utero; therefore, both

dentitions would be exposed to different levels of intrauterine hormones once the first

surge of testosterone occurs soon after testicular differentiation in males at around 7-9

weeks of gestation (Reyes et al., 1974; Knickmeyer and Baron-Cohen, 2006). Moreover,

sex hormones seem to affect each tooth dimension differently, as tooth dimensions that

form earlier, i.e. intercuspal distances, seem to display less sexual dimorphism than

dimensions that form later during tooth development (Townsend, 1985; Kondo and

Townsend, 2004; Kondo et al., 2005). Later-forming teeth also seem to present more

sexual dimorphism than teeth that form earlier, possibly as a result of increased sex

hormones produced by males than females (Gingerich, 1974).

Studies in animals have demonstrated that hormones can diffuse across the

amniotic membranes between fetuses and/or transfer through the placenta from the male

fetus into the maternal circulation (Miller, 1994; Ryan and Vandenbergh, 2002; Tapp et al.,

2011). In addition, intrauterine position seems to influence litters differently in relation to

prenatal hormone diffusion (Miller, 1994; Ryan and Vandenbergh, 2002). For example,

female rats which cohabitate the uterus with a male, or develop between males, show more

masculine effects of some physiological, behavioural and morphological traits, such as a

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decrease in the number of litters and pregnancies, increased aggressiveness and sense of

territoriality, and increased anogenital distance, when compared to normal females (Ryan

and Vandenbergh, 2002). It is suggested that the same effect occurs in humans (Miller,

1994), as females who share the uterus with a co-twin brother have been shown to display

a decrease in physiological oto-acoustic emission sensitivity (McFadden, 1993; 2011), an

increase in aggressiveness and adventure-seeking behaviour (Resnick et al., 1993; Miller

and Martin, 1995), decreased 2D:4D finger length ratios (Putz et al., 2004; van Anders et

al., 2006) and an increase in brain volumes (Peper et al., 2009) when compared to other

females. Evidence of increased permanent tooth crown size has also been reported in

females from opposite-sex dizygotic twins compared to normal females (Dempsey et al.,

1999b). However, the extent of any sex hormone contribution to differences in tooth size

between males and females is still to be established definitively, and some researchers have

indicated that the development of sex dimorphism occurs mainly due to the effects of the

sex chromosomes (Guatelli-Steinberg et al., 2008; Alvesalo, 2009).

In this chapter it is planned to describe tooth crown size in the primary and

permanent dentitions of dizygotic opposite-sex (DZOS) twins by making comparisons with

monozygotic (MZ) and dizygotic same-sex (DZSS) male and female twins from an

Australian sample of Caucasian ancestry. Dizygotic opposite-sex (DZOS) twins form a

unique twin group where male and female co-twins share the same intrauterine

environment and might be exposed to different hormonal levels, either male or female

hormones, during their development in utero compared with the other twin groups. This

prenatal environment might have an important effect on traits that develop in utero, such as

teeth, and possibly could interfere with traits that develop after birth. The general aim is to

describe the different tooth crown dimensions (mesiodistal, MB; buccolingual, BL; crown

height, CH; and intercuspal, IC) in terms of means, standard deviations (SD) and

coefficients of variation (CV) in both dentitions of DZOS co-twins, making comparisons

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between DZOS males with MZ males and DZSS males, as well as comparisons between

DZOS females with MZ females and DZSS female twins, and searching for any patterns or

trends suggestive of prenatal hormonal influence in tooth size between the sexes and

zygosities. Percentage differences between DZOS female and MZ female twin pairs as

well as between DZOS female and DZSS female twin pairs will be calculated for all tooth

dimensions studied while searching for any evidence of intrauterine hormone diffusion that

might result in increased tooth size in DZOS female twins. Percentages of sexual

dimorphism for each tooth dimension will also be calculated between males and females

from all zygosities to help understand the effects of intrauterine male hormones on tooth

dimensions by making associations with the onset and timing of formation of each tooth

dimension.

Univariate analyses have been largely used in odontometric studies to quantify

differences between different tooth crown dimensions, teeth, dentitions and sexes.

However, univariate methods assume independence between each of the variables of

interest. Measurements of different tooth crown dimensions tend to be correlated,

indicating that a multivariate analysis should be used to evaluate related groups of data

(Potter, 1972; Potter et al., 1981). Analysing data using a multivariate approach should

provide more accurate information on which crown dimension is contributing most to the

variation between the groups (Potter, 1972). In this study, taking account of the inter-

correlations between dental dimensions, multivariate analyses were also performed in

addition to the univariate analyses.

7.2. Methods

Initially, univariate analyses were used to make comparisons between male and

female twins from the different zygosities. The analyses included t-tests and F-tests with

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statistical significance set at p<0.05. The same univariate approach was used to assess

percentage differences between female twins from all zygosities as well as percentage of

sexual dimorphism between DZOS co-twins, MZ twins and DZSS twins.

Multivariate analysis (MANOVA) was also performed, analysing different tooth

crown dimensions (MD, BL, CH and IC dimensions) simultaneously. Independent

variables within the model included dentitions (primary and permanent), arches (upper and

lower), sides (right and left), and zygosities (MZ, DZSS and DZOS). The multivariate

analysis was performed on female data only because males showed no significant

differences between zygosities for any dimensions studied when univariate analyses were

performed (Chapter 6). Multivariate analysis of data from female twins from different

zygosities was performed to provide a better understanding of the possible effects of male

hormones on females from DZOS twin pairs and also to identify which crown dimensions

had contributed to this variation.

Multivariate analysis was initially performed for all crown dimensions (MD, BL,

CH and IC dimensions) simultaneously. However, when IC dimensions were included in

the multivariate analysis, there was a significant drop in the number of observations due to

the large amount of missing values for IC dimensions, thus compromising the power of the

multivariate analysis. Therefore, it was decided to exclude the IC dimensions for the

multivariate analysis in this study and only MD, BL and CH dimensions were considered.

7.3. Results

7.3.1. Univariate analysis

Sample sizes varied across all variables due to the fact that some participants did

not present with full primary or permanent dentitions when impressions were obtained.

Tooth wear and teeth that were not fully erupted also contributed to the smaller sample

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sizes for some teeth in both dentitions. Analysis of histograms showed that all variables

were normally distributed and descriptive statistics including means, standard deviations

(SD) and coefficients of variations (CV) were calculated.

Tables 7.1 – 7.4 present comparisons of MD, BL, CH and IC dimensions between

males from DZOS and MZ twins. Tables 7.5 – 7.8 present comparisons for all dimensions

between males from DZOS and DZSS twin pairs. Comparisons between females from

DZOS and MZ twin pairs are presented in Tables 7.9 – 7.12 while Tables 7.13 – 7.16

present comparisons between females from DZOS and DZSS twin pairs. Tables 7.17 –

7.20 present the percentage of sexual dimorphism for MD, BL, CH and IC dimensions

between males and females from DZOS co-twins, MZ and DZSS twins.

7.3.1.1. Comparisons between male twins from all zygosities

In general, tooth size data from males from the MZ and DZSS groups did not differ

from DZOS male twins, with most of the variables not displaying significant differences

between the three groups. No significant differences were found for MD dimensions in

both the primary and permanent dentitions between males from DZOS and MZ twins while

primary upper second molar displayed statistical significance for MD dimension between

DZOS and DZSS twin pairs (Tables 7.1 and 7.2)(p<0.05). Significant differences were

also found between males from DZOS and MZ twins for the BL dimension of the primary

upper first molars and lower left second molar while the same dimension showed statistical

significance for the permanent upper left central incisor between males from DZOS and

DZSS twin pairs (p<0.05) (Tables 7.3 and 7.4). Crown height (CH) also displayed

significant difference between males from DZOS and MZ twins for the permanent upper

left central incisor while permanent upper left first and second molars displayed significant

differences between males from DZOS and DZSS twins for the same dimension and for

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the permanent lower right second molar (Tables 7.5 and 7.6). Intercuspal (IC) dimensions

showed the highest number of statistically significant differences between males from

DZOS and MZ twin pairs as well as between males from DZOS and DZSS twin pairs and

these differences were more concentrated on the permanent dentitions (Tables 7.7 and 7.8).

Generally, DZOS males displayed smaller mean values (green highlight) for MD

dimensions compared with males from MZ twin pairs and larger mean values (red

highlight) for the same dimension compared with DZSS males, placing DZOS male twins

in an intermediate position between MZ and DZSS twins for the MD dimension. Similar

patterns were found for BL dimensions with DZOS males displaying smaller mean values

(green highlight) for BL dimensions compared with males from MZ twins and larger mean

values (red highlight) for the same dimension compared with DZSS males. No clear trend

was found for CH dimensions between both DZOS and MZ males and between DZOS and

DZSS male twins, while DZOS twins displayed smaller IC dimensions compared with

both MZ and DZSS male twins (green highlight).

Although statistically non-significant on a variable-by-variable basis, comparisons

between DZOS and MZ males indicated more “green highlights” for MD, BL, CH and IC

dimensions, suggesting that DZOS males had a tendency to display smaller tooth

dimensions than MZ males. On the other hand, comparisons between DZOS and DZSS

males showed more “red highlights” for MD, BL and CH dimensions, suggesting that

DZOS males had a tendency to display larger tooth dimensions than DZSS males. IC

dimensions displayed a similar pattern in this group. Overall, there was no clear trend to

indicate that tooth dimensions in males from DZOS pairs differed from those for males

from MZ or DZSS twin groups.

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Table 7.1: Comparison of mesiodistal (MD) dimensions in the primary and permanent dentitions of males

from dizygotic opposite-sex (DZOS) and monozygotic (MZ) twin pairs.

DZOS Males MZ Males



N mean SD CV n mean SD CV n mean SD CV n mean SD CV

Primary

Upper

i1 28 6.3* 0.39+ 6.2 30 6.3 0.44+ 7.0

29 6.4 0.40 6.2 33 6.4 0.40 6.2

i2 36 5.1* 0.38+ 7.5 32 5.1 0.37+ 7.2

34 5.1 0.30 5.8 33 5.1 0.33 6.4

c 40 6.8* 0.35+ 5.1 39 6.8 0.32+ 4.7

45 7.0 0.35 5.0 44 6.9 0.35 5.0

m1 38 7.0* 0.42+ 6.0 40 7.1 0.41+ 5.8

44 7.2 0.39 5.5 45 7.2 0.43 6.0

m2 40 8.9* 0.52+ 5.9 40 8.9 0.46+ 5.2

45 9.0 0.42 4.7 42 8.9 0.41 4.6

Lower

i1 18 4.1 0.34+ 8.3 16 4.1 0.35+ 8.6

22 4.1 0.32 7.8 22 4.1 0.32 7.7

i2 30 4.6 0.40+ 8.5 32 4.6 0.33+ 7.2

33 4.6 0.32 6.9 34 4.7 0.32 7.0

c 37 5.9 0.29+ 4.9 39 5.9 0.32+ 5.5

46 6.0 0.35 5.9 46 6.0 0.29 4.9

m1 39 7.9 0.48+ 6.0 39 7.9 0.44+ 5.6

43 8.0 0.39 4.9 45 8.0 0.41 5.1

m2 40 10.1 0.52+ 5.2 40 10.0 0.51+ 5.1

45 10.1 0.44 4.3 45 10.1 0.44 4.4

Permanent

Upper

I1 41 8.7 0.49+ 5.7 43 8.6 0.52+ 6.0

45 8.7 0.56 6.4 45 8.6 0.48 5.6

C 31 8.2 0.49+ 5.9 30 8.2 0.54+ 6.6

29 8.1 0.41 5.1 30 8.2 0.38 4.7

PM2 32 7.0 0.45+ 6.4 32 7.0 0.45+ 6.3

30 6.9 0.23 3.4 31 7.0 0.35 4.9

M1 40 10.4 0.59+ 5.6 41 10.5 0.54+ 5.2

43 10.5 0.47 4.4 44 10.5 0.53 5.0

M2 13 10.2 0.69+ 6.8 15 10.3 0.85+ 8.2

19 10.5 0.57 5.4 15 10.5 0.51 4.8

Lower

I1 42 5.4 0.33+ 6.0 42 5.4 0.31+ 5.7

43 5.4 0.39 5.2 42 5.5 0.34 6.2

I2 38 6.0 0.40+ 6.7 38 6.0 0.38+ 6.3

39 6.0 0.37 6.2 41 6.0 0.35 5.9

C 35 7.1 0.54+ 7.6 37 7.1 0.50+ 7.0

33 7.2 0.47 6.6 35 7.2 0.46 6.4

PM2 31 7.6 0.51+ 6.7 31 7.6 0.65+ 8.5

32 7.5 0.40 5.3 31 7.5 0.37 4.9

M1 38 11.3 0.75+ 6.6 39 11.3 0.68+ 6.0

39 11.4 0.66 5.8 43 11.4 0.62 5.4

M2 13 10.8 0.83+ 7.7 14 10.9 0.97+ 8.9 16 11.0 0.45 4.1 16 11.2 0.57 5.1


first molar; M2=permanent second molar; n=sample size; mean=mean values; SD=standard deviation; CV=coefficient of variation (* t-

test, + F-test: p<0.05) (red=larger mean values in DZOS males compared with MZ males, yellow=equal mean values, green=smaller mean values in DZOS males compared with MZ males).

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Table 7.2: Comparison of buccolingual (BL) dimension in the primary and permanent dentitions of males


DZOS Males MZ Males




Primary

Upper

i1 30 5.0* 0.38+ 7.5 31 5.1* 0.38+ 7.4

31 5.2 0.28 5.5 33 5.2 0.28 5.3

i2 36 4.8* 0.34+ 7.0 34 4.8* 0.37+ 7.7

36 4.9 0.40 8.1 35 4.8 0.41 8.6

c 40 6.2* 0.41+ 6.6 39 6.2* 0.43+ 6.9

44 6.3 0.34 5.4 44 6.3 0.33 5.3

m1 40 8.8* 0.45+ 5.1 40 8.7* 0.44+ 5.0

44 9.0 0.38 4.2 45 8.9 0.33 3.7

m2 40 10.0* 0.51+ 5.1 40 10.0* 0.41+ 4.1

46 10.1 0.40 3.9 45 10.1 0.41 4.1

Lower

i1 17 4.0* 0.34+ 8.7 18 3.9* 0.28+ 7.2

22 3.9 0.27 7.0 21 3.9 0.22 5.5

i2 31 4.4* 0.36+ 8.2 33 4.4* 0.33+ 7.6

33 4.4 0.28 6.4 34 4.4 0.27 6.1

c 37 5.6* 0.36+ 6.4 38 5.6* 0.38+ 6.7

46 5.7 0.33 5.8 46 5.7 0.30 5.3

m1 39 7.1* 0.36+ 5.1 39 7.2* 0.34+ 4.7

43 7.2 0.37 5.2 45 7.3 0.44 6.1

m2 40 8.7* 0.36+ 4.1 40 8.7* 0.36+ 4.1

46 8.8 0.37 4.2 45 8.9 0.38 4.3

Permanent

Upper

I1 39 7.3* 0.62+ 8.5 39 7.4* 0.57+ 7.7

45 7.3 0.58 8.0 44 7.3 0.60 8.2

C 32 8.3* 0.61+ 7.3 29 8.4* 0.70+ 8.4

26 8.4 0.61 7.3 28 8.6 0.49 5.8

PM2 32 9.6* 0.71+ 7.4 32 9.7* 0.63+ 6.5

30 9.8 0.51 5.2 31 9.8 0.49 5.0

M1 43 11.7* 0.61+ 5.2 44 11.7* 0.60+ 5.1

44 11.9 0.54 4.5 44 11.8 0.51 4.4

M2 14 11.8* 0.82+ 7.0 15 11.9* 1.04+ 8.7

19 12.0 0.63 5.3 15 12.1 0.74 6.1

Lower

I1 41 6.2* 0.48+ 7.7 41 6.2* 0.55+ 8.9

41 6.3 0.45 7.2 43 6.2 0.47 7.6

I2 38 6.5* 0.58+ 8.9 39 6.4* 0.55+ 8.7

37 6.6 0.42 6.4 38 6.5 0.40 6.1

C 33 7.6* 0.57+ 7.6 34 7.6* 0.54+ 7.1

29 7.8 0.64 8.3 34 7.8 0.64 8.2

PM2 31 8.7* 0.65+ 7.5 31 8.6* 0.60+ 6.9

32 8.7 0.44 5.0 31 8.7 0.47 5.4

M1 41 10.5* 0.51+ 4.9 41 10.5* 0.55+ 5.2

42 10.6 0.51 4.8 44 10.7 0.53 5.0

M2 16 10.6* 0.69+ 6.5 14 10.5* 0.74+ 7.0 17 10.8 0.54 5.0 16 10.8 0.47 4.4




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Table 7.3: Comparison of crown height (CH) dimensions in the primary and permanent dentitions of males


DZOS Males MZ Males




Primary

Upper

i1 12 5.6 0.47+ 8.3 9 5.8* 0.41 7.0

18 5.6 0.52 9.3 17 5.7 0.51 8.9

i2 18 5.0 0.52+ 10.3 18 5.0* 0.54 10.8

16 5.1 0.41 8.0 17 5.0 0.40 8.1

c 2 5.9 0.42+ 7.1 1 5.1* - -

3 6.3 0.96 14.9 3 6.0 0.19 3.2

m1 14 4.6 0.24+ 5.1 12 4.5* 0.39 8.8

10 4.7 0.33 7.0 6 4.8 0.33 6.9

m2 26 4.1 0.48+ 11.6 28 4.2* 0.45 10.7

19 4.2 0.38 9.0 15 4.3 0.44 10.3

Lower

i1 9 5.1 0.61+ 12.0 12 5.0* 0.58 11.6

11 5.0 0.42 8.4 11 5.0 0.54 10.7

i2 21 5.5 0.54+ 9.8 24 5.2* 0.59 11.3

20 5.3 0.43 8.1 22 5.3 0.53 10.2

c 6 6.0 0.34+ 5.6 12 6.2* 0.81 13.1

10 6.2 0.51 8.2 9 6.3 0.60 9.5

m1 17 5.1 0.47+ 9.2 16 4.9* 0.57 11.7

4 5.1 0.35 6.8 5 5.3 0.54 10.1

m2 17 4.0 0.36+ 9.1 20 3.8* 0.39 10.3

5 4.3 0.47 11.1 6 3.7 0.48 13.0

Permanent

Upper

I1 33 9.7 1.17+ 12.1 35 9.5* 1.06 11.1

40 10.0 0.79 7.9 41 10.0 0.85 8.5

C 26 9.0 1.1+ 12.2 17 9.5* 1.08 11.4

18 9.1 0.85 9.3 20 9.1 1.19 13.1

PM2 29 6.1 0.72+ 11.9 28 6.1* 0.83 13.7

24 6.3 0.76 12.0 28 6.1 0.69 11.3

M1 33 5.4 0.70+ 12.9 30 5.6* 0.77 13.7

35 5.3 0.75 14.3 30 5.4 0.70 12.9

M2 11 5.4 0.61+ 11.4 10 5.4* 0.60 11.1

13 5.1 0.66 13.0 11 5.5 0.55 10.1

Lower

I1 30 8.2 0.91+ 11.1 32 8.2* 0.93 11.3

36 8.2 0.68 8.3 35 8.2 0.83 10.1

I2 29 8.1 0.92+ 11.3 30 8.0* 0.84 10.5

27 8.1 0.82 10.1 32 8.1 0.71 8.7

C 22 8.8 1.09+ 12.3 21 9.1* 1.04 11.5

17 9.4 1.17 12.4 22 9.2 1.13 12.3

PM2 28 6.2 0.80+ 12.9 30 6.1* 0.72 11.7

23 6.4 0.78 12.3 19 6.4 0.87 13.6

M1 31 5.2 0.60+ 11.6 29 5.2* 0.57 10.9

14 5.1 0.60 11.9 12 4.9 0.67 13.9

M2 9 4.7 0.58+ 12.3 12 5.0* 0.54 10.9 7 4.7 0.59 12.6 6 4.8 0.59 12.3




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Table 7.4: Comparison of intercuspal (IC) dimensions in the primary and permanent dentitions of males


DZOS Males MZ Males




Primary

Upper

m1(ic1) 17 4.2* 0.32 7.7 15 4.2* 0.39 9.1

6 4.4 0.35 8.0 6 4.4 0.68 15.5

m2(ic1) 28 4.8* 0.42 8.8 30 4.8* 0.42 8.8

20 4.9 0.41 8.4 18 5.0 0.38 7.8

m2(ic2) 28 3.9* 0.37 9.6 30 3.9* 0.32 8.2

21 4.0 0.46 11.7 18 4.0 0.34 8.4

m2(ic3) 27 5.1* 0.56 11.0 29 4.9* 0.55 11.3

21 5.0 0.45 8.9 18 5.1 0.45 8.9

m2(ic4) 27 3.7* 0.30 8.1 29 3.7* 0.38 10.3

20 3.7 0.36 9.8 18 3.6 0.41 11.4

Lower

m1(ic1) 12 2.8* 0.33 11.8 13 2.5* 0.22 8.9

5 2.4 0.25 10.7 5 2.4 0.24 10.1

m2(ic1) 22 3.7* 0.35 9.6 20 3.7* 0.39 10.7

5 3.6 0.54 15.1 7 3.5 0.37 10.4

m2(ic2) 21 3.4* 0.29 8.7 20 3.4* 0.36 10.7

4 3.4 0.48 14.1 6 3.4 0.49 14.4

m2(ic3) 20 4.6* 0.38 8.2 21 4.6* 0.35 7.5

6 4.9 0.38 7.9 6 4.9 0.36 7.3

m2(ic4) 21 4.5* 0.46 10.2 21 4.5* 0.49 10.8

7 4.4 0.44 10.1 7 4.2 0.30 7.1

Permanent

Upper

PM2(ICP) 32 5.2* 0.53 10.2 31 5.3* 0.58 11.0

28 5.5 0.47 8.5 29 5.4 0.47 8.6

M1(IC1) 40 6.0* 0.50 8.4 40 6.0* 0.49 8.2

36 6.2 0.39 6.3 31 6.2 0.40 6.4

M1(IC2) 39 4.8* 0.39 8.2 41 4.8* 0.42 8.7

37 4.9 0.48 9.7 36 5.0 0.46 9.3

M1(IC3) 39 6.0* 0.51 8.7 39 5.9* 0.51 8.5

37 6.3 0.50 7.9 32 6.2 0.51 8.1

M1(IC4) 40 4.6* 0.39 8.5 38 4.7* 0.48 10.3

35 4.9 0.53 11.0 29 4.8 0.51 10.6

M2(IC1) 14 6.3* 0.59 9.4 15 6.4* 0.73 11.4

16 6.6 0.46 6.9 13 6.5 0.41 6.3

M2(IC2) 14 4.7* 0.59 12.5 15 4.7* 0.60 12.8

16 5.1 0.47 9.2 12 4.9 0.62 12.5

M2(IC3) 8 5.9* 0.72 12.2 9 6.1* 0.66 10.8

9 6.5 0.80 12.4 8 6.3 0.58 9.2

M2(IC4) 8 4.4* 0.79 18.1 9 4.6* 0.39 8.5

9 4.2 0.56 13.3 8 4.0 0.43 10.6

Lower

PM2(ICP) 31 4.1* 0.60 14.6 31 4.2* 0.50 11.9

27 4.4 0.54 12.2 23 4.3 0.47 10.9

M1(IC1) 33 5.0* 0.52 10.4 30 5.0* 0.57 11.5

15 5.2 0.56 10.6 10 5.0 0.49 9.8

M1(IC2) 31 4.4* 0.42 9.7 28 4.3* 0.35 8.2

12 4.9 0.61 12.5 10 4.5 0.45 10.0

M1(IC3) 34 5.7* 0.60 10.6 30 5.7* 0.61 10.7

13 5.9 0.62 10.5 15 5.8 0.41 7.0

M1(IC4) 37 5.7* 0.63 10.9 32 5.7* 0.55 9.8

17 5.7 0.56 9.8 15 5.5 0.50 9.0

M2(IC1) 15 4.8* 0.56 11.5 13 5.0* 0.65 13.1

8 4.8 0.32 6.5 8 5.3 0.35 6.6

M2(IC2) 15 5.1* 0.41 8.0 12 5.0* 0.41 8.2

7 5.2 0.32 6.2 5 5.1 0.19 3.7

M2(IC3) 13 5.2* 0.51 9.8 11 5.5* 0.47 8.5

7 5.2 0.53 10.3 5 5.4 0.67 12.4

M2(IC4) 13 5.6* 0.52 9.2 12 5.6* 0.62 11.2 7 5.9 0.24 4.1 7 5.6 0.47 8.4

m1=primary first molar; m2=primary second molar; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; (ic1)=bucco-lingual intercuspal dimension in primary first molars; (ic1)=primary mesiobuccal-mesiolingual intercuspal

dimension; (ic2)=primary mesiobuccal-distobuccal intercuspal dimension; (ic3)=primary mesiodistal-linguodistal intercuspal dimension;

(ic4)=primary mesiolingual-distolingual intercuspal dimension; (ICP)=permanent bucco-lingual intercuspal dimension; (IC1)=permanent mesiobuccal-mesiolingual intercuspal dimension; (IC2)=permanent mesiobuccal-distobuccal intercuspal dimension;

(IC3)=permanent distobuccal-distolingual intercuspal dimension; (IC4)=permanent mesiolingual-distolingual intercuspal dimension;

n=sample size; mean=mean values; SD=standard deviation; CV=coefficient of variation. (* t-test, + F-test: p<0.05) (red=larger mean values in DZOS males compared with MZ males, yellow=equal mean values, green=smaller mean values in DZOS males compared with

MZ males).

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Table 7.5: Comparison of mesiodistal (MD) dimensions in the primary and permanent dentitions of males

from dizygotic opposite-sex (DZOS) and dizygotic same-sex (DZSS) twin pairs.

DZOS Males DZSS Males




Primary

Upper

i1 28 6.3* 0.39+ 6.2 30 6.3 0.44+ 7.0

32 6.3 0.34 5.4 31 6.3 0.40 6.3

i2 36 5.1* 0.38+ 7.5 32 5.1 0.37+ 7.2

35 5.1 0.38 7.4 35 5.1 0.31 5.9

c 40 6.8* 0.35+ 5.1 39 6.8 0.32+ 4.7

42 6.8 0.42 6.1 42 6.8 0.42 6.2

m1 38 7.0* 0.42+ 6.0 40 7.1 0.41+ 5.8

42 7.0 0.49 7.0 42 7.1 0.43 6.1

m2 40 8.9* 0.52+ 5.9 40 8.9 0.46+ 5.2

41 8.7 0.54 6.2 40 8.7 0.45 5.2

Lower

i1 18 4.1* 0.34+ 8.3 16 4.1 0.35+ 8.6

23 4.0 0.26 6.3 23 4.0 0.25 6.1

i2 30 4.6* 0.40+ 8.5 32 4.6 0.33+ 7.2

34 4.6 0.33 7.0 31 4.6 0.32 6.9

c 37 5.9* 0.29+ 4.9 39 5.9 0.32+ 5.5

41 5.8 0.40 6.9 42 5.9 0.37 6.3

m1 39 7.9* 0.48+ 6.0 39 7.9 0.44+ 5.6

39 7.8 0.57 7.4 42 7.9 0.43 5.5

m2 40 10.1* 0.52+ 5.2 40 10.0 0.51+ 5.1

40 10.0 0.49 4.9 41 10.0 0.46 4.6

Permanent

Upper

I1 41 8.7* 0.49+ 5.7 43 8.6 0.52+ 6.0

40 8.7 0.47 5.4 38 8.6 0.48 5.5

C 31 8.2* 0.49+ 5.9 30 8.2 0.54+ 6.6

21 8.0 0.32 4.0 21 8.0 0.38 4.7

PM2 32 7.0* 0.45+ 6.4 32 7.0 0.45+ 6.3

26 6.8 0.36 5.3 25 6.9 0.34 4.9

M1 40 10.4* 0.59+ 5.6 41 10.5 0.54+ 5.2

37 10.3 0.55 5.3 37 10.4 0.61 5.9

M2 13 10.2* 0.69+ 6.8 15 10.3 0.85+ 8.2

4 10.0 0.53 5.3 6 10.0 0.50 4.9

Lower

I1 42 5.4* 0.33+ 6.0 42 5.4 0.31+ 5.7

38 5.4 0.26 4.8 39 5.5 0.32 5.8

I2 38 6.0* 0.40+ 6.7 38 6.0 0.38+ 6.3

38 6.0 0.32 5.4 38 5.9 0.34 5.7

C 35 7.1* 0.54+ 7.6 37 7.1 0.50+ 7.0

28 7.1 0.34 4.8 29 7.1 0.41 5.8

PM2 31 7.6* 0.51+ 6.7 31 7.6 0.65+ 8.5

28 7.4 0.39 5.3 28 7.4 0.38 5.1

M1 38 11.3* 0.75+ 6.6 39 11.3 0.68+ 6.0

38 11.2 0.60 5.3 38 11.2 0.61 5.5

M2 13 10.8* 0.83+ 7.7 14 10.9 0.97+ 8.9 3 10.4 0.81 7.8 5 10.6 0.86 8.1



test, + F-test: p<0.05) (red=larger mean values in DZOS males compared with DZSS males, yellow=equal mean values, green=smaller mean values in DZOS males compared with DZSS males).

