Biomaterials 28 (2007) 2

ic

Sd,

ity

J H

ers

nive

6; a

25

Abstract

1. Introduction

5 mm in diameter and may be up to several millimetres long,and the majority are arranged with their long axes atapproximately 901 to the enamel-dentine junction (EDJ).

The orientation of prisms in enamel has been studied in

the growth of the HA crystallites was lost. It has beenreported from grazing-incidence synchrotron X-ray dif-fraction experiments that there is a higher degree of

ARTICLE IN PRESScrystallite alignment in surface enamel compared to enamelclose to the EDJ [6]. However, only linear slices from EDJto surface were probed in these experiments. In an earlier

0142-9612/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.biomaterials.2007.02.019

Corresponding author. Tel.: +441133438331.E-mail address: m.al-jawad@leeds.ac.uk (M. Al-Jawad).Dental enamel is the most highly mineralised andhardest biological tissue. It is comprised of approximately96% mineral, 3% water, and 1% organic matter (non-collagenous protein) by weight [1]. The mineral is non-stoichiometric calcium hydroxyapatite (Ca10(PO4)6OH2)with carbonate, uoride, sodium, and magnesium ionsfrequently found within the structure. These hydroxyapa-tite (HA) crystallites are laid down as nanorods with cross-sectional dimensions of 50 nm 25 nm and up to 1mmlong [2]. Clusters of these nanorods, known as prisms,contain around 1000 crystallites. They are approximately

the past using electron microscopy. Although this is avaluable tool for nding the prism shape and size in aparticular plane of enamel, it is a qualitative technique anddoes not give detailed, quantitative information on thedegree of alignment in different parts of a tooth. Previouswork using X-ray diffraction on human dental enamel hasestablished the space group and lattice parameters as P63/m(hexagonal) and a 9.441(2) A and c 6.878(1) A respec-tively [35]. However, these values were obtained frommeasurements of powdered enamel collected from severalteeth, and as a result any information on the spatialvariation of the lattice parameters and texture relating toWe have used synchrotron X-ray diffraction to study the texture and the change in lattice parameter as a function of position in a cross

section of human dental enamel. Our study is the rst to map changes in preferred orientation and lattice parameter as a function of

position within enamel across a whole tooth section with such high resolution. Synchrotron X-ray diffraction with a micro-focused beam

spot was used to collect two-dimensional (2D) diffraction images at 150mm spatial resolution over the entire tooth crown. Contour mapsof the texture and lattice parameter distribution of the hydroxyapatite phase were produced from Rietveld renement of diffraction

patterns generated by azimuthally sectioning and integrating the 2D images. The 002 Debye ring showed the largest variation in

intensity. This variation is indicative of preferred orientation. Areas of high crystallite alignment on the tooth cusps match the expected

biting surfaces. Additionally we found a large variation in lattice parameter when travelling from the enamel surface to the enamel-

dentine junction. We believe this to be due to a change in the chemical composition within the tooth. The results provide a new insight on

the texture and lattice parameter proles within enamel.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Enamel; Hydroxyapatite; Apatite structure; Synchrotron X-ray diffraction; Texture; Preferred orientation2D mapping of texture and latt

Maisoon Al-Jawada,, Axel Steuwerb,Robert Cywinski

aLeeds Dental Institute, UniversbFaME38 at the ILL-ESRF, 6 rue

cInstitute for Materials Research, UnivdSchool of Physics and Astronomy, U

Received 19 December 200

Available online9082914

e parameters of dental enamel

usan H. Kilcoynec, Roger C. Shorea,David J. Wooda

of Leeds, Leeds, LS2 9LU, UK

orowitz, 38042 Grenoble, France

ity of Salford, Salford, M5 4WT, UK

rsity of Leeds, Leeds, LS2 9JT, UK

ccepted 16 February 2007

February 2007

www.elsevier.com/locate/biomaterials

study Hirota examined the tilting of the enamel-prismorientation in a human canine using laboratory two-dimensional (2D) X-ray diffraction [7], however only 12points within the tooth were measured and therefore theinformation obtained about the prism orientation cannotbe for the whole tooth. In this paper we aim to show for therst time how synchrotron X-ray diffraction can be used todetermine the basic crystallographic parameters of the HAphase across a whole intact tooth section, allowing us toexplore composition and texture on the sub-millimetrelength-scale. Characterising the orientation distribution ofthe anisotropic apatite crystallites of dental enamel aids thefundamental understanding of the natural growth andformation of dental enamel, and provides insights into howsynthetic enamel-like materials may be developed.

