Impact of resin composite cement on color of computer-aided design/computer-aided manufacturing ceramics

Objective: To analyze the impact of the color of a resin composite cement (RCC) on the optical properties of different computer-aided design and computer-aided manufacturing (CAD/CAM) ceramics. Materials and methods: Specimens ( N = 220, thickness: 0.9 ± 0.03 mm) were fabricated from: leucite (Initial LRF Block/IPS Empress CAD), lithium disilicate (Amber Mill/IPS e.max CAD), lithium metasilicate (Celtra Duo), and lithium alumina silicate ceramic (n!ce) in translucency levels HT and LT. All specimens were bonded with an RCC (Light + /Warm + ). Color was analyzed (spectrophotometer) initially as well as after bonding of RCC with CAD/CAM ceramics using CIELab and CIEDE2000. Kolmogorov – Smirnov test, one-way ANOVA and t test served for analyz-ing ( α = 0.05). Results: Highest impact on Δ E presented the choice of ceramic ( η p2 = 0.155/ p < 0.001), followed by translucency level (HT/LT; η p2 = 0.050/p = 0.001) as well as interaction between ceramic and translucency level ( η p2 = 0.175/p < 0.001). Δ E00 was mainly influenced by the choice of ceramic ( η p2 = 0.490/p < 0.001), the shade of resin composite ( η p2 = 0.031/p = 0.012) as well as the interaction between ceramic and resin composite ( η p2 = 0.258/p < 0.001). Conclusions: RCC shades presented differential impacts on color change of CAD/CAM ceramics. Clinical significance: Knowledge of the impact of available RCC shades on different CAD/CAM ceramics is crucial for an esthetic outcome and proper selection of ceramic restorations.


| INTRODUCTION
Silicate ceramics are frequently used to replace lost natural tooth structure, often in form of monolithic restorations, as they have good mechanical and, in particular, excellent esthetic properties. 1,2 There is a large number of different silicate-based ceramics available, fabricated either by computer-aided design and computer-aided manufacturing (CAD/CAM) technology or with the help of press technique, varying in their material characteristics. 3,4 These differences are based on their specific composition of diverse crystal structures embedded in a glass matrix, leading to variations such as leucite, lithium disilicate, lithium metasilicate, and lithium alumina silicate ceramics. 5 The translucency and the different color values of a silicate ceramic play a decisive role for an optimal esthetic result, 6,7 being usually determined using spectrophotometers. [8][9][10] The color parameters are recorded within the L*a*b* color space for CIELab color system, which was established of the Commission Internationale de L'Eclairage (CIE) related to the human color perception with three directions of the color space: L* coordinate represents the lightness, a* green-red color coordinate, and b* blue-yellow coordinate. 11,12 Another method to describe color and especially differences in color was developed more recently as CIEDE2000. Color differences calculated based on this formula correlate more properly with visually detectable color differences. 12 The final esthetic result, however, is not only determined by the silicate ceramic selected and color values itself. There are further important factors affecting the final color of the restoration, the exact knowledge of which is decisive for an optimal long-term result when restoring with silicate ceramics. These include the ceramic materials themselves, being available in various shades and translucency levels. 13 A varying layer thickness or the shade of the tooth stump can additionally influence the esthetics significantly. [14][15][16] The esthetics of diverse ceramic restoration, however, can also be influenced to some extent by the shade of the underlying resin composite cement (RCC), 14,[16][17][18][19][20][21] whereby color differences are reduced with increasing layer thickness of the ceramics. 14,19,22 The decision for cementation of a silicate-based ceramic restoration with traditional cement or bonding with a RCC is significantly influenced by the silicate ceramic material used. Leucite ceramics should always be adhesively bonded due to their reduced flexural strength in contrast to lithium silicate ceramics for example. Latter can be adhesively bonded or traditionally cemented, whereas in thinner layer thicknesses and esthetic areas, RCC should be preferred. 23,24 Adhesive bonding procedures can be performed either with light or dual-cured resin composites with slight differences in color stability, 21 whereas compounds such as reactive groups of tertiary amines and inhibitors may cause more changes in color in dual-curing materials into yellowish and reddish, especially after aging. [25][26][27][28] To authors' best knowledge, there is no information in the scientific literature on the impact of color of a RCC on the optical properties of different recently available CAD/CAM ceramics. The null hypothesis tested states there is no impact of the type of ceramic, it's translucency level and the shade of the RCC upon the final color of the tested CAD/CAM ceramics.

| MATERIALS AND METHODS
Two hundred and twenty specimens were fabricated from six CAD/CAM ceramics (Table 1, Figure 1). All specimens were bonded The intensity of the monochromatic light I0 and the light I was measured, which radiated through the specimen between wavelengths 400 nm and 700 nm (λ) with a white background. The associated software (Color Application Software V1.00, PerkinElmer Inc.) was used to carried out the color values. Color differences were calculated using the standardized CIELab formula (ΔE) and the newer CIEDE2000 formula (ΔE00) as follows: with L* coordinate representing the lightness, a* green-red color coordinate, and b* blue-yellow coordinate as well as with L*1 -L*2 representing ΔL*, a*1 -a*2 representing Δa*, and b*1 -b*2 representing Δb*with number 1 being initial values for CAD/CAM ceramic as well as number 2 being CAD/CAM ceramic combined with RCC, and

