Measurement and Analysis of Maxillary Anterior Teeth Color In The Chinese Population

DOI: https://doi.org/10.21203/rs.3.rs-2252044/v1

Abstract

Objective To measure the difference in the crown color of the maxillary anterior teeth in the Chinese population, to study its potential regularity, and to provide a reference for the colorimetry of oral anterior teeth restoration. Method Use VITA Easyshade Advance4.0 spectrophotometer to measure the color of 1/3 of maxillary anterior crowns in different age groups, use CIE-1976-L*a*b* color system to describe the color, use Spss26.0 software Statistical analysis was performed to obtain the trend of the color change of maxillary anterior teeth. 

Conclusion: 1. The color of the labial crown of maxillary anterior teeth in the Chinese population is related to different age groups and tooth positions but not gender. 2. In the Chinese population, the color of the maxillary anterior teeth on the labial side gradually decreased from the central incisor to the distal end of the dental arch while the chroma gradually increased. 3. With age increase, the L* and a* values ​​of the upper central incisors, upper lateral incisors, and upper canines gradually decrease, and the b* value gradually increases. The teeth became darker, more yellow, and redder.

1. Introduction

With the development of society, the restoration of teeth by doctors and patients is not only satisfied with the function but also the shape and position of the teeth. The color is also essential [1]. Specialists are paying more and more attention to the optical properties of teeth, and the color of the teeth is the most prominent factor in the optical properties  [2]. Patients' concern about the color matching of teeth is also an increasingly common phenomenon[3]. It is significant to strive to obtain a beautiful and natural restoration color. There are regional differences in natural tooth color in various regions. Analyzing the tooth color of the anterior teeth of the population in a specific area can better guide the clinical evaluation of the tooth color and make the correct decision. Evaluation in order to be able to produce a restoration that is aesthetically pleasing to the patient [4].

The color and appearance of teeth are complex phenomena with many factors, such as lighting conditions, translucency, opacity, light scattering, etc. [5]. It is determined by the combination of the intrinsic color produced by the interaction of light with the tooth structure and the presence of external pigments [6]. The scattering and absorption of light by enamel and dentin form the inherent color of teeth[7]. Since enamel is relatively translucent, the optical properties of dentin play an essential role in determining the overall tone and chroma of teeth[8]. The amount of light reflected and absorbed depends on the thickness and translucency of the tissue, and it is clear that the thickness of enamel and dentin affects tooth color [2].

The measurement of tooth color can be performed by various methods, including visual assessment, spectrophotometry, color chart methods, and computer analysis of digital images [9]. These methods have been used successfully with dental colorimetry [5]. Visual assessment and colorimetric methods are fast, low-cost, and traditional colorimetric methods. However, they are subject to more significant influences such as external light, the patient's makeup, experience, and visual fatigue, which are inevitable. Potential errors [10] [11], digital image colorimetry is affected by the selection of parameters, post-image processing, and other factors [12]. Due to the many disadvantages of subjective tooth color measurement, digital instruments have been introduced and continued to evolve since the late 1970s, enabling objective measurements[1]. At the same time, spectrophotometers come with a standard light source and can be protected from environmental conditions. , the influence of the operator [13], and has the function of data storage, which is conducive to collecting, recording, and analyzing data and can communicate with technicians more objectively [14].

In this study, the information collected by the VITA Easyshade Advance4.0 spectrophotometer was more accurate than the visual measurement method[15], their accuracy was improved by 33%, and the color correspondence was also improved by 93.3% [16]. In addition, the VITA Easyshade4.0 computer colorimeter has strong repeatability and high accuracy [17] [18]. The accuracy and repeatability of VITA Easyshade reached 92.6% and 96.4%, respectively[19]. Therefore, the VITA Easyshade Advance4.0 wireless computer colorimeter was selected as the experimental colorimetric device in this experiment. The measuring head was placed vertically with 1/3 of the tooth crown for data acquisition. Analyze the changing laws of tooth color so as to provide a reference for clinical colorimetry and post-restoration.

The CIE-1976-L*a*b* chromaticity system in VITA Easyshade Advance4.0 software is used for analysis, and the CIE L*a*b* color notation system of CIE-Commission International e de L'eclairage (International Commission on Illumination) is the most commonly used in dental in vivo and in vitro studies and color characterization [20] [21]. In this system L*, a*, b* represent the lightness, chroma green-red coordinates (negative a is green, positive a is red) and chroma blue-yellow coordinates (negative b is blue, positive b is yellow), which is a cylindrical color space description, and tooth color can also be described in a cylindrical color space [22]. Many researchers have reported using spectrophotometers, computer colorimeters and other instruments to measure the L*, a*, b* values of teeth to describe the color of teeth[23].