lattice parameters distribution maps. The 2y angle-range for thisexperiment was 2y 5301for comparison this corresponds to a 2y-range of 9581 on a conventional lab-based X-ray diffraction apparatuswith CuKa radiation of wavelength l 1.54 A. For our experimentalsetup, the main diffraction peak of the HA phase (2 1 1) was located in the

centre of our 2y range at 16.771. Vacuum tube slits were used to focus theX-ray beam to a diameter of 150mm on the sample. A 500mm thick toothsection was mounted in transmission geometry onto a travelling sample

platform such that the tooth could be scanned in two orthogonal

directions perpendicular to the beam. A charge-coupled device (CCD) 2D

ARTICLE IN PRESSM. Al-Jawad et al. / Biomaterials 28 (2007) 29082914 29092. Materials and methods

2.1. Specimen preparation

The sample used in this study was a section of an adult mandibular

second premolar (LR5). The tooth was collected with informed consent

from a patient undergoing routine orthodontic extraction at the Leeds

Dental Institute. The extracted tooth had its pulp removed and was

sterilised by autoclaving prior to storage at 4 1C in a thymol solution toprevent bacterial growth. A precision diamond blade cutter was used to

cut the tooth into 500mm thick longitudinal sections perpendicular to thebuccal and lingual surfaces. The sections were then polished by hand to

remove any surface roughness. A photograph of the section used for this

study is given in Fig. 1. The four arrows mark the tracks plotted in Fig. 10.

2.2. Synchrotron X-ray diffraction

The measurements were taken on the XMaS beamline [8] at the

European Synchrotron Radiation Facility (ESRF) using an X-ray

wavelength of l 0.82 A (equivalent to X-ray energies of 15 keV) and asample to detector distance of 163.09mm. The 2y angle-range for ourexperiment was limited by our experimental setup and the sample to

detector distance. We compromised on the very high-angle HA reections

in order to position the 2D detector closer to the sample and therefore

greatly reduced our counting times. This allowed us to collect more

diffraction patterns per tooth and obtain higher resolution texture and

Fig. 1. Photograph of 500mm thick tooth section from a human adult

lower right premolar. The arrows mark the tracks through the tooth

plotted in Fig. 10.detector with 2048 2048 pixel resolution was mounted behind the sampleand perpendicular to the incident beam for the collection of 2D diffraction

images. The cross-hairs of a telescope were positioned in line with the

beam centre in order to align the beam position on the tooth. A schematic

of the experimental setup with an example 2D diffraction image is shown

in Fig. 2. A single diffraction image had an exposure time of 5 s and

therefore a 150mm-resolution map of the tooth on a grid of 10mm 7mmcould be collected in approximately 8 h by moving the sample relative to

the beam in an x and y direction.

2.3. Data analysis

2D diffraction images were pre-processed with the ESRF software

Fit2D [9]. Each image was sectioned into 51 slices [10] and these integratedslices were then used to create Intensity versus 2y patterns for Rietveldrenement [11], i.e. 72 diffraction patterns per 2D image. Conventionally

only one Bragg reection in the diffraction pattern is used in texture

analysis therefore any slight changes in sample volume as a function

of position would affect the sample absorption and could affect the texture

coefcient obtained. However, using the Rietveld method minimises the

effect of variations in sample volume since all reections are used to

obtain the t. In addition, in our study, we used a scaling factor in the

renement procedure and found that changes in the scale factor were small

as a function of position within the tooth, indicating that variations in

section thickness were negligible. A total of 1095 diffraction patterns were

rened and therefore an in-house automated batching procedure was

written and used to input the patterns into the GSAS Rietveld renement

software [12]. The instrument parameters such as X-ray wavelength,

sample to detector distance, and peak-shape prole were determined using

a LaB6 standard sample. These parameters were then kept xed for

renements of the data. The scale parameter and background parameters

(four terms) were rened rst. The lattice parameters and crystallite

size (Lorentzian particle broadening term) were rened next, starting from

the values for HA taken from Young [3]. Finally the texture was rened

using a spherical harmonics function [13] and the preferred orientation

values for the 002 reection were extracted from this. The values of

preferred orientation ranged from 0 (randomly oriented) to 3.5 (strongly

textured). The quality of the renement was determined by least squares

methods where the goodness of t increased as w2 approached unitywhereby:

w2 R2wp

R2e,

where Rwp is the weighted R-factor and Re the expected R-factor.Fig. 2. Experimental setup at the XMaS beamline, ESRF, with an

example 2D diffraction image.

