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Effects of dentin and enamel porcelain layer thickness on the color of various ceramic restorations

Abstract Objective To investigate the effects of dentin and enamel porcelain layer thickness on the color of various ceramic restorations. Materials and methods Eighty specimens (shade A2 and A3, n = 10) (20 mm in length, 4 mm in width, 1.5 mm in thickness respectively) of casting ceramic (EM); alumina ceramic (AL); zirconia ceramic (ZR); and porcelain‐fused‐metal (PFM) were prepared. The color distributions of each specimen were measured at 4 places using a spectroradiometer. The dentin/enamel porcelain (D/E) layer thicknesses of the 4 places were 0.8/0.2 mm, 0.6/0.4 mm, 0.4/0.6 mm, and 0.2/0.8 mm. The color differences (ΔE00) between the specimens and the corresponding color shade tabs were calculated. Data were analyzed using three‐way repeated‐measures ANOVA and Holm–Sidak pairwise comparisons (a = 0.05). The acceptability threshold (AT) was used to analyze the results. Results The minimum ΔE00 values were 1.31 (0.6/0.4 mm for EM), 1.41 (0.8/0.2 mm for AL), and 1.92 (0.2/0.8 mm for ZR) for shade A2, and 0.93 (0.6/0.4 mm for EM), 0.89 (0.8/0.2 mm for AL), and 1.34 (0.8/0.2 mm for ZR) for shade A3. Most of them were below AT value (1.8). For AL and ZR (shade A2) and ZR (shade A3), the D/E layer thicknesses of 0.8/0.2 mm and 0.6/0.4 mm had lower ΔE00 values than 0.4/0.6 mm and 0.2/0.8 mm (p ≤ 0.001). Conclusions The dentin/enamel porcelain layer thickness that was most color‐matched to the shade tab was different for various ceramic restorations. The color of shade A2 AL and ZR and shade A3 ZR was closer to the shade tab when dentin porcelain was thicker than enamel porcelain. Clinical significance Matching the shade of ceramic restoration to the shade tab color is a great challenge in esthetic dentistry. The dentin/enamel porcelain layer thickness is an important factor to influence the improved color matching.


| INTRODUCTION
Dental porcelain, which has high wear resistance, high strength, high toughness, and excellent esthetic, is the most suitable material for replacing natural tooth tissues.1-3 However, the reproduction of selected shades with dental porcelain is not an easy task because many factors may affect the results. Several studies have shown that the thickness of the porcelain layer is a key factor in color matching, and the individual thickness of each layer and the ratio between the layers are important factors in achieving the best color match for the traditional porcelain-fused-to-metal restorations. 1,4,5 With the popularity of ceramic materials, all-ceramic restorations have been increasingly applied in clinical dentistry. All-ceramics, with their excellent biocompatibility, margin fitness, esthetics and translucency, and so forth, could lead to future esthetic dentistry. [6][7][8] To meet the demand for esthetic restorations, the use of all-ceramic crowns requires highquality shades and optical properties because these crowns do not require the use of an opaque porcelain covered metal layer. 9,10 Therefore, the thickness of the dentin porcelain layer and enamel porcelain layer is important for color match, with the disappearance of the opaque porcelain layer for all-ceramic restorations. However, all ceramic systems have different compositions, organizations, contents, and crystal phases, which may affect the optical properties of these systems. 11,12 Regarding different all-ceramic systems, there is very limited scientific literature available regarding the effects of the individual thickness of each porcelain layer upon the resultant color, and no quantitative analysis has been conducted to investigate the best color-matched layer thickness between the dentin/enamel porcelain layers for the various ceramic systems.
To obtain the color closest to the natural tooth in restorations, it is necessary to perform two different steps: (i) the use of the shade guide to select the best possible shade (ii) to use the appropriate dental material to reproduce this shade in accordance with the selected shade guide. 13 Therefore, the accuracy of the reproduction of the natural tooth color is determined by the color difference between the restoration's color and the corresponding shade tab's color. In other words, the most color-matched restoration should be the restoration with the minimum color difference.
The most space (1 mm) was occupied by enamel porcelain and dentin porcelain for metal-ceramic or all-ceramic restorations, which could mimic the complex anatomy and optical appearance of the enamel and dentin of the natural tooth. 14 The apparent color of the natural tooth is the result of light absorption, scattering, and reflectance from the enamel and dentin. 15 As a general rule, the dentin porcelain influences on chroma and hue, and the enamel porcelain influences on lightness and translucency. Different synthetic color effects should be produced when various thicknesses of dentin porcelain and enamel porcelain are placed together, which determines the resultant color of different ceramic systems. 16 Perceptibility threshold and acceptability threshold are the two major thresholds for assessing color differences. 17,18 Just perceived color difference is the smallest color difference perceived by a human observer. The color difference that 50% of observers can notice corresponds to 50:50% perceptibility threshold. Analogously, the color difference that is acceptable for 50% of observers corresponds to 50:50% acceptability threshold. 18,19 A color difference (using the color-difference formula CIEDE2000) more than 1.8 was considered as a clinically unacceptable color difference, 18 a standard which has been frequently used in previous studies. 20,21 This study compared the color differences between four ceramic systems, which have been most commonly used in current clinical dentistry, with various dentin and enamel porcelain layer thicknesses and corresponding shade tabs. 22 The purpose of this study was to evaluate the influence of different dentin and enamel porcelain layer thicknesses on the resultant colors of various ceramic systems and to investigate the most color-matched D/E layer thicknesses to the corresponding shade tabs. The hypothesis of this study is that there is influence on the color difference between the ceramic materials and the corresponding shade tabs using different ceramic systems and different D/E layer thicknesses.