Therefore, in this experiment, VITA Easyshade Advance4.0 spectrophotometer was selected for the experiment. The experiment was completed by the author alone. The instrument was calibrated every time a tooth was changed to reduce errors and provide a certain reference for clinical practice. It can provide a certain reference for the colorimetry of upper anterior teeth in China in the future. The equipment is relatively simple, has strong repeatability, high accuracy, and mature technology. It can provide data support for young doctors, clinical teaching, and student clinics.

2. Materials And Methods

2.1 Experimental subjects

In the selection method of the survey objects, 160 adult patients (88 males, 72 females, aged 20-60 years old) were randomly treated in Sanming Integrated Medicine Hospital from January 2022 to June 2022 selected as the survey objects. A total of 969 upper anterior teeth were included in the measurement. All patients lived in Sanming, Fujian for more than 5 years. All participants received written information and signed the informed consent form. This study, the informed consent form, and the experimental process and steps were approved by the Medical Ethics Committee of Sanming Integrated Medicine Hospital and met the requirements(Approval Number:2022-KY-002). All patients met the following inclusion criteria: maxillary permanent anterior teeth, normal development, no history of treatment bleaching, no caries lesions, no Restoration and dental treatment were performed, and the exclusion criteria were: abnormal dental development, teeth with fluorosis, teeth with tetracycline, pigmented teeth, calculus, etc., which could not be combined with colorimetric operations [13, 24].

The G-Power software was used to determine the sample size required for this experiment. Through literature review, the test level was set to α = 0.05, and the test power was 1-β = 0.85 [25]. The results show that the minimum sample size of this experiment is 60 people.

2.2 Colorimetric system and colorimeter

The International Commission on Illumination (Commission Internationale de l'Eclairage, CIE) 1976L*a*b* color space was used for colorimetry [26]. L* is the lightness, which means the color is from dark (black) to light (white), a* is the chroma, which means red and green −a*~ a* means the hue changes from green to red, and b* is also the chroma, represents yellow-blue, and −b*~b represents the transition of hue from blue to yellow. The larger the value, the greater the bias [27]. Color space chromaticity diagram See Figure 1[28].

Colorimeter: VITA Easyshade Advance4.0 spectrophotometric colorimeter, the instrument comes with a standard D65 light source, and the colorimeter probe diameter is 3mm.

2.3 Experimental Methods

A small brush was used to clean the subject's teeth, and the upper front teeth were fully exposed. Calibrate the colorimeter, the probes are respectively perpendicular to the middle 1/3 of the tooth surface, and the top is closely attached to the tooth surface 1/3 to avoid light leakage, record L*, a*, b* at the exact location after three tests, take the average value and record, Recalibrate the colorimeter when taking the next tooth measurement. The data from 6 upper anterior teeth were obtained, and one experimenter carried out the collection to check the operation error. See the simple method flow: Figure 2

2.4 Calculation method of color difference value [14]and inclusion criteria

That is, ΔL=L1-L2, Δa*=a*1-a*2, Δb*=b*1-b*2, total color difference ΔE=(ΔL2+Δa*2+Δb*2)1/2.

The clinically different threshold of total chromatic aberration is ΔE=1.0-3.7, and the clinically acceptable threshold is ΔE=2.72-6.8 [29] If the color difference is smaller than the threshold, it can be considered that the color difference is not recognized by the human eye.

2.5 Statistics Analysis

Spss22.0 statistical software was used to statistically analyze the L*a*b* values of the upper anterior teeth of the population in the Sanming area, and the paired t-test was used for the same name teeth in the same group, the variance of the chromaticity values of different tooth positions and different age groups was analyzed. Analysis, the test level is α = 0.05.

3. Experiment Results

3.1 Age distribution and number of teeth of the experimental population

After statistics, it was found that the distribution was relatively uniform, including 88 males and 72 females, as shown in Table 1. 