3. Results

3.1. Preferred orientation in enamel

Preferred orientation has both a magnitude and adirection. Through our analyses of the X-ray diffractiondata we have been able to quantify both these parameters.In order to obtain an overview of the preferred orientationin a tooth section the 2D X-ray scans were arranged toform a composite map of CCD images of the tooth, asshown in Fig. 3. Each small square in the image is one 2Ddiffraction pattern. The centres of adjacent diffractionpatterns are 150 mm apart. The shape of the tooth canclearly be seen from this composite image. The darkerpatterns in the middle of the tooth are from dentine and thelighter patterns covering these are from the enamel. At thesurface the enamel is thinner, therefore there is a halo ofdarker images along the edge of the tooth where there is

textured than the enamel.The intensity pattern around the Debye ring of the 002

reection was used to evaluate the texture direction. Theintensity was integrated over 3601 in a narrow bandcontaining the 002 reection and plotted versus theazimuthal angle. Fig. 5 shows a typical example of theresulting curve for one diffraction scan where there are twopronounced peaks separated by approximately 1801. Bytting these peaks to a Gaussian peak shape, the deviationangle, f, of the crystallite axis relative to vertical wasdetermined. By applying this procedure to each of thediffraction images, a map of the local orientation of thetexture in the 002 direction in the enamel is obtained.Fig. 6 shows a map of the orientation angles overlaid

onto the composite image of the tooth. In Fig. 6, forclarity, only every fourth value of f has been drawn. It canbe seen from this that the texture direction in the 002reection is approximately perpendicular to, and followsthe contour of the EDJ. From our knowledge of thestructure of dental enamel it would appear that thepreferred orientation in the 002 direction approximatelyfollows the direction of the enamel prism arrangement.

3.2. Rietveld refinement

The extent of preferred orientation in the tooth sectionhas been quantied using Rietveld renement. An example

ARTICLE IN PRESSM. Al-Jawad et al. / Biomater2910partial air scattering. This can also be seen in the ssurewhere there is a gap between the two cusps. Additionally, itcan be seen that below the ssure there is a circular regionof enamel which is darker than the surrounding patterns. Itis likely that this is caused by a ssure caries lesion whichhas partially demineralised the enamel in that area.In Figs. 4ad four individual diffraction scans from

different parts of the tooth are shown. Figs. 4a, c and dillustrate the change in texture direction in the 002 plane atdifferent positions within the enamel. Variations inintensity around diffraction rings are indicative of texturein the tooth enamel. The strongest texture (the mostextreme variation in intensity) was found in the 002reection (2y 13.71)labelled in Fig. 4a. A line throughzero degree has also been marked in Fig. 4a. Fig. 4b showsthe diffraction pattern from dentine. Here the peaks aremuch broader indicating that the crystallites are smaller.There is also much less variation in intensity around theFig. 3. Composite of 2714 diffraction images arranged spatially to show

the outline of the tooth specimen.Debye rings indicating that the dentine is much less

Fig. 4. (a), (c) and (d) Illustrate the change in texture direction at

difference positions within the enamel. (b) Shows the poorly crystalline

nature of dentine.

ials 28 (2007) 29082914of a typical 1D Intensity versus 2y diffraction patterntogether with the calculated pattern is shown in Fig. 7. The

open circles are the observed data points, and the solid lineis the calculated diffraction pattern. Below the pattern is aplot of the difference (observedcalculated), and beneathare the tick marks for the 2y peak positions for thecalculated diffraction pattern of HA. The difference plotshows that the agreement between observed and calculateddata is generally very good with a typical value for w2 of1.5. The nal parameters obtained from this renementare given in Table 1. Similar renements were carried outon all diffraction patterns.A contour map showing the change in magnitude of

preferred orientation in the 002 diffraction peak has beenplotted in Fig. 8. The 002 preferred orientation parametershave been extracted for the zero degree slice of each 2D

ARTICLE IN PRESS

Fig. 6. Texture direction of the 002 reection of hydroxyapatite crystal-

lites in enamel, calculated using 2D diffraction images.

Fig. 5. Typical intensity versus Azimuthal angle curve for the 002

reection showing the pronounced texture in this sample. The left hand

peak has been tted to a Gaussian.