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Table 7.6: Comparison of buccolingual (BL) dimensions in the primary and permanent dentitions of males

from dizygotic opposite-sex (DZOS) and diygotic same-sex (DZSS) twin pairs.





Primary

Upper

i1 30 5.0 0.38 7.5 31 5.1* 0.38 7.4

35 5.0 0.31 6.2 33 5.1 0.34 6.7

i2 36 4.8 0.34 7.0 34 4.8* 0.37 7.7

36 4.8 0.38 7.9 36 4.8 0.35 7.4

c 40 6.2 0.41 6.6 39 6.2* 0.43 6.9

42 6.1 0.49 8.0 42 6.0 0.50 8.4

m1 40 8.8 0.45 5.1 40 8.7* 0.44 5.0

42 8.6 0.47 5.5 42 8.6 0.42 4.8

m2 40 10.0 0.51 5.1 40 10.0* 0.41 4.1

41 9.9 0.45 4.6 42 9.9 0.45 4.6

Lower

i1 17 4.0 0.34 8.7 18 3.9* 0.28 7.2

24 3.8 0.26 6.8 26 3.8 0.23 6.2

i2 31 4.4 0.36 8.2 33 4.4* 0.33 7.6

36 4.3 0.33 7.6 37 4.3 0.31 7.2

c 37 5.6 0.36 6.4 38 5.6* 0.38 6.7

41 5.6 0.40 7.1 41 5.6 0.39 7.0

m1 39 7.1 0.36 5.1 39 7.2* 0.34 4.7

40 7.0 0.44 6.3 42 7.1 0.41 5.8

m2 40 8.7 0.36 4.1 40 8.7* 0.36 4.1

42 8.6 0.41 4.8 41 8.6 0.41 4.7

Permanent

Upper

I1 39 7.3 0.62 8.5 39 7.4* 0.57 7.7

40 7.1 0.63 8.9 40 7.1 0.60 8.5

C 32 8.3 0.61 7.3 29 8.4* 0.70 8.4

21 8.1 0.55 6.7 21 8.1 0.67 8.2

PM2 32 9.6 0.71 7.4 32 9.7* 0.63 6.5

26 9.5 0.55 5.8 25 9.5 0.48 5.0

M1 43 11.7 0.61 5.2 44 11.7* 0.60 5.1

37 11.6 0.54 4.7 38 11.6 0.51 4.4

M2 14 11.8 0.82 7.0 15 11.9* 1.04 8.7

3 11.9 0.56 4.7 6 11.7 0.81 6.9

Lower

I1 41 6.2 0.48 7.7 41 6.2* 0.55 8.9

40 6.0 0.47 7.8 41 6.0 0.52 8.7

I2 38 6.5 0.58 8.9 39 6.4* 0.55 8.7

38 6.3 0.53 8.4 38 6.3 0.52 8.2

C 33 7.6 0.57 7.6 34 7.6* 0.54 7.1

24 7.5 0.74 10.0 26 7.5 0.65 8.7

PM2 31 8.7 0.65 7.5 31 8.6* 0.60 6.9

28 8.7 0.56 6.5 28 8.6 0.53 6.2

M1 41 10.5 0.51 4.9 41 10.5* 0.55 5.2

39 10.5 0.47 4.4 38 10.5 0.43 4.1

M2 16 10.6 0.69 6.5 14 10.5* 0.74 7.0 5 10.3 0.68 6.6 7 10.3 0.76 7.4



test, + F-test: p<0.05) (red=larger mean values in DZOS males compared with DZSS males, yellow=equal mean values, green=smaller mean values in DZOS males compared with DZSS males).

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Table 7.7: Comparison of crown height (CH) dimensions in the primary and permanent dentitions of males






Primary

Upper

i1 12 5.6* 0.47+ 8.3 9 5.8* 0.41+ 7.0

12 6.0 0.65 10.9 15 5.9 0.53 9.0

i2 18 5.0* 0.52+ 10.3 18 5.0* 0.54+ 10.8

19 5.0 0.64 12.8 17 5.0 0.47 9.5

c 2 5.9* 0.42+ 7.1 1 5.1* - -

2 5.6 0.47 8.4 3 6.0 0.63 10.5

m1 14 4.6* 0.24+ 5.1 12 4.5* 0.39+ 8.8

11 4.6 0.28 6.0 7 4.5 0.47 10.3

m2 26 4.1* 0.48+ 11.6 28 4.2* 0.45+ 10.7

17 4.1 0.44 10.9 16 4.0 0.45 11.1

Lower

i1 9 5.1* 0.61+ 12.0 12 5.0* 0.58+ 11.6

7 4.7 0.33 7.0 12 4.7 0.51 10.9

i2 21 5.5* 0.54+ 9.8 24 5.2* 0.59+ 11.3

16 5.4 0.40 7.5 25 5.2 0.47 8.9

c 6 6.0* 0.34+ 5.6 12 6.2* 0.81+ 13.1

5 5.9 0.38 6.3 8 6.2 0.66 10.7

m1 17 5.1* 0.47+ 9.2 16 4.9* 0.57+ 11.7

5 5.5 0.74 13.3 6 5.2 0.50 9.6

m2 17 4.0* 0.36+ 9.1 20 3.8* 0.39+ 10.3

9 4.0 0.28 7.0 5 4.0 0.23 5.8

Permanent

Upper

I1 33 9.7* 1.17+ 12.1 35 9.5* 1.06+ 11.1

31 9.8 1.00 10.2 29 9.9 0.99 10.0

C 26 9.0* 1.10+ 12.2 17 9.5* 1.08+ 11.4

10 9.3 0.85 9.1 10 9.4 1.19 12.6

PM2 29 6.1* 0.72+ 11.9 28 6.1* 0.83+ 13.7

24 5.9 0.70 11.9 24 5.9 0.78 13.1

M1 33 5.4* 0.70+ 12.9 30 5.6* 0.77+ 13.7

27 5.1 0.86 17.0 27 5.2 0.65 12.5

M2 11 5.4* 0.61+ 11.4 10 5.4* 0.60+ 11.1

3 5.1 0.61 12.0 3 5.2 0.58 11.2

Lower

I1 30 8.2* 0.91+ 11.1 32 8.2* 0.93+ 11.3

13 8.5 1.02 12.0 14 8.3 0.98 11.8

I2 29 8.1* 0.92+ 11.3 30 8.0* 0.84+ 10.5

16 8.0 0.73 9.2 14 8.1 0.72 8.9

C 22 8.8* 1.09+ 12.3 21 9.1* 1.04+ 11.5

8 9.1 0.74 8.1 14 8.7 0.91 10.5

PM2 28 6.2* 0.80+ 12.9 30 6.1* 0.72+ 11.7

23 5.8 0.64 11.0 24 6.1 0.66 10.9

M1 31 5.2* 0.60+ 11.6 29 5.2* 0.57+ 10.9

8 5.1 0.63 12.4 6 5.4 0.47 8.6

M2 9 4.7* 0.58+ 12.3 12 5.0* 0.54+ 10.9 2 4.1 0.08 1.9 2 4.7 0.27 5.7



test, + F-test: p<0.05) (red=larger mean values in DZOS males compared with DZSS males, yellow=equal mean values, green=smaller mean values).

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Table 7.8: Comparison of intercuspal (IC) dimensions in the primary and permanent dentitions of males from

dizygotic opposite-sex (DZOS) and dizygotic same-sex (DZSS) twin pairs.





Primary

Upper

m1(ic1) 17 4.2* 0.32+ 7.7 15 4.2* 0.39+ 9.1

11 4.4 0.27 6.1 7 4.5 0.20 4.5

m2(ic1) 28 4.8* 0.42+ 8.8 30 4.8* 0.42+ 8.8

15 4.9 0.37 7.5 16 5.0 0.32 6.4

m2(ic2) 28 3.9* 0.37+ 9.6 30 3.9* 0.32+ 8.2

16 3.9 0.30 7.8 17 4.0 0.45 11.4

m2(ic3) 27 5.1* 0.56+ 11.0 29 4.9* 0.55+ 11.3

17 5.1 0.33 6.5 16 5.0 0.39 8.0

m2(ic4) 27 3.7* 0.30+ 8.1 29 3.7* 0.38+ 10.3

16 3.6 0.29 8.0 15 3.7 0.44 12.1

Lower

m1(ic1) 12 2.8* 0.33+ 11.8 13 2.5* 0.22+ 8.9

3 2.5 0.43 17.3 3 2.8 0.15 5.4

m2(ic1) 22 3.7* 0.35+ 9.6 20 3.7* 0.39+ 10.7

8 3.7 0.24 6.4 4 3.9 0.28 7.2

m2(ic2) 21 3.4* 0.29+ 8.7 20 3.4* 0.36+ 10.7

7 3.7 0.41 11.1 4 3.4 0.28 8.4

m2(ic3) 20 4.6* 0.38+ 8.2 21 4.6* 0.35+ 7.5

7 4.9 0.39 8.0 5 5.0 0.44 8.7

m2(ic4) 21 4.5* 0.46+ 10.2 21 4.5* 0.49+ 10.8

8 4.9 0.27 5.5 5 4.7 0.36 7.7

Permanent

Upper

PM2(ICP) 32 5.2* 0.53+ 10.2 31 5.3* 0.58+ 11.0

25 5.5 0.36 6.6 24 5.4 0.39 7.4

M1(IC1) 40 6.0* 0.50+ 8.4 40 6.0* 0.49+ 8.2

26 6.2 0.49 7.9 28 6.2 0.51 8.3

M1(IC2) 39 4.8* 0.39+ 8.2 41 4.8* 0.42+ 8.7

25 4.9 0.54 11.0 28 4.8 0.48 9.9

M1(IC3) 39 6.0* 0.51+ 8.7 39 5.9* 0.51+ 8.5

26 6.2 0.58 9.4 27 6.1 0.60 9.8

M1(IC4) 40 4.6* 0.39+ 8.5 38 4.7* 0.48+ 10.3

26 4.7 0.53 11.2 27 4.7 0.57 12.0

M2(IC1) 14 6.3* 0.59+ 9.4 15 6.4* 0.73+ 11.4

2 6.2 0.25 4.0 3 6.3 0.41 6.6

M2(IC2) 14 4.7* 0.59+ 12.5 15 4.7* 0.60+ 12.8

3 5.2 0.65 12.4 4 5.0 0.21 4.3

M2(IC3) 8 5.9* 0.72+ 12.2 9 6.1* 0.66+ 10.8

1 6.7 - - 3 6.5 0.71 11.1

M2(IC4) 8 4.4* 0.79+ 18.1 9 4.6* 0.39+ 8.5

1 4.9 - - 2 4.8 0.05 1.0

Lower

PM2(ICP) 31 4.1* 0.60+ 14.6 31 4.2* 0.50+ 11.9

24 4.5 0.49 10.9 27 4.3 0.46 10.7

M1(IC1) 33 5.0* 0.52+ 10.4 30 5.0* 0.57+ 11.5

7 5.2 0.34 6.5 7 5.3 0.18 3.3

M1(IC2) 31 4.4* 0.42+ 9.7 28 4.3* 0.35+ 8.2

6 4.4 0.34 7.6 6 4.6 0.29 6.3

M1(IC3) 34 5.7* 0.60+ 10.6 30 5.7* 0.61+ 10.7

7 5.9 0.33 5.6 6 6.0 0.45 7.6

M1(IC4) 37 5.7* 0.63+ 10.9 32 5.7* 0.55+ 9.8

8 5.9 0.53 9.0 7 6.0 0.42 7.0

M2(IC1) 15 4.8* 0.56+ 11.5 13 5.0* 0.65+ 13.1

3 5.0 0.34 6.8 1 5.1 - -

M2(IC2) 15 5.1* 0.41+ 8.0 12 5.0* 0.41+ 8.2

3 5.3 0.59 11.2 1 5.0 - -

M2(IC3) 13 5.2* 0.51+ 9.8 11 5.5* 0.47+ 8.5

3 5.6 0.61 10.9 2 5.6 0.54 9.7

M2(IC4) 13 5.6* 0.52+ 9.2 12 5.6* 0.62+ 11.2 3 6.0 0.46 7.7 2 6.0 0.21 3.4





n=sample size; mean=mean values; SD=standard deviation; CV=coefficient of variation. (* t-test, + F-test: p<0.05) (red=larger mean values in DZOS males compared with DZSS males, yellow=equal mean values, green=smaller mean values).

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7.3.1.2. Comparisons between female twins from all zygosities

7.3.1.2.1. Comparison between females from dizygotic opposite-sex (DZOS)

and monozygotic (MZ) twin pairs

Overall, females from DZOS twins displayed larger mean values for MD, BL and

CH dimensions in the primary and permanent dentitions compared with MZ females (red

highlight). However, the same pattern was not evident for IC dimension (Tables 7.9 -

7.12). Statistically significant differences between DZOS and MZ female twins were

found for MD dimensions in the primary dentition for the lower left central incisors, lower

lateral incisors and for permanent upper right canines, upper right second premolars, upper

right first molars, and lower right first molars (Table 7.9). Significant differences were

also found for BL dimensions between females from DZOS and MZ twins in the primary

dentition for the lower left lateral incisors and lower second molars while the same

dimension displayed statistical significance in the permanent dentition for the upper right

canines, and upper second molars, and for the lower right lateral incisors and lower first

molars (Table 7.10). Crown height (CH) dimensions also displayed statistically significant

differences between females from DZOS and MZ twins in the primary dentition for the

right lateral incisors and left second molars in the upper arch and for the second molars in

the lower arch. Statistical significance was also found for CH dimensions in the permanent

dentition in the upper arch for the right canines, first molars and second molars while the

lower arch displayed statistical significance for the right second premolar and first molars

(Table 7.11). Intercuspal (IC) dimensions displayed no significant differences in this

group, except for the ic2 dimension in primary upper left second molar (Table 7.12).

Although the patterns of differences varied across dentitions, there was a definite

trend for females from DZOS pairs to display large tooth dimensions than females from

MZ pairs, except for IC dimensions. IC dimensions in DZOS females displayed a mixture

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of being smaller, equal to and larger than corresponding values in MZ female twins. The

early-forming IC dimensions seemed to display a different pattern compared with the other

dimensions studied as more similar mean values were found in the primary dentition

(yellow highlight) while the permanent dentition displayed a combination of smaller mean

values (green highlight) and equal mean values (yellow highlight). Given that IC

dimensions are the first dimensions to form during odontogenesis, it is expected that these

dimensions in DZOS female co-twins might be exposed to male hormone for a shorter

period of time during development.

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Table 7.9: Comparison of mesiodistal (MD) dimensions in the primary and permanent dentitions of females


DZOS Females MZ Females




Primary

Upper

i1 25 6.2* 0.41+ 6.5 29 6.2* 0.46+ 7.5

40 6.2 0.39 6.3 41 6.2 0.37 6.0

i2 32 5.2* 0.40+ 7.8 34 5.2* 0.37+ 7.2

44 5.1 0.33 6.5 45 5.1 0.31 6.2

c 40 6.7* 0.34+ 5.1 40 6.7* 0.35+ 5.3

51 6.6 0.44 6.6 52 6.6 0.52 7.9

m1 39 6.8* 0.35+ 5.2 39 6.9* 0.41+ 5.9

51 6.8 0.36 5.3 52 6.9 0.39 5.7

m2 39 8.7* 0.40+ 4.7 39 8.7* 0.44+ 5.1

52 8.5 0.36 4.3 52 8.6 0.36 4.2

Lower

i1 15 4.0* 0.26+ 6.5 17 4.1* 0.28+ 7.0

27 3.9 0.29 7.4 24 3.8 0.26 6.9

i2 27 4.6* 0.22+ 4.7 28 4.6* 0.29+ 6.3

41 4.4 0.33 7.5 38 4.4 0.33 7.5

c 40 5.8* 0.31+ 5.4 40 5.8* 0.33+ 5.6

51 5.8 0.27 4.7 51 5.7 0.30 5.3

m1 40 7.8* 0.46+ 6.0 40 7.7* 0.47+ 6.1

51 7.6 0.35 4.7 49 7.6 0.38 5.0

m2 42 9.9* 0.48+ 4.9 40 9.8* 0.45+ 4.6

51 9.8 0.37 3.8 51 9.7 0.37 3.8

Permanent

Upper

I1 42 8.5* 0.58+ 6.9 42 8.5* 0.56+ 6.7

52 8.4 0.53 6.2 51 8.4 0.50 5.9

IC 27 7.8* 0.47+ 6.0 32 7.7* 0.47+ 6.1

33 7.6 0.34 4.5 33 7.6 0.40 5.3

PM2 28 6.9* 0.48+ 6.9 30 6.9* 0.45+ 6.6

38 6.7 0.32 4.8 36 6.7 0.36 5.3

M1 42 10.3* 0.52+ 5.0 41 10.2* 0.53+ 5.2

50 10.0 0.47 4.6 51 10.1 0.42 4.2

M2 15 10.0* 0.41+ 4.1 9 10.0* 0.55+ 5.6

9 9.7 0.54 5.3 9 9.6 0.40 4.1

Lower

I1 35 5.4* 0.34+ 6.2 37 5.3* 0.35+ 6.6

49 5.3 0.29 5.5 48 5.3 0.29 5.4

I2 32 5.9* 0.32+ 5.5 36 5.9* 0.41+ 6.9

48 5.8 0.37 6.3 49 5.8 0.34 5.9

C 32 6.8* 0.42+ 6.3 32 6.7* 0.40+ 5.9

38 6.6 0.37 5.7 39 6.6 0.35 5.4

PM2 28 7.3* 0.55+ 7.6 26 7.3* 0.48+ 6.6

36 7.1 0.40 5.6 35 7.2 0.40 5.5

M1 35 11.0* 0.59+ 5.4 36 11.0* 0.65+ 5.9

45 10.7 0.58 5.4 50 10.8 0.45 4.5

M2 9 10.1* 0.60+ 5.9 12 10.5* 0.76+ 7.3 15 10.2 0.37 3.7 16 10.2 0.48 4.7



test, + F-test: p<0.05) (red=larger mean values in DZOS females compared with MZ females, yellow=equal mean values, green=smaller mean values).

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Table 7.10: Comparison of buccolingual (BL) dimensions in the primary and permanent dentitions of females






Primary

Upper

i1 27 5.0* 0.34+ 7.0 29 5.0* 0.41+ 8.1

43 4.9 0.33 6.8 41 5.0 0.31 6.3

i2 33 4.9* 0.37+ 7.6 34 4.8* 0.35+ 7.3

45 4.7 0.33 7.1 46 4.8 0.34 7.2

c 40 6.1* 0.40+ 6.4 40 6.1* 0.36+ 5.8

52 6.1 0.41 6.8 52 6.1 0.40 6.7

m1 41 8.5* 0.38+ 4.4 41 8.6* 0.41+ 4.8

52 8.5 0.35 4.1 52 8.5 0.32 3.7

m2 40 9.7* 0.43+ 4.4 41 9.7* 0.42+ 4.3

52 9.7 0.38 3.9 52 9.6 0.39 4.0

Lower

i1 17 3.8* 0.24+ 6.4 18 3.8* 0.21+ 5.6

26 3.6 0.33 9.0 25 3.7 0.27 7.4

i2 27 4.4* 0.27+ 6.2 29 4.4* 0.26+ 5.9

40 4.3 0.28 6.6 39 4.3 0.28 6.6

c 39 5.7* 0.40+ 7.0 40 5.6* 0.36+ 6.4

51 5.6 0.38 6.8 51 5.6 0.39 7.0

m1 40 7.0* 0.44+ 6.3 40 7.0* 0.37+ 5.3

51 6.8 0.41 6.0 50 7.0 0.37 5.4

m2 41 8.6* 0.37+ 4.4 40 8.5* 0.35+ 4.1

51 8.2 0.42 5.1 51 8.4 0.39 4.6

Permanent

Upper

I1 41 7.1* 0.55+ 7.7 41 7.2* 0.57+ 8.0

48 7.1 0.46 6.4 47 7.1 0.52 7.4

C 23 8.1* 0.57+ 7.0 30 8.0* 0.62+ 7.8

31 7.8 0.40 5.2 33 7.8 0.45 5.8

PM2 28 9.4* 0.63+ 6.7 30 9.3* 0.65+ 7.0

38 9.2 0.34 3.7 36 9.3 0.33 3.6

M1 43 11.4* 0.52+ 4.5 43 11.4* 0.51+ 4.5

50 11.1 0.48 4.3 51 11.1 0.47 4.3

M2 15 11.4* 0.82+ 7.2 13 11.3* 0.77+ 6.9

11 11.0 0.41 3.8 12 10.9 0.39 3.6

Lower

I1 35 6.1* 0.52+ 8.6 36 6.1* 0.47+ 7.7

48 5.9 0.38 6.4 45 6.0 0.38 6.3

I2 35 6.5* 0.51+ 7.9 36 6.4* 0.43+ 6.7

49 6.2 0.45 7.3 47 6.2 0.44 7.0

C 29 7.2* 0.48+ 6.7 31 7.4* 0.53+ 7.2

37 7.2 0.48 6.7 38 7.3 0.49 6.7

PM2 28 8.5* 0.62+ 7.3 26 8.5* 0.56+ 6.6

36 8.3 0.52 6.3 35 8.3 0.53 6.4

M1 38 10.2* 0.42+ 4.1 37 10.3* 0.46+ 4.4

49 9.9 0.48 4.8 50 9.9 0.45 4.5

M2 15 10.2* 0.67+ 6.5 15 10.2* 0.62+ 6.1 22 9.8 0.54 5.6 23 9.9 0.54 5.4




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Table 7.11: Comparison of crown height (CH) dimensions in the primary and permanent dentitions of

females from dizygotic opposite-sex (DZOS) and monozygotic (MZ) twin pairs.





Primary

Upper

i1 16 5.4* 0.48+ 9.0 16 5.3* 0.40+ 7.5

35 5.3 0.66 12.5 36 5.3 0.68 12.9

i2 21 5.0* 0.48+ 9.5 24 5.0* 0.52+ 10.5

43 4.7 0.57 12.2 44 4.7 0.60 12.8

c 4 5.5* 0.51+ 9.3 8 5.2* 0.43+ 8.3

42 5.4 0.62 11.6 39 5.4 0.60 11.1

m1 17 4.5* 0.38+ 8.6 16 4.5* 0.49+ 10.9

47 4.2 0.40 9.5 49 4.3 0.48 11.2

m2 30 4.0* 0.38+ 9.6 27 4.2* 0.37+ 8.9

48 3.8 0.38 10.0 49 3.9 0.36 9.3

Lower

i1 11 4.9* 0.29+ 6.0 12 4.8* 0.40+ 8.3

24 4.8 0.64 13.3 25 4.8 0.63 13.2

i2 22 5.4* 0.46+ 8.5 22 5.2* 0.64+ 12.3

40 5.1 0.58 11.3 37 5.1 0.53 10.6

c 13 5.9* 0.55+ 9.3 14 5.9* 0.40+ 6.7

45 5.8 0.49 8.4 43 5.9 0.55 9.3

m1 18 4.9* 0.44+ 9.0 20 4.9* 0.48+ 9.9

49 4.7 0.43 9.2 46 4.7 0.43 9.1

m2 16 3.9* 0.46+ 11.7 20 4.0* 0.44+ 11.2

48 3.6 0.47 12.8 48 3.6 0.52 14.2

Permanent

Upper

I1 39 9.4* 0.88+ 9.4 39 9.4* 0.91+ 9.6

48 9.1 0.74 8.1 51 9.2 0.72 7.8

C 26 8.8* 0.98+ 11.1 25 9.0* 0.85+ 9.5

25 8.1 0.82 10.1 28 8.4 0.76 9.0

PM2 27 5.9* 0.78+ 13.2 28 5.8* 0.68+ 11.6

31 5.7 0.67 11.7 35 5.6 0.65 11.7

M1 34 5.3* 0.67+ 12.7 40 5.1* 0.65+ 12.6

46 4.7 0.65 13.9 49 4.7 0.60 12.7

M2 8 5.5* 0.67+ 12.3 11 5.2* 0.49+ 9.3

9 4.7 0.62 13.1 11 4.7 0.56 11.9

Lower

I1 35 8.0* 0.91+ 11.3 36 7.9* 0.79+ 10.1

41 7.8 0.75 9.6 42 7.9 0.71 9.0

I2 35 7.7* 0.94+ 12.1 30 8.0* 0.84+ 10.5

42 7.5 0.70 9.4 42 7.4 0.77 10.3

C 25 8.5* 1.05+ 12.4 30 8.5* 1.01+ 12.0

37 8.3 0.77 9.3 36 8.3 0.86 10.3

PM2 24 6.1* 0.72+ 11.8 24 5.9* 0.62+ 10.6

35 5.7 0.67 11.8 31 5.7 0.73 12.8

M1 29 5.2* 0.54+ 10.5 28 5.0* 0.54+ 10.9

43 4.7 0.65 13.9 45 4.6 0.70 15.3

M2 7 4.6* 0.59+ 12.8 6 4.9* 0.69+ 13.9 17 4.3 0.64 14.9 17 4.5 0.55 12.3




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Table 7.12: Comparison of intercuspal (IC) dimensions in the primary and permanent dentitions of females






Primary

Upper

m1(ic1) 16 4.1 0.38+ 9.1 18 4.2* 0.37+ 8.7

44 4.1 0.30 7.3 47 4.3 0.31 7.2

m2(ic1) 29 4.8 0.33+ 6.9 31 4.8* 0.36+ 7.5

47 4.8 0.45 9.2 47 4.9 0.46 9.4

m2(ic2) 28 3.9 0.32+ 8.3 31 3.9* 0.31+ 7.9

47 4.0 0.36 8.9 46 4.0 0.37 9.1

m2(ic3) 28 5.0 0.48+ 9.6 30 5.0* 0.50+ 9.9

48 5.0 0.47 9.4 48 4.9 0.41 8.3

m2(ic4) 28 3.6 0.32+ 8.8 30 3.6* 0.36+ 10.0

47 3.7 0.36 9.8 48 3.6 0.31 8.4

Lower

m1(ic1) 18 2.5 0.33+ 13.3 17 2.5* 0.34+ 13.6

42 2.6 0.38 14.8 39 2.5 0.36 14.5

m2(ic1) 23 3.7 0.33+ 8.9 23 3.7* 0.31+ 8.4

45 3.6 0.40 11.4 46 3.6 0.34 9.5

m2(ic2) 23 3.3 0.33* 10.0 22 3.3* 0.28+ 8.5

45 3.3 0.35 10.5 46 3.3 0.36 10.7

m2(ic3) 24 4.7 0.43+ 9.1 21 4.7* 0.47+ 9.4

46 4.7 0.43 9.2 46 4.7 0.50 10.6

m2(ic4) 24 4.5 0.53+ 11.7 22 4.4* 0.44+ 10.1

47 4.5 0.36 8.1 45 4.5 0.37 8.4

Permanent

Upper

PM2(ICP) 28 5.2 0.43+ 8.2 30 5.2* 0.43+ 8.2

38 5.3 0.48 9.1 36 5.4 0.48 9.0

M1(IC1) 40 6.0 0.51+ 8.6 41 6.0* 0.56+ 9.3

50 6.0 0.56 9.3 51 6.0 0.48 7.9

M1(IC2) 38 4.6 0.43+ 9.3 41 4.8* 0.38+ 8.0

50 4.8 0.48 10.1 51 4.8 0.46 9.6

M1(IC3) 38 6.1 0.52+ 8.5 40 6.0* 0.65+ 11.0

51 6.0 0.61 10.3 51 6.0 0.53 8.8

M1(IC4) 39 4.5 0.44+ 9.7 39 4.6* 0.51+ 11.1

51 4.5 0.38 8.3 51 4.6 0.38 8.3

M2(IC1) 15 6.1 0.80+ 13.1 13 6.2* 0.59+ 9.6

13 6.0 0.49 8.2 14 6.0 0.33 5.5

M2(IC2) 14 4.7 0.53+ 11.3 13 4.7* 0.61+ 12.8

13 4.8 0.36 7.5 14 4.9 0.29 6.0

M2(IC3) 5 6.1 0.86+ 14.1 9 5.8* 0.80+ 13.9

8 6.0 0.48 7.9 9 5.8 0.55 9.5

M2(IC4) 8 4.3 0.58+ 13.7 9 4.2* 0.38+ 9.1

8 4.1 0.55 13.6 9 4.4 0.51 11.6

Lower

PM2(ICP) 27 4.1 0.54+ 13.0 26 4.2* 0.52+ 12.3

36 4.2 0.51 12.2 35 4.2 0.52 12.4

M1(IC1) 33 5.0 0.46+ 9.2 30 4.8* 0.41+ 8.6

45 5.0 0.47 9.4 45 5.0 0.55 11.0

M1(IC2) 32 4.3 0.46+ 10.6 27 4.3* 0.39+ 9.1

44 4.3 0.45 10.6 44 4.2 0.45 10.6

M1(IC3) 30 5.4 0.64+ 11.8 29 5.6* 0.67+ 11.9

43 5.6 0.63 11.2 46 5.7 0.59 10.4

M1(IC4) 32 5.6 0.57+ 10.1 31 5.6* 0.54+ 9.6

45 5.7 0.37 6.6 49 5.6 0.44 7.8

M2(IC1) 12 4.8 0.48+ 9.9 13 4.6* 0.41+ 9.0

21 4.8 0.47 9.9 22 4.9 0.71 14.5

M2(IC2) 12 4.8 0.52+ 10.8 14 4.8* 0.49+ 10.3

21 4.7 0.50 10.7 23 4.7 0.38 8.2

M2(IC3) 12 5.0 0.63+ 12.6 13 4.9* 0.58+ 11.8

21 5.0 0.58 11.6 22 4.9 0.56 11.4

M2(IC4) 13 5.5 0.68+ 12.3 14 5.3* 0.60+ 11.2 21 5.2 0.60 11.5 21 5.1 0.61 11.9





n=sample size; mean=mean values; SD=standard deviation; CV=coefficient of variation. (* t-test, + F-test: p<0.05) (red=larger mean values in DZOS females compared with MZ females, yellow=equal mean values, green=smaller mean values).