| MATERIALS AND METHODS
A porcelain-fused-metal system (PFM) and three all-ceramic systems, including one hot pressure casting ceramic (EM), one glassinfiltrated alumina ceramic (AL), and one CAD-CAM zirconia ceramic (ZR), were included in the study to represent metalceramic or all-ceramic restorations with various dentin and enamel porcelain thicknesses and colors. The information about these systems is listed in Table 1.

| Preparation of specimens-PFM and allceramic specimens
A total of 80 cuboid specimens (40 shade A2 and 40 shade A3) (20 mm in length, 4 mm in width, 1.5 mm in thickness respectively) were fabricated (n = 10). 23 The specimens were fabricated according to routine laboratory procedures: two firings of opaque layer, one firing of dentine layer and enamel layer, and finally, self-glazing. The thickness of each layer was adjusted using wet silicon carbide paper

| Preparation of specimens-Composite background specimens
Cuboid composite specimens (20 mm in length, 4 mm in width, 4 mm in thickness) were fabricated and served as the background colors to mimic the prepared tooth substrate. To standardize the shapes and thicknesses of the composites, a special mold using silicone impression material was designed. Composite resins were packed into the T A B L E 1 Information of layer materials from the experimental groups used in the present study

| Calculating the color differences
The color differences (4E 00 ) between the specimens and corresponding color shade tabs were calculated using the colordifference formula CIEDE2000.

| Statistical analysis
The three-way repeated-measures ANOVA was used to observe the effect of shade, ceramic system, and dentin/enamel porcelain (D/E) layer thickness on color differences (ΔE 00 ). In the model, the shade  The results of three-way repeated-measures ANOVA showed that the shade, the ceramic system, and the D/E layer thickness have significant influences on color differences (ΔE 00 ) between specimens and corresponding color shade tabs, and significant interactions between the three factors (p < 0.001) ( Table 3).

| RESULTS
The results of pairwise comparisons for the statistically significant differences among the different ceramic systems within the same shade and D/E layer thickness were shown in Table 2. On shade A2,  AL and ZR and shade A3 ZR more than corresponding shade PFM and EM when enamel porcelain was thicker than dentin porcelain.