| CIEDE2000: ΔE00 calculation-global analysis
The choice of CAD/CAM ceramic (η p 2 = 0.490/p < 0.001) showed the highest impact on ΔE00, followed by the color of RCC (η p 2 = 0.031/p = 0.012) and the interaction between ceramic and RCC (η p 2 = 0.258/p < 0.001; Table 3). Note: a,b,c letters show significant differences between CAD/CAM ceramics within translucency level LT and resin composite cement color Light+ with regard to ceramics. a,b,c letters show significant differences between CAD/CAM ceramics within translucency level LT and resin composite cement color Warm+ with regard to CAD/CAM ceramics. A,B,C letters show significant differences between CAD/CAM ceramics within translucency level HT and resin composite cement color Light+ with regard to CAD/ CAM ceramics. A,B,C letters show significant differences between CAD/CAM ceramics within translucency level HT and resin composite cement color Light+ with regard to CAD/CAM ceramics. z,y letters show significant differences between translucency levels LT and HT with regard to resin composite cement. I,II, III letters show significant differences between resin composite cements with regard to translucency levels LT and HT.

| CIEDE2000: ΔE00 calculation-analysis within RCC shade
Within the Light+ shade, higher ΔE00 values were analyzed for the leucite ceramic LRF for translucency level HT, the leucite ceramic IPR for translucency level LT, and the lithium disilicate ceramic ABM for translucency level LT (p ≤ 0.038).
Within the Warm+ shade, higher values presented the leucite ceramic LRF for translucency level HT, the lithium disilicate ceramic IEM for both translucency levels LT/HT, and the lithium metasilicate ceramic NIC for translucency level LT (P ≤ 0.047; Table 3).

| CIE lab: ΔE calculation-analysis within translucency level
Within the translucency level LT, the lithium metasilicate ceramic CEL, the leucite ceramic LRF, and the lithium disilicate ceramic IEM pres-  Table 3).

| DISCUSSION
The null hypothesis stating that there is no impact of the type of ceramic, it's translucency level and the shade of the RCC upon the final color of the tested CAD/CAM ceramics could be rejected.
The reasons are discussed in the following sections.
In the present study, the choice of CAD/CAM ceramics generally showed the greatest impact on the results for both ΔE and should not be forgotten that mathematically determined shade differences are not necessarily visible to the naked eye, whereas several investigations stated that the CIEDE2000 formula provided results better fitting to the color differences perceived by the human eye than the CIELab formula. [30][31][32] The CIEDE2000 formula also revealed better indicators of human perceptibility and acceptability of color change. 31 It must be noted that when color differences between two different materials are detected by 50% of the observers, this corresponds to the 50:50% perceptibility threshold. If the color differences are considered acceptable by 50% of the observers, this corresponds to the 50:50% acceptability threshold.
Consequently, a perceptible color match or an acceptable color match is a color below the specific threshold, and several studies have already shown the differences in threshold values between ΔE and ΔE00. 12,33 These results can be confirmed in the present investigation. In the literature, the perceptibility level for ΔE was between 1.0 and 3.7 and the acceptability level between 2.7 and 6.8, whereas the level for ΔE00 was found to be 1.2 for the perceptibility level and 2.7 for the acceptability level, respectively. 12 the values were similar in tendency, but as in the case of the lithium alumina silicate ceramic they can also give contrary results, which might be caused by the significantly different calculation formula. Different outcomes of the two formulas have already been analyzed in the literature as mentioned before. 13,33 The CAD/CAM ceramics with highest values could achieve the greatest color changes in combination with the RCC shades. This means that minimal changes in the CAD/CAM ceramic shade could be achieved with the adhesive bonding procedure in order to find the optimum esthetic result.
In the present study the color measurements were mechanically performed using a spectrophotometer (Lambda 35), being a sensitive device that reacts to even the slightest inaccuracies as dust or dirt particles in the interior of the spectrophotometer leading to deviations in the measured values. The advantage of the spectrophotometer used, however, is the determination of the color values, which belong to a universally applicable and internationally used color system. For many years now, the analysis using the CIEDE2000 formula has been increasingly accepted. In the present study, therefore both formulas were used to better compare the analyzed values. Both color systems could be an obstacle for clinical application, since the values cannot be interpreted without consulting a color scale.
A limitation of the present study is the lack of clinical geometries, which may have led to changes in the results. A further limitation is the analysis of just one RCC from one manufacturer and just one thickness for all ceramic specimens; hence results only refer to this one product and ceramic thickness.

| CONCLUSION
Within the limitations of this study, following conclusions could be drawn: F I G U R E 4 Differences in light transmission of all tested computer-aided design and computer-aided manufacturing (CAD/CAM) ceramics with translucency levels LT at the top and HT in the bottom row in the following order: LRF, IPR, ABM, IEM, CEL, NIC, and VES • The choice of ceramic showed an impact on color change of the analyzed CAD/CAM ceramics (ΔE0 and ΔE00).
• The translucency level showed an impact on color change of the analyzed CAD/CAM ceramics (ΔE).
• The RCC shade (Warm+/Light+) showed an impact on color change of the analyzed CAD/CAM ceramics (ΔE00).

ACKNOWLEDGEMENTS AND DISCLOSURE
The authors would like to thank all companies for providing the materials analyzed. The authors do not have any financial interest in the companies whose materials are included in this article. This research did not receive any specific grant from funding agencies in the public, commercial, or not-profit sectors. Open access funding enabled and organized by Projekt DEAL.

DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.