Table 1 Age and number of teeth of experimental population

Age group(years)

number of people

Number of teeth (pcs)

20~29

38

228

30~39

42

252

40~49

41

246

50~59

39

234

3.2 Chromaticity value of each tooth position

The obtained chromaticity values were statistically analyzed, and it was obtained that each chromaticity value obeyed the ordinary distribution law and was described by the mean ± standard deviation (`x±s). The results are shown in Table 2 and Table 3. There was no statistical difference in the chromaticity values of teeth with the same name between different genders (P > 0.05).

Table 2 Measurement results of chromaticity value of each tooth position( ±s)

tooth

number of teeth(pcs)

L*

a*

b*

right central incisor

160

72.30±4.26

-0.57±0.39

14.35±2.93

right lateral incisors

160

69.53±4.83

0.17±0.69

18.37±3.35

right canine

160

64.71±4.20

1.35±0.66

23.71±3.25

left central incisor

160

73.03±4.58

-0.51±0.31

14.65±3.51

left lateral incisors

160

70.39±4.55

0.27±0.56

18.82±4.37

left canine

160

65.57±4.20

1.44±0.56

23.57±3.37

Table 3 The measurement results of the chromaticity value of each tooth position in different genders(±s)

Gender

tooth

number of teeth(pcs)

L*

a*

b*

Men

right central incisor

88

72.52 ±4.11

-0.63 ±0.45

14.68± 2.02


right lateral incisors

88

69.15± 4.62

0.03±0.76

18.33±2.67


right canine

88

64.05±4.21

1.18±0.73

23.72±2.99


left central incisor

88

73.18±6.22

-0.48±0.33

14.92±3.29


left lateral incisors

88

69.59±4.7

0.22±0.56

17.94±3.80


left canine

88

65.70±4.10

1.52±0.62

23.31±3.53

Women

right central incisor

72

72.74±4.6

-0.45±0.31

14.53±3.43


right lateral incisors

72

69.86±4.84

0.16±0.56

18.03±3.98


right canine

72

66.59±4.21

1.43±0.53

23.03±3.41


left central incisor

72

73.26±4.47

-0.53±0.32

14.41±3.37


left lateral incisors

72

69.95±4.73

0.24±0.55

19.92±4.41


left canine

72

65.96±4.04

1.40±0.51

24.20±2.98

3.3 Left-right symmetrical interdental chromaticity values

The L*, a*, b* chromaticity values of the left and right symmetrical teeth with the same name were analyzed by paired t-test, and the results are shown in Table 4. There was no significant difference in the L*, a*, b* chromaticity values of the left and right symmetrical teeth with the same name (P > 0.05). Obtain the chromaticity difference (ΔL*, Δa*, Δb*) of the corresponding chromaticity values of the tooth with the same name, and obtain the chromaticity value of the left-right symmetrical tooth with the same name ΔE < 0.9, it can be considered that the color difference is not recognized by the human eye and cannot be recognized by the human eye. The difference in chromaticity between the left and right teeth with the same name.

Table 4 Statistics of chromaticity difference of chromaticity values of left-right symmetrical teeth with the same name

tooth

ΔL*

Δa*

Δb*

central incisor

0.73

0.06

0.1

lateral incisors

0.86

0.1

0.45

canine

0.86

0.09

0.14

3.4 Comparison of chromaticity values of different tooth positions 

The mean ± standard deviation(`x±s) description, variance analysis, and paired t-test analysis of the chromaticity values of different tooth positions are shown in Table 5. There was no significant difference in the chromaticity values of the left and right symmetrical teeth with the same name (P > 0.05). Among them, 1) The order of lightness L* value is central incisor>lateral incisor>canine, the order of chroma a* value is: canine>lateral incisor>central incisor, the size of chroma b* value The order is: canine>lateral incisor>central incisor.

Table 5 Comparison of chromaticity values of each tooth position (±s)

tooth

number of teeth(pcs)

L*

a*

b*

central incisor

320

73.02±4.41 #*

-0.54±0.35 #*

14.50±3.23#*

lateral incisor

320

69.96±4.70 #

0.22±0.63 #

18.60±3.94 #

canine

320

65.14±4.21 *

1.40±0.62 *

23.64±3.30 *

Compared with the same group of middle canines, #P < 0.05, compared with the same group of lateral incisors, *P < 0.05

