الفهرس 
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Abstract
In vitro study was designed to evaluate the masking ability and translucency of two ceramic materials with different thicknesses when used as monolithic restoration. A total of square shaped thirty-six all ceramic specimen were constructed with dimensions of 10*10 with three different thicknesses of 0.5, 0.9 and 1.2 for the two tested materials. Eighteen specimens of lithium disilicate and eighteen specimens of resinous hybrid ceramic divided into 3 groups according to thickness so each subgroup consist of six specimens. Sectioning of the specimens was carried out using Buehler ISOMET 4000 Linear Precision Saw and polished for further standardization of surface characters. Outer surface of specimens was glazed for lithium disilicate specimens and polished for resinous hybrid ceramic specimens according to manufacture instructions, while inner surface of both materials was hydrofluoric acid etched and silanated. Black and white tiles used to represent dark and light backgrounds respectively, tooth shaded composite dies of dimensions 10*10*5 was constructed using vacuum formed sheet for assessment of masking ability of different materials and different thicknesses against darker tooth structure.
The colour coordinates (CIE L*a*b*) were measured Using spectral reflectance spectrophotometer (Cary 5000 Agilent Technologies (USA), First measurement was carried out to asses translucency parameter (TP) of all specimens Calculated by using spectrophotometer to determine CIE LAB value over Black and White tiles
TP={(Lb-Lw)2 + (ab-aw)2 +(bb-bw)2 }
Second measurement aimed to measure the contrast ratio (CR) of thirty-six specimens calculated by measuring Reflectance Value of specimens over Black and White tiles (figure 26).
CR = Reflectance value over black/ Reflectance value over white
Third measurement aimed to measure Masking power of the specimens through measuring Shade Difference when specimens placed against 4 tooth shaded composite resin die (A2, A3.5, A4, A4) then CIE LAB value obtained by spectrophotometer was calculated and the difference between L*, a* and b* for control group and comparison groups will be expressed by single value ΔE* which is calculated using formula:
ΔE*={(L*1-L*ref)2 + (a*1-a*ref)2 +(b*1-b*ref)2 }0.5
Two-way ANOVA was used to study the effect of different tested materials and thicknesses and their interaction. Comparison of main and simple effects were done utilizing pairwise t-tests with bonferroni correction. Spearman rank order correlation coefficient was used to study the correlation between the two different materials in different groups and translucency parameter. The results showed that TP values of Lithium disilicate (Dental Lithium Disilicate) Showed higher values ranged from 14.35 to 25.69 when compared with Resinous hybrid ceramic (Hyramic) which showed TP values ranged from 12.33 to 20.32. translucency increases with decrease in ceramic thickness for both materials. Lithium disilicate (Dental Lithium Disilicate) Showed higher (ΔE) values ranged from 1.34 to 0.64 when compared to Resinous hybrid ceramic (Hyramic) which showed ΔE values ranged from 1.13 to 0.48.
L* values decreased when thickness increased, while both a* and b* values was observed to be increased with increasing ceramic thickness.