Separate contribution of enamel and dentine to overall toothcolour change in tooth bleaching

Xiao Maa, Rong Li a, Yue Sa, Shanshan Liang, Lili Sun, Tao Jiang, Yining Wang *

Key Laboratory for Oral Biomedical Engineering, Ministry of Education, School and Hospital of Stomatology, Wuhan University,

Wuhan, PR China

j o u r n a l o f d e n t i s t r y 3 9 ( 2 0 1 1 ) 7 3 9 – 7 4 5

a r t i c l e i n f o

Article history:

Received 19 March 2011

Received in revised form

7 August 2011

Accepted 8 August 2011

Keywords:

Bleaching

Enamel

Dentine

Colour

Contribution

s u m m a r y

Objectives: The aim of this study was to evaluate the separate contribution of enamel (E) and

dentine (D) to the colour change of tooth which subjected to 10% carbamide peroxide (CP)

gels using a novel recombined enamel–dentine (Recombined-ED) study model.

Methods: 120 enamel–dentine (ED) samples (four homogeneous premolar ED from each

patient; total = 30 � 4 ED) were involved in the present study. Two homogeneous ED samples

were bleached with 10% CP and the other two ones were stored in artificial saliva for one,

two or four weeks. After treatment, four kinds of layers were prepared from each four

homogeneous ED samples by removing enamel or dentine part: bleached-enamel (BE),

bleached-dentine (BD), control-enamel (CE) and control-dentine (CD). Initial and final colour

records of samples were taken with a spectrophotometer in CIELab system. The contribu-

tion of enamel/dentine to the colour change of tooth (CTCC) was calculated by measuring

the colour difference DE between two different enamel–dentine combinations (DE between

BE/BD and CE/BD for enamel; DE between BE/BD and BE/CD for dentine). Translucency

parameter (TP) was obtained by calculating the colour difference between enamel on black

and white backings.

Results: ED and recombined-ED were significantly correlated in L*a*b* values both for un-

bleached samples and bleached samples. Bleaching resulted in a significant colour change

(DE) of E, D and ED samples. The TP of BE was significantly lower than that of CE. The CTCC of

enamel was significantly higher than that of dentine all through the time points.

Conclusions: Enamel played a more important role than dentine in tooth bleaching due to the

changes in translucency and colour.

# 2011 Elsevier Ltd. All rights reserved.

avai lab le at www . s c ien c edi r ect . co m

journal homepage: www.intl.elsevierhealth.com/journals/jden

1. Introduction

Tooth bleaching has gained popular acceptance in recent

years, especially after the introduction of night-guard vital

bleaching. Nowadays this technique has been recognized

as an efficacious and safe method to treat discoloured

teeth.1

* Corresponding author at: Key Laboratory for Oral Biomedical EngineWuhan University, 237 Luoyu Road, Wuhan 430079, PR China. Tel.: +

E-mail address: wang.yn@whu.edu.cn (Y. Wang).a These authors contributed equally to the work.

0300-5712/$ – see front matter # 2011 Elsevier Ltd. All rights reserveddoi:10.1016/j.jdent.2011.08.005

Numerous scientists and dentists have investigated the

efficacy of tooth bleaching in the laboratory2,3 and clinic.1,4

Although most studies have demonstrated teeth experi-

enced good whitening effects immediately5 and several

months6 after bleaching treatments, the exact optical

mechanism of tooth bleaching has not been fully revealed

yet.7 It has been proved that bleaching agents lighten the

tooth by penetrating into enamel and dentine.8,9 The optical

ering, Ministry of Education, School and Hospital of Stomatology,86 27 87686246; fax: +86 27 87873260.