3.5 Comparison of chromaticity values of the same tooth position in different age groups

The mean ± standard deviation (`x±s)description, variance analysis, and paired t-test analysis results of chromaticity values L*, a*, b* at different ages are shown in Table 6. L* value between different age groups: L* value of central incisors were significantly different (P<0.05), L* value of lateral incisors: there was no statistical difference between the 20-year-old group and the 30-year-old group, and between the 30-year-old group and the 40-year-old group, and there were statistical differences in the other age groups (P<0.05), L* value of canine group: there is a statistical difference between the 20-year-old group and the 30-year-old group (P<0.05), and there is no statistical difference between the other age groups. A* value between different age groups: The a* value of central incisors: 20-year-old group and 40-year-old group, 20-year-old group and 50-year-old group were statistically different (P<0.05), and there was no statistical difference between the other age groups, The a* value of lateral incisors: there was no statistical difference between each age group, a* value of canine teeth: there was a statistical difference between the 20-year-old group and the 50-year-old group (P<0.05), and the other age groups were compared in pairs no statistical difference. B* value between different age groups: The b* value of central incisors: the 20-year-old group was significantly different from other groups (P<0.05), and there was no statistical difference between the other age groups, The b* value of lateral incisors: there are statistical differences between the 20-year-old group and the 40-year-old group, the 20-year-old group and the 50-year-old group (P<0.05), and there is no statistical difference between the other age groups, And canine b* value: there were statistical differences between the 20-year-old group and the 50-year-old group, the 30-year-old group and the 50-year-old group (P<0.05), and there was no statistical difference between the other age groups.

Table 6 Comparison of chromaticity values for each age group

Age group(years)

tooth

number of teeth(pcs)

L*

a*

b*


central incisor

76

77.45±2.65*&

-0.51±0.60&

13.19±2.69*&

20~29

lateral incisor

73.23±2.83&

0.42±0.68

17.53±3.12&


canine

68.30±2.69&

1.67±0.63

23.18±2.96


central incisor


75.81±2.55#&

-0.68±0.66

14.17±2.57#

30~39

lateral incisor

84

71.62±8.48

0.37±0.58

18.69±2.57


canine


67.41±3.13&

1.59±0.65

23.75±3.17


central incisor


74.59±3.02#*

-0.75±0.65#

14.31±2.81#

40~49

lateral incisor

82

69.49±3.36#

0.35±0.47

19.19±2.87#


canine


64.20±3.27#*

1.44±0.63*

24.10±3.04


central incisor


72.63±3.01#*&

-0.83±0.72#

14.53±2.58#

50~59

lateral incisor

78

67.77±2.97#*&

0.30±0.75

19.38±3.21#


canine


62.45±2.87#*&

1.33±0.68#

24.48±2.55#*

Compared with the same tooth position 20-29 years old, #P < 0.05, compared with the same tooth position 30-39 years old, *P < 0.05, compared with the same tooth position 40-49 years old, &P < 0.05

3.6 Mean distribution of different tooth positions in different age groups

The mean distribution of L* values in different tooth positions in different age groups is shown in Figure 3. It can be seen that the L* value tends to decrease with the increase of age. The L* value represents the lightness, indicating that the younger the age, the higher the lightness. As age increases, the value of lightness decreases, and the color develops from white to black. No matter at which age, the L* value decreases from the central incisor to the distal end of the dental arch to the canine, indicating that from the central incisor to the distal end of the dental arch, The lightness gradually decreases from end to canine.

The mean distribution of a* values for different tooth positions in different age groups is shown in Figure 4. It can be seen that the a* value tends to decrease with age. The a* value represents the chroma, representing red and green, and −a*~ a* represents the hue from green to red. With the increase of age, the color develops from red to green. At the same time, no matter at which age, the a* value increases from the central incisor to the distal end of the dental arch to the canine, indicating that from the central incisor to the distal end of the dental arch to the canine Canine chroma develops towards red. With the increase of age, the color develops from red to green. At the same time, no matter at which age, the a* value increases from the central incisor to the distal end of the dental arch to the canine, indicating that from the central incisor to the distal end of the dental arch to the canine Canine chroma develops towards red.

The mean distribution of b* values for different tooth positions in different age groups is shown in Figure 5. It can be seen that the b* value tends to increase with age. The b* value represents the chroma, representing yellow and blue, and −b*~b represents the hue from blue to yellow, and the b value increases with age. , the color develops from blue to yellow, no matter at which age the b* value increases from the central incisor to the distal end of the dental arch to the canine, and the color develops from blue to yellow.