Fig. 7. Typical diffraction pattern including the raw data (circles), the calculate

2y peak positions for the calculated diffraction pattern of hydroxyapatite.

M. Al-Jawad et al. / Biomaterials 28 (2007) 29082914 2911diffraction image (see Fig. 4a). Areas with higher values ofpreferred orientation parameter are more strongly texturedi.e. the crystallites are more aligned to the zero degreedirection in these areas. Areas with low texture coefcienthave less well-aligned crystallites.

3.3. Change in lattice parameters

Both the a- and c-lattice parameters of HA were renedin each diffraction pattern and after carrying out Rietveldrenements of 1095 data sets it was noticed that neither thea- nor c-lattice parameters were constant as a function ofposition. Note that the variation was not due to X-raywavelength drift as a function of time, but was clearlydependent on the position within the tooth. Trends in thelattice-parameter changes have been plotted in Fig. 9relative to the average lattice parameters. The relativepercentage change in the a-lattice parameter (Ra) across thetooth section was calculated using:

Ra an a0

a0

100%,

where an is the rened lattice parameter for diffractionpattern n, and a0 is the overall average lattice parameter.The same type of equation was used for dening thec-lattice parameter variation, Rc. Both Ra and Rc have beend diffraction pattern (solid line), the difference, and the tick marks for the

ARTICLE IN PRESS

el

a b 901, g 1201535.49(8)

7.83(9)

1.9(1)

1.5

(SH) is the 002 spherical harmonic preferred orientation term.

aterTable 1

Rened structural parameters for typical diffraction pattern of dental enam

Space group

a (A)

c (A)

a, b, gV (A3)

Y(particle)002(SH)w2

Y(particle) is the coefcient for Lorentzian particle size broadening, and 002

M. Al-Jawad et al. / Biom2912plotted as contour plots in Figs. 9a and b. It can be seenfrom these gures that the a- and c-lattice parameters varybetween 0.6% and +0.3% of their average values. In allcases the uncertainty in the lattice parameters did notexceed 1 103, and the values for the average latticeparameters were a0 9.5165(6) A and c0 6.9394(2) A.These plots clearly reveal that there is a systematicvariation of the lattice parameters as a function of positionwithin the enamel.Figs. 10ad shows plots of Ra and Rc as a function of

distance from the surface for the four tracks through theenamel indicated in Fig. 1. In all regions, Ra (lledsymbols) decreases with increasing distance from the toothsurface, especially around the cusps. In Figs. 10b and cthere is second peak in the Ra curve at around 600 mm fromthe surface indicating a region of enamel which has ahigher a-lattice parameter than the surrounding enamel.This region can be seen clearly in the contour plot in Fig.9a. Along each track, the c-lattice parameter curves (opensymbols) are much atter. This indicates that Rc is lessdependent on the distance from the enamel surface thanRa. In both the a- and c-lattice parameter values, there isalso a difference between the lingual and the buccal sides ofthe tooth.

Fig. 8. Texture distribution map generated from the calculated texture

coefcient via Rietveld renement.(HA)

Phase hydroxyapatite

P63/m (#176)

9.4660(9)

6.9018(3)

ials 28 (2007) 290829144. Discussion

It has been seen previously that there is a higher degreeof crystallite alignment in surface enamel compared toenamel close to the EDJ [6]. However, in that work, onlylinear slices from EDJ to surface were probed. The resultsfrom our study show that the texture distribution ismuch more complex than previously thought. We can see

Fig. 9. (a) a-lattice parameter and (b) c-lattice parameter contour maps

showing the change in lattice parameter value at different positions

around the tooth.

ARTICLE IN PRESSaterin Fig. 8 that HA crystallites are most aligned in the cuspalregions: on both sides of the buccal cusp and on the innerside of the lingual cusp HA crystallites are highly aligned.Conversely, along the sides of the tooth away from thecusps generally the crystallites are less ordered. It isinteresting to note that the areas of high crystallitealignment match the expected occlusal surfaces of a lowersecond premolar [14]. This may be an evolutionarydevelopment of enamel so that the regions of enamelwhich are exposed to the largest load are the strongest. It isinteresting to note that a strong correlation betweenfunctionality and texture in human bone has been reportedby Bacon [15] where he observed that the living conditions

Fig. 10. Four tracks through the tooth section going from enamel surface

to EDJ showing the change in lattice parameter as a function of distance

from the enamel surface. The tracks are indicated in Fig. 1.