.

j o u r n a l o f d e n t i s t r y 3 9 ( 2 0 1 1 ) 7 3 9 – 7 4 5740

properties of a tooth are influenced by both enamel and

dentine, and tooth colour is the result of diffuse reflectance

from the inner dentine through the outer translucent

enamel layer.10

Although the opinion that both enamel and dentine

influenced the colour of teeth has been approved by lots of

studies,3,11 whether enamel or dentine contributed more to

the tooth colour change during tooth bleaching still remains

unclear. Some researchers believed that the colour change of

the bleached teeth mainly resulted from alterations in the

colour of subsurface dentine.12,13 However, others argued that

the majority of colour change of tooth crowns after bleaching

was because of the colour and translucency change in

enamel.11,14

Moreover, most of studies compared the contribution of

enamel and dentine through calculating respective colour

changes. However, the simple comparison of respective colour

changes would be not enough to determine the contribution of

translucent enamel and underlying dentine for they could

influence each other and act together on the tooth colour.

Therefore, the optical interactions between enamel and

dentine should be mostly retained in the investigation of

the function of enamel and dentine.

In the present study, we developed a novel recombined

enamel–dentine model to study the separate contribution of

enamel and dentine to the tooth colour change in tooth

bleaching. The hypothesis was that enamel might contribute

more than dentine in tooth bleaching due to the masking

effects of enamel on the underlying dentine.

2. Materials and methods

2.1. Sample preparation and bleaching

Thirty pairs of intact premolars were obtained from ortho-

dontic departments and then stored in 0.1% thymol solution at

room temperature until required.

Four enamel–dentine (ED) samples were prepared from

the labial surface of each pair of tooth by means of a low-

speed saw (Isomet, Buehler Ltd., Lake Bluff, IL, USA) allowed

a homogeneous distribution amongst the experimental

groups with respect to baseline colour values of the ED

specimens. Make sure the colour difference among every

four homogeneous samples did not exceed 0.9, which was

reported as the mean colour difference between contralat-

eral natural teeth.15 The thickness of enamel and dentine

was mostly reserved (total � 2 mm � 3 mm � 4 mm, average

thickness of enamel = 0.88 � 0.07 mm), and homogeneous

ED samples were trimmed to the same thickness. The labial

and pulpal surfaces of each ED specimen were serially

ground flat with water-cooled SiC paper 500–2000 grits and

then polished with cloth and diamond polishing paste (1–

0.5 mm). After preparation, the specimens were stored in

artificial saliva.16

The flow chart of treatments was shown in Fig. 1. Two of

each four homogeneous ED specimens were firstly bleached

with 10% carbamide peroxide (pH � 6.8; Ultradent Products

Inc., South Jordan, UT, USA) for 8 h per day, by covering the

enamel surfaces with 1 mm thickness of bleaching gel. Then

the bleaching gel was removed under running distilled water,

and the specimens were individually kept in artificial saliva for

the remaining 16 h. The other two were stored in artificial

saliva (control) for 24 h per day. This procedure was repeated

for 7 days, 14 days and 28 days with each n = 10 � 4 ED. During

these cycles, the specimens were kept in a humid atmosphere

at 37 8C and the artificial saliva was replaced daily. Baseline

L*a*b* values of ED specimens were assessed according to the

CIE-Lab system.

2.2. Separation and recombination of enamel and dentinesections

Due to the limited size of the crown, the enamel–dentine unit

could not be separated into enamel and dentine slabs

perfectly. Therefore, careful removal of the enamel or dentine

by grinding and polishing was necessary to obtain pure

dentine or enamel slabs.

The above separation of each pair of teeth ended in four

kinds of sections: bleached enamel (BE), bleached dentine (BD),

control enamel (CE) and control dentine (CD). Recombined

enamel–dentine specimens (Recombined-ED) were obtained

by recombining different kinds of enamel and dentine sections

(Enamel/Dentine: BE/BD, CE/BD, BE/BD and BE/CD).

To obtain recombined-ED samples, glycerol was firstly

applied on the contact surfaces of the enamel and dentine

sections, then enamel and dentine sections were impacted

tightly by a consistent pressure. Make sure the gap between

enamel and dentine was filled with glycerol to avoid great

changes of refractive index caused by air.

2.3. Colour measurement

The colour coordinates (L*, a*, b*) of each specimen were

measured with a spectrophotometer (Spectrascan PR650,

Photo Research, CA, USA) for it was accurate and objective

in colour measurement.17 A D65 illuminant was used with a

45-degree entrance angle and 0-degree observation angle

geometry. Before the measurement, the spectrophotometer

was calibrated according to the manufacturer’s protocol.

