Development of a customized whiteness index for dentistry based on CIELAB color space

Highlights

  • No adequate CIELAB-based indices are available for whiteness evaluation in dentistry.

  • Visual data optimization methods allowed the development of a new whiteness index-WI D .

  • WI D is the first CIELAB-based index developed to evaluate whiteness in dentistry.

  • The new index WI D was validated under laboratory and typical clinical conditions.

  • WI D has great clinical and industrial applications for dental materials.

Abstract

Objectives

To develop a customized CIELAB-based whiteness index for dentistry that accurately correlates to perception of tooth whiteness.

Methods

Four psychophysical experiments (PE1-4) were conducted by three panels of observers (OP1-3) under diffuse/0° observation/measuring conditions and under typical clinical viewing conditions. Nine whiteness indices (WI, Z %, WIC, WIO, W31, W64, W , WLAB, W *), two yellowness indices (YID1925, YIE313) and tint of white in the CIELAB color system ( T ) were compared with regard to their ability to measure the perceived whiteness of human teeth. Determination coefficient ( R 2 ) and ‘% wrong decision’ (%WD) method were used as direct measures of the quality of the indices for whiteness perception in dentistry.

Results

CIELAB-based whiteness index (WI D = 0.511 L * −2.324 a * −1.100 b *) was developed through optimization from the data obtained in PE1. The proposed WI D performed better than all the CIELAB and CIE1931 XYZ-based indices under laboratory and clinical conditions (only WIO was comparable to WI D in PE2 and PE4).

Conclusion

The validation experiments under laboratory and typical clinical conditions revealed that the proposed index WI D outperformed previous indices, being the only CIELAB-based index developed for evaluation of whiteness in dentistry.

Introduction

Currently, the CIE L * a * b * (CIELAB, CIE76) color notation system of the CIE-Commission Internationale de l’Eclairage (International Commission on Illumination) is the most frequently used in both in vivo and in vitro research and color characterization in dentistry, with the rare usage of CIEDE2000 (Δ E 00 ) color difference formula . In addition, it is important to point out that color measuring devices currently used in dentistry almost exclusively use the CIELAB color space for color measurements. Moreover, chromatic changes in clinical practice situations and research are usually assessed by means of the color differences associated with this color space .

In addition to color measurement, proper measurement of whiteness is important for research and manufacturing of dental materials, and for the clinical practice, including tooth whitening monitoring (effectiveness of different bleaching agents). In spectral terms, a white material is the material whose reflectance across the entire visible wavelength range is constant and high (near to 100%). In terms of CIELAB color space, this type of spectral behavior is translated into a very high lightness and very low (ideally zero) saturation.

Some whiteness indices based on the CIE 1931 XYZ color notation system, such as the CIE whiteness index WIC and the WI whiteness index according to ASTM E-313-73 , have been used in dental research . Within this context, it is worth mentioning the optimized whiteness index, WIO , developed specifically for evaluation of tooth whiteness. WIO maintains the functional form of the CIE WIC index and has been proven adequate for tooth whitening research and monitoring .

Given the need to establish a CIELAB-based whiteness index that exhibits good correlation with visual evaluation, Ganz and Pauli performed a lineal approximation of the previous CIE whiteness and tint formulas in the CIELAB color space, thus developing the W and T indices . Although these formulas characterized the whiteness appearance well, they poorly correlated with visual evaluation and often failed in the evaluation of white specimens. These results, together with the great industrial applicability of the CIELAB color notation system, led to the development of a new whiteness index (WLAB) which had better correlation with visual evaluations of observers and also had better results in terms of uniformity and applicability than the WIC index. However, none of these indices was used so far in dentistry and their applicability in dental practice has not been evaluated.

Although the CIELAB system is currently used routinely in color research in dentistry, with whiteness indices based on this color notation system occurs the opposite. The following parameters have been predominantly used in evaluating whiteness efficacy: the overall color difference ( <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='ΔEab*’>ΔEabΔEab*
Δ E ab *
, Δ E 00 ), changes in color coordinates (Δ L *, lightness; Δ a *, green–red coordinate; and Δ b *, blue–yellow coordinate) , Δ b * coordinate and the W* index, described by Gelarch et al. and based on the distance of a color value from a nominal white point, represented in CIELAB color space as L * = 100, a * = 0 and b * = 0.

