الفهرس 
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Abstract
Purpose: The purpose of this study was to evaluate and compare the fracture resistance
and failure mode of occlusal veneers constructed from lithium disilicate (Rosetta) and hybrid
ceramic (VITA Enamic) materials with two preparation designs.
Materials and Methods: Two mandibular first molars of a typodont were chosen to
perform the master preparations; one for the Conventional Planar Occlusal veneer design and
the second for the modified design, which consisted of occlusal veneer preparation with a 1
mm circumferential chamfer finish line. The preparations were duplicated into twenty epoxy
resin dies over which the occlusal veneers were adhesively cemented using a standard bonding
protocol. Each group of epoxy resin dies was subdivided randomly into two equal subgroups
according to the ceramic material to be tested; lithium disilicate ”Rosetta ” and the hybrid
ceramic ”VITA ENAMIC”. All the occlusal veneers were fabricated using the CAD/CAM
Cerec Soft Wear 5.2 System. Then the fracture resistance and failure mode of occlusal veneers
were evaluated.
Results: The results showed that the CAD/CAM restorative materials used in this study
had no statistically significant effect on mean fracture resistance regardless of preparation
design. Additionally, preparation design had no statistically significant effect on mean fracture
resistance regardless of the type of material used. Regarding the mode of failure, the
comparison between different groups (group 1 and group II) , demonstrated that 100% of
specimens made from hybrid ceramic (VITA ENAMIC) in both groups showed a repairable
mode of failure (Mode 1), while 40% of specimens made from lithium disilicate (Rosetta) with
a conventional design (group 1, subgroup A) and 17% of specimens made of lithium disilicate (Rosetta) with a modified design (group II, subgroup A) showed a repairable mode of failure
(Mode 1). 20% of specimens made from lithium disilicate (Rosetta) with a conventional design
(group 1, subgroup A), and 33% of specimens made from lithium disilicate (Rosetta) with a
modified design (group II, subgroup A) showed a catastrophic failure (Mode 3).
Conclusion: All the tested CAD/CAM materials showed fracture loads above the
recommended minimum fracture strength for posterior restorations with different preparation
designs. Hybrid ceramic (VITAENAMIC) occlusal veneer can be the first choice in cases with
increased occlusal stresses as its failure mode was favorable than lithium disilicate (Rosetta).