4. Discussion

There are many methods for evaluating tooth color, but the subjective color is often included. VITA Easyshade Advance4.0 spectrophotometer is easy to operate and has relatively few error sources. The mature color system can obtain tooth color data objectively, and the experimental data is reliable[30].

It can be seen from the experimental results: 1. There is no statistical difference in the chromaticity values of the teeth with the same name of different genders (P > 0.05), and the chromatic difference value of the left-right symmetrical teeth with the same name is ΔE < 0.9, which is not recognized by the human eye and cannot be recognized by the human eye The difference in chromaticity between the left and correct teeth with the same name. The age factor can be excluded. The color of teeth has nothing to do with gender, so there is no need to consider gender when selecting tooth color. The color difference between the left and correct teeth with the same name is not recognized by the naked eye, so the opposite side teeth with the same name can be used as a reference when choosing the color. It is consistent with the research of scholars such as Alma Dozić [31], Yong-Keun Lee[32], Farhad Tabatabaian[33], Tahir Karaman[34].

2. Statistical differences exist in the chromaticity values between different positions of the maxillary anterior teeth. The lightness value of the central incisors to the canines gradually decreases while the chroma gradually increases, and the color gradually becomes darker and yellower[35]. It shows that 1) in the clinical colorimetric analysis of the upper anterior teeth when the contralateral tooth with the same name is missing at the same time, the adjacent teeth need to be used as reference teeth, and the different brightness and chroma between the upper central incisors, lateral incisors and canines should be compared. Changes are taken into consideration, 2) If the maxillary anterior teeth are all missing, the color selection during the simultaneous restoration should be differentiated and excessive.

3. The chromaticity value of maxillary anterior teeth gradually decreases with age, and the L* and a* values of upper central incisors, upper lateral incisors, and upper canines decrease progressively, and the b* value gradually increases. Darker, more yellow, and redder[36]. Therefore, age should be taken into account in the selection of color for maxillary anterior teeth restoration, and the color selection should not be carried out with a uniform standard for all age groups, which is consistent with the research of most scholars [34, 37] [38].

In this experiment, the Chinese population is selected as the experimental population, which has relative limitations. In future experiments, more populations can be included in the practical scope to obtain more comprehensive empirical data. The measurement site selected in this experiment is the middle 1/3 of the crown. Whether there is a difference in color between the incisal 1/3 of the crown and the cervical 1/3 remains to be studied, and this experiment only examines the ipsilateral and The color difference between the contralateral teeth and the difference in brightness and chromaticity between the opposite teeth can be further expanded in future research.

5. Conclusions

1. The color of the labial crowns of the maxillary anterior teeth in the Chinese population is related to different age groups and tooth positions and has nothing to do with gender. 2. The color difference between the Chinese population's left and correct teeth of the same name cannot be recognized by the naked eye, and the colors are very close. 3. The color of the maxillary anterior teeth on the labial side of the Chinese population gradually decreased from the central incisor to the distal end of the dental arch. At the same time, the chroma gradually increased, and the color gradually became darker and yellower. 4. With age, the L* and a* values of upper central incisors, upper lateral incisors, and upper canines gradually decrease, and the b* value gradually increases. The teeth become darker, yellow, and redder with age, the L*lightness value is the coordinate that has the most significant relationship with tooth color in the aging process.The measurement site selected in this experiment is the middle 1/3 of the crown. Whether there is a difference in the color between the incisal 1/3 of the crown and the neck 1/3 remains to be studied, and this experiment only studies the ipsilateral and The color difference between the contralateral teeth and the difference in brightness and chromaticity between the opposite teeth can be further expanded in future research.

Abbreviations

CIE:Commission Internationale de l'Eclairage

Declarations

Author contributions

CHENGLU RUAN, LIN WANG designated the research methodology, project management, resources, supervision and validation. All authors were responsible for writing the original draft, writing the final article and reviewing it. 

Availability of data and materials

All the data generated or analyzed during this study are included in this published article. 

Funding 

This study was supported by Fujian Science and Technology Commissioner Fund and Research on Sustainable Influencing Factors of Social Innovation Design (Grant No. 113-KC21056S)

Acknowledgements

Thanks to Sanming Integrated Medicine Hospital supported the study. Thanks to all the patients who participated in this experiment. Thanks for the financial support of Fujian Science and Technology Special Commissioner Fund and Research on Sustainable Influencing Factors of Social Innovation Design (Approval No.: 113-KC21056S).

