The luting agents were applied and pressed onto the inner (non-glazed) surface between the ceramic and a glass slide using a micrometer (Mitutoyo, Neuss, Germany), producing a cement thickness of 0.2 mm. Color evaluation The evaluation of the color parameters was determined using the CIE Lab system of colors using a colorimeter (Color-Guide?, BYK-Gardner, meanwhile Columbia, MD, USA). In the CIE Lab system, the place of a color in the space is defined in terms of 3 coordinates: L*, a*, and b*. The L* coordinate is a measure of the lightness-darkness of the specimen, on a numerical scale from 0 (black) to 100 (white). The measure of the total difference of color between 2 objects is described by ��E. Color differences above 3.7 units are visually detected. The formula used to calculate the ��E was: ��Eab* = [(��L *)2 + (��a *)2 + (��b *)2]1/2.

The ��E values were graded as follows: ��E < 1 = not appreciable, ��E < 2 = clinically acceptable, ��E > 2 = clinically unacceptable, ��E > 3.7 = clinically unacceptable with very poor match.17,21,22 The L*a*b* color notation for each specimen was measured 3 times consecutively, and an average was calculated to give the initial color of all substrata. A ��E >3.7 was considered to be clinically unacceptable. Statistical analysis Statistical analyses were performed employing a significance level fixed at 5%. The null hypotheses assumed no differences among ceramic thickness and luting agents. Data that violated the assumptions of equality of variances and normal distribution of errors were transformed and analyzed by three-way ANOVA, followed by the Tukey test.

The influence of the different ceramic thicknesses, luting agents, and their interaction on the final color of the ceramic veneers were analyzed. RESULTS Three-way ANOVA showed that the mean values of L*, a*, and b* were often significantly influenced by the ceramic brand, luting agent, thickness, and interaction terms of the 3 variables (Table 1, P<.001). For all the ceramics tested, in general, the thickness of 1.5 and 2 mm resulted in higher a*, b* and L* values when compared with the 1 mm thickness, regardless of the luting agent applied (Figures 1, ,22 and and3).3). The use of an opaque cement resulted in higher a*, b*, and L* values for all thicknesses and ceramics tested. Figure 1. a* values. Figure 2. b* values. Figure 3. L* values. Table 1.

P-values of 3-way ANOVA in accordance to with color parameters. Lower a* values were obtained in the 1 mm thickness ceramics without the opaque, while the use of the opaque cement with 1.5 and 2 mm thicknesses resulted in higher a* values (P<.05). No difference occurred between the 1.5 Drug_discovery and 2 mm thickness when the opaque cement was applied (Figure 1). Following the same trend as the a* values, higher b* values (P<.001) were obtained when the opaque was applied, except for the ceramics Vintage Halo and Vision Esthetic, which did not result in differences in b* values (P>.05).