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7.3.1.2.2. Comparison between females from dizygotic opposite-sex (DZOS)

and dizygotic same-sex (DZSS) twin pairs

Overall, MD, BL and CH dimensions displayed larger mean values in females from

DZOS twin pairs compared with females from DZSS twin pairs and this was evident in

both primary and permanent dentitions. As with the comparison in 7.3.1.2.1, IC

dimensions failed to present the same pattern. Mesiodistal (MD) dimensions displayed

significant differences for the permanent lower right second premolars and lower right first

molars (Table 7.13). Statistically significant differences between females from DZOS and

DZSS twins were found for BL dimension in the primary dentition for the upper left

canine, lower lateral incisors, lower right canines, and lower second molars. The

permanent dentition also displayed statistical significance in the lower arch for the right

lateral incisors, left second premolars, and right first molars (Table 7.14). Statistical

significance for CH dimensions was found in the primary dentition for the lower central

incisors and lower lateral incisors while the permanent dentition displayed significance for

the upper right first molar, lower lateral incisors and lower right first molars (Table 7.15).

Intercuspal (IC) dimensions failed to display evidence of larger mean values between

females from DZOS and DZSS twin pairs, except for the ic3 dimension in the upper left

second molar and ic1 dimension which displayed statistical significance in the lower right

second molar in the primary dentition (Table 7.16).

Similar to comparisons of females from DZOS compared with MZ twin pairs,

females from DZOS twins also displayed larger mean values (red highlight) for MD, BL

and CH dimensions compared with DZSS female twins. Both the primary and permanent

dentitions were affected even though the pattern varied across dentitions and dimensions.

Intercuspal (IC) dimensions seemed to be affected differently in females from DZOS twins

as more similar (yellow highlight) and smaller (green highlight) values were found in both

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dentitions. The development of IC dimensions seems to be similar in females from all

zygosities with no, or little, evidence of a possible male hormone influence from the male

co-twin in the development of this dimension in DZOS female twins.

Although the differences between MD, BL and CH dimensions in DZOS females

and MZ females were not statistically significant for many of the dimensions studied, it

appears that there is a trend for larger tooth size in DZOS females indicating that these

tooth crown dimensions might have developed under the influence of male hormone in

utero from the male co-twin. Moreover, this possible intrauterine hormonal influence does

not seem to affect the IC dimensions in DZOS females since these are the first dental

dimensions to form during tooth development, being established at the beginning of crown

calcification.

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Table 7.13: Comparison of mesiodistal (MD) dimensions in the primary and permanent dentitions of females


DZOS Females DZSS Females




Primary

Upper

i1 25 6.2* 0.41+ 6.5 29 6.2 0.46 7.5

30 6.3 0.45 7.2 31 6.3 0.46 7.4

i2 32 5.2* 0.40+ 7.8 34 5.2 0.37 7.2

35 5.2 0.36 6.9 35 5.2 0.36 6.9

c 40 6.7* 0.34+ 5.1 40 6.7 0.35 5.3

39 6.8 0.36 5.3 38 6.7 0.35 5.2

m1 39 6.8* 0.35+ 5.2 39 6.9 0.41 5.9

39 6.9 0.49 7.1 38 7.0 0.46 6.6

m2 39 8.7* 0.40+ 4.7 39 8.7 0.44 5.1

39 8.7 0.44 5.1 36 8.7 0.46 4.9

Lower

i1 15 4.0* 0.26+ 6.5 17 4.1 0.28 7.0

25 4.0 0.30 7.6 24 3.9 0.30 7.7

i2 27 4.6* 0.22+ 4.7 28 4.6 0.29 6.3

33 4.5 0.38 8.4 30 4.5 0.35 7.7

c 40 5.8* 0.31+ 5.4 40 5.8 0.33 5.6

39 5.7 0.34 6.0 39 5.7 0.34 5.9

m1 40 7.8* 0.46+ 6.0 40 7.7 0.47 6.1

38 7.7 0.38 5.0 39 7.7 0.41 5.4

m2 42 9.9* 0.48+ 4.9 40 9.8 0.45 4.6

39 9.8 0.45 4.6 39 9.8 0.47 4.8

Permanent

Upper

I1 42 8.5* 0.58+ 6.9 42 8.5 0.56 6.7

38 8.4 0.52 6.2 37 8.4 0.49 5.8

C 27 7.8* 0.47+ 6.0 32 7.7 0.47 6.1

24 7.8 0.62 8.0 26 7.7 0.49 6.4

PM2 28 6.9* 0.48+ 6.9 30 6.9 0.45 6.6

20 6.6 0.50 7.6 22 6.7 0.52 7.7

M1 42 10.3* 0.52+ 5.0 41 10.2 0.53 5.2

36 10.0 0.48 4.8 37 10.0 0.48 4.8

M2 15 10.0* 0.41+ 4.1 9 10.0 0.55 5.6

8 9.8 0.65 6.6 12 9.9 0.64 6.5

Lower

I1 35 5.4* 0.34+ 6.2 37 5.3 0.35 6.6

37 5.3 0.37 6.9 34 5.2 0.34 6.5

I2 32 5.9* 0.32+ 5.5 36 5.9 0.41 6.9

36 5.8 0.39 6.8 32 5.7 0.35 6.2

C 32 6.8* 0.42+ 6.3 32 6.7 0.40 5.9

28 6.7 0.43 6.4 28 6.7 0.50 7.5

PM2 28 7.3* 0.55+ 7.6 26 7.3 0.48 6.6

25 7.1 0.46 6.4 25 7.2 0.54 7.6

M1 35 11.0* 0.59+ 5.4 36 11.0 0.65 5.9

34 10.8 0.66 6.1 35 10.8 0.63 5.8

M2 9 10.1* 0.60+ 5.9 12 10.5 0.76 7.3 12 10.4 0.66 6.3 13 10.6 0.65 6.2



test, + F-test: p<0.05) (red=larger mean values in DZOS females compared with DZSS females, yellow=equal mean values, green=smaller mean values).

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Table 7.14: Comparison of buccolingual (BL) dimensions in the primary and permanent dentitions of females






Primary

Upper

i1 27 5.0* 0.34 7.0 29 5.0* 0.41 8.1

30 4.9 0.40 8.2 31 5.0 0.41 8.2

i2 33 4.9* 0.37 7.6 34 4.8* 0.35 7.3

36 4.7 0.39 8.4 36 4.7 0.42 9.0

c 40 6.1* 0.40 6.4 40 6.1* 0.36 5.8

39 6.0 0.36 6.0 38 6.0 0.34 5.7

m1 41 8.5* 0.38 4.4 41 8.6* 0.41 4.8

38 8.6 0.35 4.1 39 8.5 0.36 4.2

m2 40 9.7* 0.43 4.4 41 9.7* 0.42 4.3

39 9.6 0.44 4.6 38 9.6 0.42 4.3

Lower

i1 17 3.8* 0.24 6.4 18 3.8* 0.21 5.6

25 3.8 0.25 6.7 25 3.7 0.24 6.3

i2 27 4.4* 0.27 6.2 29 4.4* 0.26 5.9

32 4.2 0.28 6.7 31 4.3 0.30 7.0

c 39 5.7* 0.40 7.0 40 5.6* 0.36 6.4

39 5.5 0.33 6.0 39 5.5 0.32 5.9

m1 40 7.0* 0.44 6.3 40 7.0* 0.37 5.3

38 6.8 0.33 4.8 39 6.8 0.34 4.9

m2 41 8.6* 0.37 4.4 40 8.5* 0.35 4.1

39 8.4 0.36 4.4 39 8.4 0.38 4.5

Permanent

Upper

I1 41 7.1* 0.55 7.7 41 7.2* 0.57 8.0

36 6.9 0.61 8.9 31 6.9 0.58 8.4

C 23 8.1* 0.57 7.0 30 8.0* 0.62 7.8

23 7.9 0.67 8.4 24 8.1 0.55 6.8

PM2 28 9.4* 0.63 6.7 30 9.3* 0.65 7.0

21 9.2 0.61 6.6 22 9.2 0.60 6.5

M1 43 11.4* 0.52 4.5 43 11.4* 0.51 4.5

35 11.2 0.59 5.3 38 11.2 0.50 4.5

M2 15 11.4* 0.82 7.2 13 11.3* 0.77 6.9

12 11.1 0.59 5.3 14 11.0 0.49 4.4

Lower

I1 35 6.1* 0.52 8.6 36 6.1* 0.47 7.7

36 5.9 0.49 8.3 34 5.9 0.47 8.0

I2 35 6.5* 0.51 7.9 36 6.4* 0.43 6.7

35 6.2 0.58 9.4 34 6.2 0.52 8.3

C 29 7.2* 0.48 6.7 31 7.4* 0.53 7.2

27 7.2 0.54 7.6 28 7.2 0.67 9.3

PM2 28 8.5* 0.62 7.3 26 8.5* 0.56 6.6

24 8.2 0.53 6.5 25 8.2 0.49 6.0

M1 38 10.2* 0.42 4.1 37 10.3* 0.46 4.4

36 9.9 0.50 5.0 38 10.1 0.50 4.9

M2 15 10.2* 0.67 6.5 15 10.2* 0.62 6.1 15 10.1 0.56 5.6 14 10.2 0.53 5.3




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Table 7.15: Comparison of crown height (CH) dimensions in the primary and permanent dentitions of

females from dizygotic opposite-sex (DZOS) and dizygotic same-sex (DZSS) twin pairs.





Primary

Upper

i1 16 5.4* 0.48 9.0 16 5.3* 0.40+ 7.5

31 5.3 0.58 11.0 28 5.3 0.52 9.8

i2 21 5.0* 0.48 9.5 24 5.0* 0.52+ 10.5

35 4.8 0.52 10.8 35 4.7 0.59 12.6

c 4 5.5* 0.51 9.3 8 5.2* 0.43+ 8.3

29 5.5 0.57 10.2 30 5.4 0.59 10.9

m1 17 4.5* 0.38 8.6 16 4.5* 0.49+ 10.9

34 4.3 0.48 11.0 33 4.4 0.39 8.9

m2 30 4.0* 0.38 9.6 27 4.2* 0.37+ 8.9

38 3.9 0.53 13.4 37 4.0 0.45 11.3

Lower

i1 11 4.9* 0.29 6.0 12 4.8* 0.40+ 8.3

24 4.5 0.43 9.6 23 4.6 0.45 9.9

i2 22 5.4* 0.46 8.5 22 5.2* 0.64+ 12.3

32 5.0 0.45 9.0 30 5.0 0.39 7.8

c 13 5.9* 0.55 9.3 14 5.9* 0.40+ 6.7

28 5.9 0.37 6.2 28 5.9 0.44 7.5

m1 18 4.9* 0.44 9.0 20 4.9* 0.48+ 9.9

36 4.8 0.38 7.9 32 4.7 0.52 10.9

m2 16 3.9* 0.46 11.7 20 4.0* 0.44+ 11.2

33 3.8 0.45 12.0 30 3.7 0.44 11.7

Permanent

Upper

I1 39 9.4* 0.88 9.4 39 9.4* 0.91+ 9.6

36 9.1 0.70 7.7 35 9.1 0.83 9.1

C 26 8.8* 0.98 11.1 25 9.0* 0.85+ 9.5

24 8.7 0.99 11.4 23 8.9 1.18 13.2

PM2 27 5.9* 0.78 13.2 28 5.8* 0.68+ 11.6

20 5.5 0.73 13.1 20 5.6 0.61 11.1

M1 34 5.3* 0.67 12.7 40 5.1* 0.65+ 12.6

34 4.9 0.65 13.4 34 5.1 0.78 15.4

M2 8 5.5* 0.67 12.3 11 5.2* 0.49+ 9.3

10 5.2 0.63 12.1 12 5.4 0.67 12.5

Lower

I1 35 8.0* 0.91 11.3 36 7.9* 0.79+ 10.1

36 7.7 0.95 12.3 33 7.9 0.76 9.6

I2 35 7.7* 0.94 12.1 30 8.0* 0.84+ 10.5

36 7.2 0.92 12.7 34 7.3 0.76 10.5

C 25 8.5* 1.05 12.4 30 8.5* 1.01+ 12.0

28 8.6 1.07 12.4 26 8.4 0.90 10.7

PM2 24 6.1* 0.72 11.8 24 5.9* 0.62+ 10.6

24 5.7 0.65 11.4 22 5.9 0.73 12.4

M1 29 5.2* 0.54 10.5 28 5.0* 0.54+ 10.9

33 4.8 0.60 12.5 30 4.8 0.62 12.9

M2 7 4.6* 0.59 12.8 6 4.9* 0.69+ 13.9 9 4.5 0.57 12.8 9 4.5 0.60 13.4




120

79

Table 7.16: Comparison of intercuspal (IC) dimensions in the primary and permanent dentitions of females






Primary

Upper

m1(ic1) 16 4.1* 0.38+ 9.1 18 4.2* 0.37+ 8.7

33 4.1 0.31 7.6 33 4.2 0.29 7.1

m2(ic1) 29 4.8* 0.33+ 6.9 31 4.8* 0.36+ 7.5

36 4.7 0.37 7.9 36 4.7 0.31 6.6

m2(ic2) 28 3.9* 0.32+ 8.3 31 3.9* 0.31+ 7.9

36 3.9 0.35 8.9 34 3.9 0.33 8.5

m2(ic3) 28 5.0* 0.48+ 9.6 30 5.0* 0.50+ 9.9

35 4.8 0.46 9.5 33 4.8 0.45 9.5

m2(ic4) 28 3.6* 0.32+ 8.8 30 3.6* 0.36+ 10.0

35 3.6 0.29 8.0 35 3.6 0.33 9.3

Lower

m1(ic1) 18 2.5* 0.33+ 13.3 17 2.5* 0.34+ 13.6

31 2.4 0.34 14.0 29 2.4 0.26 10.8

m2(ic1) 23 3.7* 0.33+ 8.9 23 3.7* 0.31+ 8.4

30 3.5 0.32 9.4 32 3.6 0.32 9.1

m2(ic2) 23 3.3* 0.33+ 10.0 22 3.3* 0.28+ 8.5

31 3.4 0.32 9.3 32 3.3 0.46 13.8

m2(ic3) 24 4.7* 0.43+ 9.1 21 4.7* 0.47+ 9.4

33 4.6 0.44 9.6 32 4.5 0.43 9.5

m2(ic4) 24 4.5* 0.53+ 11.7 22 4.4* 0.44+ 10.1

32 4.6 0.46 10.1 32 4.4 0.42 9.6

Permanent

Upper

PM2(ICP) 28 5.2* 0.43+ 8.2 30 5.2* 0.43+ 8.2

21 5.2 0.6 11.6 22 5.3 0.45 8.5

M1(IC1) 40 6.0* 0.51+ 8.6 41 6.0* 0.56+ 9.3

35 5.8 0.44 7.5 39 5.9 0.46 7.8

M1(IC2) 38 4.6* 0.43+ 9.3 41 4.8* 0.38+ 8.0

36 4.8 0.44 9.1 39 4.7 0.43 9.1

M1(IC3) 38 6.1* 0.52+ 8.5 40 6.0* 0.65+ 11.0

36 6.0 0.48 8.0 37 6.0 0.56 9.3

M1(IC4) 39 4.5* 0.44+ 9.7 39 4.6* 0.51+ 11.1

35 4.5 0.39 8.7 37 4.5 0.39 8.8

M2(IC1) 15 6.1* 0.80+ 13.1 13 6.2* 0.59+ 9.6

12 5.9 0.5 8.5 14 6.0 0.58 9.7

M2(IC2) 14 4.7* 0.53+ 11.3 13 4.7* 0.61+ 12.8

12 4.7 0.61 12.8 14 4.8 0.52 10.9

M2(IC3) 5 6.1* 0.86+ 14.1 9 5.8* 0.80+ 13.9

4 6.0 0.78 13.0 6 6.2 0.92 14.9

M2(IC4) 8 4.3* 0.58+ 13.7 9 4.2* 0.38+ 9.1

4 4.1 0.49 11.8 6 4.5 0.35 7.8

Lower

PM2(ICP) 27 4.1* 0.54+ 13.0 26 4.2* 0.52+ 12.3

24 4.0 0.52 12.9 23 4.0 0.53 13.4

M1(IC1) 33 5.0* 0.46+ 9.2 30 4.8* 0.41+ 8.6

37 4.8 0.46 9.5 37 4.8 0.36 7.5

M1(IC2) 32 4.3* 0.46+ 10.6 27 4.3* 0.39+ 9.1

36 4.3 0.30 7.0 38 4.2 0.49 11.7

M1(IC3) 30 5.4* 0.64+ 11.8 29 5.6* 0.67+ 11.9

36 5.4 0.56 10.3 37 5.6 0.59 10.6

M1(IC4) 32 5.6* 0.57+ 10.1 31 5.6* 0.54+ 9.6

37 5.7 0.54 9.4 36 5.6 0.51 9.1

M2(IC1) 12 4.8* 0.48+ 9.9 13 4.6* 0.41+ 9.0

16 4.9 0.49 10.0 15 4.7 0.60 12.9

M2(IC2) 12 4.8* 0.52+ 10.8 14 4.8* 0.49+ 10.3

15 4.8 0.41 8.5 15 5.1 0.47 9.2

M2(IC3) 12 5.0* 0.63+ 12.6 13 4.9* 0.58+ 11.8

15 5.0 0.74 15.0 15 4.9 0.77 15.7

M2(IC4) 13 5.5* 0.68+ 12.3 14 5.3* 0.60+ 11.2 15 5.7 0.46 8.0 15 5.6 0.75 13.6





n=sample size; mean=mean values; SD=standard deviation; CV=coefficient of variation. (* t-test, + F-test: p<0.05) (red=larger mean values in DZOS females compared with DZSS females, yellow=equal mean values, green=smaller mean values).

121

79

7.3.1.3. Percentage differences in tooth size between female twins from all

zygosities

By comparing tooth crown dimensions between DZOS females with MZ female

and DZSS female twins it was possible to quantify the percentage increases in size

between different dimensions, teeth and dentitions (Tables 7.17 - 7.24).

Overall, DZOS females displayed larger percentage differences for MD, BL and

CH dimensions when compared with MZ female twins, while smaller or negative

percentage differences were found for IC dimensions in the same group. This pattern was

evident for both the primary and permanent dentitions. The primary dentition displayed

smaller percentage differences, on average, between females from DZOS and MZ twins

compared with the permanent dentition (Figure 7.17). In the primary dentition, upper

central incisors and upper canines displayed no differences between DZOS females and

MZ females for MD dimensions while primary upper canines displayed no percentage

differences in the same group for BL dimensions. In general, CH displayed larger

percentage differences, on average, compared with MD and BL dimensions in both the

primary and permanent dentitions.

A similar pattern was found between females from DZOS and females from DZSS

twin pairs (Tables 7.21 – 7.24). Dizygotic opposite-sex females displayed larger

percentage values for MD, BL and CH dimensions compared with DZSS females. On

average, smaller percentage values were found in the primary dentition compared with the

permanent dentition in this group. Crown height (CH) displayed the largest percentage

values compared with the other dimensions studied while IC dimensions displayed small or

negative percentage values for the permanent dentition in the same group (Figure 7.2).

122

Table 7.17: Percentage increase in size for mesiodistal (MD) dimensions in the primary and permanent dentitions between dizygotic

opposite-sex (DZOS) and monozygotic (MZ) female twins.

DZOS Females MZ Females % increase in size

Right (MD)

Left (MD)


n mean SD CV n mean SD CV n mean SD CV n mean SD CV Right Left

Primary

Upper

i1 25 6.2 0.41 6.5

29 6.2 0.46 7.5

40 6.2 0.39 6.3

41 6.2 0.37 6.0

0.00 0.00

i2 32 5.2 0.40 7.8

34 5.2 0.37 7.2

44 5.1 0.33 6.5

45 5.1 0.31 6.2

1.96 1.96

c 40 6.7 0.34 5.1

40 6.7 0.35 5.3

51 6.6 0.44 6.6

52 6.6 0.52 7.9

1.52 1.52

m1 39 6.8 0.35 5.2

39 6.9 0.41 5.9

51 6.8 0.36 5.3

52 6.9 0.39 5.7

0.00 0.00

m2 39 8.7 0.40 4.7

39 8.7 0.44 5.1

52 8.5 0.36 4.3

52 8.6 0.36 4.2

2.35 1.16

Lower

i1 15 4.0 0.26 6.5

17 4.1 0.28 7.0

27 3.9 0.29 7.4

24 3.8 0.26 6.9

2.56 7.89

i2 27 4.6 0.22 4.7

28 4.6 0.29 6.3

41 4.4 0.33 7.5

38 4.4 0.33 7.5

4.55 4.55

c 40 5.8 0.31 5.4

40 5.8 0.33 5.6

51 5.8 0.27 4.7

51 5.7 0.30 5.3

0.00 1.75

m1 40 7.8 0.46 6.0

40 7.7 0.47 6.1

51 7.6 0.35 4.7

49 7.6 0.38 5.0

2.63 1.32

m2 42 9.9 0.48 4.9

40 9.8 0.45 4.6

51 9.8 0.37 3.8

51 9.7 0.37 3.8

1.02 1.03

Average 1.66 2.12

Permanent

Upper

I1 42 8.5 0.58 6.9

42 8.5 0.56 6.7

52 8.4 0.53 6.2

51 8.4 0.50 5.9

1.19 1.19

C 27 7.8 0.47 6.0

32 7.7 0.47 6.1

33 7.6 0.34 4.5

33 7.6 0.40 5.3

2.63 1.32

PM2 28 6.9 0.48 6.9

30 6.9 0.45 6.6

38 6.7 0.32 4.8

36 6.7 0.36 5.3

2.99 2.53

M1 42 10.3 0.52 5.0

41 10.2 0.53 5.2

50 10.0 0.47 4.6

51 10.1 0.42 4.2

3.00 0.99

M2 15 10.0 0.41 4.1

9 10.0 0.55 5.6

9 9.7 0.54 5.3

9 9.6 0.40 4.1

3.09 4.17

Lower

I1 35 5.4 0.34 6.2

37 5.3 0.35 6.6

49 5.3 0.29 5.5

48 5.3 0.29 5.4

1.89 0.00

I2 32 5.9 0.32 5.5

36 5.9 0.41 6.9

48 5.8 0.37 6.3

49 5.8 0.34 5.9

1.72 1.72

C 32 6.8 0.42 6.3

32 6.7 0.40 5.9

38 6.6 0.37 5.7

39 6.6 0.35 5.4

3.03 1.52

PM2 28 7.3 0.55 7.6

26 7.3 0.48 6.6

36 7.1 0.40 5.6

35 7.2 0.40 5.5

2.82 1.39

M1 35 11.0 0.59 5.4

36 11.0 0.65 5.9

45 10.7 0.58 5.4

50 10.8 0.45 4.5

2.80 1.85

M2 9 10.1 0.60 5.9 12 10.5 0.76 7.3

15 10.2 0.37 3.7

16 10.2 0.48 4.7

-0.98 2.94

Average 2.20 1.78

i1=primary central incisor; i2=primary lateral incisor; c=primary canine; m1=primary first molar; m2=primary second molar; I1=permanent central incisor; I2=permanent

lateral incisor; C=permanent canine; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; n=sample size; mean=mean values; SD=standard deviation; CV=coefficient of variation.

123

Table 7.18: Percentage increase in size for buccolingual (BL) dimensions in the primary and permanent dentitions between dizygotic

opposite-sex (DZOS) and monozygotic (MZ) female twins.