In this study, a psychophysical experiment was performed, aiming to develop and assess a new whiteness index (WI D ), designed specifically for dentistry and based on the CIELAB color notation system. It was expected that WI D would correlate with visual findings, and that it would perform well compared with other whiteness indices based on both CIELAB and the CIE 1931 XYZ color notation systems. Consequently, the null hypothesis was that the newly proposed WI D whiteness index did not differ in performance compared to previous indices.

Materials and methods

Specimens

Three different sets of specimens were used, as follows:

  • Specimen Set A (SSA) : 29 tabs of a VITA Linearguide 3D-MASTER (Vita Zahnfabrik, Bad Säckingen, Germany) shade guide;

  • Specimen Set B (SSB) : 16 tabs of a Vitapan Classical (Vita Zahnfabrik, Bad Säckingen, Germany) shade guide;

  • Specimen Set C (SSC) : 25 experimental (laboratory manufactured) resin composite specimens within the color range of natural teeth in the CIELAB color space (10 mm in diameter, 1-mm thick).

The experimental resin composites were custom fabricated as a mixture of organic matrix, inorganic filler, photo activator and other components in minor quantities and four types of pigments in various mixtures in order to generate composites with different colors . Specimens in SSC were made on a glass plate (Knittel GLASER, Bielefeld, Germany) and clear plastic sheet (Acrylite Plus Clear, Tap Plastics, Dublin, CA, USA) on the top and bottom of the jig. After placement of composites, another glass plate was pressed onto the top of the specimen to standardize the thickness. All specimens were light-cured during 40 s using a using a LED light-curing-unit (Bluephase Ivoclar Vivadent, Shaan, Liechtenstein) operating on standard mode and emitting 1100 mW/cm 2 .

Color measurements

A non-contact spectroradiometer (SpectraScan PR-704, Photo Research, Chatsworth, USA) was used to measure the spectral reflectance of shade guide tabs and resin composite specimens. The specimens were placed in the center of a viewing cabinet (VeriVide CAC60, VeriVide Limited, Leicester, United Kingdom) on a 45° tilted base and a light source simulating the spectral relative irradiance of CIE D65 standard illuminant was used to provide consistent illuminating/viewing conditions. The shade guide tabs and the experimental resin composites were positioned 40 cm away from the spectrorradiometer and measured at 0° (corresponding to diffuse/0° illuminanting/measuring geometry) and the CIE 1931 2° Standard Colorimetric Observer was used to calculate the CIE tristimulus values from the reflectance factors. As the oral cavity is dark, a black ( L * = 3.1, a * = 0.7 and b * = 2.4) 50 mm × 50 mm ceramic tile background (Ceram, Staffordshire, United Kingdom) was used for measurements in this study. A triangular stand was built to hold specimens and avoid the specular reflection from the glossy surface .

The range of the color coordinates of the experimental resin composites specimens were L * = 62.88–79.38, a * = −2.31 to 7.52 and b * = 4.01–27.81. All discs were within the color range of central and lateral incisor and canine teeth according to previously published studies .

Whiteness metrics

Table 1 shows the original metrics included in this study. Nine whiteness indices (WI, Z %, WIC, WIO, W 31 , W 64 , W , WLAB, W *), two yellowness indices (YID1925, YIE313) and tint of white in the CIELAB color system ( T ) were evaluated . The values corresponding to all of these indices were calculated from measured CIE tristimulus (XYZ) values of specimens in the three sets.

