الفهرس 
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Abstract
Purpose: The purpose of the present, in-vitro, study was to evaluate the effect of different ceramic types and thicknesses on the color parameters of monolithic lithium disilicate and monolithic zirconia. Materials and methods: Three specially designed molds were fabricated with same diameter and different thicknesses (1 mm, 1.5 mm and 2 mm) and used to yield30 ceramic discs. The specimens were divided into two equal groups(n=15) according to the type of ceramic systems: monolithic lithium disilicate glass-ceramic material using lost wax-and heat pressed technique and monolithic zirconia using CAD/CAM technology. Each group was subdivided into three equal subgroups (n=5) according to the ceramic thickness: (1 mm, 1.5 mm and 2 mm). All ceramic discs were cemented to composite background (shadeA3) using an adhesive resin cement. The colors of ceramic discs were measured with a spectrophotometer and the average values of the color differences (∆E) were calculated. The data were analyzed with One-way ANOVA for comparing color parameters for each material at different thickness, and Tukey’s test was used for pair-wise comparison between groups of different thickness. Student T-test was used for comparing color parameters between the two materials at each thickness. Results: Color change of this study showed thatthe highest mean value of ∆E was for e.max press at all thickness also, there were statistical significant difference between e.max press and zirconia. An increase in ceramic thickness resulted in a significant decrease in ∆E values (P<.01). Conclusions: The type of ceramic materials and its thickness has definite effect on the final color of tested materials. The color changes at (ΔE) of all specimens grouping were considered clinically acceptable except at 1mm thickness of e.max press (If ΔE <3.7 is regarded as a clinically acceptable color change).