DZOS Females MZ Females % increase in

size

Right (BL)

Left (BL)

Right (BL)

Left (BL)


Primary

Upper i1 27 5.0 0.34 7.0

29 5.0 0.41 8.1

43 4.9 0.33 6.8

41 5.0 0.31 6.3

2.04 0.00

i2 33 4.9 0.37 7.6

34 4.8 0.35 7.3

45 4.7 0.33 7.1

46 4.8 0.34 7.2

4.26 0.00

c 40 6.1 0.40 6.4

40 6.1 0.36 5.8

52 6.1 0.41 6.8

52 6.1 0.40 6.7

0.00 0.00

m1 41 8.5 0.38 4.4

41 8.6 0.41 4.8

52 8.5 0.35 4.1

52 8.5 0.32 3.7

0.00 1.18

m2 40 9.7 0.43 4.4

41 9.7 0.42 4.3

52 9.7 0.38 3.9

52 9.6 0.39 4.0

0.00 1.04

Lower

i1 17 3.8 0.24 6.4

18 3.8 0.21 5.6

26 3.6 0.33 9.0

25 3.7 0.27 7.4

5.56 2.70

i2 27 4.4 0.27 6.2

29 4.4 0.26 5.9

40 4.3 0.28 6.6

39 4.3 0.28 6.6

2.33 2.33

c 39 5.7 0.40 7.0

40 5.6 0.36 6.4

51 5.6 0.38 6.8

51 5.6 0.39 7.0

1.79 0.00

m1 40 7.0 0.44 6.3

40 7.0 0.37 5.3

51 6.8 0.41 6.0

50 7.0 0.37 5.4

2.94 0.00

m2 41 8.6 0.37 4.4

40 8.5 0.35 4.1

51 8.2 0.42 5.1

51 8.4 0.39 4.6

4.88 1.19

Average 2.38 0.84

Permanent

Upper I1 41 7.1 0.55 7.7

41 7.2 0.57 8.0

48 7.1 0.46 6.4

47 7.1 0.52 7.4

0.00 1.41

C 23 8.1 0.57 7.0

30 8.0 0.62 7.8

31 7.8 0.40 5.2

33 7.8 0.45 5.8

3.85 2.56

PM2 28 9.4 0.63 6.7

30 9.3 0.65 7.0

38 9.2 0.34 3.7

36 9.3 0.33 3.6

2.17 0.00

M1 43 11.4 0.52 4.5

43 11.4 0.51 4.5

50 11.1 0.48 4.3

51 11.1 0.47 4.3

2.70 2.70

M2 15 11.4 0.82 7.2

13 11.3 0.77 6.9

11 11.0 0.41 3.8

12 10.9 0.39 3.6

3.64 3.67

Lower

I1 35 6.1 0.52 8.6

36 6.1 0.47 7.7

48 5.9 0.38 6.4

45 6.0 0.38 6.3

3.39 1.67

I2 35 6.5 0.51 7.9

36 6.4 0.43 6.7

49 6.2 0.45 7.3

47 6.2 0.44 7.0

4.84 3.23

C 29 7.2 0.48 6.7

31 7.4 0.53 7.2

37 7.2 0.48 6.7

38 7.3 0.49 6.7

0.00 1.37

PM2 28 8.5 0.62 7.3

26 8.5 0.56 6.6

36 8.3 0.52 6.3

35 8.3 0.53 6.4

2.41 2.41

M1 38 10.2 0.42 4.1

37 10.3 0.46 4.4

49 9.9 0.48 4.8

50 9.9 0.45 4.5

3.03 4.04

M2 15 10.2 0.67 6.5

15 10.2 0.62 6.1

22 9.8 0.54 5.6

23 9.9 0.54 5.4

4.08 3.03

Average 2.74 2.37

i1=primary central incisor; i2=primary lateral incisor; c=primary canine; m1=primary first molar; m2=primary second molar; I1=permanent central incisor; I2=permanent lateral incisor; C=permanent canine; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; n=sample size; mean=mean values;

SD=standard deviation; CV=coefficient of variation.

124

Table 7.19: Percentage increase in size for crown height (CH) dimensions in the primary and permanent dentitions between dizygotic opposite-

sex (DZOS) and monoygotic (MZ) female twins.

DZOS Females MZ Females % increase in

size

Right (CH)

Left (CH)

Right (CH)

Left (CH)


Primary

Upper i1 16 5.4 0.48 9.0

16 5.3 0.40 7.5

35 5.3 0.66 12.5

36 5.3 0.68 12.9

1.89 0.00

i2 21 5.0 0.48 9.5

24 5.0 0.52 10.5

43 4.7 0.57 12.2

44 4.7 0.60 12.8

6.38 6.38

c 4 5.5 0.51 9.3

8 5.2 0.43 8.3

42 5.4 0.62 11.6

39 5.4 0.60 11.1

1.85 -3.70

m1 17 4.5 0.38 8.6

16 4.5 0.49 10.9

47 4.2 0.40 9.5

49 4.3 0.48 11.2

7.14 4.65

m2 30 4.0 0.38 9.6

27 4.2 0.37 8.9

48 3.8 0.38 10.0

49 3.9 0.36 9.3

5.26 6.92

Lower

i1 11 4.9 0.29 6.0

12 4.8 0.40 8.3

24 4.8 0.64 13.3

25 4.8 0.63 13.2

2.08 0.00

i2 22 5.4 0.46 8.5

22 5.2 0.64 12.3

40 5.1 0.58 11.3

37 5.1 0.53 10.6

5.88 1.96

c 13 5.9 0.55 9.3

14 5.9 0.40 6.7

45 5.8 0.49 8.4

43 5.9 0.55 9.3

1.72 0.00

m1 18 4.9 0.44 9.0

20 4.9 0.48 9.9

49 4.7 0.43 9.2

46 4.7 0.43 9.1

4.26 4.26

m2 16 3.9 0.46 11.7

20 4.0 0.44 11.2

48 3.6 0.47 12.8

48 3.6 0.52 14.2

8.33 11.11

Average 4.48 3.16

Permanent

Upper I1 39 9.4 0.88 9.4

39 9.4 0.91 9.6

48 9.1 0.74 8.1

51 9.2 0.72 7.8

3.30 2.17

C 26 8.8 0.98 11.1

25 9.0 0.85 9.5

25 8.1 0.82 10.1

28 8.4 0.76 9.0

8.64 6.67

PM2 27 5.9 0.78 13.2

28 5.8 0.68 11.6

31 5.7 0.67 11.7

35 5.6 0.65 11.7

2.79 4.13

M1 34 5.3 0.67 12.7

40 5.1 0.65 12.6

46 4.7 0.65 13.9

49 4.7 0.60 12.7

12.37 8.97

M2 8 5.5 0.67 12.3

11 5.2 0.49 9.3

9 4.7 0.62 13.1

11 4.7 0.56 11.9

15.92 10.38

Lower

I1 35 8.0 0.91 11.3

36 7.9 0.79 10.1

41 7.8 0.75 9.6

42 7.9 0.71 9.0

2.56 0.00

I2 35 7.7 0.94 12.1

30 8.0 0.84 10.5

42 7.5 0.70 9.4

42 7.4 0.77 10.3

2.67 8.11

C 25 8.5 1.05 12.4

30 8.5 1.01 12.0

37 8.3 0.77 9.3

36 8.3 0.86 10.3

2.41 2.41

PM2 24 6.1 0.72 11.8

24 5.9 0.62 10.6

35 5.7 0.67 11.8

31 5.7 0.73 12.8

7.02 3.33

M1 29 5.2 0.54 10.5

28 5.0 0.54 10.9

43 4.7 0.65 13.9

45 4.6 0.70 15.3

9.77 8.95

M2 7 4.6 0.59 12.8

6 4.9 0.69 13.9

17 4.3 0.64 14.9

17 4.5 0.55 12.3

6.98 11.01

Average 6.77 6.01

i1=primary central incisor; i2=primary lateral incisor; c=primary canine; m1=primary first molar; m2=primary second molar; I1=permanent central incisor; I2=permanent lateral incisor; C=permanent canine; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; n=sample size; mean=mean values; SD=standard

deviation, CV=coefficient of variation..

125

Table 7.20: Percentage increase in size for intercuspal (IC) dimensions in the primary and permanent dentitions between dizygotic opposite-sex (DZOS) and

monozygotic (MZ) female twins.

DZOS Females MZ Females % increase in size

Right (IC)

Left (IC)



Primary

Upper m1(ic1) 16 4.1 0.38 9.1

18 4.2 0.37 8.7

44 4.1 0.30 7.3

47 4.3 0.31 7.2

0.00 -2.33

m2(ic1) 29 4.8 0.33 6.9

31 4.8 0.36 7.5

47 4.8 0.45 9.2

47 4.9 0.46 9.4

0.00 -2.04

m2(ic2) 28 3.9 0.32 8.3

31 3.9 0.31 7.9

47 4.0 0.36 8.9

46 4.0 0.37 9.1

-2.50 -2.50

m2(ic3) 28 5.0 0.48 9.6

30 5.0 0.50 9.9

48 5.0 0.47 9.4

48 4.9 0.41 8.3

0.00 2.04

m2(ic4) 28 3.6 0.32 8.8

30 3.6 0.36 10.0

47 3.7 0.36 9.8

48 3.6 0.31 8.4

-2.70 0.00

Lower m1(ic1) 18 2.5 0.33 13.3

17 2.5 0.34 13.6

42 2.6 0.38 14.8

39 2.5 0.36 14.5

-3.85 0.00

m2(ic1) 23 3.7 0.33 8.9

23 3.7 0.31 8.4

45 3.6 0.40 11.4

46 3.6 0.34 9.5

2.78 2.78

m2(ic2) 23 3.3 0.33 10.0

22 3.3 0.28 8.5

45 3.3 0.35 10.5

46 3.3 0.36 10.7

0.00 0.00

m2(ic3) 24 4.7 0.43 9.1

21 4.7 0.47 9.4

46 4.7 0.43 9.2

46 4.7 0.50 10.6

0.00 0.00

m2(ic4) 24 4.5 0.52 11.7

22 4.4 0.44 10.1

47 4.5 0.36 8.1

45 4.5 0.37 8.4

0.00 -2.22

Average -0.63 -0.43

Permanent

Upper PM2(ICP) 28 5.2 0.43 8.2

30 5.2 0.43 8.2

38 5.3 0.48 9.1

36 5.4 0.48 9.0

-1.89 -3.70

M1(IC1) 40 6.0 0.51 8.6

41 6.0 0.56 9.3

50 6.0 0.56 9.3

51 6.0 0.48 7.9

0.00 0.00

M1(IC2) 38 4.6 0.43 9.3

41 4.8 0.38 8.0

50 4.8 0.48 10.1

51 4.8 0.46 9.6

-4.17 0.00

M1(IC3) 38 6.1 0.52 8.5

40 6.0 0.65 11.0

51 6.0 0.61 10.3

51 6.0 0.53 8.8

1.67 0.00

M1(IC4) 39 4.5 0.44 9.7

39 4.6 0.51 11.1

51 4.5 0.38 8.3

51 4.6 0.38 8.3

0.00 0.00

M2(IC1) 15 6.1 0.80 13.1

13 6.2 0.59 9.6

13 6.0 0.49 8.2

14 6.0 0.33 5.5

1.67 3.33

M2(IC2) 14 4.7 0.53 11.3

13 4.7 0.61 12.8

13 4.8 0.36 7.5

14 4.9 0.29 6.0

-2.08 -4.08

M2(IC3) 5 6.1 0.86 14.1

9 5.8 0.80 13.9

8 6.0 0.48 7.9

9 5.8 0.55 9.5

1.67 0.00

M2(IC4) 8 4.3 0.58 13.7

9 4.2 0.38 9.1

8 4.1 0.55 13.6

9 4.4 0.51 11.6

4.88 -4.55

Lower PM2(ICP) 27 4.1 0.54 13.0

26 4.2 0.52 12.3

36 4.2 0.51 12.2

35 4.2 0.52 12.4

-2.38 0.00

M1(IC1) 33 5.0 0.46 9.2

30 4.8 0.41 8.6

45 5.0 0.47 9.4

45 5.0 0.55 11.0

0.00 -4.00

M1(IC2) 32 4.3 0.46 10.6

27 4.3 0.39 9.1

44 4.3 0.45 10.6

44 4.2 0.45 10.6

0.00 2.38

M1(IC3) 30 5.4 0.64 11.8

29 5.6 0.67 11.9

43 5.6 0.63 11.2

46 5.7 0.59 10.4

-3.57 -1.75

M1(IC4) 32 5.6 0.57 10.1

31 5.6 0.54 9.6

45 5.7 0.37 6.6

49 5.6 0.44 7.8

-1.75 0.00

M2(IC1) 12 4.8 0.48 9.9

13 4.6 0.41 9.0

21 4.8 0.47 9.9

22 4.9 0.71 14.5

0.00 -6.12

M2(IC2) 12 4.8 0.52 10.8

14 4.8 0.49 10.3

21 4.7 0.50 10.7

23 4.7 0.38 8.2

2.13 2.13

M2(IC3) 12 5.0 0.63 12.6

13 4.9 0.58 11.8

21 5.0 0.58 11.6

22 4.9 0.56 11.4

0.00 0.00

M2(IC4) 13 5.5 0.68 12.3

14 5.3 0.6 11.2

21 5.2 0.60 11.5

21 5.1 0.61 11.9

5.77 3.92

Average 0.11 -0.69

m1=primary first molar; m2=primary second molar; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; (ic1)=bucco-lingual intercuspal dimension in primary first molars; (ICP)=permanent bucco-lingual intercuspal dimension; (IC1)=permanent mesiobuccal-mesiolingual intercuspal dimension; (IC2)=permanent mesiobuccal-distobuccal

intercuspal dimension; (IC3)=permanent distobuccal-distolingual intercuspal dimension; (IC4)=permanent mesiolingual-distolingual intercuspal dimension; n=sample size; mean=mean values;

SD=standard deviation.

126

Table 7.21: Percentage increase in size for mesiodistal (MD) dimensions in the primary and permanent dentitions between dizygotic opposite-sex

(DZOS) and dizygotic same-sex (DZSS) female twins.

DZOS Females DZSS Females % increase in size

Right (MD)

Left (MD)



Primary

Upper

i1 25 6.2 0.41 6.5

29 6.2 0.46 7.5

30 6.3 0.45 7.2

31 6.3 0.46 7.4

-1.59 -1.59

i2 32 5.2 0.40 7.8

34 5.2 0.37 7.2

35 5.2 0.36 6.9

35 5.2 0.36 6.9

0.00 0.00

c 40 6.7 0.34 5.1

40 6.7 0.35 5.3

39 6.8 0.36 5.3

38 6.7 0.35 5.2

-1.47 0.00

m1 39 6.8 0.35 5.2

39 6.9 0.41 5.9

39 6.9 0.49 7.1

38 7.0 0.46 6.6

-1.45 -1.43

m2 39 8.7 0.40 4.7

39 8.7 0.44 5.1

39 8.7 0.44 5.1

36 8.7 0.46 4.9

0.00 0.00

Lower

i1 15 4.0 0.26 6.5

17 4.1 0.28 7.0

25 4.0 0.30 7.6

24 3.9 0.30 7.7

0.00 5.13

i2 27 4.6 0.22 4.7

28 4.6 0.29 6.3

33 4.5 0.38 8.4

30 4.5 0.35 7.7

2.22 2.22

c 40 5.8 0.31 5.4

40 5.8 0.33 5.6

39 5.7 0.34 6.0

39 5.7 0.34 5.9

1.75 1.75

m1 40 7.8 0.46 6.0

40 7.7 0.47 6.1

38 7.7 0.38 5.0

39 7.7 0.41 5.4

1.30 0.00

m2 42 9.9 0.48 4.9

40 9.8 0.45 4.6

39 9.8 0.45 4.6

39 9.8 0.47 4.8

1.02 0.00

Average 0.18 0.61

Permanent

Upper

I1 42 8.5 0.58 6.9

42 8.5 0.56 6.7

38 8.4 0.52 6.2

37 8.4 0.49 5.8

1.19 1.19

C 27 7.8 0.47 6.0

32 7.7 0.47 6.1

24 7.8 0.62 8.0

26 7.7 0.49 6.4

0.00 0.00

PM2 28 6.9 0.48 6.9

30 6.9 0.45 6.6

20 6.6 0.50 7.6

22 6.7 0.52 7.7

4.55 2.99

M1 42 10.3 0.52 5.0

41 10.2 0.53 5.2

36 10.0 0.48 4.8

37 10.0 0.48 4.8

3.00 2.00

M2 15 10.0 0.41 4.1

9 10.0 0.55 5.6

8 9.8 0.65 6.6

12 9.9 0.64 6.5

2.04 1.01

Lower

I1 35 5.4 0.34 6.2

37 5.3 0.35 6.6

37 5.3 0.37 6.9

34 5.2 0.34 6.5

1.89 1.92

I2 32 5.9 0.32 5.5

36 5.9 0.41 6.9

36 5.8 0.39 6.8

32 5.7 0.35 6.2

1.72 3.51

C 32 6.8 0.42 6.3

32 6.7 0.40 5.9

28 6.7 0.43 6.4

28 6.7 0.50 7.5

1.49 0.00

PM2 28 7.3 0.55 7.6

26 7.3 0.48 6.6

25 7.1 0.46 6.4

25 7.2 0.54 7.6

2.82 1.39

M1 35 11.0 0.59 5.4

36 11.0 0.65 5.9

34 10.8 0.66 6.1

35 10.8 0.63 5.8

1.85 1.85

M2 9 10.1 0.60 5.9 12 10.5 0.76 7.3

12 10.4 0.66 6.3

13 10.6 0.65 6.2

-2.88 -0.94

Average 1.61 1.36


C=permanent canine; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; n=sample size; mean=mean values; SD=standard deviation; CV=coefficient of variation.

127

Table 7.22: Percentage increase in size for buccolingual (BL) dimensions in the primary and permanent dentitions between dizygotic opposite-sex



Right (BL)

Left (BL)

Right (BL)

Left (BL)


Primary

Upper i1 27 5.0 0.34 7.0

29 5.0 0.41 8.1

30 4.9 0.40 8.2

31 5.0 0.41 8.2

2.04 0.00

i2 33 4.9 0.37 7.6

34 4.8 0.35 7.3

36 4.7 0.39 8.4

36 4.7 0.42 9.0

4.26 2.13

c 40 6.1 0.40 6.4

40 6.1 0.36 5.8

39 6.0 0.36 6.0

38 6.0 0.34 5.7

1.67 1.67

m1 41 8.5 0.38 4.4

41 8.6 0.41 4.8

38 8.6 0.35 4.1

39 8.5 0.36 4.2

-1.16 1.18

m2 40 9.7 0.43 4.4

41 9.7 0.42 4.3

39 9.6 0.44 4.6

38 9.6 0.42 4.3

1.04 1.04

Lower

i1 17 3.8 0.24 6.4

18 3.8 0.21 5.6

25 3.8 0.25 6.7

25 3.7 0.24 6.3

0.00 2.70

i2 27 4.4 0.27 6.2

29 4.4 0.26 5.9

32 4.2 0.28 6.7

31 4.3 0.30 7.0

4.76 2.33

c 39 5.7 0.40 7.0

40 5.6 0.36 6.4

39 5.5 0.33 6.0

39 5.5 0.32 5.9

3.64 1.82

m1 40 7.0 0.44 6.3

40 7.0 0.37 5.3

38 6.8 0.33 4.8

39 6.8 0.34 4.9

2.94 2.94

m2 41 8.6 0.37 4.4

40 8.5 0.35 4.1

39 8.4 0.36 4.4

39 8.4 0.38 4.5

2.38 1.19

Average 2.16 1.70

Permanent

Upper I1 41 7.1 0.55 7.7

41 7.2 0.57 8.0

36 6.9 0.61 8.9

31 6.9 0.58 8.4

2.90 4.35

C 23 8.1 0.57 7.0

30 8.0 0.62 7.8

23 7.9 0.67 8.4

24 8.1 0.55 6.8

2.53 -1.23

PM2 28 9.4 0.63 6.7

30 9.3 0.65 7.0

21 9.2 0.61 6.6

22 9.2 0.60 6.5

2.17 1.09

M1 43 11.4 0.52 4.5

43 11.4 0.51 4.5

35 11.2 0.59 5.3

38 11.2 0.50 4.5

1.79 1.79

M2 15 11.4 0.82 7.2

13 11.3 0.77 6.9

12 11.1 0.59 5.3

14 11.0 0.49 4.4

2.70 2.73

Lower

I1 35 6.1 0.52 8.6

36 6.1 0.47 7.7

36 5.9 0.49 8.3

34 5.9 0.47 8.0

3.39 3.39

I2 35 6.5 0.51 7.9

36 6.4 0.43 6.7

35 6.2 0.58 9.4

34 6.2 0.52 8.3

4.84 3.23

C 29 7.2 0.48 6.7

31 7.4 0.53 7.2

27 7.2 0.54 7.6

28 7.2 0.67 9.3

0.00 2.78

PM2 28 8.5 0.62 7.3

26 8.5 0.56 6.6

24 8.2 0.53 6.5

25 8.2 0.49 6.0

3.66 3.66

M1 38 10.2 0.42 4.1

37 10.3 0.46 4.4

36 9.9 0.50 5.0

38 10.1 0.50 4.9

3.03 1.98

M2 15 10.2 0.67 6.5

15 10.2 0.62 6.1

15 10.1 0.56 5.6

14 10.2 0.53 5.3

0.99 0.00

Average 2.55 2.16

i1=primary central incisor; i2=primary lateral incisor; c=primary canine; m1=primary first molar; m2=primary second molar; I1=permanent central incisor; I2=permanent lateral incisor; C=permanent canine; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; n=sample size; mean=mean values; SD=standard deviation;

CV=coefficient of variation.

128

Table 7.23: Percentage increase in size for crown height (CH) dimensions in the primary and permanent dentitions between dizygotic opposite-sex



Right (CH)

Left (CH)

Right (CH)

Left (CH)


Primary

Upper i1 16 5.4 0.48 9.0

16 5.3 0.40 7.5

31 5.3 0.58 11.0

28 5.3 0.52 9.8

1.89 0.00

i2 21 5.0 0.48 9.5

24 5.0 0.52 10.5

35 4.8 0.52 10.8

35 4.7 0.59 12.6

4.17 6.38

c 4 5.5 0.51 9.3

8 5.2 0.43 8.3

29 5.5 0.57 10.2

30 5.4 0.59 10.9

0.00 -3.70

m1 17 4.5 0.38 8.6

16 4.5 0.49 10.9

34 4.3 0.48 11.0

33 4.4 0.39 8.9

4.65 2.27

m2 30 4.0 0.38 9.6

27 4.2 0.37 8.9

38 3.9 0.53 13.4

37 4.0 0.45 11.3

2.56 4.25

Lower

i1 11 4.9 0.29 6.0

12 4.8 0.40 8.3

24 4.5 0.43 9.6

23 4.6 0.45 9.9

8.89 4.35

i2 22 5.4 0.46 8.5

22 5.2 0.64 12.3

32 5.0 0.45 9.0

30 5.0 0.39 7.8

8.00 4.00

c 13 5.9 0.55 9.3

14 5.9 0.40 6.7

28 5.9 0.37 6.2

28 5.9 0.44 7.5

0.00 0.00

m1 18 4.9 0.44 9.0

20 4.9 0.48 9.9

36 4.8 0.38 7.9

32 4.7 0.52 10.9

2.08 4.26

m2 16 3.9 0.46 11.7

20 4.0 0.44 11.2

33 3.8 0.45 12.0

30 3.7 0.44 11.7

2.63 6.95

Average 3.49 2.88

Permanent

Upper I1 39 9.4 0.88 9.4

39 9.4 0.91 9.6

36 9.1 0.70 7.7

35 9.1 0.83 9.1

3.30 3.30

C 26 8.8 0.98 11.1

25 9.0 0.85 9.5

24 8.7 0.99 11.4

23 8.9 1.18 13.2

1.15 0.67

PM2 27 5.9 0.78 13.2

28 5.8 0.68 11.6

20 5.5 0.73 13.1

20 5.6 0.61 11.1

7.27 3.57

M1 34 5.3 0.67 12.7

40 5.1 0.65 12.6

34 4.9 0.65 13.4

34 5.1 0.78 15.4

7.55 0.59

M2 8 5.5 0.67 12.3

11 5.2 0.49 9.3

10 5.2 0.63 12.1

12 5.4 0.67 12.5

4.60 -3.34

Lower

I1 35 8.0 0.91 11.3

36 7.9 0.79 10.1

36 7.7 0.95 12.3

33 7.9 0.76 9.6

3.90 0.00

I2 35 7.7 0.94 12.1

30 8.0 0.84 10.5

36 7.2 0.92 12.7

34 7.3 0.76 10.5

6.94 9.59

C 25 8.5 1.05 12.4

30 8.5 1.01 12.0

28 8.6 1.07 12.4

26 8.4 0.90 10.7

-1.16 1.19

PM2 24 6.1 0.72 11.8

24 5.9 0.62 10.6

24 5.7 0.65 11.4

22 5.9 0.73 12.4

7.02 0.17

M1 29 5.2 0.54 10.5

28 5.0 0.54 10.9

33 4.8 0.60 12.5

30 4.8 0.62 12.9

8.61 3.53

M2 7 4.6 0.59 12.8

6 4.9 0.69 13.9

9 4.5 0.57 12.8

9 4.5 0.60 13.4

2.68 10.76

Average 4.71 2.73

i1=primary central incisor; i2=primary lateral incisor; c=primary canine; m1=primary first molar; m2=primary second molar; I1=permanent central incisor; I2=permanent lateral incisor; C=permanent canine; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; n=sample size; mean=mean values; SD=standard deviation;

CV=coefficient of variation.

129

Table 7.24: Percentage increase in size for intercuspal (IC) dimensions in the primary and permanent dentitions between dizygotic opposite-sex (DZOS) and

dizygotic same-sex (DZSS) female twins.


Right (IC)

Left (IC)



Primary

Upper m1(ic1) 16 4.1 0.38 9.1

18 4.2 0.37 8.7

33 4.1 0.31 7.6

33 4.2 0.29 7.1

0.00 0.00

m2(ic1) 29 4.8 0.33 6.9

31 4.8 0.36 7.5

36 4.7 0.37 7.9

36 4.7 0.31 6.6

2.13 2.13

m2(ic2) 28 3.9 0.32 8.3

31 3.9 0.31 7.9

36 3.9 0.35 8.9

34 3.9 0.33 8.5

0.00 0.00

m2(ic3) 28 5.0 0.48 9.6

30 5.0 0.50 9.9

35 4.8 0.46 9.5

33 4.8 0.45 9.5

4.17 4.17

m2(ic4) 28 3.6 0.32 8.8

30 3.6 0.36 10.0

35 3.6 0.29 8.0

35 3.6 0.33 9.3

0.00 0.00

Lower m1(ic1) 18 2.5 0.33 13.3

17 2.5 0.34 13.6

31 2.4 0.34 14.0

29 2.4 0.26 10.8

4.17 4.17

m2(ic1) 23 3.7 0.33 8.9

23 3.7 0.31 8.4

30 3.5 0.32 9.4

32 3.6 0.32 9.1

5.71 2.78

m2(ic2) 23 3.3 0.33 10.0

22 3.3 0.28 8.5

31 3.4 0.32 9.3

32 3.3 0.46 13.8

-2.94 0.00

m2(ic3) 24 4.7 0.43 9.1

21 4.7 0.47 9.4

33 4.6 0.44 9.6

32 4.5 0.43 9.5

2.17 4.44

m2(ic4) 24 4.5 0.52 11.7

22 4.4 0.44 10.1

32 4.6 0.46 10.1

32 4.4 0.42 9.6

-2.17 0.00

Average 1.32 1.77

Permanent

Upper PM2(ICP) 28 5.2 0.43 8.2

30 5.2 0.43 8.2

21 5.2 0.60 11.6

22 5.3 0.45 8.5

0.00 -1.89

M1(IC1) 40 6.0 0.51 8.6

41 6.0 0.56 9.3

35 5.8 0.44 7.5

39 5.9 0.46 7.8

3.45 1.69

M1(IC2) 38 4.6 0.43 9.3

41 4.8 0.38 8.0

36 4.8 0.44 9.1

39 4.7 0.43 9.1

-4.17 2.13

M1(IC3) 38 6.1 0.52 8.5

40 6.0 0.65 11.0

36 6.0 0.48 8.0

37 6.0 0.56 9.3

1.67 0.00

M1(IC4) 39 4.5 0.44 9.7

39 4.6 0.51 11.1

35 4.5 0.39 8.7

37 4.5 0.39 8.8

0.00 2.22

M2(IC1) 15 6.1 0.80 13.1

13 6.2 0.59 9.6

12 5.9 0.50 8.5

14 6.0 0.58 9.7

3.39 3.33

M2(IC2) 14 4.7 0.53 11.3

13 4.7 0.61 12.8

12 4.7 0.61 12.8

14 4.8 0.52 10.9

0.00 -2.08

M2(IC3) 5 6.1 0.86 14.1

9 5.8 0.80 13.9

4 6.0 0.78 13.0

6 6.2 0.92 14.9

1.67 -6.45

M2(IC4) 8 4.3 0.58 13.7

9 4.2 0.38 9.1

4 4.1 0.49 11.8

6 4.5 0.35 7.8

4.88 -6.67

Lower PM2(ICP) 27 4.1 0.54 13.0

26 4.2 0.52 12.3

24 4.0 0.52 12.9

23 4.0 0.53 13.4

2.50 5.00

M1(IC1) 33 5.0 0.46 9.2

30 4.8 0.41 8.6

37 4.8 0.46 9.5

37 4.8 0.36 7.5

4.17 0.00

M1(IC2) 32 4.3 0.46 10.6

27 4.3 0.39 9.1

36 4.3 0.30 7.0

38 4.2 0.49 11.7

0.00 2.38

M1(IC3) 30 5.4 0.64 11.8

29 5.6 0.67 11.9

36 5.4 0.56 10.3

37 5.6 0.59 10.6

0.00 0.00

M1(IC4) 32 5.6 0.57 10.1

31 5.6 0.54 9.6

37 5.7 0.54 9.4

36 5.6 0.51 9.1

-1.75 0.00

M2(IC1) 12 4.8 0.48 9.9

13 4.6 0.41 9.0

16 4.9 0.49 10.0

15 4.7 0.60 12.9

-2.04 -2.13

M2(IC2) 12 4.8 0.52 10.8

14 4.8 0.49 10.3

15 4.8 0.41 8.5

15 5.1 0.47 9.2

0.00 -5.88

M2(IC3) 12 5.0 0.63 12.6

13 4.9 0.58 11.8

15 5.0 0.74 15.0

15 4.9 0.77 15.7

0.00 0.00

M2(IC4) 13 5.5 0.68 12.3

14 5.3 0.6 11.2

15 5.7 0.46 8.0

15 5.6 0.75 13.6

-3.51 -5.36

Average 0.57 -0.76

m1=primary first molar; m2=primary second molar; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; (ic1)=bucco-lingual intercuspal dimension in primary

first molars; (ICP)=permanent bucco-lingual intercuspal dimension; (IC1)=permanent mesiobuccal-mesiolingual intercuspal dimension; (IC2)=permanent mesiobuccal-distobuccal intercuspal

dimension; (IC3)=permanent distobuccal-distolingual intercuspal dimension; (IC4)=permanent mesiolingual-distolingual intercuspal dimension; n=sample size; mean=mean values; SD=standard deviation..