Table 1
Description of the whiteness and yellowness indices considered in the study.
Index Formula Notes
WIC Y + 800( x n x ) + 1700( y n y ) CIE proposed whiteness index, where x n and y n are the chromaticity coordinates of the perfect diffuser under CIE D65 Illuminant
WI <SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='4Z%−3YYn100′>4Z%3YYn1004Z%−3YYn100
4 Z % − 3 Y Y n 100
Whiteness index according to the ASTM E-313-73
Z % <SPAN role=presentation tabIndex=0 id=MathJax-Element-3-Frame class=MathJax style="POSITION: relative" data-mathml='ZZn100′>ZZn100ZZn100
Z Z n 100
Developed to assess progress in bleaching and efficiency of bleaching process
W 31 WIC − 2( T W31 ) 2
where
T W31 = 1000( x n x ) − 650( y n y )
T W31 is the CIE1931 tint index
40 < WIC 31 < 5Y 31 < 275
W 64 WIC − 2( T W64 ) 2
where
T W64 = 900( x n x ) − 650( y n y )
T W64 is the CIE 1964 tint index
40 < WIC 64 < 5Y 64 < 275
WIO y + 1075.012( x n x ) + 145.516( y n y ) A modified version of the WIC whiteness index, shown to be acceptable since it was proved to be as reliable as the average human observer in dentistry
W 2.41 L * − 4.45 b* [1 − 0.0090( L* − 96)] − 141.4 Approximation in the CIELAB color space of the CIE whiteness formula
T −1.58 a * − 0.38 b* Approximation in the CIELAB color space of the CIE tint formula
WLAB <SPAN role=presentation tabIndex=0 id=MathJax-Element-4-Frame class=MathJax style="POSITION: relative" data-mathml='Wab−2(Tab)2if40&lt;Wab&lt;3.37L10*−191′>Wab2(Tab)2if40<Wab<3.37L10191Wab−2(Tab)2if40<Wab<3.37L10*−191
W ab − 2 ( T ab ) 2 if 40 < W ab < 3.37 L 10 * − 191

<SPAN role=presentation tabIndex=0 id=MathJax-Element-5-Frame class=MathJax style="POSITION: relative" data-mathml='Pab−2(Tab)2ifWab&gt;3.37L10*−191′>Pab2(Tab)2ifWab>3.37L10191Pab−2(Tab)2ifWab>3.37L10*−191
P ab − 2 ( T ab ) 2 if W ab > 3.37 L 10 * − 191

where:
<SPAN role=presentation tabIndex=0 id=MathJax-Element-6-Frame class=MathJax style="POSITION: relative" data-mathml='Wab=L10*−0.1131a10*−1.6772b10*’>Wab=L100.1131a101.6772b10Wab=L10*−0.1131a10*−1.6772b10*
W ab = L 10 * − 0.1131 a 10 * − 1.6772 b 10 *

<SPAN role=presentation tabIndex=0 id=MathJax-Element-7-Frame class=MathJax style="POSITION: relative" data-mathml='Pab=5.74L10*+0.1131a10*+1.6772b10*’>Pab=5.74L10+0.1131a10+1.6772b10Pab=5.74L10*+0.1131a10*+1.6772b10*
P ab = 5.74 L 10 * + 0.1131 a 10 * + 1.6772 b 10 *

<SPAN role=presentation tabIndex=0 id=MathJax-Element-8-Frame class=MathJax style="POSITION: relative" data-mathml='Tab=−1.4965a10*−0.4224b10*’>Tab=1.4965a100.4224b10Tab=−1.4965a10*−0.4224b10*
T ab = − 1.4965 a 10 * − 0.4224 b 10 *
When sample’s whiteness WLAB is less than 40, it is considered that the sample is not White color

W * [ L * − 100) 2 + ( a *) 2 + ( b *) 2 ] 1/2 Whiteness index described based on the distance between a specified color value and a nominal white, represented in the CIELAB color space as L * = 100, a * = 0 and b * = 0
YID1925 <SPAN role=presentation tabIndex=0 id=MathJax-Element-9-Frame class=MathJax style="POSITION: relative" data-mathml='1001.3013X−1.1498ZY’>(100(1.3013X1.1498Z)Y)1001.3013X−1.1498ZY
100 1.3013 X − 1.1498 Z Y
Developed for evaluating the yellowness index of plastics for D65/10°
YIE313 <SPAN role=presentation tabIndex=0 id=MathJax-Element-10-Frame class=MathJax style="POSITION: relative" data-mathml='1001−0.847ZY’>100(10.847ZY)1001−0.847ZY
100 1 − 0.847 Z Y
Used for evaluation of whiteness and yellowness indexes of almost white or opaque materials
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Nov 23, 2017 | Posted by in Dental Materials | Comments Off on Development of a customized whiteness index for dentistry based on CIELAB color space
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