الفهرس 
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Abstract
Aim: The aim of this study was to determine the influence of different preparation designs on
fracture resistance of CAD / CAM lithium disilicate occlusal veneers.
Methodology: A Mandibular first molar of a typodont model was prepared to receive glass ceramic
(lithium disilicate) occlusal veneer to manufacture 27 occlusal veneers. The occlusal veneers were
separated into three groups (n=9) based on the preparation design: group A: (chamfer finish line
preparation), group B: inclined plane (planar design), group C:(flat occlusal surface preparation).
The prepared typodont tooth was scanned then occlusal veneers of the three groups were designed
and milled using the CAD/CAM Cerec In Lab SW 4.2.5 System, then checked for seating under
magnification. The 27 occlusal veneers were bonded by dual cure resin according to the
manufacturer instructions’ to their corresponding epoxy resin dies which were duplicated from the
prepared typodont tooth using rubber base duplicating material to obtain 27 epoxy resin dies. Then
by using a universal testing machine samples were loaded along the long axis of the teeth until they
fractured and the maximum breaking load was recorded. The load required to fracture was recorded
in Newton and tabulated to be statistically analyzed. The tested samples were then visually
inspected under magnification to detect the mode of failure.
Results: Comparison between the 3 groups revealed significant difference between different groups
(p=0.003). The highest value was found in chamfer finish line preparation (1378.67±184.22),
followed by inclined planes (1164.70±253.79), while the lowest value was found in flat preparation
(1026.34±135.11).
Conclusions: It was concluded that (1) occlusal veneers with chamfer finish line have superior
fracture resistance than the other two occlusal veneer groups. (2) All the tested occlusal veneers
revealed fracture load values that exceeded the maximum occlusal force.(3)Most of the fractured
occlusal veneers showed favorable failure mode pattern.
Keywords: Fracture Resistance,Cad /Cam,occlusal veneers,Lithium Disilicate, preparation designs.