A circular area with 1.0 mm in diameter was measured at

the middle third region of the specimen. The measurement

was repeated three times for each specimen and the values

were averaged to get the final reading. Wet cotton pellets were

used to inhibit dehydration of samples.

The colour of D and ED samples were measured directly,

and the colour of enamel was measured over an A3 coloured

resin background (3M, St. Paul, MN, USA) with the medium of

glycerol.

The colour differences were calculated between bleached

ones and respective controls by the following expression:

DE ¼ ðDL�2 þ Da�2 þ Db�2Þ1=2

where DL*, Da*, and Db* are the respective differences in the L*,

a* and b* colour parameters between two colours.18

Translucency of a single enamel slab was expressed by

translucency parameter (TP). The colour of enamel slab was

firstly measured over a black background and a white

background separately, and then translucency parameter

(TP) was obtained by calculating the colour difference between

Fig. 1 – The flow chart of study.

j o u r n a l o f d e n t i s t r y 3 9 ( 2 0 1 1 ) 7 3 9 – 7 4 5 741

enamel on the backings. Glycerol was also used between

enamel sections and backgrounds.

TP ¼ ½ðL�B � L�WÞ2 þ ða�B � a�WÞ

2 þ ðb�B � b�WÞ2�1=2

The subscript B refers to the colour coordinates of enamel

slabs over the black background and the subscript W refers to

those over the white background.

TP difference (DTP) was calculated by the following

expression:DTP ¼ TPBleached � TPControl

2.4. Validation of recombined-ED model

To validate the efficacy of recombined-ED model,

Pearson correlation test was used to discover the relation-

ships between L*a*b* values of ED specimens and recom-

bined-ED specimens both for bleached and unbleached

specimens.

2.5. Calculation of contribution of enamel and dentine

After enamel–dentine samples being bleached, either enamel

or dentine layers were replaced with non-bleached controls,

and then the colour change DE of recombined-ED samples

were calculated before and after replacement. The DE was

considered to be able to indirectly reflect the influence of the

replaced layers on the colour expression of bleached enamel–

dentine samples. Therefore, an index was developed in the

present study to describe respective roles of enamel and

dentine in tooth bleaching, and it was calculated by measuring

the colour difference DE between two different enamel–

dentine combinations (DE between BE/BD and CE/BD for

enamel; DE between BE/BD and BE/CD for dentine). The index

was named the contribution of enamel/dentine to the colour

change of tooth (CTCC).

The CTCC of enamel was calculated by measuring the

colour difference between bleached enamel and control

enamel over bleached dentine.

Table 2 – The L*a*b* values of ED, E and D samples at three time points.

1st week 2nd week 4th week

Bleached Control P Bleached Control P Bleached Control P

ED L* 71.48 � 1.15 67.83 � 1.40 <0.001** 73.68 � 1.74 68.22 � 1.41 <0.001** 75.77 � 1.08 69.89 � 0.95 <0.001**

a* 1.61 � 0.43 2.49 � 0.35 <0.001** �0.03 � 0.26 1.05 � 0.28 <0.001** 0.90 � 0.15 2.78 � 0.66 <0.001**

b* 12.84 � 2.43 20.87 � 2.20 <0.001** 12.03 � 1.97 20.78 � 3.59 <0.001** 11.55 � 1.14 21.76 � 0.83 <0.001**

E L* 70.05 � 2.82 64.68 � 1.60 <0.001** 70.53 � 2.10 65.23 � 1.23 <0.001** 69.57 � 1.23 64.07 � 0.77 <0.001**

a* �1.23 � 0.12 �1.25 � 0.23 0.749 �0.94 � 0.29 �1.00 � 0.30 0.423 �1.03 � 0.31 �1.29 � 0.22 0.001

b* 4.02 � 0.88 10.16 � 1.24 <0.001** 2.91 � 1.67 9.12 � 4.00 <0.001** 2.92 � 0.59 10.13 � 1.80 <0.001**