Ethics approval and consent to participate

1、Confirmation that all methods were performed in accordance with the relevant guidelines and regulations, in compliance with the Declaration of Helsinki.

2、The experimental procedures and steps involved in the study were approved by the Medical Ethics Committee of Sanming Integrated Medicine Hospital and met the requirements(Approval Number:2022-KY-002).

3、All participants received written information and signed the informed consent form.

4、The Medical Ethics Committee approved this study and the informed consent form of Sanming Integrated Medicine Hospital, and the experimental procedures and procedures met the requirements(Approval Number:2022-KY-002)

Consent for publication 

Not applicable

Competing interests 

The authors declare that they have no competing interests

References

  1. Gómez-Polo C, Gómez-Polo M, Celemin-Viñuela A, Martínez Vázquez De Parga JA: Differences between the human eye and the spectrophotometer in the shade matching of tooth colour. J Dent 2014, 42(6):742-745.
  2. Turgut S, Kilinc H, Eyupoglu GB, Bagis B: Color relationships of natural anterior teeth: An In vivo study. Niger J Clin Pract 2018, 21(7):925-931.
  3. Joiner A, Luo W: Tooth colour and whiteness: A review. Journal of Dentistry 2017, 67:S3-S10.
  4. Pan QQ, Westland S: Tooth color and whitening - digital technologies. Journal of Dentistry 2018, 74:S42-S46.
  5. Joiner A: Tooth colour: a review of the literature. J Dent 2004, 32 Suppl 1:3-12.
  6. Eimar H, Marelli B, Nazhat SN, Abi Nader S, Amin WM, Torres J, de Albuquerque RF, Jr., Tamimi F: The role of enamel crystallography on tooth shade. J Dent 2011, 39 Suppl 3:e3-10.
  7. Joiner A, Luo W: Tooth colour and whiteness: A review. J Dent 2017, 67s:S3-s10.
  8. Karamouzos A, Papadopoulos MA, Kolokithas G, Athanasiou AE: Precision of in vivo spectrophotometric colour evaluation of natural teeth. Journal of oral rehabilitation 2007, 34(8):613-621.
  9. Magsumova OA, Postnikov MA, Trunin DA, Filippova MD: [Modern aspects of determining dental color in aesthetic dentistry]. Stomatologiia 2021, 100(5):102-109.
  10. Meireles SS, Demarco FF, dos Santos Ida S, Dumith Sde C, Bona AD: Validation and reliability of visual assessment with a shade guide for tooth-color classification. Oper Dent 2008, 33(2):121-126.
  11. Dozić A, Kleverlaan CJ, El-Zohairy A, Feilzer AJ, Khashayar G: Performance of five commercially available tooth color-measuring devices. J Prosthodont 2007, 16(2):93-100.
  12. Tabatabaian F, Beyabanaki E, Alirezaei P, Epakchi S: Visual and digital tooth shade selection methods, related effective factors and conditions, and their accuracy and precision: A literature review. J Esthet Restor Dent 2021, 33(8):1084-1104.
  13. Brandt J, Nelson S, Lauer HC, von Hehn U, Brandt S: In vivo study for tooth colour determination-visual versus digital. Clinical oral investigations 2017, 21(9):2863-2871.
  14. Weyhrauch M, Igiel C, Pabst AM, Wentaschek S, Scheller H, Lehmann KM: Interdevice agreement of eight equivalent dental color measurement devices. Clinical oral investigations 2015, 19(9):2309-2318.
  15. Liberato WF, Barreto IC, Costa PP, de Almeida CC, Pimentel W, Tiossi R: A comparison between visual, intraoral scanner, and spectrophotometer shade matching: A clinical study. The Journal of prosthetic dentistry 2019, 121(2):271-275.
  16. Paul S, Peter A, Pietrobon N, Hämmerle CH: Visual and spectrophotometric shade analysis of human teeth. J Dent Res 2002, 81(8):578-582.
  17. Knezović D, Zlatarić D, Illeš I, Alajbeg M, Žagar: In Vivo and in Vitro Evaluations of Repeatability and Accuracy of VITA Easyshade® Advance 4.0 Dental Shade-Matching Device. Acta stomatologica Croatica 2015, 49(2):112-118.