M. Al-Jawad et al. / Biom(either on a steep hillside, or on the at) of two Neolithictribes radically affected the HA crystallite growth andalignment on the lower front edge of the tibia. Althoughdental enamel cannot regenerate itself as bone can, it islikely that through evolution the degree of crystallitealignment in different regions of a tooth has beenoptimised for the function of the tooth.Changes in lattice parameter can be indicative of changes

in enamel crystal chemistry as well as changes in the stress/strain-state of a material. Separating these two possibleeffects or even using the lattice parameters to determinecompositional changes in biological apatites is notstraightforward as changes in crystal chemistry can bethe result of several ionic substitutions, such as Na, Mg, Clor F, as well as variations in the carbonate content and theCa/P ratio. However, the magnitude of the changes seenacross our tooth suggest that the changes in latticeparameters plotted in Figs. 9 and 10 arise predominantlyfrom changes in the chemical composition of the enamel indifferent regions of the tooth. Chemical analysis of thedistribution of uoride, carbonate and magnesium inenamel has been carried out previously by Robinson etal. By dissecting tooth sections into pieces weighing2050 mg, the amount of uoridated apatite was determinedby etching the tooth sections and analysing the uorideconcentration in the post-etched buffer solution [16], theconcentration of carbonate was determined by dissolvingeach piece in acid and measuring the volume of CO2emitted [17], and concentration of magnesium was foundusing an atomic absorption spectrophotometer [18]. Theyfound that uoride concentrations decreased in going fromthe enamel surface to the EDJ, while carbonate andmagnesium concentrations increased (from 2% to 46%,and from 0.2% to 0.5% respectively) across the samedistance [19]. In dentine, changes in the a-lattice parameterof up to 0.5% have been reported in going from the EDJinto the centre of the dentine [20]. This trend has beenexplained principally as a result of the increased substitu-tion of CO3

2 for PO43 associated with the more immature

dentine crystallites. Our results are of a comparable orderof magnitude and we believe them to be the result ofcompositional change. Comparing Figs. 9a and b it can beseen that there is more variation in the a-lattice parameterthan in c. This trend has also been seen in dentine [20]where the a-lattice parameter decreased by 0.5% withincreasing distance from the EDJ into the dentine, whereasthe c-lattice parameter only decreased by 0.1% over thesame distance. In addition to a decrease in latticeparameters going from the surface enamel to the EDJ,there is a difference in the lattice parameters on the buccaland lingual sides of the tooth indicating a change in crystalchemistry on the different sides of the tooth. This couldeither be due to the different functions of the two sides ofthe tooth or due to their slightly different oral environ-ments, or a combination of both.Cuy et al. have generated 2D distribution maps for the

hardness (H) and Youngs modulus (E) of enamel usingnanoindendation [21]. In the molars they investigated, theyfound that the cuspal regions had higher hardness andYoungs modulus. They report values ranging fromH46GPa to Ho3GPa and E4115GPa and Eo70Gpa,respectively, going from the enamel surface to the EDJ.The contour maps they generated of H and E show similarfeatures to our lattice-parameter distribution maps, in-dicating that the crystallographic and mechanical proper-ties of enamel are closely linked, therefore anunderstanding of both is necessary in order to fullyunderstand the function of enamel in different parts of atooth.

5. Conclusions

Using spatially resolved synchrotron X-ray diffractionwe have quantied the changes in texture and latticeparameters in dental enamel as a function of positionwithin the tooth. With this technique, in a few hours ofdata collection, we have generated 2D distribution maps ofboth texture and lattice-parameter changes in enamel with

ials 28 (2007) 29082914 2913150 mm resolution. This has given detailed quantitativeinformation on the degree of crystallite alignment in

different regions of tooth enamel not previously reported.It has also shown that the lattice parameters maps, relatedto changes in crystal chemistry, are more complicated thatpreviously thought indicating that understanding hetero-geneities within a single tooth is as important as realisingthe differences between teeth. We have shown thatcharacterising the crystallographic properties of dentalenamel is crucial in order to design optimised dentalrestorative materials. Finally, we have shown through thiswork that synchrotron X-ray diffraction is a powerfultechnique in the study of the crystallography and micro-structure of dental enamel and it could be equallysuccessful in the study of other biological hard tissues, inthe study of synthetic biomaterials, and in the study of bio-synthetic complexes.