D L* 72.82 � 2.16 73.90 � 2.45 0.053 77.52 � 2.78 75.47 � 2.26 0.153 79.96 � 1.95 79.62 � 4.03 0.689

a* �2.08 � 0.55 �2.21 � 0.34 0.219 �1.72 � 0.33 �1.74 � 0.27 0.679 �2.32 � 0.59 �2.19 � 0.41 0.150

b* 13.98 � 2.31 14.17 � 2.47 0.747 12.52 � 3.11 14.79 � 2.10 0.004** 14.27 � 4.10 15.55 � 4.07 0.051

**Significant different at level of p = 0.01.

0

1

2

3

4

5

6

7

8

Day0 Da y7 Da y14 Da y28

Tooth

Enam el

Den�ne

Fig. 2 – DL* of E, D and ED samples with time.

Table 1 – Correlation test between ED and Recombined-ED.

Group Parameter ED Recombined-ED T-test Correlation test

P P r

Control L* 71.00 � 3.09 68.17 � 2.61 <0.01 <0.01 0.84

a* 1.35 � 1.44 �0.57 � 0.45 <0.01 <0.01 0.67

b* 19.89 � 3.95 12.89 � 3.70 <0.01 <0.01 0.91

Bleached L* 75.61 � 3.43 71.27 � 3.29 <0.01 <0.01 0.80

a* 0.49 � 0.84 �0.70 � 0.39 <0.01 0.01 0.55

b* 11.50 � 3.51 6.02 � 2.72 <0.01 <0.01 0.86

Significantly different at level of p = 0.01.

j o u r n a l o f d e n t i s t r y 3 9 ( 2 0 1 1 ) 7 3 9 – 7 4 5742

CTCC of enamel ¼ ½DL�2ðBE&BD�CE&BDÞ þ Da�2ðBE&BD�CE&BDÞ

þ Db�2ðBE&BD�CE&BDÞÞ�1=2

The CTCC of dentine was calculated by measuring the DE

between bleached dentine and control dentine under bleached

enamel.

CTCC of dentine ¼ ½DL�2ðBE&BD�BE&CDÞ þ Da�2ðBE&BD�BE&CDÞ

þ Db�2ðBE&BD�BE&CDÞÞ�1=2

2.6. Statistical analysis

We conducted a paired T-test to examine the colour and TP

differences between bleached E, D and ED specimens and

respective controls. T test was also used to compare the CTCC

of enamel and dentine. P < 0.05 was considered significantly

different.

3. Results

Before treatment, the L*a*b* values of ED specimens at three

time points were detected no significant difference. The

correlation test certified that the recombined-ED sample was

highly correlated with original ED samples in L*a*b* values both

for bleached and control ones (P � 0.01) (Table 1), although the

L*a*b* values were significantly different between them.

Bleaching treatment led to a significant colour change of

bleached-ED and bleached-E specimens compared to control

values (P < 0.01, Table 2). For all bleached ED and E samples,

significant increase of L* values and decrease of b* values could

be observed, indicating a shift in the direction of more white

and less yellow (Figs. 2 and 3). Compared with controls, colour

of bleached D samples was not significantly improved by

bleaching agents except for the b* values at the second week.

Significant overall colour change DE of bleached-ED samples

were also detected after 7, 14, 28 days bleaching (8.91 � 0.85,

10.51 � 1.7 and 12.02 � 1.54, respectively; P < 0.05). DE of ED

and E samples were significantly higher than that of D samples

(Fig. 4).

Besides colour change, the TP values of bleached-E samples

were sharply decreased in the period 0–14 days and were

significantly different from that of controls (P < 0.05).

The contribution of enamel/dentine to tooth colour change

(CTCC) at three time points (day 7, 14, and 28) of the bleaching

period were presented in Fig. 5. The CTCC of E samples were

0

2

4

6

8

10

12

14

Day7 Da y14 Da y28

CTCC -Enam el

CTCC -Den�ne

Fig. 5 – The CTCC of enamel and dentine at three time

points.

-14

-12

-10

-8

-6

-4

-2

0

2

Day0 Day7 Day14 Day28

Tooth

Enam el

Den�ne

Fig. 3 – Db* of E, D and ED samples with time.

j o u r n a l o f d e n t i s t r y 3 9 ( 2 0 1 1 ) 7 3 9 – 7 4 5 743

consistently higher than that of dentine all through the

treating period (P < 0.01).