130

129

Figure 7.1: Percentage difference of mesiodistal (MD), buccolingual (BL), crown height (CH), and

intercuspal (IC) dimensions in the primary and permanent dentitions between females from dizygotic

opposite-sex (DZOS) and monozygotic (MZ) twin pairs. (positive values=DZOS larger).

Figure 7.2: Percentage difference of mesiodistal (MD), buccolingual (BL), crown height (CH), and

intercuspal (IC) dimensions in the primary and permanent dentitions between females from dizygotic

opposite-sex (DZOS) and dizygotic same-sex (DZSS) twin pairs. (positive values=DZOS larger).

-2

-1

0

1

2

3

4

5

6

7

8

MD BL CH IC MD BL CH IC

Right (mm)

Left (mm)

Primary dentition Permanent dentition

-2

-1

0

1

2

3

4

5

MD BL CH IC MD BL CH IC

Right (mm)

Left (mm)

Primary dentition Permanent dentition

131

129

7.3.1.4. Percentage of sexual dimorphism between dizygotic opposite-sex

(DZOS) co-twins, monozygotic (MZ) twins and dizygotic same-sex (DZSS) twins

Differences between males and females were evident in all twin zygosities studied,

with males displaying, on average, larger mean values for all dimensions (MD, BL, CH

and IC) and in both dentitions compared with females from the same zygosity groups.

Percentages of sexual dimorphism were calculated for MD, BL, CH and IC dimensions

between DZOS co-twins, MZ twins and DZSS twins and are presented in Tables 7.25 –

7.28. Overall, DZOS co-twins displayed the smallest percentage values of sexual

dimorphism for MD, BL and CH dimensions in the primary dentition and permanent

dentitions while IC dimensions displayed small percentages values just in the permanent

dentition of DZOS co-twins compared with the other twin groups studied.

The smaller percentage of sexual dimorphism found in DZOS co-twins is

consistent with the suggestions that tooth dimensions in females from this group might

have been increased due to the effects of male hormone in utero. Tooth dimensions seem

to have been affected differently in both dentitions, with the primary dentition being less

affected compared with the permanent dentition. Higher percentages of sexual

dimorphism were evident in later-forming tooth dimensions, BL and CH, (Tables 7.26 and

7.27) compared with early-forming IC dimensions, suggesting that different tooth

dimensions might be exposed to different amounts of male hormone during their

development and/or might be exposed to male hormone in utero for a longer period of

time.

132

129

Table 7.25: Percentage of sexual dimorphism for mesiodistal (MD) dimension between males and females

from dizygotic opposite-sex (DZOS) co-twins, monozygotic (MZ), and dizygotic same-sex (DZSS) twins.

DZOS co-twins MZ twins DZSS twins




n % n % n % n % n % n %

Primary

Upper i1 53 1.6 59 1.6

69 3.2 74 3.2

62 0.0 62 0.0

i2 68 -1.9 66 -1.9

78 0.0 78 0.0

70 -1.9 70 -1.9

c 80 1.5 79 1.5

96 6.1 96 4.5

81 0.0 80 1.5

m1 77 2.9 79 2.9

95 5.9 97 4.3

81 1.4 80 1.4

m2 79 2.3 79 2.3

97 5.9 94 3.5

80 0.0 76 0.0

Lower

i1 33 2.5 33 0.0

49 5.1 46 7.9

48 0.0 47 2.6

i2 57 0.0 60 0.0

74 4.5 72 6.8

67 2.2 61 2.2

c 77 1.7 79 1.7

97 3.4 97 5.3

80 1.8 81 3.5

m1 79 1.3 79 2.6

94 5.3 94 5.3

77 1.3 81 2.6

m2 82 2.0 80 2.0

96 3.1 96 4.1

79 2.0 80 2.0

Average 1.4 1.3 4.2 4.5 0.7 1.4

Permanent

Upper I1 83 2.4 85 1.2

97 3.6 96 2.4

78 3.6 75 2.4

C 58 5.1 62 6.5

62 6.6 63 7.9

45 2.6 47 3.9

PM2 60 1.4 62 1.4

68 3.0 67 4.0

46 3.0 47 3.0

M1 82 1.0 82 2.9

93 5.0 95 4.0

73 3.0 74 4.0

M2 28 2.0 24 3.0

28 8.2 24 9.4

12 2.0 18 1.0

Lower

I1 77 0.0 79 1.9

92 1.9 90 3.8

75 1.9 73 5.8

I2 70 1.7 74 1.7

87 3.4 90 3.4

74 3.4 70 3.5

C 67 4.4 69 6.0

71 9.1 74 9.1

56 6.0 57 6.0

PM2 59 4.1 57 4.1

68 5.6 66 4.2

53 4.2 53 2.8

M1 73 2.7 75 2.7

84 6.5 93 5.6

72 3.7 73 3.7

M2 22 6.9 26 3.8 31 7.8 32 9.8

15 0.0 18 0.0

Average 2.9 3.2 5.5 5.8 3.0 3.3

n=number of males and females; %=percentage of sexual dimorphism; i1=primary central incisor; i2=primary lateral incisor; c=primary canine; m1=primary first molar; m2=primary second molar; I1=permanent central incisor; I2=permanent lateral incisor;

C=permanent canine; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar.

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Table 7.26: Percentage of sexual dimorphism for buccolingual (BL) dimension between males and females






n % n % n % n % n % n %

Primary

Upper

i1 57 0.0 60 2.0

74 6.1 74 4.0

65 2.0 64 2.0

i2 69 -2.0 68 0.0

81 4.3 81 0.0

72 2.1 72 2.1

c 80 1.6 79 1.6

96 3.3 96 3.3

81 1.7 80 0.0

m1 81 3.5 81 1.2

96 5.9 97 4.7

80 0.0 81 1.2

m2 80 3.1 81 3.1

98 4.1 97 5.2

80 3.1 80 3.1

Lower

i1 34 5.3 36 2.6

48 8.3 46 5.4

49 0.0 51 2.7

i2 58 0.0 62 0.0

73 2.3 73 2.3

68 2.4 68 0.0

c 76 -1.8 78 0.0

97 1.8 97 1.8

80 1.8 80 1.8

m1 79 1.4 79 2.9

94 5.9 95 4.3

78 2.9 81 4.4

m2 81 1.2 80 2.4

97 7.3 96 6.0

81 2.4 80 2.4

Average 1.2 1.6 4.9 3.7 1.8 2.0

Permanent

Upper

I1 80 2.8 80 2.8

93 2.8 91 2.8

76 2.9 71 2.9

C 55 2.5 59 5.0

57 7.7 61 10.3

44 2.5 45 0.0

PM2 60 2.1 62 4.3

68 6.5 67 5.4

47 3.3 47 3.3

M1 86 2.6 87 2.6

94 7.2 95 6.3

72 3.6 76 3.6

M2 29 3.5 28 5.3

30 9.1 27 11.0

15 7.2 20 6.4

Lower

I1 76 1.6 77 1.6

89 6.8 88 3.3

76 1.7 75 1.7

I2 73 0.0 75 0.0

86 6.5 85 4.8

73 1.6 72 1.6

C 62 5.6 65 2.7

66 8.3 72 6.8

51 4.2 54 4.2

PM2 59 2.4 57 1.2

68 4.8 66 4.8

52 6.1 53 4.9

M1 79 2.9 78 1.9

91 7.1 94 8.1

75 6.1 76 4.0

M2 31 3.9 29 2.9 39 10.2 39 9.1

20 2.0 21 1.0

Average 2.7 2.8 7.0 6.6 3.7 3.0

n=number of males and females; %=percentage of sexual dimorphism; i1=primary central incisor; i2=primary lateral incisor; c=primary canine; m1=primary first molar; m2=primary second molar; I1=permanent central incisor; I2=permanent lateral

incisor; C=permanent canine; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar.

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Table 7.27: Percentage of sexual dimorphism for crown height (CH) dimension between males and females






n % n % n % n % n % n %

Primary

Upper

i1 28 3.7 25 9.4

53 5.7 53 7.5

43 13.2 43 11.3

i2 39 0.0 42 0.0

59 8.5 61 6.4

54 4.2 52 6.4

c 6 7.3 9 -1.9

45 16.7 42 11.1

31 1.8 33 11.1

m1 31 2.2 28 0.0

57 11.9 55 11.6

45 7.0 40 2.3

m2 56 2.5 55 0.5

67 10.5 64 9.2

55 5.1 53 0.0

Lower

i1 20 4.1 24 4.2

35 4.2 36 4.2

31 4.4 35 2.2

i2 43 1.9 46 0.0

60 3.9 59 3.9

48 8.0 55 4.0

c 19 1.7 26 5.1

55 6.9 52 6.8

33 0.0 36 5.1

m1 35 4.1 36 0.0

53 8.5 51 12.8

41 15.2 38 10.6

m2 33 1.5 40 -5.0

53 19.4 54 2.8

42 5.3 35 7.0

Average 2.9 1.2 9.6 7.6 6.4 6.0

Permanent

Upper

I1 72 3.2 74 1.1

88 9.9 92 8.7

67 7.7 64 8.8

C 52 2.3 42 6.4

43 12.3 48 8.3

34 6.9 33 5.6

PM2 56 2.9 56 4.3

55 10.1 63 9.5

44 7.3 44 5.4

M1 67 3.0 70 10.4

81 11.9 79 15.4

61 4.1 61 3.0

M2 19 -1.8 21 4.0

22 7.2 22 16.1

13 -2.3 15 -3.5

Lower

I1 65 2.5 68 3.8

77 5.1 77 4.2

49 10.4 47 5.1

I2 64 5.2 60 0.0

69 8.5 74 9.2

52 11.1 48 11.0

C 47 3.5 51 7.1

54 13.6 58 10.4

36 5.8 40 3.6

PM2 52 1.6 54 4.1

58 12.5 50 12.4

47 1.8 46 3.6

M1 60 0.0 57 3.6

57 7.6 57 5.9

41 6.1 36 12.0

M2 16 2.6 18 0.8 24 9.1 9 8.3

11 -8.5 11 4.9

Average 2.3 4.1 9.8 9.9 4.6 5.4

n=number of males and females; %=percentage of sexual dimorphism; i1=primary central incisor; i2=primary lateral incisor; c=primary canine; m1=primary first molar; m2=primary second molar; I1=permanent central incisor; I2=permanent lateral

incisor; C=permanent canine; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar.

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Table 7.28: Percentage of sexual dimorphism for intercuspal (IC) dimension between males and

females from dizygotic opposite-sex (DZOS) co-twins, monozygotic (MZ), and dizygotic same-sex

(DZSS) twins.





n % n % n % n % n % n %

Primary

Upper

m1(ic1) 33 2.4 33 0.0

50 7.3 53 2.3

44 6.1 40 7.1

m2(ic1) 57 0.0 61 0.0

67 2.1 65 2.0

51 4.3 52 6.4

m2(ic2) 56 0.0 61 0.0

68 0.0 64 0.0

52 0.0 51 2.6

m2(ic3) 55 2.0 59 -2.0

69 0.0 66 4.1

52 6.3 49 4.2

m2(ic4) 55 2.8 59 2.8

67 0.0 66 0.0

51 0.0 50 2.8

Lower

m1(ic1) 30 12.0 30 0.0

47 -7.7 44 -4.0

34 4.2 32 16.7

m2(ic1) 45 0.0 43 0.0

50 0.0 53 -2.8

38 5.7 36 8.3

m2(ic2) 44 3.0 42 3.0

49 3.0 52 3.0

38 8.8 36 3.0

m2(ic3) 44 -2.1 42 -2.1

52 4.3 52 4.3

40 6.5 37 11.1

m2(ic4) 45 0.0 43 2.3

54 -2.2 52 -6.7

40 6.5 37 6.8

Average 2.0 0.4 0.7 0.2 4.8 6.9

Permanent

Upper

PM2(ICP) 60 0.0 61 1.9

66 3.8 65 0.0

46 5.8 46 1.9

M1(IC1) 80 0.0 81 0.0

86 3.3 82 3.3

61 6.9 67 5.1

M1(IC2) 77 4.3 82 0.0

87 2.1 87 4.2

61 2.1 67 2.1

M1(IC3) 77 -1.6 79 -1.7

88 5.0 83 3.3

62 3.3 64 1.7

M1(IC4) 79 2.2 77 2.2

86 8.9 80 4.3

61 4.4 64 4.4

M2(IC1) 29 3.3 28 3.2

29 10.0 27 8.3

14 5.1 17 5.0

M2(IC2) 28 0.0 28 0.0

29 6.3 26 0.0

15 10.6 18 4.2

M2(IC3) 13 -3.3 18 5.2

17 8.3 17 8.6

5 11.7 9 4.8

M2(IC4) 16 2.3 18 9.5

17 2.4 17 -9.1

5 19.5 8 6.7

Lower

PM2(ICP) 58 0.0 57 0.0

63 4.8 58 2.4

48 12.5 50 7.5

M1(IC1) 66 0.0 60 4.2

60 4.0 55 0.0

44 8.3 44 10.4

M1(IC2) 63 2.3 55 0.0

56 14.0 54 7.1

42 2.3 44 9.5

M1(IC3) 64 5.6 59 1.8

56 5.4 61 1.8

43 9.3 43 7.1

M1(IC4) 69 1.8 63 1.8

62 0.0 64 -1.8

45 3.5 43 7.1

M2(IC1) 27 0.0 26 8.7

29 0.0 30 8.2

19 2.0 16 8.5

M2(IC2) 27 6.3 26 4.2

28 10.6 28 8.5

18 10.4 16 -2.0

M2(IC3) 25 4.0 24 12.2

28 4.0 27 10.2

18 12.0 17 14.3

M2(IC4) 26 1.8 26 5.7 28 13.5 28 9.8 18 5.3 17 7.1

Average 1.6 3.3 5.9 3.8 7.5 5.9

n=number of males and females; %=percentage of sexual dimorphism; m1=primary first molar; m2=primary second molar;

PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; (ic1)=bucco-lingual intercuspal dimension in primary first molars; (ic1)=primary mesiobuccal-mesiolingual intercuspal dimension; (ic2)=primary mesiobuccal-

distobuccal intercuspal dimension; (ic3)=primary mesiodistal-linguodistal intercuspal dimension; (ic4)=primary mesiolingual-

distolingual intercuspal dimension; (ICP)=permanent bucco-lingual intercuspal dimension; (IC1)=permanent mesiobuccal-mesiolingual intercuspal dimension; (IC2)=permanent mesiobuccal-distobuccal intercuspal dimension; (IC3)=permanent

distobuccal-distolingual intercuspal dimension; (IC4)=permanent mesiolingual-distolingual intercuspal dimension.

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7.3.2. Multivariate analysis

Univariate analyse have often been used in odontometric studies to quantify

differences between different tooth crown dimensions, teeth, dentitions and sexes.

However, univariate methods assume independence between each variable of interest.

Measurements of different tooth crown dimensions tend to be correlated,

suggesting that a multivariate analysis should be used to evaluate related groups of data

(Potter, 1972; Potter et al., 1981). Moreover, analysing the data using a multivariate

approach should provide a more sophisticated understanding of the influence of zygosity

on crown dimensions per se (Potter, 1972).

In this study, a multivariate analysis (MANOVA) was performed, analysing

different tooth crown dimensions (MD, BL, CH and IC) simultaneously. Independent

variables within the model included dentitions (primary and permanent), arches (upper and

lower), sides (right and left), and zygosities (MZ, DZSS and DZOS). The multivariate

analysis was performed on female data only because males showed no significant

differences between zygosities for any dimensions studied when univariate analyses were

performed (Chapter 6). Multivariate analysis of data from female twins of all zygosities

should lead to a better understanding of the possible effects of male hormones in females

from DZOS twin pairs, and also help to identify which crown dimensions have contributed

to this variation.

Multivariate analysis was initially performed for all crown dimensions (MD, BL,

CH and IC dimensions) simultaneously. However, when IC dimensions were included in

the multivariate analysis, there was a significant drop in the number of observations due to

the large amount of missing values for IC dimensions, thus compromising the power of the

multivariate analysis. Therefore, it was decided to exclude the IC dimensions for the

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multivariate analysis and only MD, BL and CH dimensions were considered for the

multivariate approach.

The final model included a random effect for twin pair, and the fixed effects of

zygosity (3), dentition (2), quadrant (4) and tooth (7), as well as all two- and three-way

interactions between zygosity, dentition, and dimension.

The MANOVA test statistics for the effect of zygosity on tooth dimensions were all

significant (p<0.05). Examination of least squares means showed that the DZOS females

had the largest dimensions, MZ females had the smallest dimensions, and DZSS females

were intermediate.

The interaction between zygosity and dentition was responsible for the most

interesting effects in the model. The effect of zygosity was greatest for CH dimensions,

followed by BL dimensions and MD dimensions. Figure 7.3 presents the least squares

mean values and standard errors (in millimetres) for all crown dimensions in the primary

and permanent dentitions of MZ, DZSS and DZOS twin pairs.

Figure 7.3: Least squares mean values (in mm) and standard errors (SE) for mesiodistal (MD), buccolingual

(BL) and crown height (CH) dimensions in the primary and permanent dentitions of monozygotic (MZ),

dizygotic same-sex (DZSS) and dizygotic opposite-sex (DZOS) female twins.

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

DZOS DZSS MZ DZOS DZSS MZ

Primary Dentition Permanent Dentition

MD

BL

CH

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Different trends in least squares mean were found between primary and permanent

dentitions in all crown dimensions, thus the dentitions are presented separately. Small

standard errors were found for all crown dimensions in the primary and permanent

dentitions. In the primary dentition, MD dimensions were similar between zygosities,

while BL dimensions were the same between DZOS and MZ females, but smaller in DZSS

female. Although a slight decrease in CH dimensions was evident from DZOS to MZ

females, it was not statistically significant.

In the permanent dentition, similar to the primary dentition, there was no significant

difference in MD dimensions between zygosities. BL dimensions were also similar in size

between zygosities. A significant decrease in CH dimensions from DZOS to MZ was

observed. This result supports the hypothesis that in females from DZOS twin pairs, later-

forming tooth crown dimensions such as crown height might have developed under more

prolonged exposure intrauterine male hormone from the co-twin brother.

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7.4. Discussion

Previous studies have indicated that sex hormones are likely to be important in

explaining the differences observed between males and females in various physical

features (Cohen-Bendahan et al., 2005a; Hines, 2006; Knickmeyer and Baron-Cohen,

2006; Vuoksimaa et al., 2010b). However, studies of the human dentition have suggested

that the dimorphism in tooth size occurs mainly due to the effects of the sex chromosomes

(Guatelli-Steinberg et al., 2008; Alvesalo, 2009).

Studies on levels of steroid hormones in normal males have shown that three surges

of testosterone occur during development. The first surge occurs in utero soon after

testicular differentiation, around 7-9 weeks of gestation. The levels of testosterone are

highest between weeks 10 and 20 post-conception, peaking around week 14, and the values

are comparable to the levels in normal adult males (Reyes et al., 1974; Knickmeyer and

Baron-Cohen, 2006). A second testosterone surge occurs in males soon after birth due to

the inhibition of estrogen levels produced by the placenta, and a third surge in testosterone

occurs at puberty during adolescence (Larsen et al., 2003).

In this study, comparisons between DZOS males with MZ males, as well as

comparisons between DZOS males and DZSS males, showed no differences or only a few

statistically significant differences for MD, BL, CH and IC dimensions, suggesting that

teeth from both dentitions in males from all zygosities have the same development pattern

and DZOS males seem to display no definite influence of female hormones from the

female co-twin during odontogenesis, even though smaller IC dimensions were found in

DZOS males suggesting a possible female interference on IC dimension development in

males. On the other hand, females from DZOS twins displayed increased MD, BL and CH

dimensions compared with the other female twins studied, suggesting that this increase in

size might have occurred due to the influence of intrauterine male hormones produced by

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the male co-twin during tooth development. No trend was found for intercuspal (IC)

dimensions in DZOS females possibly because this is the first dimension to form during

odontogenesis. It also forms over a short period of time, therefore, intercuspal dimensions

might not be exposed to the same amount of intrauterine male hormone compared with the

other dimensions studied.

The percentage differences found in the primary and permanent dentitions between

DZOS females and MZ females as well as between DZOS females and DZSS females

suggest that the earlier-forming primary dentition might be exposed to smaller amounts of

male hormone than the permanent dentition that forms later. The primary dentition starts

to form just before the peak of testosterone surge in utero, around 4-6 weeks post

conception, while the permanent dentition starts to develop at around 16-18 weeks post

conception, which is when the levels of circulating testosterone in utero are high. This

might explain the smaller percentage differences in tooth size found in the primary

dentition compared with the permanent dentition between DZOS females and the other

female twin groups (Tables 7.17 - 7.24).

Different tooth dimensions start to form at different times in utero and they develop

progressively over different periods of time, thus each tooth dimension might be exposed

to different levels of male hormone in utero. Intercuspal (IC) dimensions are the first

crown dimensions to form during odontogenesis and, in this study, this dimension did not

display any evidence of male hormone effect in utero in DZOS female twins compared

with the other female twin groups (Tables 7.12 and 7.16). Small percentage differences for

IC dimensions were also found between DZOS females and females from MZ and DZSS

twins, supporting the evidence of no or little effect of testosterone diffusion in utero on this

tooth dimension in DZOS female co-twins. (Tables 7.20 and 7.24, Figures 7.1 and 7.2).

The other dimensions (MD, BL and CH dimensions) studied displayed increases in size in

DZOS female twins compared with females from MZ and DZSS twin pairs, suggesting

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that they may have been exposed to higher levels of male hormone during their formation

in utero. However, it seems that male hormones might affect tooth dimensions differently,

as dimensions that develop over longer periods of time, such as BL and CH dimensions,

displayed larger percentage differences in size than dimensions that form over shorter

periods of time, such as the MD dimension (Tables 7.9 – 7.11 and 7.13 – 7.15). Increased

percentage differences were found for BL and CH dimensions in DZOS females compared

with the other female groups studied (Figures 7.1 and 7.2).

Overall, small percentages of sexual dimorphism were found between DZOS co-

twins compared with MZ and DZSS twin pairs. This places DZOS females in an

intermediate position between males from all zygosities and females from MZ and DZSS

twin pairs (Tables 7.25 – 7.28). This trend was evident for MD, BL and CH dimensions in

the primary and permanent dentitions, as well as for the IC dimensions in the permanent

dentition. This is a strong indication that increased tooth size in DZOS females might have

occurred due to the influence of male hormones in utero during tooth development.

Multivariate analyses confirmed that tooth size in DZOS females was significantly

greater than in females from the other zygosity groups, taking account of the correlations

between dental dimensions. Multivariate analyses also enabled identification of those

dental crown dimensions which contributed most to differences found between DZOS

females and the other female twin groups. These analyses showed that later-forming CH

dimensions in both the primary and permanent dentitions displayed the largest least

squares mean values in DZOS females compared with MZ and DZSS female twins,

supporting the hypothesis that crown dimensions that required a longer period of time to be

established are likely to be exposed to male hormones for a longer period of time in utero

from their male co-twin.

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8. Dental asymmetries

8.1. Introduction

Bilateral structures, such as human teeth, develop essentially as mirror images

reflecting similar genetic input to antimeric pairs (Adams and Niswander, 1967).

However, small discrepancies in tooth size and shape between right and left sides are

evident in different populations (Townsend and Brown, 1980; Sharma et al., 1986). These

dental asymmetries are an indication of genetic and/or environmental disturbances during

tooth formation and they are assumed to reflect an organism’s inability to “buffer” against

developmental interferences (Van Valen, 1962; Bailit et al., 1970; Potter and Nance,

1976). Dental asymmetries have been reported to be increased in some genetic disorders

and syndromes (Garn et al., 1970; Townsend, 1983; Kieser, 1990; Pirttiniemi et al., 1998;

Pirila-Parkkinen et al., 2001), as well as in adverse prenatal and maternal environmental

conditions (Bailit and Sung, 1968; Bailit et al., 1970; Kieser et al., 1997).

Consistent discrepancies between antimeric pairs of teeth are classified as

directional asymmetries (DA), where one side is consistently larger than the other. In

contrast, small, random variations in size between antimeric pairs of teeth are classified as

fluctuating asymmetries (FA) (Harris and Nweeia, 1980; Townsend and Brown, 1980;

Kieser et al., 1990). Males have been reported by some researchers to display increased

dental asymmetries compared with females (Garn et al., 1966a; 1967a), but similar levels

of dental asymmetry have also been noted in males and females (Bailit et al., 1970).

Patterns of asymmetries have been reported to differ between twins and singletons, with

twins displaying less dental asymmetry than singletons (Boklage, 1987b), even though it is

assumed that twins develop under more competitive and stressful environmental conditions

than singletons. A study of dental asymmetries in twins reported that MZ twins displayed

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greater dental asymmetry than DZSS twins (Sharma et al., 1986) while others found

greater dental asymmetry in DZ twins compared with MZ twin pairs (Potter and Nance,

1976) or even that there were no significant differences between twins and singletons

(Townsend et al., 1999). Overall, there is a lack of information on dental asymmetries,

either DA or FA, in both dentitions of male and female twins from all zygosities.

Cerebral lateralization is an example of directional asymmetry (DA). Males tend to

display greater right cerebral hemisphere dominance compared with females and it has

been proposed that this could lead to differences in many body traits in males, such as

greater brain volume, increased left handedness and decreased language ability

(Geschwind and Galaburda, 1987). Male hormones seem to be essential for brain

development and formation in males (Geschwind and Galaburda, 1987). A study on brain

volumes reported that females who share development in utero with a male co-twin tend to

display larger brain volumes than females who share the intrauterine space with a female

co-twin (Peper et al., 2009). Geschwind and Galaburda’s theory states that intrauterine

male hormones stimulate right-hemisphere development of the brain, leading to more left-

handedness (Geschwind and Galaburda, 1987). Contrary to Geschwind and Galaburda’s

theory, the callosal theory suggests that exposure to lower levels of male hormones in utero

alters male brain development resulting in a larger isthmus of the corpus callosum and to

less cerebral functional asymmetry, and therefore increased left-handedness (Witelson,

1991). A recent study that reported decreased left-handedness in DZOS females compared

with the other female twins, provided some support for the callosal theory (Vuoksimaa et

al., 2010a). Left handedness has been reported to be more prevalent in twins than in

singletons (Boklage, 1987a; Derom et al., 1996), and a study of handedness in a sample of

twins enrolled in the ongoing study at the School of Dentistry, University of Adelaide, also

confirmed that left handedness was more common in twins than the general population

(Dempsey et al., 1999a), but there was no clear trend for differences between zygosity

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groups. Therefore, the possible associations between cerebral lateralization, the effects of

intrauterine male hormones, handedness and dental asymmetries in twins still need further

investigation.