  18. Czigola A, Róth I, Vitai V, Fehér D, Hermann P, Borbély J: Comparing the effectiveness of shade measurement by intraoral scanner, digital spectrophotometer, and visual shade assessment. J Esthet Restor Dent 2021, 33(8):1166-1174.
  19. Kim-Pusateri S, Brewer JD, Davis EL, Wee AG: Reliability and accuracy of four dental shade-matching devices. The Journal of prosthetic dentistry 2009, 101(3):193-199.
  20. Pérez Mdel M, Ghinea R, Rivas MJ, Yebra A, Ionescu AM, Paravina RD, Herrera LJ: Development of a customized whiteness index for dentistry based on CIELAB color space. Dental materials : official publication of the Academy of Dental Materials 2016, 32(3):461-467.
  21. Lee YK: Correlation between three color coordinates of human teeth. Journal of biomedical optics 2014, 19(11):115006.
  22. Eiffler C, Cevirgen E, Helling S, Zornek J, Pritsch M, Hassel AJ: Differences in lightness, chroma, and hue in the anterior teeth of quinquagenarians and septuagenarians. Clinical oral investigations 2010, 14(5):587-591.
  23. Joiner A, Hopkinson I, Deng Y, Westland S: A review of tooth colour and whiteness. J Dent 2008, 36 Suppl 1:S2-7.
  24. Kim HK: Evaluation of the repeatability and matching accuracy between two identical intraoral spectrophotometers: an in vivo and in vitro study. The journal of advanced prosthodontics 2018, 10(3):252-258.
  25. McLeod C, Fields HW, Hechter F, Wiltshire W, Rody W, Jr., Christensen J: Esthetics and smile characteristics evaluated by laypersons. The Angle orthodontist 2011, 81(2):198-205.
  26. Masaoka K, Nishida Y: Metric of color-space coverage for wide-gamut displays. Optics express 2015, 23(6):7802-7808.
  27. Karaagaclioglu L, Terzioglu H, Yilmaz B, Yurdukoru B: In vivo and in vitro assessment of an intraoral dental colorimeter. J Prosthodont 2010, 19(4):279-285.
  28. 曲云鹏, 任丽丽, 刘建国: 某地区苗族中青年上前牙牙冠颜色的研究 %J 中国医药指南. 2020, 18(20):137-138.
  29. Paravina RD, Westland S, Imai FH, Kimura M, Powers JM: Evaluation of blending effect of composites related to restoration size. Dental materials : official publication of the Academy of Dental Materials 2006, 22(4):299-307.
  30. Chang JY, Chen WC, Huang TK, Wang JC, Fu PS, Chen JH, Hung CC: Evaluating the accuracy of tooth color measurement by combining the Munsell color system and dental colorimeter. The Kaohsiung journal of medical sciences 2012, 28(9):490-494.
  31. Dozić A, Kleverlaan CJ, Aartman IH, Feilzer AJ: Relations in color among maxillary incisors and canines. Dental materials : official publication of the Academy of Dental Materials 2005, 21(3):187-191.
  32. Lee YK: Color Correlations among Six Types of Permanent Anterior Teeth. J Esthet Restor Dent 2016, 28 Suppl 1:S5-s13.
  33. Tabatabaian F, Ourang SA, Khezri AS, Namdari M: Are there any color match and color correlation between maxillary anterior teeth? J Esthet Restor Dent 2022, 34(2):397-404.
  34. Karaman T, Altintas E, Eser B, Talo Yildirim T, Oztekin F, Bozoglan A: Spectrophotometric Evaluation of Anterior Maxillary Tooth Color Distribution According to Age and Gender. J Prosthodont 2019, 28(1):e96-e102.
  35. Pop-Ciutrila IS, Colosi HA, Dudea D, Badea ME: Spectrophotometric color evaluation of permanent incisors, canines and molars. A cross-sectional clinical study. Clujul medical (1957) 2015, 88(4):537-544.
  36. Yang DL, Qian HX, Gan H, Qian CH, Zhang XY: [Color distribution of the anterior teeth in 137 elder people]. Shanghai kou qiang yi xue = Shanghai journal of stomatology 2008, 17(3):300-303.
  37. Falcone ME, Kelly JR, Rungruanganut P: In Vivo Color Relationships Between the Maxillary Central Incisors and Canines as a Function of Age. The International journal of prosthodontics 2016, 29(5):496-502.
  38. Gómez-Polo C, Montero J, Gómez-Polo M, de Parga JA, Celemin-Viñuela A: Natural Tooth Color Estimation Based on Age and Gender. J Prosthodont 2017, 26(2):107-114.