4. Discussion

To allow for direct measurements of the optical changes of

enamel and dentine with bleaching, careful separation was

performed on the ED samples. As expected, pronounced

changes of L* and b* values were observed in E and D samples,

which reflected the increase of lightness and reduction in

yellowness by bleaching treatment. This was consistent with

the results of former researches,3,11 which demonstrated both

enamel and dentine could be lightened by bleaching agents in

different degrees. Moreover, the results also demonstrated

that enamel experienced larger colour improvement than

dentine, indicating enamel was easier to be bleached in direct

contact with bleaching gel.

As we know, once a light falls on tooth surfaces, a

multitude of interactions between enamel and dentine, such

as transmission, reflection, scattering and refraction, may

occur simultaneously. Separation of single enamel and

dentine from a tooth would obviously detach these interac-

tions. Therefore, in the present study we developed a novel

enamel–dentine model by recombining enamel layer and

dentine layer which were cut from bleached or unbleached

-15

-10

-5

0

5

10

15

Day0 Da y7 Da y14 Da y28

Tooth

Enamel

Den�ne

∆TP

Fig. 4 – DE of E, D and ED samples and DTP of enamel with

time.

tooth specimens. The method of ‘‘separation and recombina-

tion’’ has been successfully utilized before in two former

studies to evaluate bleaching effects. Wiegand et al.3 obtained

single enamel and dentine samples by grinding and polishing

and then recombined them into enamel–dentine samples to

measure separate and combined colour parameters. The

similar pattern has been also adopted in an in vitro study

aiming to bleach tetracycline-stained rat teeth by attaching a

rat tooth to the human enamel layer.19

The thickness of enamel has been proved to be highly

correlated with its translucency.20 Previous studies revealed

that the natural enamel varies greatly in thickness among

individuals and types of teeth.21,22,23 In the present study, be

respect to original thickness of enamel, we did not create an

uniform thickness of enamel but keep original thickness of

enamel almost unchanged (despite the thickness of 200 mm by

necessary abrasion). The varied thickness of enamel would

reflect the real covering effect of enamel on the dentine.

Through recombining enamel and dentine, the model

mostly preserved the optical interactions between them,24 and

encouraged us to investigate the optical changes of combined

ED samples after separation of enamel and dentine. The

optical contact between E samples and D samples was

improved by an interfacing layer of glycerol with a refractive

index (n) of approximately 1.5, which approached that of

enamel (n � 1.65). Obviously this model would not perform

like non-separated tooth samples and the results showed that

the colour parameters (L*, a*, b*) were different between

original ED and recombined-ED. However, the correlation tests

showed that ED samples and recombined-ED samples were

highly correlated although the absolute values of L*a*b* values

were different. The results suggested that this model could

provide a relatively reliable study model for investigating the

mechanism of tooth bleaching.

Vieira25 and Ma et al.14 reported that the bleaching

procedure significantly changed the enamel translucency,

making it more opaque. In the enamel–dentine system, along

with the decrease in enamel translucency, more light was

reflected within the enamel, leading to more light reflected to

the human eye. And more importantly, enamel acted as a light

filter for dentine. The present study indicated that, when

j o u r n a l o f d e n t i s t r y 3 9 ( 2 0 1 1 ) 7 3 9 – 7 4 5744

bleaching resulted in a decrease in enamel translucency

(which means ‘‘more opaque’’), less light would fall on dentine

and less light from dentine would be reflected to human eye.

Consequently, the influence of dentine colour on tooth colour

would be decreased. In other words, the object would seem

lighter.25

Enamel consisted of large amount of inorganic materials

and very small amount of organic phase such as protein and

water.26 The decrease in translucency of enamel sample

would be related to partial removal of mineralized tissues and

organic matrix, which might be affected by the etching and

oxidization of bleaching gels. A study carried out by Li et al.27

showed the density of enamel was detected significantly

decreased after treatment with 30% hydrogen peroxide (HP)

using m-CT. Jiang et al.28 analysed Raman scattering and laser-

induced fluorescence of enamel subjected to 30% HP, and

found that HP may have adverse effects on the mineral and the

organic matter of human tooth enamel. As the matter was

destroyed by HP, the distance among enamel crystals

increased and thus the distribution of enamel crystals was

less compact than before, which could increase the refractive

index of enamel.