Some researchers have found no evidence of DA in the human dentition (Garn et

al., 1966a; 1967a) while others have reported a tendency for teeth on the right side to be

larger than those on the left (Sharma et al., 1986), or that laterality is reversed in opposing

arches (Harris, 1992; Townsend et al., 1999). Although there is limited evidence of DA in

the human dentition, researchers are still looking for a link between cerebral

lateralization/brain hemisphere dominance and the effects of male hormone diffusion in

utero on the magnitude and patterns of DA in the primary and permanent dentitions of

humans. In this study, it was hypothesised that females from DZOS twins would display

more DA in the dentition compared with MZ females and DZSS females due to the effects

of testosterone associated with sharing the uterine environment with a male co-twin.

Fluctuating asymmetries (FA), or small, random variations in size between right

and left antimeric pairs of teeth, are assumed to be a host response to environmental

disturbances during odontogenesis (Garn et al., 1966a; 1967a; Bailit et al., 1970; Kieser,

1990). FA is evident in both the primary and permanent dentitions (Guatelli-Steinberg et

al., 2006). In humans, increased FA levels noted in dermatoglyphic traits in males have

raised questions about whether intrauterine male hormones might also increase FA in other

body traits, such as the dentition, in females from DZOS twin pairs (Jamison et al., 1993;

Benderlioglu, 2010). Following on from this mark, we hypothesise that females from

DZOS twins would display more FA in their dentitions compared with MZ females and

DZSS females.

Dental asymmetries seem to follow the morphogenetic field patterns described by

Butler (1939) and extended by Dahlberg (1945), with the “key” or more stable tooth of

each class of teeth displaying less dental asymmetry than the more posterior or distal teeth

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(Perzigian, 1977; Townsend and Brown, 1980). Moreover, different tooth dimensions

seem to display different amounts of FA, with intercuspal dimensions displaying more FA

than other tooth crown dimensions (Townsend et al., 2003a).

This chapter aims to describe dental asymmetries in the primary and permanent

dentitions of monozygotic (MZ), dizygotic same-sex (DZSS), and dizygotic opposite-sex

(DZOS) male and female twins from an Australian twin sample of Caucasian ancestry. In

particular, it aims to quantify the magnitude and patterns of DA between antimeric pairs of

teeth by analysing the data according to tooth dimension, upper and lower arches,

dentitions, sexes and twin zygosities. It also aims to quantify the amount of FA in the

different dental crown dimensions of males and females from all zygosities, searching for

any evidence of increased DA and/or FA levels in females from DZOS twins that might

reflect the effects of intrauterine hormone diffusion from the male co-twin.

8.2. Methods

A critical issue in the study of dental asymmetries is the large number of different

formulae available to quantify FA, which makes comparisons between results from

different studies difficult (Guatelli-Steinberg et al., 2006). In this study, the approach

reported by Harris and Nweeia (1980) was used to calculate DA and FA for all tooth

dimensions measured and for all twin zygosity groups. Differences in size between

antimeric pairs of teeth (R-L) were assessed by using a paired t-test with significance set at

p<0.05. Negative values indicated that the left side was larger than the right side. This

approach to quantify DA is limited as it does not remove errors of the method and it is

sensitive to outliers.

FA was quantified according to the formula: , where

R=right side values and L=left side values. A mixed linear model analysis, with statistical

146

129

significance set at p<0.05, was used to identify patterns of FA between zygosity groups

and dentitions.

8.3. Results

8.2.1. Directional asymmetry

Tables 8.1 – 8.4 present summary statistics of directional asymmetry (DA) for MD,

BL, CH and IC dimensions for the primary and permanent dentitions of males and females

from MZ, DZSS and DZOS twin pairs. Similar distributions of positive and negative mean

values of the differences between right and left sides (R-L) were found for all dimensions

studied, suggesting that there was no side predominance in any of the twin groups studied.

Some statistically significant differences between antimeric pairs of teeth were

found in this study, but no systematic trend was evident in the directionality of DA across

tooth crown dimensions, teeth, arches, dentitions, sexes and zygosities (Tables 8.1 – 8.4).

The number of dental dimensions displaying statistically significant differences between

sides slightly exceeded the 5% normally accepted as due to chance or type I errors, but

errors of measurement were not taken into account in the analysis. Mean values of the

differences between right and left sides were small in magnitude, ranging from -0.17mm to

0.11mm for MD dimensions, -0.21mm to 0.14mm for BL dimensions, -0.34mm to 0.20mm

for CH dimensions, and from -0.29mm to 0.37mm for IC dimensions (Tables 8.1 - 8.4).

A similar pattern of DA was evident for all crown dimensions. MD dimensions

displayed 10 significant values from a total of 126 values, BL dimensions displayed 17

significant values from a total of 126 values, CH dimensions displayed 8 significant values

from a total of 124 values and IC dimensions displayed 13 significant values from a total

of 162 values. DA was evident for all teeth in the primary and permanent dentitions,

except for the primary lower central incisors (i1) and permanent lower central (I1) and

147

129

lateral (I2) incisors, which did not display any evidence of DA for any crown dimensions

studied.

In general, males displayed more statistically significant differences between

antimeric pairs of teeth (DA) compared with females for MD and CH dimensions, while

females displayed more DA than males for BL and IC dimensions. When male and female

values from all zygosities were pooled together, there were a total of 48 significant DA

values from a total of 546 values. Of these 48 significant DA values, MZ twins displayed

the highest proportion, a total of 21 out of 48, or 43.8%, while DZSS displayed the least

number of significant DA values, 10 out of 48 or 20.8%. DZOS twins presented an

intermediate position between MZ and DZSS twins, with 17 significant values or 35.4%.

These findings do not support the hypothesis that male hormone increases DA in DZOS

females compared with the other female twins studied, or that DZOS males display less

DA than the other male twins due to sharing the uterine environment with a female co-

twin.

148

Table 8.1: Summary statistics of directional asymmetry (DA) for mesiodistal (MD) dimensions in the primary and permanent dentitions of males and females

from monozygotic (MZ), dizygotic same-sex (DZSS) and dizygotic opposite-sex (DZOS) twin pairs.

MZ (MD) DZSS (MD) DZOS (MD)

Males

Females

Males

Females

Males

Females

n mean+ SD+ n mean+ SD+ n mean+ SD+ n mean+ SD+ n mean+ SD+ n mean+ SD+

Primary

Upper

i1 (R-L) 28 0.02* 0.15

39 0.01* 0.12

29 0.05* 0.32

30 0.03 0.20

28 0.01* 0.20

24 0.04* 0.24

i2 (R-L) 31 0.02* 0.15

44 -0.01* 0.19

34 -0.05* 0.24

34 -0.04 0.17

32 0.02* 0.16

32 0.01* 0.16

c (R-L) 43 0.04* 0.13

51 0.03* 0.29

42 0.03* 0.16

38 0.01 0.20

39 0.03* 0.17

40 -0.01* 0.18

m1 (R-L) 44 -0.02* 0.17

51 -0.05* 0.15

42 -0.09* 0.31

38 -0.04 0.22

38 -0.09* 0.16

38 -0.06* 0.16

m2 (R-L) 41 0.01* 0.17

52 -0.03* 0.19

41 -0.08* 0.49

36 0.04 0.23

40 0.04* 0.30

39 -0.03* 0.19

Lower

i1(R-L) 22 0.02* 0.12

24 -0.01* 0.14

21 0.00* 0.08

24 0.00 0.24

15 -0.05* 0.13

14 0.01* 0.08

i2 (R-L) 33 0.06* 0.15

38 -0.03* 0.15

30 0.02* 0.16

30 0.03 0.17

29 -0.03* 0.29

24 0.03* 0.15

c (R-L) 46 -0.04* 0.21

51 -0.05* 0.17

41 0.05* 0.15

39 0.01 0.23

37 0.02* 0.18

40 0.02* 0.35

m1 (R-L) 42 -0.03* 0.21

49 -0.01* 0.23

39 0.11* 0.40

38 -0.05 0.18

39 0.01* 0.25

39 -0.03* 0.22

m2 (R-L) 44 -0.01* 0.20

51 -0.02* 0.18

40 -0.02* 0.23

39 -0.04 0.19

40 -0.03* 0.20

40 -0.06* 0.18

Permanent upper

I1 (R-L) 44 0.05* 0.30

51 0.02* 0.23

38 0.03* 0.21

37 0.05 0.38

41 0.07* 0.20

42 0.00* 0.24

C (R-L) 27 -0.01* 0.20

31 0.00* 0.18

20 -0.08* 0.16

24 0.10 0.37

29 -0.01* 0.17

27 0.06* 0.20

PM2 (R-L) 29 -0.11* 0.23

36 0.00* 0.18

24 -0.06* 0.18

19 -0.06 0.24

30 -0.04* 0.24

28 0.03* 0.19

M1 (R-L) 42 0.09* 0.29

49 -0.01* 0.21

36 -0.03* 0.28

34 -0.03 0.19

38 -0.08* 0.29

40 0.02* 0.25

M2 (R-L) 14 -0.07* 0.43

7 0.02* 0.32

4 -0.17* 0.34

8 0.10 0.27

13 -0.03* 0.32

9 0.01* 0.24

lower I1 (R-L) 40 0.06* 0.22

48 0.05* 0.18

36 0.00* 0.17

33 -0.04 0.16

41 0.03* 0.18

34 -0.05* 0.20

I2 (R-L) 37 0.00* 0.17

46 -0.04* 0.21

35 -0.01* 0.21

31 -0.05 0.25

35 -0.01* 0.19

30 0.02* 0.14

C (R-L) 33 0.04* 0.24

37 -0.02* 0.20

27 -0.02* 0.29

28 -0.02 0.46

35 -0.02* 0.21

32 -0.03* 0.13

PM2 (R-L) 30 0.03* 0.20

34 0.10* 0.24

27 0.05* 0.20

24 0.09 0.45

31 0.08* 0.55

25 0.01* 0.21

M1 (R-L) 39 0.00* 0.29

44 0.04* 0.28

37 -0.04* 0.20

34 0.02 0.24

37 0.02* 0.24

33 0.01* 0.26

M2 (R-L) 15 0.11* 0.32 13 0.08* 0.34 3 0.20* 0.38 11 0.16 0.30 12 0.10* 0.35 7 0.01* 0.23


C=permanent canine; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; n=number of antimeric tooth pairs; mean+=mean difference between antimeric teeth; SD+=standard deviation of mean+ difference; R=right side; L=left side. Negative sign=left antimere larger than right. *Significant difference between right and left sides: p<0.05.

149

Table 8.2: Summary statistics of directional asymmetry (DA) of buccolingual (BL) dimensions in the primary and permanent dentitions of males

and females from monozygotic (MZ), dizygotic same-sex (DZSS) and dizygotic opposite-sex (DZOS) twin pairs.

MZ (BL) DZSS (BL) DZOS (BL)

Males

Females

Males

Females

Males

Females


Primary

Upper

i1 (R-L) 30 -0.01* 0.17

41 -0.05* 0.15

33 -0.07* 0.18

30 -0.03* 0.12

30 -0.01* 0.13

26 -0.02* 0.25

i2 (R-L) 35 0.09* 0.16

44 -0.06* 0.19

36 0.02* 0.20

36 -0.04* 0.25

34 0.04* 0.16

33 0.06* 0.20

c (R-L) 42 -0.01* 0.20

52 0.02* 0.17

42 0.07* 0.18

38 0.02* 0.19

39 -0.01* 0.18

40 0.00* 0.16

m1 (R-L) 44 0.02* 0.20

52 0.01* 0.19

42 -0.01* 0.31

38 0.03* 0.18

40 0.02* 0.18

41 -0.03* 0.14

m2 (R-L) 45 0.06* 0.23

52 0.09* 0.17

41 0.01* 0.23

38 0.05* 0.23

40 0.05* 0.24

40 -0.03* 0.16

Lower i1(R-L) 21 -0.01* 0.16

25 0.05* 0.17

24 -0.03* 0.11

25 -0.04* 0.13

15 0.02* 0.14

17 0.01* 0.10

i2 (R-L) 33 -0.03* 0.22

38 -0.02* 0.12

36 0.01* 0.29

30 0.04* 0.16

31 -0.03* 0.23

26 0.02* 0.21

c (R-L) 46 -0.02* 0.21

51 0.01* 0.20

40 0.00* 0.18

39 0.06* 0.21

36 0.03* 0.18

39 -0.06* 0.28

m1 (R-L) 42 0.14* 0.29

50 0.14* 0.22

40 0.08* 0.28

38 0.00* 0.18

39 0.06* 0.19

39 0.05* 0.19

m2 (R-L) 45 0.06* 0.22

51 0.14* 0.19

41 -0.03* 0.23

39 0.00* 0.17

40 0.00* 0.22

39 -0.08* 0.19

Permanent

upper

I1 (R-L) 44 0.00* 0.32

45 -0.03* 0.25

40 0.05* 0.29

31 0.05* 0.29

38 -0.04* 0.29

40 -0.05* 0.24

C (R-L) 23 -0.11* 0.29

30 -0.09* 0.29

20 0.03* 0.44

23 -0.21* 0.55

29 -0.05* 0.36

23 0.05* 0.21

PM2 (R-L) 29 0.01* 0.23

36 -0.04* 0.18

24 0.01* 0.16

20 -0.03* 0.18

30 0.04* 0.21

28 0.10* 0.21

M1 (R-L) 43 0.09* 0.19

49 0.08* 0.23

37 0.03* 0.17

34 0.07* 0.22

43 0.03* 0.20

42 0.05* 0.23

M2 (R-L) 14 -0.15* 0.37

11 0.07* 0.23

3 -0.02* 0.03

12 0.07* 0.29

14 0.00* 0.39

13 0.11* 0.23

lower

I1 (R-L) 41 -0.05* 0.31

45 0.04* 0.23

40 -0.04* 0.28

32 -0.02* 0.19

40 0.02* 0.29

34 0.03* 0.22

I2 (R-L) 35 -0.04* 0.23

45 0.00* 0.22

37 -0.03* 0.28

32 -0.01* 0.31

38 -0.13* 0.22

33 -0.06* 0.22

C (R-L) 28 0.08* 0.44

36 0.10* 0.24

22 0.09* 0.34

27 0.01* 0.43

33 0.08* 0.39

28 0.14* 0.28

PM2 (R-L) 30 0.04* 0.27

34 0.05* 0.26

27 -0.07* 0.22

23 -0.04* 0.18

31 -0.03* 0.42

25 0.02* 0.30

M1 (R-L) 42 0.05* 0.22

48 0.09* 0.24

38 0.05* 0.19

36 0.12* 0.27

40 -0.02* 0.23

36 0.03* 0.25

M2 (R-L) 16 0.00* 0.29 21 0.08* 0.22 5 0.05* 0.22 13 0.14* 0.26 14 0.00* 0.21 14 -0.06* 0.27

i1=primary central incisor; i2=primary lateral incisor; c=primary canine; m1=primary first molar; m2=primary second molar; I1=permanent central

incisor; I2=permanent lateral incisor; C=permanent canine; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second

molar; n=number of antimeric tooth pairs; mean+=mean difference between antimeric teeth; SD

+=standard deviation of mean

+ difference; R=right

side; L=left side. Negative sign=left antimere larger than right. *Significant difference between right and left sides: p<0.05.

150

Table 8.3: Summary statistics of directional asymmetry (DA) of crown height (CH) dimensions in the primary and permanent dentitions of males and females from

monozygotic (MZ), dizygotic same-sex (DZSS) and dizygotic opposite-sex (DZOS) twin pairs.

MZ (CH) DZSS (CH) DZOS (CH)

Males

Females

Males

Females

Males

Females


Primary Upper

i1 (R-L) 13 0.04* 0.35

33 -0.01* 0.23

12 0.07* 0.48

28 -0.06* 0.39

6 -0.10* 0.24

13 0.13* 0.49

i2 (R-L) 14 0.13* 0.29

41 -0.08* 0.34

14 -0.03* 0.41

34 0.05* 0.34

14 -0.01* 0.28

20 0.08* 0.15

c (R-L) 2 -0.34* 0.16

37 0.05* 0.63

27 0.08* 0.49

3 0.34* 0.34

m1 (R-L) 6 -0.02* 0.30

46 -0.05* 0.32

7 0.13* 0.47

32 -0.06* 0.40

11 0.08* 0.27

13 -0.04* 0.35

m2 (R-L) 13 0.14* 0.27

48 -0.06* 0.33

13 0.08* 0.46

37 -0.08* 0.36

22 -0.03* 0.32

23 -0.13* 0.34

Lower

i1(R-L) 9 0.07* 0.53

23 0.00* 0.32

7 0.05* 0.21

23 0.04* 0.29

9 0.07* 0.17

10 -0.06* 0.22

i2 (R-L) 19 -0.02* 0.31

36 -0.01* 0.29

16 0.03* 0.21

29 -0.07* 0.36

20 -0.21* 0.42

18 -0.18* 0.40

c (R-L) 9 0.14* 0.36

40 0.02* 0.35

4 0.61* 0.24

27 0.01* 0.28

5 -0.05* 0.41

9 -0.02* 0.57

m1 (R-L) 4 0.01* 0.24

46 0.01* 0.32

3 0.36* 0.21

32 -0.16* 0.46

14 -0.20* 0.39

15 -0.08* 0.21

m2 (R-L) 4 -0.24* 0.33

47 0.00* 0.33

5 0.04* 0.21

29 -0.06* 0.30

15 -0.18* 0.28

16 0.05* 0.40

Permanent

upper

I1 (R-L) 38 -0.15* 0.44

48 -0.05* 0.34

28 -0.18* 0.46

34 -0.03* 0.59

33 0.12* 0.48

37 0.03* 0.39

C (R-L) 13 -0.10* 1.21

21 -0.14* 0.38

9 -0.26* 0.70

23 -0.20* 0.73

16 -0.29* 0.63

24 -0.09* 0.51

PM2 (R-L) 22 0.10* 0.50

30 0.08* 0.33

23 -0.04* 0.56

18 0.04* 0.28

25 0.10* 0.53

26 0.10* 0.36

M1 (R-L) 29 -0.12* 0.52

44 -0.04* 0.45

23 -0.22* 0.43

30 -0.14* 0.44

29 -0.10* 0.44

33 0.01* 0.43

M2 (R-L) 8 -0.41* 0.75

7 -0.17* 0.49

3 -0.12* 0.03

10 -0.26* 0.56

9 0.22* 0.41

7 0.19* 0.29

lower I1 (R-L) 32 -0.13* 0.52

40 0.05* 0.50

8 -0.24* 0.45

31 0.00* 0.65

28 0.02* 0.50

33 -0.11* 0.35

I2 (R-L) 24 -0.06* 0.49

40 -0.06* 0.60

13 0.05* 0.64

34 -0.05* 0.50

26 -0.17* 0.51

32 0.10* 0.35

C (R-L) 15 -0.12* 0.79

33 0.05* 0.63

6 0.00* 0.85

26 -0.30* 0.70

15 0.18* 0.73

24 0.02* 0.45

PM2 (R-L) 17 -0.10* 0.40

30 0.01* 0.50

20 0.19* 0.42

21 0.16* 0.67

27 0.01* 0.62

21 -0.10* 0.35

M1 (R-L) 8 -0.10* 0.42

40 -0.07* 0.39

5 0.30* 0.67

28 0.01* 0.47

26 0.04* 0.68

26 -0.08* 0.28

M2 (R-L) 5 -0.02* 0.71 14 0.12* 0.61 2 0.59* 0.19 6 0.20* 0.24 9 0.31* 0.57 5 0.33* 0.37

i1=primary central incisor; i2=primary lateral incisor; c=primary canine; m1=primary first molar; m2=primary second molar; I1=permanent central incisor; I2=permanent lateral incisor; C=permanent canine; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; n=number of antimeric tooth pairs; mean+=mean difference between antimeric

teeth; SD+=standard deviation of mean+ difference; R=right side; L=left side. Negative sign=left antimere larger than right. *Significant difference between right and left sides: p<0.05

.

151

Table 8.4: Summary statistics of directional asymmetry (DA) of intercuspal (IC) dimensions in the primary and permanent dentitions of males and females from monozygotic

(MZ), dizygotic same-sex (DZSS), and dizygotic opposite-sex (DZOS) twin pairs.

MZ (IC) DZSS (IC) DZOS (IC)

Males

Females

Males

Females

Males

Females

n Mean+ SD+

n Mean+ SD+

n Mean+ SD+

n Mean+ SD+

n Mean+ SD+

n Mean+ SD+

Primary

Upper m1(ic1) (R-L) 5 0.12* 0.33

43 -0.10* 0.32

7 0.00 0.14

33 -0.09* 0.26

14 -0.08* 0.32

14 -0.06* 0.16

m2(ic1) (R-L) 15 -0.01* 0.20

45 -0.07* 0.44

13 -0.03 0.20

35 0.04* 0.29

27 -0.02* 0.39

27 -0.04* 0.27

m2(ic2) (R-L) 16 -0.06* 0.42

45 -0.01* 0.42

14 -0.02 0.25

34 -0.02* 0.31

27 0.02* 0.33

27 -0.02* 0.31

m2(ic3) (R-L) 16 -0.06* 0.25

47 0.08* 0.29

14 0.13 0.25

33 0.08* 0.29

25 0.08* 0.32

26 -0.12* 0.31

m2(ic4) (R-L) 15 0.13* 0.29

46 0.03* 0.41

13 0.00 0.46

34 0.00* 0.28

25 -0.04* 0.27

26 -0.03* 0.27

Lower m1(ic1) (R-L) 4 0.12* 0.16

37 -0.11* 0.24

28 -0.04* 0.33

8 -0.22* 0.18

15 -0.12* 0.28

m2(ic1) (R-L) 4 0.11* 0.27

44 0.00* 0.27

4 0.18 0.36

30 0.08* 0.26

19 0.02* 0.26

19 0.07* 0.30

m2(ic2) (R-L) 3 0.04* 0.14

45 0.00* 0.41

4 -0.26 0.40

31 -0.07* 0.46

19 0.03* 0.33

18 0.00* 0.30

m2(ic3) (R-L) 5 0.06* 0.28

46 0.01* 0.33

5 0.03 0.45

32 -0.08* 0.49

18 0.03* 0.29

18 0.07* 0.28

m2(ic4) (R-L) 6 -0.06* 0.33

45 -0.04* 0.35

5 -0.20 0.20

31 -0.20* 0.37

18 0.00* 0.37

19 -0.08* 0.38

Permanent

Upper PM2(ICP) (R-L) 26 0.13* 0.39

36 -0.08* 0.37

23 0.13 0.30

20 -0.09* 0.33

29 -0.11* 0.35

28 0.00* 0.31

M1(IC1) (R-L) 29 0.04* 0.28

49 0.00* 0.39

26 0.03 0.25

35 0.01* 0.45

38 -0.01* 0.41

39 -0.02* 0.35

M1(IC2) (R-L) 34 -0.03* 0.35

49 0.01* 0.39

25 0.09 0.41

36 0.12* 0.42

38 -0.06* 0.36

37 -0.13* 0.34

M1(IC3) (R-L) 29 0.01* 0.37

50 0.01* 0.39

25 0.09 0.41

34 0.02* 0.38

37 0.00* 0.31

36 0.08* 0.32

M1(IC4) (R-L) 26 0.03* 0.37

50 -0.08* 0.46

25 0.07 0.51

33 0.02* 0.33

37 -0.05* 0.50

36 -0.10* 0.38

M2(IC1) (R-L) 12 0.04* 0.28

13 0.05* 0.43

12 -0.06* 0.33

14 -0.06* 0.40

13 0.05* 0.56

M2(IC2) (R-L) 11 0.19* 0.49

13 -0.06* 0.42

3 0.18 0.40

12 -0.10* 0.37

14 0.06* 0.39

13 0.01* 0.43

M2(IC3) (R-L) 7 0.09* 0.36

8 0.15* 0.43

4 -0.39* 0.28

8 -0.19* 0.84

5 0.16* 0.51

M2(IC4) (R-L) 7 0.18* 0.64

8 -0.29* 0.34

4 -0.38* 0.73

8 -0.19* 0.61

7 0.14* 0.47

Lower PM2(ICP) (R-L) 22 0.01* 0.33

34 0.02* 0.50

23 -0.11 0.36

22 0.00* 0.42

31 0.09* 0.39

24 0.09* 0.40

M1(IC1) (R-L) 8 -0.02* 0.28

39 -0.10* 0.44

5 0.03 0.42

36 -0.03* 0.36

28 -0.01* 0.51

30 -0.20* 0.38

M1(IC2) (R-L) 6 -0.08* 0.65

39 -0.08* 0.38

4 0.23 0.40

36 -0.09* 0.45

27 -0.11* 0.48

27 0.05* 0.52

M1(IC3) (R-L) 10 0.05* 0.35

40 0.10* 0.42

5 -0.05 0.26

36 0.19* 0.34

30 -0.02* 0.43

27 0.21* 0.31

M1(IC4) (R-L) 13 -0.07* 0.23

43 -0.09* 0.28

6 -0.01 0.37

36 -0.06* 0.36

31 -0.01* 0.38

29 0.00* 0.30

M2(IC1) (R-L) 6 0.48* 0.39

19 0.08* 0.75

14 -0.15* 0.55

13 0.17* 0.44

12 -0.23* 0.33

M2(IC2) (R-L) 4 0.12* 0.41

20 0.07* 0.44

13 0.21* 0.55

12 -0.08* 0.47

12 -0.15* 0.41

M2(IC3) (R-L) 3 0.18* 0.54

19 -0.15* 0.41

2 0.29 0.68

13 -0.08* 0.38

9 0.37* 0.33

12 -0.05* 0.29

M2(IC4) (R-L) 4 -0.18* 0.58 18 -0.11* 0.43 2 -0.10 0.85 13 -0.12* 0.69 10 0.02* 0.48 13 -0.13* 0.41

m1=primary first molar; m2=primary second molar; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; (ic1)=bucco-lingual intercuspal dimension in primary first molars;

(ic1)=primary mesiobuccal-mesiolingual intercuspal dimension; (ic2)=primary mesiobuccal-distobuccal intercuspal dimension; (ic3)=primary mesiodistal-linguodistal intercuspal dimension; (ic4)=primary mesiolingual-

distolingual intercuspal dimension; (ICP)=permanent bucco-lingual intercuspal dimension; (IC1)=permanent mesiobuccal-mesiolingual intercuspal dimension; (IC2)=permanent mesiobuccal-distobuccal intercuspal dimension; (IC3)=permanent distobuccal-distolingual intercuspal dimension; (IC4)=permanent mesiolingual-distolingual intercuspal dimension; n=number of antimeric tooth pairs; mean+=mean difference between

antimeric teeth; SD+=standard deviation of mean+ difference; R=right side; L=left side. Negative sign=left antimere larger than right. *Significant difference between right and left sides: p<0.05.

152

151

8.2.2. Fluctuating asymmetry

Tables 8.5 – 8.8 present estimates of FA for MD, BL, CH and IC dimensions in the

primary and permanent dentitions of males and females from MZ, DZSS and DZOS twin

pairs. Figures 8.1 – 8.4 also display FA estimates and standard errors (SE) for MD, BL,

CH and IC dimensions in the primary and permanent dentitions of MZ, DZSS and DZOS

twin pairs. Figure 8.5 presents comparisons of FA estimates and standard errors for MD,

BL, CH and IC dimensions in the primary and permanent dentitions of males and females

from MZ, DZSS and DZOS twin pairs.

Small random variations in crown size between antimeric pairs of teeth, so-called

FA, were found for all dimensions, dentitions, sexes and in all twin zygosities studied.

However, the magnitude of FA varied between crown dimensions, with CH and IC

dimensions displaying higher FA compared with MD and BL dimensions and this pattern

was evident for males and females from all zygosities (Tables 8.5 – 8.8). No distinct

trends in FA were found between males and females from MZ twins, DZSS twins and

DZOS co-twins, except for CH dimensions in the permanent dentition of DZOS twins,

which showed a tendency for males to display higher FA compared with their female co-

twins (Table 8.7).

Levels of FA generally followed the morphogenetic field theory, as increased FA

levels were found for the primary upper lateral incisors compared with the upper central

incisors, and this was evident for MD and BL dimensions in males and females from all

zygosities. The same pattern was found for the upper and lower primary molars, with first

molars displaying more FA than second molars in the primary dentitions. Morphogenetic

field patterns were also evident in the permanent dentition, with upper and lower second

molars displaying more FA, in general, than their adjacent first molars and this was evident

in both males and females from all zygosities, except for females from DZOS twin pairs,

153

151

where no differences were noted between these two teeth (M1 and M2) (Table 8.5). FA in

the MD dimensions of lower permanent incisors did not follow the morphogenetic field

concept described by Butler (1939), as no consistent pattern of FA was found for these

dimensions in males and females from any of the zygosities studied.

