الفهرس 
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Abstract
Abstract Statement of problem : Different preparation designs and materials play an important role in the success of the endocrown restoration, but the studies that evaluated the effect of the designs and materials on the fracture resistance are inadequate. Purpose : This study evaluated the fracture resistance of endocrowns prepared with two preparation designs and manufactured with two CAD/CAM ceramic materials. Materials and Methods : Forty endodontically treated maxillary 1st molar teeth were prepared to receive endocrown restorations and divided into two main groups according to the preparation design: group B (n=20) endocrowns with butt joint design; group F (n=20) endocrowns with ferrule design. The main groups were subdivided into two subgroups according to the CAD/CAM material used: Subgroup E (n=10) endocrowns manufactured from lithium disilicate glass ceramic (LD); Subgroup C (n=10) endocrowns manufactured from hybrid nano-ceramic (HN). The endocrown restoration were cemented to the prepared teeth using dual cured self-adhesive resin cement. The specimens were subjected to thermal aging up to 10.000 cycles in thermocycling paths with 5oC and 55oC. All specimens were subjected to a compressive force until fracture occurs by using universal testing machine. The maximum compressive force is recorded for all specimens; then the failure modes were examined using scanning electron microscope. Results: The mean value of fracture resistance of BC group was statistically and significantly higher than the other tested groups (BC group, 4361.01±554.32 N) and the lowest mean value was recorded with FE group with (2693.04±638.69 N). B group showed significant higher fracture resistance than F group (B group, 3707.09±1156.06 N; F group, 2724.34±601.96 N). C subgroup showed significant higher fracture resistance compared to that of E subgroup (C subgroups, 3558.33±995.92 N; E subgroups, 2873.10±983.95 N). All groups showed a high rate of catastrophic failures but at loads greater than normal maximum masticatory forces. Conclusions : Butt joint design endocrowns exhibited significantly higher fracture resistance than ferrule design endocrowns. Hybrid nano-ceramic endocrowns showed higher fracture resistance than lithium disilicate endocrowns. Endocrown showed a high percent of irreparable failure mode but at loads larger than normal masticatory function.