The background was also an important aspect to consider.

A translucent material placed against two distinct different

coloured backgrounds gave two different visual expressions.

The light went through the translucent material and fell on the

background. Then the light returned to the observer, carrying

colour information both of the translucent layer and the

background. In the present study, results demonstrated that

the colour of dentine was less affected by bleaching agents.

This result would be partly owing to the consumption of

bleaching agents when penetrating through enamel.

The index ‘‘CTCC’’, integrating the colour and translucency

change of tooth and simplifying the complicated optical

interactions between enamel and dentine, successfully

assessed the contribution through measuring final colour

expression of different combinations of ED samples. The

results of the present study showed that the contribution

(CTCC) of enamel was higher than that of dentine all through

the time points. Compared with dentine, enamel functioned in

tooth bleaching not only by the colour changes, but also by the

decrease of its translucency. Although tooth colour has been

proved to be mainly determined by dentine colour,21 overall

tooth colour change during tooth bleaching was strongly

influenced by enamel. Taking all the previous information into

consideration, it could be said that enamel and dentine colour

as well as enamel translucency determine the colour of

teeth.25

It has been pointed out that the results were condition

dependent. Most of the time enamel and dentine are both

treated simultaneously in vivo and the optical changes are

both acting together. Additionally, variations in the age of

tooth samples and the initial colour changes of teeth caused

by grinding and polishing would also influence the final

results.7 Therefore, the results of present study could not fully

reflect the real situation of intra-tooth changes in clinical

tooth bleaching process. Within the limitation of the present

study and based on the results of CTCC, it could be speculated

that the teeth with more transparent enamel might be easier

to be whitened than the teeth with less transparent enamel.

5. Conclusions

This finding indicated that both enamel and dentine contrib-

uted to the tooth colour change and enamel played a more

important role than dentine in tooth bleaching.

Acknowledgements

This work was supported by the Natural Science Foundation of

China (No. 81071190), the Youth Chenguang Project of Science

and Technology of Wuhan City (No. 200950431186), the

Fundamental Research Funds for the central Universities

(No. 4103003) and the Self-Research Program for Doctoral

Candidates of Wuhan University.

r e f e r e n c e s

1. Matis BA, Wang Y, Jiang T, Eckert GJ. Extended at-homebleaching of tetracycline-stained teeth with differentconcentrations of carbamide peroxide. QuintessenceInternational 2002;33:645–55.

2. Sulieman M, Addy M, MacDonald E, Rees JS. The effect ofhydrogen peroxide concentration on the outcome of toothwhitening: an in vitro study. Journal of Dentistry 2004;32:295–9.

3. Wiegand A, Drebenstedt S, Roos M, Magalhaes AC, Attin T.12-Month color stability of enamel, dentine, and enamel–dentine samples after bleaching. Clinical Oral Investigations2008;12:303–10.

4. Kihn PW, Barnes DM, Romberg E, Peterson K. A clinicalevaluation of 10 percent vs 15 percent carbamide peroxidetooth-whitening agents. Journal of the American DentalAssociation 2000;131:1478–84.

5. Ontiveros JC, Paravina RD. Color change of vitalteeth exposed to bleaching performed with andwithout supplementary light. Journal of Dentistry2009;37:840–7.

6. Meireles SS, Santos IS, Bona AD, Demarco FF. A double-blindrandomized clinical trial of two carbamide peroxide toothbleaching agents: 2-year follow-up. Journal of Dentistry2010;38:956–63.

7. Joiner A. The bleaching of teeth: a review of the literature.Journal of Dentistry 2006;34:412–9.

8. Sulieman M, Addy M, Macdonald E, Rees JS. The bleachingdepth of a 35% hydrogen peroxide based in-office product: astudy in vitro. Journal of Dentistry 2005;33:33–40.