FA for BL dimensions seemed to follow Butler’s morphogenetic field theory. In

the primary dentition, an incisor morphogenetic field was evident in the maxillary arch,

with the upper central incisors displaying less FA than the upper lateral incisors, except for

DZSS males. Primary molars in the lower arch also followed field theory, with first

molars displaying higher levels of FA than second molars (Table 8.6). In the permanent

dentition, a molar field was evident in the upper arch with first molars displaying less FA

compared with second molars. An interesting finding for BL dimensions of the lower

incisors was that lateral incisors displayed less FA than central incisors in males, whereas

females displayed the opposite pattern (Table 8.6). CH dimensions also displayed

evidence of a morphogenetic field effect in levels of FA in the permanent dentition, with

lower first molars displaying less FA than second molars and this was evident in males and

females from all zygosities. Levels of FA for CH dimension in the primary dentition failed

to display a consistent pattern, except for FA levels in the lateral incisors which were

higher than in the central incisors for DZSS and DZOS males and for MZ and DZSS

females (Table 8.7).

Analysis of FA scores using a mixed linear model approach showed that, overall,

the primary and permanent dentitions displayed similar patterns and magnitudes of FA

levels for MD and BL dimensions (Figures 8.1 – 8.2). Similar patterns of FA between

primary and permanent dentitions were also found for CH and IC dimensions, but these

two dimensions displayed greater magnitudes of FA compared with MD and BL

dimensions (Figures 8.3 and 8.4). Estimates of FA were not possible for the primary upper

canine antimeres due to the fact that many of these teeth showed some evidence of wear at

154

151

the time the impressions were made and therefore they were excluded from the study

(Figure 8.3). Similar to the other dimensions, IC dimensions displayed the same patterns

of FA between primary and permanent dentitions and their magnitudes were comparable to

CH dimensions (Figures 8.4 and 8.5). In summary, no clear differences in the patterns of

FA were evident between primary and permanent dentitions for each of the dimensions

studied, but an increased magnitude of FA was found for CH and IC dimensions compared

with MD and BL dimensions. These findings do not support the hypotheses that

intrauterine male hormone increases the levels of FA in females from DZOS twins

compared with the other female twins studied, or that males from DZOS twins present less

FA than the other male twins through sharing the uterine environment with a female co-

twin.

155

Table 8.5: Estimates of fluctuating asymmetry (FA) for mesiodistal (MD) dimension in the primary and permanent dentitions of males and females from


MZ (MD) DZSS (MD) DZOS (MD)

Males

Females

Males

Females

Males

Females

n mean* SD* n mean* SD* n mean* SD* n mean* SD* n mean* SD* n mean* SD*

Primary

Upper

i1 28 0.017 0.016

35 0.014 0.014

29 0.028 0.047

30 0.022 0.025

28 0.024 0.019

24 0.023 0.037

i2 31 0.024 0.019

40 0.029 0.022

34 0.031 0.038

34 0.025 0.022

32 0.023 0.024

32 0.025 0.021

c 43 0.017 0.011

45 0.027 0.040

42 0.017 0.017

38 0.021 0.021

39 0.019 0.015

40 0.024 0.016

m1 44 0.020 0.012

45 0.017 0.015

42 0.025 0.041

38 0.023 0.022

38 0.019 0.018

38 0.020 0.013

m2 41 0.015 0.013

46 0.016 0.015

39 0.024 0.056

36 0.017 0.020

40 0.021 0.026

38 0.018 0.014

Lower

i1 22 0.024 0.019

22 0.026 0.021

21 0.014 0.014

24 0.033 0.049

15 0.025 0.018

14 0.015 0.011

i2 33 0.028 0.019

35 0.026 0.024

30 0.027 0.023

30 0.026 0.033

29 0.037 0.046

24 0.023 0.022

c 46 0.025 0.025

45 0.024 0.021

41 0.022 0.016

39 0.026 0.028

37 0.025 0.017

40 0.040 0.045

m1 42 0.017 0.022

43 0.025 0.016

39 0.027 0.048

38 0.020 0.013

39 0.026 0.020

39 0.023 0.020

m2 44 0.016 0.012

45 0.014 0.012

40 0.018 0.016

39 0.017 0.012

40 0.015 0.014

40 0.016 0.011

Permanent

upper I1 44 0.023 0.025

45 0.022 0.016

38 0.019 0.015

37 0.025 0.040

41 0.018 0.016

42 0.024 0.014

C 27 0.019 0.015

28 0.019 0.013

20 0.016 0.014

24 0.033 0.032

29 0.016 0.013

27 0.022 0.015

PM2 29 0.029 0.021

32 0.021 0.017

24 0.022 0.017

19 0.027 0.024

30 0.027 0.020

28 0.022 0.016

M1 42 0.022 0.017

43 0.017 0.013

36 0.021 0.015

34 0.014 0.013

38 0.021 0.020

40 0.020 0.013

M2 14 0.033 0.026

7 0.026 0.018

4 0.033 0.010

8 0.023 0.018

13 0.028 0.012

9 0.020 0.012

lower

I1 40 0.031 0.029

42 0.026 0.026

36 0.024 0.020

33 0.025 0.021

41 0.025 0.022

34 0.028 0.025

I2 37 0.022 0.018

41 0.027 0.026

35 0.025 0.024

31 0.032 0.030

35 0.025 0.019

30 0.018 0.016

C 33 0.023 0.024

33 0.023 0.021

27 0.025 0.030

28 0.031 0.056

35 0.023 0.017

32 0.015 0.012

PM2 30 0.021 0.017

31 0.028 0.024

27 0.022 0.016

24 0.030 0.051

31 0.030 0.062

25 0.024 0.015

M1 39 0.018 0.018

40 0.021 0.017

37 0.013 0.013

34 0.018 0.012

37 0.016 0.013

33 0.018 0.014

M2 15 0.024 0.018 13 0.028 0.017 3 0.031 0.016 11 0.023 0.021 12 0.025 0.019 7 0.018 0.013

i1=primary central incisor; i2=primary lateral incisor; c=primary canine; m1=primary first molar; m2=primary second molar; I1=permanent central incisor; I2=permanent lateral incisor; C=permanent

canine; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; n=number of antimeric pairs; mean*=absolute values of mean difference between antimeric teeth;

SD*=standard deviation of mean*.

156

Table 8.6: Estimates of fluctuating asymmetry (FA) for buccolingual (BL) dimension in the primary and permanent dentitions of males and females from monozygotic

(MZ), dizygotic same-sex (DZSS) and dizygotic opposite-sex (DZOS) twin pairs.

MZ (BL) DZSS (BL) DZOS (BL)

Males

Females

Males

Females

Males

Females

n mean* SD* n mean* SD* n mean* SD* n mean* SD* n mean* SD* N mean* SD*

Primary Upper

i1 30 0.027 0.018

37 0.024 0.021

33 0.029 0.023

30 0.022 0.016

30 0.020 0.015

26 0.028 0.038

i2 35 0.032 0.025

39 0.033 0.025

36 0.031 0.029

36 0.043 0.038

34 0.029 0.023

33 0.037 0.021

c 42 0.022 0.021

46 0.023 0.017

42 0.023 0.021

38 0.025 0.019

39 0.025 0.018

40 0.022 0.015

m1 44 0.016 0.016

46 0.016 0.015

42 0.018 0.034

38 0.016 0.013

40 0.017 0.012

41 0.012 0.010

m2 45 0.018 0.017

46 0.015 0.013

41 0.014 0.020

38 0.016 0.020

40 0.017 0.017

40 0.014 0.021

Lower i1 21 0.027 0.031

22 0.032 0.035

24 0.020 0.021

25 0.022 0.029

15 0.028 0.021

17 0.018 0.019

i2 33 0.039 0.033

35 0.023 0.017

36 0.038 0.050

30 0.022 0.027

31 0.032 0.037

26 0.033 0.037

c 46 0.028 0.020

45 0.027 0.025

40 0.025 0.022

39 0.029 0.029

36 0.029 0.019

39 0.024 0.040

m1 42 0.026 0.032

44 0.031 0.023

40 0.026 0.031

38 0.019 0.018

39 0.023 0.015

39 0.023 0.017

m2 45 0.020 0.017

45 0.024 0.015

41 0.015 0.021

39 0.015 0.013

40 0.018 0.016

39 0.019 0.014

Permanent upper I1 44 0.028 0.036

40 0.029 0.021

40 0.032 0.025

31 0.029 0.031

38 0.031 0.025

40 0.026 0.023

C 23 0.028 0.025

27 0.031 0.025

20 0.040 0.040

23 0.049 0.059

29 0.035 0.025

23 0.018 0.021

PM2 28 0.018 0.015

32 0.016 0.012

24 0.014 0.010

20 0.016 0.012

30 0.017 0.015

28 0.019 0.016

M1 43 0.014 0.011

44 0.018 0.014

37 0.012 0.009

34 0.016 0.012

43 0.013 0.011

42 0.015 0.014

M2 14 0.026 0.020

11 0.018 0.013

3 0.003 0.001

12 0.021 0.015

14 0.024 0.024

13 0.019 0.010

lower I1 41 0.031 0.036

39 0.026 0.030

40 0.035 0.033

32 0.025 0.023

40 0.035 0.030

34 0.026 0.028

I2 35 0.026 0.023

39 0.027 0.024

37 0.032 0.029

32 0.036 0.032

38 0.032 0.023

33 0.027 0.022

C 28 0.036 0.042

32 0.030 0.021

22 0.036 0.032

27 0.038 0.050

33 0.040 0.034

28 0.036 0.022

PM2 30 0.022 0.021

31 0.024 0.021

27 0.020 0.018

23 0.019 0.013

31 0.032 0.035

25 0.029 0.020

M1 42 0.017 0.013

42 0.021 0.015

38 0.014 0.012

36 0.023 0.019

40 0.017 0.014

36 0.020 0.014

M2 16 0.020 0.017 20 0.018 0.014 5 0.015 0.013 13 0.022 0.019 14 0.014 0.013 14 0.022 0.015

i1=primary central incisor; i2=primary lateral incisor; c=primary canine; m1=primary first molar; m2=primary second molar; I1=permanent central incisor; I2=permanent lateral incisor; C=permanent canine; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; n=number of antimeric pairs; mean*=absolute values of mean difference between antimeric teeth;


157

Table 8.7: Estimates of fluctuating asymmetry (FA) for crown height (CH) dimension in the primary and permanent dentitions of males and females from


MZ (CH) DZSS (CH) DZOS (CH)

Males

Females

Males

Females

Males

Females


Primary

Upper

i1 13 0.053 0.036

29 0.035 0.029

12 0.059 0.054

28 0.045 0.061

6 0.035 0.020

13 0.066 0.064

i2 14 0.047 0.031

38 0.058 0.043

14 0.064 0.055

34 0.063 0.038

14 0.040 0.036

20 0.028 0.020

c 2 0.079 0.029

33 0.080 0.084

0 - -

27 0.073 0.054

0 - -

3 0.066 0.053

m1 6 0.066 0.041

40 0.062 0.041

7 0.082 0.079

32 0.075 0.058

11 0.036 0.035

13 0.068 0.038

m2 14 0.073 0.125

42 0.070 0.051

13 0.080 0.098

37 0.067 0.062

25 0.066 0.054

26 0.073 0.050

Lower

i1 9 0.066 0.080

20 0.047 0.040

7 0.037 0.025

23 0.039 0.047

9 0.038 0.016

10 0.028 0.033

i2 19 0.047 0.038

33 0.044 0.033

16 0.028 0.021

29 0.060 0.040

20 0.044 0.067

18 0.073 0.049

c 9 0.050 0.032

35 0.049 0.036

4 0.094 0.040

27 0.039 0.028

5 0.051 0.045

9 0.075 0.061

m1 4 0.028 0.023

40 0.053 0.045

4 0.054 0.033

32 0.071 0.095

14 0.073 0.062

15 0.039 0.032

m2 6 0.106 0.056

41 0.075 0.053

5 0.038 0.024

34 0.083 0.071

17 0.066 0.059

16 0.084 0.064

Permanent

upper I1 38 0.038 0.025

43 0.032 0.019

28 0.037 0.034

34 0.051 0.042

33 0.043 0.028

37 0.034 0.026

C 13 0.098 0.121

18 0.040 0.026

9 0.061 0.049

23 0.059 0.064

18 0.054 0.044

24 0.045 0.042

PM2 22 0.065 0.050

30 0.051 0.031

23 0.074 0.052

18 0.036 0.032

29 0.076 0.052

26 0.049 0.037

M1 29 0.074 0.065

42 0.075 0.065

26 0.080 0.066

36 0.081 0.060

35 0.077 0.060

35 0.072 0.047

M2 8 0.118 0.111

11 0.078 0.068

3 0.024 0.009

12 0.080 0.064

12 0.065 0.045

11 0.069 0.049

lower

I1 32 0.052 0.040

34 0.050 0.040

8 0.053 0.024

31 0.061 0.049

28 0.049 0.038

33 0.039 0.023

I2 24 0.049 0.036

34 0.064 0.059

13 0.062 0.057

34 0.057 0.039

26 0.053 0.040

32 0.039 0.027

C 15 0.064 0.061

29 0.058 0.047

6 0.083 0.027

26 0.065 0.062

15 0.068 0.051

24 0.040 0.035

PM2 17 0.055 0.030

29 0.071 0.071

20 0.065 0.041

22 0.097 0.071

28 0.071 0.077

21 0.049 0.039

M1 8 0.059 0.053

39 0.075 0.068

5 0.117 0.064

37 0.095 0.072

29 0.107 0.081

29 0.046 0.030

M2 5 0.119 0.055 18 0.128 0.064 2 0.133 0.038 13 0.200 0.420 13 0.132 0.124 12 0.095 0.058

i1=primary central incisor; i2=primary lateral incisor; c=primary canine; m1=primary first molar; m2=primary second molar; I1=permanent central incisor; I2=permanent lateral incisor; C=permanent

canine; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; n=number of antimeric pairs; mean*=absolute values of mean difference between antimeric teeth;


158

Table 8.8: Estimates of fluctuating asymmetry (FA) for intercuspal (IC) dimension in the primary and permanent dentitions of males and females from monozygotic (MZ),

dizygotic same-sex (DZSS) and dizygotic opposite-sex (DZOS) twin pairs.

MZ (IC) DZSS (IC) DZOS (IC)

Males

Females

Males

Females

Males

Females


Primary Upper m1(ic1) 5 0.069 0.033

37 0.060 0.052

7 0.024 0.021

33 0.055 0.039

14 0.057 0.049

14 0.033 0.022

m2(ic1) 15 0.032 0.021

39 0.064 0.063

13 0.034 0.024

35 0.051 0.037

27 0.064 0.052

27 0.044 0.029

m2(ic2) 16 0.089 0.072

39 0.085 0.064

14 0.054 0.040

34 0.069 0.044

27 0.069 0.050

27 0.063 0.044

m2(ic3) 16 0.040 0.034

41 0.050 0.035

14 0.047 0.030

33 0.051 0.041

25 0.058 0.032

26 0.050 0.035

m2(ic4) 15 0.085 0.050

40 0.089 0.074

13 0.076 0.079

34 0.057 0.050

25 0.064 0.037

26 0.061 0.041

Lower m1(ic1) 4 0.062 0.067

33 0.090 0.052

1 0.114 -

28 0.094 0.096

8 0.093 0.059

15 0.094 0.063

m2(ic1) 4 0.062 0.063

39 0.061 0.047

4 0.095 0.068

30 0.068 0.047

19 0.061 0.042

19 0.067 0.056

m2(ic2) 3 0.032 0.021

40 0.096 0.070

4 0.104 0.034

31 0.086 0.087

19 0.061 0.060

18 0.069 0.057

m2(ic3) 5 0.046 0.046

41 0.058 0.035

5 0.079 0.053

32 0.071 0.084

18 0.049 0.038

18 0.056 0.027

m2(ic4) 6 0.069 0.025

40 0.064 0.047

5 0.051 0.041

31 0.068 0.058

18 0.070 0.051

19 0.072 0.051

Permanent

Upper PM2(ICP) 26 0.060 0.046

32 0.055 0.039

23 0.048 0.032

20 0.053 0.040

29 0.057 0.041

28 0.045 0.035

M1(IC1) 29 0.036 0.029

43 0.051 0.042

26 0.032 0.025

35 0.060 0.051

38 0.053 0.040

39 0.046 0.037

M1(IC2) 34 0.050 0.048

43 0.067 0.047

25 0.066 0.059

36 0.077 0.053

38 0.059 0.046

37 0.065 0.041

M1(IC3) 29 0.048 0.032

44 0.050 0.038

25 0.053 0.042

34 0.047 0.040

37 0.042 0.031

36 0.043 0.041

M1(IC4) 26 0.057 0.050

44 0.084 0.057

25 0.078 0.068

33 0.057 0.046

37 0.072 0.080

36 0.072 0.049

M2(IC1) 12 0.033 0.028

13 0.061 0.036

1 0.089 -

12 0.046 0.029

14 0.051 0.039

13 0.077 0.044

M2(IC2) 11 0.077 0.079

13 0.069 0.049

3 0.071 0.009

12 0.063 0.038

14 0.070 0.046

13 0.072 0.050

M2(IC3) 7 0.046 0.029

8 0.061 0.039

1 0.082 -

4 0.059 0.034

8 0.111 0.087

5 0.063 0.046

M2(IC4) 7 0.124 0.080

8 0.072 0.087

1 0.031 -

4 0.116 0.145

8 0.113 0.089

7 0.087 0.067

Lower PM2(ICP) 22 0.063 0.042

31 0.100 0.076

23 0.071 0.048

22 0.084 0.062

31 0.081 0.063

24 0.086 0.050

M1(IC1) 8 0.046 0.036

33 0.071 0.056

5 0.052 0.056

36 0.057 0.046

28 0.073 0.065

30 0.067 0.059

M1(IC2) 6 0.104 0.080

34 0.065 0.058

4 0.060 0.086

36 0.088 0.068

27 0.077 0.083

27 0.099 0.070

M1(IC3) 10 0.043 0.043

35 0.060 0.048

5 0.029 0.031

36 0.058 0.039

30 0.056 0.053

27 0.054 0.040

M1(IC4) 13 0.034 0.027

37 0.041 0.032

6 0.053 0.025

36 0.052 0.039

31 0.053 0.040

29 0.045 0.028

M2(IC1) 6 0.095 0.075

18 0.087 0.113

1 0.072 -

14 0.081 0.081

13 0.077 0.064

12 0.073 0.035

M2(IC2) 4 0.048 0.065

19 0.080 0.060

1 0.096 -

13 0.086 0.073

12 0.072 0.057

12 0.060 0.067

M2(IC3) 3 0.066 0.067

18 0.074 0.057

2 0.087 0.073

13 0.063 0.045

9 0.081 0.045

12 0.052 0.023

M2(IC4) 4 0.082 0.045 17 0.070 0.055 2 0.099 0.020 1 0.003 - 10 0.066 0.060 13 0.058 0.047

m1=primary first molar; m2=primary second molar; PM2=permanent second premolar; M1=permanent first molar; M2=permanent second molar; (ic1)=bucco-lingual intercuspal dimension in primary first molars;

(ic1)=primary mesiobuccal-mesiolingual intercuspal dimension; (ic2)=primary mesiobuccal-distobuccal intercuspal dimension; (ic3)=primary mesiodistal-linguodistal intercuspal dimension; (ic4)=primary mesiolingual-

distolingual intercuspal dimension; (ICP)=permanent bucco-lingual intercuspal dimension; (IC1)=permanent mesiobuccal-mesiolingual intercuspal dimension; (IC2)=permanent mesiobuccal-distobuccal intercuspal dimension; (IC3)=permanent distobuccal-distolingual intercuspal dimension; (IC4)=permanent mesiolingual-distolingual intercuspal dimension; n=number of antimeric tooth pairs; mean*=absolute values of mean

difference between antimeric teeth; SD*=standard deviation of mean*.

159

158

Figure 8.1: Estimates of fluctuating asymmetry (FA) and standard errors (SE) for mesiodistal (MD) dimensions in

the primary and permanent dentitions for males and females from all zygosities.

MD=mesiodistal dimension; FA between antimeric teeth: i1.ic1=primary central incisors; i2.i2=primary lateral incisors;

c.c=primary canines; m1.m1=primary first molars; m2.m2=primary second molars; I1.I1=permanent central incisors;

I2.I2=permanent lateral incisors; C.C=permanent canines; PM2.PM2=permanent second premolars; M1.M1=permanent

first molars; M2.M2=permanent second molars.

Figure 8.2: Estimates of fluctuating asymmetry (FA) and standard errors (SE) for buccolingual (BL) dimensions

in the primary and permanent dentitions for males and females from all zygosities.

BL=buccolingual dimension; FA between antimeric teeth: i1.i1=primary central incisors; i2.i2=primary lateral incisors;




0

0.005

0.01

0.015

0.02

0.025

0.03

0.035i1

.i1 c.c

m2

.m2

i1.i1 c.

c

m2

.m2

I1.I

1

PM

2.P

M2

M2

.M2

I1.I

1

C.C

M1

.M1


Primary Dentition (MD) Permanent Dentition (MD)

FA estimates

FA estimates

00.005

0.010.015

0.020.025

0.030.035

0.04

i1.i1

i2.i2 c.

cm

1.m

1m

2.m

2

i1.i1

i2.i2 c.

cm

1.m

1m

2.m

2

I1.I

1C

.CP

M2

.PM

2M

1.M

1M

2.M

2

I1.I

1I2

.I2

C.C

PM

2.P

M2

M1

.M1

M2

.M2


Primary Dentition (BL) Permanent Dentition (BL)

FA estimate

FA estimate

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158

Figure 8.3: Estimates of fluctuating asymmetry (FA) and standard errors (SE) for crown height (CH) dimensions

in the primary and permanent dentitions for males and females from all zygosities.

CH=crown height dimension; FA between antimeric teeth: i1.i1=primary central incisors; i2.i2=primary lateral incisors;




Figure 8.4: Estimates of fluctuating asymmetry (FA) and standard errors (SE) for intercuspal (IC) dimensions in

the primary and permanent dentitions for males and females from all zygosities.

IC=intercuspal dimension; FA between antimeric teeth: ic1.ic1=primary mesiobuccal-mesiolingual intercuspal

dimension; ic2.ic2=primary mesiobuccal-distobuccal intercuspal dimenion; ic3.ic3=primary distobuccal-distolingual

intercuspal dimension; ic4.ic4=primary mesiolingual-distolingual intercuspal dimension; IC1.IC1=permanent

mesiobuccal-mesiolingual intercuspal dimension; IC2.IC2=permanent mesiobuccal-distobuccal intercuspal dimenion;

IC3.IC3=permanent distobuccal-distolingual intercuspal dimension; IC4.IC4=permanent mesiolingual-distolingual

intercuspal dimension.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2i1

.i1i2

.i2 c.c

m1

.m1

m2

.m2

i1.i1

i2.i2 c.

cm

1.m

1m

2.m

2

I1.I

1C

.CP

M2

.PM

2M

1.M

1M

2.M

2

I1.I

1I2

.I2

C.C

PM

2.P

M2

M1

.M1

M2

.M2


Primary Dentition (CH) Permanent Dentition (CH)

FA estimate

FA estimate

0

0.02

0.04

0.06

0.08

0.1

0.12

ic1

.ic1

ic2

.ic2

ic3

.ic3

ic4

.ic4

ic1

.ic1

ic2

.ic2

ic3

.ic3

ic4

.ic4

IC1

.IC

1

IC2

.IC

2

IC3

.IC

3

IC4

.IC

4

IC1

.IC

1

IC2

.IC

2

IC3

.IC

3

IC4

.IC

4


Primary Dentition (IC) Permanent Dentition (IC)

FA estimate

FA estimate

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Figure 8.5: Comparisons of fluctuating asymmetry (FA) estimates and standard errors (SE) for mesiodistal (MD), buccolingual (BL), crown height (CH), and intercuspal (IC) dimensions

in the primary and permanent dentitions of males and females from all zygosities.

MD=mesiodistal dimensions; BL=buccolingual dimensions; CH=crown height dimensions; IC intercuspal dimensions. FA between antimeric teeth in the primary and permanent dentitions:

i1.i1=central incisors; i2.i2=lateral incisors; c.c=canines; m1.m1=first molars; m2.m2=second molars; I1.I1=central incisors; I2.I2=lateral incisors; C.C=canines; PM2.PM2=second premolars;

M1.M1=first molars; M2.M2=second molars, ic1.ic1=mesiobuccal-mesiolingual intercuspal dimension; ic2.ic2=mesiobuccal-distobuccal intercuspal dimenion; ic3.ic3=distobuccal-distolingual

intercuspal dimension; ic4.ic4=mesiolingual-distolingual intercuspal dimension; IC1.IC1=mesiobuccal-mesiolingual intercuspal dimension; IC2.IC2=mesiobuccal-distobuccal intercuspal dimenion;

IC3.IC3=distobuccal-distolingual intercuspal dimension; IC4.IC4=mesiolingual-distolingual intercuspal dimension

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

i1.i

1

c.c

m2

.m2

i1.i

1

c.c

m2

.m2

I1.I

1

PM

2.P

M2

M2

.M2

I1.I

1

C.C

M1

.M1

i1.i

1

c.c

m2

.m2

i1.i

1

c.c

m2

.m2

I1.I

1

PM

2.P

M2

M2

.M2

I1.I

1

C.C

M1

.M1

ic1

.ic1

ic3

.ic3

IC1

.IC

1

IC3

.IC

3

Upper Lower Upper Lower Upper Lower Upper Lower Upper Lower Upper Lower Upper Lower Upper Lower

Primary (MD) Permanent (MD) Primary (BL) Permanent (BL) Primary (CH) Permanent (CH) Primary (IC) Permanent (IC)

MD BL CH IC

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8.3. Discussion

Asymmetries in different tooth dimensions in both the primary and permanent

dentitions have been reported by many researchers (Garn et al., 1967a; Townsend and

Brown, 1980; Sharma et al., 1986; Hershkovitz et al., 1987; Kieser, 1990; Harris, 1992;

Kieser et al., 1997; Townsend et al., 1999). Although complete directionality, where the

teeth on one side of the dental arch are consistently larger than the other, is not evident in

the human dentition, some teeth and tooth crown dimensions seem to display this

phenomenon (Harris and Nweeia, 1980; Townsend and Brown, 1980; Hershkovitz et al.,

1987).

In this study, statistically significant differences between some crown dimensions

from the right and left sides, or DA, were evident in both dentitions of males and females

from MZ, DZSS and DZOS twin pairs, even though no definite patterns of DA were

identified across tooth crown dimensions, upper and lower arches, dentitions, sexes and

zygosities. Statistical analysis showed that the number of dimensions that showed DA in

this study (a total of 48 out of 546 possible variables) slightly exceeded 5%, normally

accepted as being due to chance.

Some researchers have stated that DA in many body traits may be associated with

cerebral lateralization and right hemisphere dominance. Cerebral lateralization develops

within 8 days after conception and may be influenced by male hormones either in utero or

post-natally (Geschwind and Galaburda, 1987). If male hormones lead to more cerebral

lateralization in males, females from opposite-sex twin pairs might be expected to display

more DA in the dentition than the other female groups due to the fact that they share the

uterine environment with a co-twin brother, and therefore may develop under the effect of

higher concentrations of male hormone. In this study, females from DZOS twins did not

display more DA compared with females from MZ and DZSS twins, failing to support the

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Geschwind and Galaburda’s hypothesis that intrauterine male hormone increases DA in the

dentition in females from DZOS twin pairs. It seems that the timing of sex-related

differentiation is trait specific and depends on the moment that the trait differentiates (Tapp

et al., 2011).