| DISCUSSION
The Commission International de l'E´clairage (CIE) defined the color space (L*a*b*) and color-difference formula CIELAB in 1976 to describe and quantify color. 30 To improve the accuracy of visual color assessment in industrial applications, the CIE has recently proposed a CIELAB-based color difference formula (CIEDE2000). 31 The CIEDE2000 performs a specific correction on the nonuniformity of the CIELAB space (weighting functions S L , S C, and S H ), and considers the parameters of the effects of illumination and observation conditions in the color difference evaluation (parameter factors K L , K C, and K H ). The interactive term (R T ) between chroma and hue differences is introduced in the CIEDE2000 to improve the performance of the blue, as well as the scaling factor for the CIELAB a * scale used to improve the gray performance. 26 The formula is officially called the latest color difference equation, and based on small color differences, is considered more accurate in experimental data than the CIELAB formula. [31][32][33] To measure the color of translucent samples, the spectroradiometer and the optical fiber optic cable were located at approximately 45 from the vertical plane to provide an optical arrangement of 0 observation and 45 illumination for the objects in this study. 24 In addition, the spectroradiometer provided a larger area of illumination and a relatively small viewing area to avoid "edge loss". Meanwhile, the custom jigs with a pointer were used to provide repeatable and suitable color measurements for samples.  Figure 3). It is noted that compared with the metal-ceramic system, increasing the dentin porcelain thickness can acquire a more colormatched effect to the corresponding shade tabs for glass-infiltrated alumina ceramic restorations and CAD-CAM zirconia ceramic restorations. The possible explanation is that the apparent color of natural teeth is the result of the reflectance from the dentin modified by the absorption, scattering, and thickness of the enamel. 15 The higher transparent and better optical quality of all-ceramic material could enhance the effect of the dentin layer on the sophisticated blending of final color. This result agreed with a previous study, which suggested increased porcelain thickness (particularly increased dentin layer) and increased porcelain opacity resulted in decreased ΔE and better masking ability of the dental backgrounds.40 In this study, flattened specimens were used as the models for color evaluation to accurately control the thickness of ceramic layers, which were different from the curving specimens such as veneers and crowns. The color of prosthetic material is mainly determined by its inherent optical properties, however, external factors such as surface curvature and texture may also influence the final color. The spectroradiometer could measure a circular area with 1.5 mm in diameter, which is small enough to avoid the color of natural teeth or restorative materials as determined by surface curvature or texture.
The Vitapan Classical shade tabs were used to represent the color of natural teeth because they are the most popular and classical shade guide system in commercial dentistry. 13 The thickness of the middle third of these shade tabs was different from that of the ceramic samples bonded on the composite background samples used in this study.
However, shade tabs were the most commonly used reference for shade matching in dental clinical practice. Moreover, the thickness of both had reached the requirements of infinite optical thickness. 41,42 The limitations of this study included that only shade A2 and shade A3 specimens, which represented the normal color rather than discolored tooth substrate and restorations, were investigated in the study. The translucent shade adhesive agent was applied into the interspace between ceramic specimen and composite specimen to avoid the influence of adhesive agent color on the final color in this study. The application of different color luting composites seems to be a promising approach for adjusting the resultant color. Therefore, further investigation into the effect of various shades of ceramic systems and adhesive systems will be conducted in our next study, and it is hoped that the study will add to the overall clinical picture.

| CONCLUSIONS
Within the limitations of this study, the following conclusions can be drawn: (1) The dentin/enamel porcelain layer thickness that was most color-matched to the shade tab was 0.6/0.4 mm for EM, and 0.8/0.2 mm for shade A2 AL and shade A3 ZR.
(2) When dentin porcelain was thicker than enamel porcelain，the color of shade A2 AL and ZR and shade A3 ZR was closer to the shade tab.

ACKNOWLEDGMENTS AND DISCLOSURE
The project supported by Program for Innovation Team Building at The author does not have any financial interest in the companies whose materials are included in this article.

DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available from the corresponding author upon reasonable request.