The effect of ceramic thickness and number of firings on the color of two all-ceramic systems

Statement of problem. All-ceramic restorations have been advocated for superior esthetics, and various materials have been used to improve ceramic core strength, but there is a lack of information on how color is affected by different core substructures and fabrication procedures. Purpose. The purpose of this study was to evaluate the effects of various dentin ceramic thicknesses and repeated firings on the color of lithium disilicate glass-ceramic (IPS e.max Press) and zirconium-oxide (DC-Zirkon) all-ceramic systems. Material and methods. Thirty disc-shaped specimens, 4 mm in diameter with a 1-mm core thickness, and 0.5-, 1-, or 1.5-mm dentin ceramic thicknesses, were made from each of 2 ceramic systems (n = 10). Repeated firings (3, 5, 7, or 9) were performed, and the color of the specimens was compared with the color after the initial Firing. Color differences among ceramic specimens were measured using a spectro photometer (VITA Easyshade), and data were expressed in CIELAB system coordinates. Repeated measures ANOVA was used to analyze the data (number of firings, ceramic composition, and ceramic thickness) for significant differences. The Tukey HSD test and paired 2-tailed tests were used to perform multiple comparisons (alpha = .05). Results. L*a*b* values of the ceramic systems were affected by the number of firings (3, 5, 7, or 9 firings) (P < .001 ceramic composition (DC-Zirkon or IPS e.max Press) (P < .001), and ceramic thickness (0.5, 1, or 1.5 mm) (P < .001). Significant interactions were present in L*a*b* values between the number of firings and ceramic composition (P < .001) and between the number of firings and ceramic thickness (P < .001). Significant interactions were present between the number of Firings and ceramic thickness and ceramic composition (P < .05) in L* and b* values, but not for the a* value (P = .068). As the ceramic thickness increased, significant reductions in L* values (P < .01) were recorded for I PS e.max Press and DC-Zirkon specimens. For I PS e.max Press specimens, there were significant increases in a* and b* values (P < .01). For DC-Zirkon specimens, significant increases in a* values were observed; however, no significant differences were recorded for b* values. Conclusions. The analysis revealed that there were significant changes in L*a*b* color data as the number of firings increased, which resulted in perceptual color changes in L*a*b* color parameters.