[5] Wilson RM, Elliott JC, Dowker SEP, Smith RI. Rietveld structure

renement of precipitated carbonate apatite using neutron diffraction

data. Biomaterials 2004;25(11):220513.

[6] Low IM. Depth-proling of crystal structure, texture, and micro-

hardness in a functionally graded tooth enamel. J Am Ceram Soc

2004;87(11):212531.

[7] Hirota F. Prism arrangement in human cusp enamel deduced by

X-ray diffraction. Arch Oral Biol 1982;27(11):9317.

[8] Brown SD, Bouchenoire L, Bowyer D, Kervin J, Laundy D,

Longeld MJ, et al. The XMaS beamline at ESRF: instrumental

developments and high resolution diffraction studies. J Synchrotron

Radiat 2001;8(6):117281.

[9] Hammersley AP. FIT2D: An Introduction and Overview. ESRF

Internal Report 1997; ESRF97HA02T.

[10] Hammersley AP, Svensson SO, Hanand M, Fitch AN, Hausermann

D. Two-dimensional detector software: from real detector to

idealised image or two-theta scan. High Pressure Res 1996;14(46):

23548.

ARTICLE IN PRESSM. Al-Jawad et al. / Biomaterials 28 (2007) 290829142914Acknowledgments

This work was performed on the EPSRC-funded CRGbeamline (XMaS BM28) at the ESRF. We are grateful toL. Bouchenoire and J. Wright (ESRF) for their invaluableassistance and to S. Beaufoy for additional administrativesupport. Also, we would like to thank the FaME38 facilityfor providing the VAMAS approved precise samplemounting system. Thanks to C. Sullivan at Leeds DentalInstitute for producing the photograph in Fig. 1. Thisresearch was funded by the UK Medical Research Council.

References

[1] Deakins M, Volker IF. Amount of organic matter in enamel from

several types of human teeth. J Dent Res 1941;20(2):11721.

[2] Johansen E. In: StackMV, Fearnhead RW, editors. Tooth enamel: its

composition, properties, and fundamental structure. Bristol: I Wright

and Sons; 1965. p. 17781.

[3] Young RA, Mackie PE. Crystallography of human tooth enamel:

Initial structure renement. Mat Res Bull 1980;15(1):1729.

[4] Wilson RM, Elliott JC, Dowker SEP. Rietveld renement of the

crystallographic structure of human dental enamel apatites. Am

Mineral 1999;84(9):140614.structures. J Appl Crystallogr 1969;2(2):6571.

[12] Larson AC, Von Dreele RB. General Structure Analysis System

(GSAS). Los Alamos National Laboratory Report 2004;LAUR

86-748.

[13] Von Dreele RB. Quantitative texture analysis by Rietveld renement.

J Appl Crystallogr 1997;30(4):51725.

[14] Berkovitz BKB, Holland GR, Moxham BJ. Oral anatomy, embry-

ology and histology. 3rd ed. New York: Mosby; 2002.

[15] Bacon GE. The dependence of human bone texture on life-style.

Philos Trans Roy Soc B 1990;240(1298):36370.

[16] Weatherell JA, Robinson C, Hallsworth AS. Changes in the uoride

concentration of the labial enamel surface with age. Caries Res

1972;6(1):31224.

[17] Robinson C, Weatherell JA, Hallsworth AS. Variation in composi-

tion of dental enamel within thin ground sections. Caries Res 1971;

5(1):4457.

[18] Robinson C, Weatherell JA, Hallsworth AS. Distribution of

magnesium in mature human-enamel. Caries Res 1981;15(1):707.

[19] Robinson C, Shore RC, Brookes SJ, Strafford S, Wood SR, Kirkham

J. The chemistry of enamel caries. Crit Rev Oral Biol M 2000;

11(4):48195.

[20] Zioupos P, Rogers KD. Complementary physical and mechanical

techniques to characterise tooth: a bone-like tissue. J Bionic Eng

2006;3(1):1931.

[21] Cuy JL, Mann AB, Livi KJ, Teaford MF, Weihs TP. Nanoindenta-

tion mapping of the mechanical properties of human molar tooth

enamel. Arch Oral Biol 2002;47(4):28191.[11] Rietveld HM. A prole renement method for nuclear and magnetic

2D mapping of texture and lattice parameters of dental enamelIntroductionMaterials and methodsSpecimen preparationSynchrotron X-ray diffractionData analysis

ResultsPreferred orientation in enamelRietveld refinementChange in lattice parameters

DiscussionConclusionsAcknowledgmentsReferences