Male hormones are essential for brain development, function and growth. The

growth of brain tissue seems to be associated with circulating male hormone in utero or

post-natally, as researchers have found evidence of larger human total brain volumes in

females from DZOS twin pairs (Peper et al., 2009). Taking into account that the dentitions

of females tended to be more symmetrical than males (Garn et al., 1966a; 1967a), it was

expected that MZ and DZSS females would present less DA than DZOS females due to the

fact that DZOS females are likely to be more exposed to male hormones in utero from their

male co-twin. In fact, measurements of DA in this study placed DZOS females in an

intermediate position between MZ and DZSS female twins, suggesting that intrauterine

male hormone has no influence on the development of DA in the dentition of DZOS

females.

In this study, DA did not display any clear side predominance for any of the

dimensions studied, differing from a study that reported that one side tended to be larger,

on average, than the other in opposing dental arches (Townsend et al., 1999). Moreover,

primary central incisors, as well as permanent lower central and lateral incisors, did not

show evidence of DA in any crown dimension. The possibility of a link between brain

development and DA in the human dentition, as well as the possible influence of male

hormone either in utero or post-natally in the expression of DA between antimeric pairs of

teeth, are still to be elucidated and further investigation of these issues is needed.

The small, non-directional random differences in size found between right and left

sides, or FA, were assessed in this study. Although some studies have stated that males are

more likely to be asymmetric for tooth size than females (Garn et al., 1966a; 1967a), this

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study showed no clear significant differences between males and females for all

dimensions, in both dentitions and in all zygosities studied, being consistent with the

findings of Khalaf et al. (2005b) who used a 2D image photography system. This study

suggests that males and females react similarly in terms of FA during tooth formation,

regardless of the environmental conditions.

In this study, when data from both sexes were compared, DZOS females displayed

less FA than females from other zygosities while DZOS males displayed more FA

compared with males from MZ and DZSS twins. In fact, to support the male hormone

hypothesis, the opposite result would be expected, i.e., that DZOS females would display

more FA compared with MZ and DZSS females because they have shared the uterine

environment with a male co-twin. If females are better buffered against developmental

disturbances during tooth formation due to having a paired X-chromosome, then female

twins from same-sex pairs might display less FA than females from DZOS twin pairs. The

same thoughts would be valid for males, with DZOS males displaying less FA than males

from MZ and DZSS twin pairs due to sharing intrauterine environment with a female co-

twin. Indeed, the findings of this study showed the opposite, that females from DZOS

twins showed lower levels of FA than females from the other zygosities and that DZOS

males showed more FA than males from MZ and DZSS twin pairs. These findings do not

support increased FA levels in DZOS females as a result of increased circulating male

hormone in utero from the male co-twin. It seems that possible male hormone diffusion in

utero has little or no influence on the levels of FA in the dentition, even though a positive

correlation between male hormone and increase FA levels have been found in other

morphological traits such as dermatoglyphics (Jamison et al., 1993; Benderlioglu, 2010).

In this study, FA generally followed Butler’s (1939) morphogenetic field theory,

with the most stable or mesial tooth in each tooth class tending to display less FA than the

more distal tooth (Butler, 1939; Dahlberg, 1945). The findings are also consistent with a

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study on dental asymmetries that used the same measurement system as the present study

(Khalaf et al., 2005a). The primary and permanent dentitions displayed the same pattern of

FA in all dimensions studied. However, an increased magnitude in FA levels was found in

CH and IC dimensions compared with MD and BL dimensions, possibly due to difficulties

in precisely locating the landmarks when the measurements were obtained (IC) or due to

the position of the gingival tissues when the impressions were obtained (CH). This

increased magnitude in FA levels of IC dimensions is consistent with studies suggesting

that environmental influences may interfere with the positioning of enamel knots during

odontogenesis (Townsend et al., 2003a; Hunter et al., 2010).

A fundamental point in studying the patterns and magnitude of FA on dental crown

traits is the diversity of statistical approaches used to analyse the data, which makes

comparisons with published data for different populations challenging (Palmer and

Strobeck, 1986). The approach used to quantify FA in this study was based on the absolute

difference between right and left sides following Harris and Nweeia (1980). Moreover, the

small sample sizes for some dimensions might have led to type II errors.

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9. General discussion

Knowledge of the effects of genetic, environmental and epigenetic influences on

variability of tooth number, size and shape is essential for many fields of study, such as

physical anthropology, forensic odontology, auxology and clinical dentistry (Townsend et

al., 2012a). The study of dental crown morphology within and between populations, as

well as the study of sexual dimorphism of dental crowns, has shown that variation in tooth

size reflects a multifactorial mode of inheritance with complex interactions between

genetic, epigenetic and environmental factors during odontogenesis (Bailit, 1975; Kabban

et al., 2001; Brook, 2009; Brook et al., 2009b; Townsend et al., 2012a).

Studies of related individuals have confirmed that genetic factors play a major role

in tooth size variation (Garn et al., 1965b; Townsend and Brown, 1978a), but

environmental factors also contribute to variation (Kabban et al., 2001). More recently,

researchers have suggested that epigenetic factors are important during dental

development, perhaps explaining why phenotypic differences can be noted between

monozygotic co-twins (Townsend et al., 2005; Brook, 2009; Townsend et al., 2009c;

Townsend et al., 2012a).

Sliding calipers have been the traditional method of measuring tooth crown

dimensions (Moorrees et al., 1957) and they have been used mainly to obtain linear

measurements of dental crowns, such as MD and BL dimensions (Garn et al., 1966b;

1967a; Harris and Lease, 2005). More recently, new technologies, such as 2D and 3D

image analysis systems, have been used to measure dental crown dimensions with

accuracy and precision, allowing the study of new dental phenotypes such as intercuspal

dimensions (IC), crown heights (CH), crown areas, perimeters, angles and volumes (Smith

et al., 2009). The study of dental asymmetries, both DA and FA, also help researchers to

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better understand the ability of individuals to “buffer” against developmental disruptions

during tooth development (Van Valen, 1962).

Alvesalo and colleagues studied the influence of the sex chromosomes on dental

growth by examining patients with chromosomal aneuploidies and their findings indicated

that both the X and Y chromosomes are responsible for dental growth, with the Y-

chromosome promoting the development of both enamel and dentin, while the X-

chromosome influences the development of enamel (Alvesalo et al., 1975; Alvesalo and

Kari, 1977; Alvesalo and Varrela, 1980; Alvesalo and Tammisalo, 1981; Alvesalo, 2009).

These researchers have proposed that sexual dimorphism in the human dentition is due

mainly to the effects of the sex chromosomes. Although sex chromosomes seem to be

responsible for a large amount of the variation found in the dentition between males and

females, other environmental factors, such as the sex hormones, should also be considered

when assessing sexual dimorphism in tooth crown size. A comprehensive study of sexual

dimorphism in different dental crown dimensions of male and female twins has been

presented in this thesis to improve our understanding of how genetic and environmental

factors influence dental crown size.

Studies in animals have shown that females which develop between males in utero

display more masculinising effects of some physiological, behavioural and morphological

traits than females which develop between females in utero, possibly due to intrauterine

male hormone diffusion (Miller, 1994; Ryan and Vandenbergh, 2002; Banszegi et al.,

2010). In humans, studies have also demonstrated that females who share the uterine

environment with a co-twin brother display more masculinising effects of some traits than

females who develop in the utero with a female co-twin (McFadden, 1993; Miller, 1994;

Dempsey et al., 1999b; Lummaa et al., 2007; Peper et al., 2009; McFadden, 2011). It is

proposed that females who share the uterine environment with a male co-twin are more

exposed to intrauterine male hormones than females who develop with another female in

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utero, and this high concentration of male hormone might influence the development of

many body traits, including the dentition (Miller, 1994). Gingerich (1974), studying sexual

dimorphism in many fossil mammals, stated that bones and teeth are subject to hormonal

effects during development with later-forming teeth displaying more sexual dimorphism

than earlier-forming teeth. Moreover, research using pregnant Rhesus monkeys treated

with testosterone has shown that their female offspring display larger tooth crown

dimensions compared with non-treated female Rhesus monkeys (Zingeser and Phoenix,

1978), consistent with male hormone diffusion in utero and an influence on the dentition.

Guatelli-Steinberg and colleagues (2008) examined Gingerich’s (1974) hormonal

hypothesis on the degree of sexual dimorphism in tooth size and also a modified version of

this hormonal hypothesis introduced by Kondo and Townsend (2004) and Kondo et al.

(2005), searching for any measurable evidence to support the hormonal hypothesis. They

based their predictions on enamel initiation and formation times provided by Reid and

Dean (2006) and tested these predictions using mesiodistal dimensions of permanent tooth

size data previously collected from seven populations (Table 9.1). They claimed that their

findings supported the hypothesis that sexual dimorphism in the human dentition is mainly

related to the growth-promoting effect of the Y-chromosome as previously proposed by

Alvesalo and colleagues (2009). They also argued that sex hormones have no effect or

only a minor influence on dental crown formation (Guatelli-Steinberg et al., 2008).

Guatelli-Steinberg and co-workers based their predictions on postnatal testosterone

surges and underestimated the importance of the intrauterine male hormone surge

described by Reyes et al. (1974) and Knickmeyer and Baron-Cohen (2006). Furthermore,

the data of Reid and Dean (2006) that were used in Guatelli-Steinberg’s study were based

on histological sections of tooth crowns obtained in a cross-sectional study design.

Guatelli-Steinberg and co-workers appears to have overlooked the fact that teeth from both

dentitions start to form in utero and that each tooth passes through different stages of soft

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tissue formation (thickening and specialization of dental lamina, bud, cap and bell stages)

before calcification of dentine and enamel commences (Nanci, 2003). Indeed, it is during

this “soft tissue” phase of tooth formation that the process of morphogenesis, epithelial

histogenesis and cell differentiation occurs. I would argue that this is the time when the

dental tissues are most likely to be susceptible to the influence of sex hormones in utero.

Guatelli-Steinberg et al.’s (2008) predictions were based on critical intervals from the age

at initiation of enamel calcification and age at which maximum mesiodistal dimensions are

formed. For example, permanent upper central incisors start enamel calcification at 0.35

years of age and reach its mesiodistal breadth at 1.34 years of age (Table 9.1), but this does

not take into account the soft tissue phase of odontogenesis.

To estimate the effects of the sex hormones on tooth size it is necessary to conduct

a refined longitudinal study including both the primary and permanent dentitions of the

same individuals, allowing an evaluation of the effects of the sex hormones between tooth

classes and dentitions. Moreover, it is necessary to incorporate different dental dimensions

obtained by using new technology systems, such as 2D and 3D image analysis systems,

which cover the entire crown formation period to evaluate the influence of sex hormones.

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Table 9.1: Table of predictions of sexual dimorphism from two models presented by Guatelli-Steinberg and co-workers (2008).

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This study was designed to cover a major gap in our current knowledge of

hormonal influences on tooth size by measuring different dental crown dimensions (MD,

BL, CH and IC dimensions) on primary and permanent dentitions of males and females

from MZ, DZ same-sex and DZOS twin pairs. Opposite-sex dizygotic twin pairs provided

a unique model to analyse the influence of sex hormones on dental crown size. It was

possible to compare data obtained from males and females from DZOS twins with same-

sex males and females from MZ and DZSS twin pairs. The uniqueness of this opposite-sex

twin model is that males and females from DZOS twins share the same intrauterine

environment and most likely develop under different concentrations of sex hormones than

singletons or same-sex twins (Miller, 1994).

Human males experience three hormonal surges from conception until the end of

puberty. The first hormonal surge occurs in utero soon after testicular differentiation,

around 7-9 weeks post-conception with a peak around 14 weeks of gestation. The second

surge occurs soon after birth, guided by the suppression of placental hormones, and a peak

occurs around the 3rd

and 4th

month after birth. The third hormonal surge occurs during the

adolescent growth spurt and is driven by puberty (Reyes et al., 1974; Knickmeyer and

Baron-Cohen, 2006).

The primary and permanent dentitions start to form in utero at different times, with

the primary dentition starting to develop around 4-6 weeks post conception and the

permanent dentition starting around 16 weeks of gestation. Moreover, each tooth crown

dimension starts to form at different times: intercuspal dimensions of the crown being the

first dimensions to develop in the position where the secondary enamel knots are located;

mesiodistal dimensions are established when tooth crown formation reaches its greatest

convexity in size; buccolingual dimensions are formed when the dental crown is almost

completed, while crown height dimensions are completed when the dental crown is fully

formed and calcification of the root commences.

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In this study, descriptive statistics showed no differences between males from all

zygosities, dentitions and tooth crown dimensions studied (Chapter 6), suggesting that no

special hormonal influence was evident in males from DZOS pairs, regardless of having

shared the uterine environment with a co-twin sister. On the other hand, significant

differences were found in the female group, with DZOS females displaying larger tooth

crown dimensions compared with MZ and DZSS female twins. These differences were

evident in both dentitions.

In DZOS females, the primary dentition seemed to be less influenced by the

possible effect of male hormone from the male co-twin than the permanent dentition. This

may be due to: 1) the “soft tissue” phase in the primary dentition occurs when male

hormones are starting to be released by the male co-twin testis and, by the time the male

hormones reach their peak, around 14 weeks post conception, most of the primary teeth

have already passed through the “soft tissue” phase and are close to commencing

calcification; 2) the developmental processes in the primary dentition occur within a

shorter period of time compared with the permanent dentition and, therefore, this dentition

is less exposed to environmental factors, such as intrauterine male hormones, than the

permanent dentition. On the other hand, teeth in the permanent dentition commence their

“soft tissue” phase around 16 weeks post conception, soon after the peak of intrauterine

male hormone surge. Moreover, permanent teeth undertake a longer period of

development before dental crown formation is completed, therefore being potentially more

exposed to male hormone in utero than the primary dentition. This is a possible

explanation for the differences in sexual dimorphism in both dentitions found between

DZOS females and the other female groups studied, which places DZOS females in an

intermediate position between male twins from all zygosities and females from MZ and

DZSS twin pairs (Figures 7.1 and 7.2).

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Figure 9.1: Representation of dental crown measurements: A) intercuspal dimensions (IC); B) mesiodistal

dimension (MD); C) buccolingual dimension (BL); D) crown height dimension (CH).

Different dental crown dimensions may be exposed to different amounts of sex

hormones depending on the time it takes for each crown dimension to form (Figure 9.1).

In this study, dental crown dimensions that form over a short period of time, i.e. IC

dimensions, displayed smaller percentages of sexual dimorphism than dental crown

dimensions that require a longer period of time to develop, such as CH dimensions (Tables

7.25 – 7.28). It is possible that earlier-forming dental crown dimensions are less exposed

to environmental factors, such as intrauterine male hormones, than later-forming crown

dimensions. Figure 9.2 shows a schematic representation of the stages of formation of the

primary and permanent upper central incisors and the surges in testosterone production

(Ribeiro et al., 2012). The effects of intrauterine male hormones seem to be a possible

explanation for the increased tooth crown size found in both dentitions of females from

DZOS twins. No systematic trend was found in either dentition of males from any of the

zygosity groups.

A B

C D

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Figure 9.2: Schematic representation of the stages of formation of the upper central incisor and surges in

testosterone production.

Dental asymmetries are assumed to be an indication of genetic and/or

environmental disturbances during odontogenesis. In this study, DA in females from

DZOS twins was not increased compared with females from MZ and DZSS pairs or DZOS

males, therefore not supporting Geschwind and Galaburda’s theory. Furthermore, the

patterns and magnitudes of FA in the dentition of females from DZOS twins did not seem

to differ from males from DZOS twins. Although it has been reported that sex hormones

can interfere with FA levels in various morphological, behavioural, and physiological traits

(Benderlioglu, 2010), we were unable to find a similar effect in the dentition. The possible

associations between pre-natal male hormones and dental asymmetries in twins still need

further investigation.

It is important to note that the finding of this thesis of increased tooth size in

females from DZOS pairs compared with females from MZ or DZSS pairs only provides

indirect evidence of a possible effect of testosterone during development. Indeed, there

have not been any studies so far that have confirmed directly the occurrence of pre-natal

transfer of testosterone in human twins. Transfer could potentially occur via the feto-fetal

route or via the maternal-fetal route, and the findings of a study by Cohen-Bendahan et al.

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(2005b) indicate that the maternal route is unlikely, although the testosterone samples were

obtained relatively late during pregnancy in this study (Cohen-Bendahan et al., 2005b). It

has been proposed that amniotic fluid is likely to be a better source to study the effects of

testosterone in utero and, hopefully, findings such as the ones in this thesis will lead to

further investigations in this field.

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10. Conclusions

The findings of this thesis support the hypothesis that females from DZOS twin

pairs display larger tooth crown dimensions in both dentitions than females from either MZ

or DZSS pairs, possibly reflecting intrauterine male hormone diffusion.

No systematic trend was found for any differences in tooth size in either dentition

of males from all twin zygosities, suggesting that intrauterine female hormone has no

measurable effect on dental crown size of male twins.

Different dental crown dimensions displayed different amounts of sexual

dimorphism depending on the time required for each dimension to develop, with IC

dimensions displaying the smallest percentage of sexual dimorphism, followed by MD

dimensions, BL dimensions and CH dimensions.

There was no evidence that the patterns or extent of DA or FA were altered in

females from DZOS twin pairs.

The findings of this thesis add to our current knowledge on the effects of the sex

hormones in the human dentition, indicating that male hormones are likely to have a small

but significant effect on sexual dimorphism in addition to the effects of the sex

chromosomes. Further investigation of the roles of male hormonal action on dental

development appears to be warranted in both humans and in experimental animals, with

the application of 3D imaging systems enabling new phenotypes to be defined.

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Adler CJ and Donlon D (2010). Sexual dimorphism in deciduous crown traits of a

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AlQahtani SJ, Hector MP, and Liversidge HM (2010). Brief communication: the London

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Alvesalo L, Osborne RH, and Kari M (1975). The 47,XYY male, Y chromosome, and

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Alvesalo L and Kari M (1977). Sizes of deciduous teeth in 47,XYY males. American

Journal of Human Genetics, 29:486-489.

Alvesalo L and Varrela J (1980). Permanent tooth size in 46,XY females. American

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Alvesalo L and Tammisalo E (1981). Enamel thickness in 45,X females' permanent teeth.

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Alvesalo L, Tammisalo E, and Therman E (1987). 47,XXX females, sex chromosomes,

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Alvesalo L, Tammisalo E, and Townsend G (1991). Upper central incisor and canine tooth

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12. Appendices

Appendix 1 – List of achievements and professional development activities of Daniela

Ribeiro during PhD candidature 2008-2012

Research Scholarship

International Postgraduate Research Scholarship (IPRS) granted by the Australian

Government from January 2009 to February 2012 to develop the research proposed

in this thesis.

Research Grants

Ribeiro DC, Sampson W, Hughes T and Townsend G (2009) Australian Dental Research

Foundation (ADRF). Are dental crown dimensions increased in females from

opposite-sex dizygotic twin pairs reflecting possible male hormone diffusion in

utero? ($2,800)

Travel Awards

Ribeiro DC (2009) Herbert Gill-Williams Scholarship, School of Dentistry, The University

of Adelaide, traineeship at Imaging Laboratory of the School of Dental Sciences,

University of Liverpool, UK. (Reimbursement for cost of airfares).

Ribeiro DC (2010) Herbert Gill-Williams Scholarship, School of Dentistry, The University

of Adelaide, 7th

International Orthodontic Conference, Sydney, NSW.

(Reimbursement for airfares, accommodation and conference registration).

Ribeiro DC (2011) School of Dentistry, The University of Adelaide,

IUAES/AAS/ASAANZ Conference 2011, University of Western Australia, WA.

(Reimbursement for airfares and conference registration).

Ribeiro DC (2011) Australian Twin Registry Research Travel Grant, 15th

International

Symposium on Dental Morphology, Newcastle Upon Tyne, UK. (Reimbursement

of airfares and symposium registration).

Ribeiro DC (2011) Faculty of Health Science Postgraduate Travelling Fellowship, The

University of Adelaide, 15th

International Symposium on Dental Morphology,

Newcastle Upon Tyne, UK. (Reimbursement of airfares, accommodation and

symposium registration).

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170

Published paper relevant to the thesis

Ribeiro DC, Sampson W, Hughes T, Brook A, Townsend G (2012). Sexual dimorphism in

the primary and permanent dentitions of twins: an approach to clarifying the role of

hormonal factors. In: New directions in dental anthropology, paradigms,

methodology and outcomes. G Townsend, E Kanazawa and H Takayama editors.

University of Adelaide Press, Adelaide, Australia. pp. 47-66.

Published abstracts for conference presentations during candidature

Ribeiro DC, Sampson W, Hughes, T, Brook A, Townsend G. Increased dental crown

dimension in opposite-sex dizygotic twin pairs as a possible result of intrauterine

hormone diffusion. Abstract submitted and oral presentation performed at the 23rd

Australian Orthodontic Congress, Perth Convention Exhibition Centre, 10 – 14th

February 2012.

Ribeiro DC, Sampson W, Hughes T, Brook A, Townsend G. Is tooth size altered in

opposite-sex dizygotic twin pairs, reflecting possible hormone diffusion in utero?

Abstract submitted an oral presentation performed at the 15th

International

Symposium on Dental Morphology, 24 – 27th

August 2011, Northumbria

University, Newcastle upon Tyne, UK.

Ribeiro DC, Sampson W, Hughes T, Brook A, Townsend G. Altered tooth crown

dimensions in opposite-sex dizygotic twin pairs possibly reflecting prenatal

hormone diffusion. Abstract submitted and oral presentation performed at the

International Union of Anthropological and Ethnological Sciences (IUAES), the

Australian Anthropological Society (AAS) and the Association of Social

Anthropologists of Aotearoa / New Zealand (ASAANZ), The University of

Western Australia, Perth, WA, July 2011.

Ribeiro DC, Sampson W, Hughes T, Brook A, Townsend G. Is there any evidence of pre-

natal hormonal influences on tooth size in opposite sex dizygotic twins? Abstract

submitted and oral presentation performed at the School of Dentistry Research Day

2010, The University of Adelaide, August 2010.

Ribeiro DC, Sampson W, Hughes T, Brook A, Townsend G. Are dental crown dimensions

altered in opposite-sex dizygotic twin pairs, reflecting possible hormone diffusion

in utero? Abstract submitted and oral presentation performed at the 7th

International

Orthodontic Congress, Sydney, 2010.


increased in females from opposite-sex dizygotic twin pairs reflecting possible

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170

male hormone diffusion in utero? Abstract submitted and poster presented at the

School of Dentistry Research Day 2009, The University of Adelaide, August 2009.




Postgraduate Research Expo 2009, Adelaide, 2009.




Australian Society for Medical Research, ASMR SA Division, Annual Scientific

Meeting, Adelaide, 2009.

Other professional development activities

Visited the School of Dental Sciences, The University of Liverpool, Liverpool, UK

(from 7th

of June 2009 to 12th

of July 2009).

- First contact with the 2D and 3D image analysis system.

- Trained by Dr Tom Coxon and Mr James Hibbard on how to used the 2D

image analysis system.

- Carried out double determinations (intra- and inter-operator double

determinations).

Visited the School of Dentistry, University of Oulu, Oulu, Finland (from 28th

of

August 2011 to 2nd

of September 2011).

- Gave an invited seminar to the Orthodontic staff of the School of Dentistry,

the University of Oulu

- Had a discussion with Prof Pertti Pirttiniemi (Head of Orthodontic

Department), Associate Professor Raija Lahdesmaki (Orthodontic

Department) about possible future research collaborations and post-

doctorate fellowship.

- Received comments and suggestions from Dr Tuomo Heikkinen

(Orthodontic Department) in regards to research on hormonal influence in

opposite-sex dizygotic twins.

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170

Appendix 2 – Bland and Altman graphs of intra- and inter-operator measurements

obtained in the School of Dental Sciences, the University of Liverpool, Liverpool, UK.

Graph 2.1: Differences between inter-operator measurements for mesiodistal (MD) dimension of permanent

upper right central incisors.

Graph 2.2: Differences between inter-operator measurements for buccolingual (BL) dimension of permanent


5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5

0.2

0.1

0.0

-0.1

-0.2

-0.3

-0.4

-0.5

-0.6

Average of repeat measurements (mm)

Mean

-0.23

-1.96 SD

-0.50

+1.96 SD

0.04

7.0 7.5 8.0 8.5 9.0 9.5 10.0

0.3

0.2

0.1

0.0

-0.1

-0.2

-0.3

-0.4


Difference between measurements (mm)

Mean

-0.03

-1.96 SD

-0.28

+1.96 SD

0.21


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170


upper left central incisors.

Graph 2.4: Differences between inter-operator measurements for buccolingual (BL) dimension of permanent


5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0

0.4

0.2

0.0

-0.2

-0.4

-0.6

-0.8


Mean

-0.20

-1.96 SD

-0.58

+1.96 SD

0.17

7.5 8.0 8.5 9.0 9.5 10.0

0.2

0.1

0.0

-0.1

-0.2

-0.3

-0.4


Mean

-0.10

-1.96 SD

-0.36

+1.96 SD

0.16



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170


upper right first molars.

Graph 2.6: Differences between inter-operator measurements for buccolingal (BL) dimensions of permanent

upper right first molars.

9.5 10.0 10.5 11.0 11.5 12.0 12.5

0.6

0.4

0.2

0.0

-0.2

-0.4

-0.6

-0.8



Mean

-0.09

-1.96 SD

-0.63

+1.96 SD

0.46

8.5 9.0 9.5 10.0 10.5 11.0 11.5

0.6

0.4

0.2

-0.0

-0.2

-0.4

-0.6


Mean

-0.09

-1.96 SD

-0.53

+1.96 SD

0.36


206

170

Graph 2.7: Differences between inter-operator measurements for mesiodistal (MD) dimensions of permanent

upper left first molars.

Graph 2.8: Differences between inter-operator measurements for buccolingual (BL) dimensions of permanent


9 10 11 12 13

0.6

0.4

0.2

-0.0

-0.2

-0.4

-0.6



Mean

-0.07

-1.96 SD

-0.54

+1.96 SD

0.41

9.0 9.5 10.0 10.5 11.0 11.5

0.6

0.4

0.2

-0.0

-0.2

-0.4

-0.6


Mean

0.02

-1.96 SD

-0.41

+1.96 SD

0.46


207

170

Graph 2.9: Differences between intra-operator measurements for mesiodistal (MD) dimensions of permanent


Graph 2.10: Differences between intra-operator measurements for buccolingual (BL) dimensions of permanent


5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0

0.3

0.2

0.1

0.0

-0.1

-0.2

-0.3



Mean

-0.03

-1.96 SD

-0.19

+1.96 SD

0.14

7.5 8.0 8.5 9.0 9.5 10.0

0.2

0.1

0.0

-0.1

-0.2


Mean

0.01

-1.96 SD

-0.11

+1.96 SD

0.12


208

170





5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0

0.2

0.1

0.0

-0.1

-0.2

-0.3

-0.4



Mean

-0.08

-1.96 SD

-0.32

+1.96 SD

0.17

7.5 8.0 8.5 9.0 9.5 10.0

0.2

0.1

0.0

-0.1

-0.2



Mean

0.00

-1.96 SD

-0.12

+1.96 SD

0.13

209

170


upper right first molar.


upper right first molar.

9.5 10.0 10.5 11.0 11.5 12.0 12.5

0.5

0.4

0.3

0.2

0.1

0.0

-0.1

-0.2

-0.3

-0.4

-0.5



Mean

0.03

-1.96 SD

-0.28

+1.96 SD

0.33

8.5 9.0 9.5 10.0 10.5 11.0 11.5

0.5

0.4

0.3

0.2

0.1

0.0

-0.1

-0.2

-0.3

-0.4

-0.5


Mean

-0.01

-1.96 SD

-0.37

+1.96 SD

0.36


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170

Graph 2.15: Differences between intra-operator measurements for mesiodistal (MD) dimenions of permanent




9 10 11 12 13

0.3

0.2

0.1

0.0

-0.1

-0.2

-0.3

-0.4

-0.5


Mean

-0.04

-1.96 SD

-0.33

+1.96 SD

0.24

9.0 9.5 10.0 10.5 11.0 11.5

0.3

0.2

0.1

0.0

-0.1

-0.2


Mean

0.05

-1.96 SD

-0.09

+1.96 SD

0.20



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170

Appendix 3 - Ethical approval

This thesis is part of a major project named “Dentofacial variation in twins:

genetics and environmental determinants”, submitted to the Human Research Ethics

Committee at the The University of Adelaide, SA, Australia and approved under project

number: H-07-1984A.

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