الفهرس 
Title : Effect of type and thickness of machinable ceramics on color masking of external and internal enamel discolorations
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Abstract
This in vitro study was carried out to assess the masking ability of two different veneering materials of two thickness to conceal the extrinsic and intrinsic discoloration. Non-carious 40 sound bovine incisors were selected for the study. The labial surfaces of teeth were flattened within the enamel surface. The teeth were divided to two equal groups; each of 20 teeth according to the type of discoloration whether extrinsic or intrinsic. Each group was subdivided to two equal groups according to type of material used [IPS Empress (pure ceramic) and Lava ultimate (hybrid resin material)]. Further division of two groups (5 teeth each) was done according to the thickness of the veneering material used (0.6 mm and 1 mm). The discoloration of all groups were performed using tea solution. Extrinsic stained teeth with closed apex were immersed in tea solution for 6 days. While for intrinsic discoloration, the crown portion was sectioned mesio-distally to access the pulp chamber, then teeth were immersed in tea solution for 24 hrs. The veneers were cemented using a translucent resin cement. Using the spectrophotometer, the color of the teeth was recorded at baseline as control, after teeth staining and after the veneers cementation. The color difference (xE) was also calculated. Statistical analysis was performed using t-test to detect the difference between the {u2206}E after discoloration and after veneering. The results of this study showed that there was no significant difference between the materials in color masking for both extrinsic and intrinsic discoloration, while significant difference was monitored regarding the veneers thickness, where it was found that the thicker veneers produced more color shift from the discoloration than the thinner ones