9. Torres CR, Wiegand A, Sener B, Attin T. Influence ofchemical activation of a 35% hydrogen peroxide bleachinggel on its penetration and efficacy – in vitro study. Journal ofDentistry 2010;38:838–46.

10. O’Brien WJ. Double layer effect and other opticalphenomena related to esthetics. Dental Clinics of NorthAmerica 1985;29:667–72.

11. Kugel G, Petkevis J, Gurgan S, Doherty E. Separate whiteningeffects on enamel and dentin after fourteen days. Journal ofEndodontics 2007;33:34–7.

12. Wiegand A, Vollmer D, Foitzik M, Attin R, Attin T. Efficacy ofdifferent whitening modalities on bovine enamel anddentin. Clinical Oral Investigations 2005;9:91–7.

13. Markovic L, Fotouhi K, Lorenz H, Jordan RA, Gaengler P,Zimmer S. Effects of bleaching agents on human enamellight reflectance. Operative Dentistry 2010;35:405–11.

j o u r n a l o f d e n t i s t r y 3 9 ( 2 0 1 1 ) 7 3 9 – 7 4 5 745

14. Ma X, Jiang T, Sun L, Wang Z, Zhou Y, Wang Y. Effects oftooth bleaching on the color and translucency properties ofenamel. American Journal of Dentistry 2009;22:324–8.

15. Ishikawa-Nagai S, Yoshida A, Sakai M, Kristiansen J, DaSilva JD. Clinical evaluation of perceptibility of colordifferences between natural teeth and all-ceramic crowns.Journal of Dentistry 2009;37:e57–63.

16. Rodrigues JA, Erhardt MCG, Marchi GM, Pimenta LAF,Ambrosano GMB. Association effect of in-office andnightguard vital bleaching on dental enamelmicrohardness. Brazilian Journal of Oral Sciences 2003;2:365–9.

17. Chu SJ, Trushkowsky RD, Paravina RD. Dental colormatching instruments and systems. Review of clinical andresearch aspects. Journal of Dentistry 2010:e2–16.

18. Johnston WM. Color measurement in dentistry. Journal ofDentistry 2009;37:e2–6.

19. Shin DH, Summitt JB. The whitening effect of bleachingagents on tetracycline-stained rat teeth. Operative Dentistry2002;27:66–72.

20. O’Brien WJ, Johnston WM, Fanian F. Double-layer coloreffects in porcelain systems. Journal of Dental Research1985;64:940–3.

21. ten Bosch JJ, Coops JC. Tooth color and reflectance as relatedto light scattering and enamel hardness. Journal of DentalResearch 1995;74:374–80.

22. Murray PE, Stanley HR, Matthews JB, Sloan AJ, Smith AJ.Age-related odontometric changes of human teeth. OralSurgery Oral Medicine Oral Pathology Oral Radiology &Endodontics 2002;93:474–82.

23. Ferrari M, Patroni S, Balleri P. Measurement of enamelthickness in relation to reduction for etched laminateveneers. The International Journal of Periodontics & RestorativeDentistry 1992;12:407–13.

24. Ragain Jr JC, Johnston WM. Accuracy of Kubelka–Munkreflectance theory applied to human dentin and enamel.Journal of Dental Research 2001;80:449–52.

25. Vieira GF, Arakaki Y, Caneppele TM. Spectrophotometricassessment of the effects of 10% carbamide peroxideon enamel translucency. Brazilian Oral Research 2008;22:90–5.

26. He LH, Swain MV. Enamel—A functionally graded naturalcoating. Journal of Dentistry 2009;37:596–603.

27. Li Q, Xu BT, Li R, Yu H, Wang YN. Quantitative evaluation ofcolour regression and mineral content change of bleachedteeth. Journal of Dentistry 2010;38:253–60.

28. Jiang T, Ma X, Wang Y, Tong H, Shen X, Hu Y, et al.Investigation of the effects of 30% hydrogen peroxide onhuman tooth enamel by Raman scattering and laser-induced fluorescence. Journal of Biomedical Optics2008;13:014019.