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Abstract This study was designed to evaluate the effect of different preparation designs including non anatomical and anatomical with different bonding substrate on fracture resistance of poymer infiltrated ceramic occlusal veneer restorations. Twenty eight extracted human mandibular molars without caries or visible fracture lines were selected based on the teeth having similar bucco-lingual (BL) and mesio-distal(MD) dimensions. The selected teeth were divided into 2 main groups according to the occlusal preparation designs and then each group of teeth was subdivided into 2 subgroups according to the different bonding substrate. For all teeth, the CNC milling machine was adjust to reduce fourteen teeth with 90º butt margin (flat) and other fourteen follow the occlusal anatomy of the teeth with an angle 150º between cusps. Occlusal veneers were designed by CAD software Exocad from VITA ENAMIC blocks. The restorations were checked and polished. Surface treatment of the restorations was performed with hydrofluoric acid and sialne before cementation with resin cement. Surface treatment of the teeth was performed by etching then bonding agent. Thermal cycling was applied and all samples were loaded in a universal testing machine until fracture occurred. The fractured specimens and fragments were collected and mode of fracture were examined visually and under stereomicroscope at magnification (6x-8x) to determine different fracture modes. Two Way ANOVA test was used to examine the effect of different occlusal preparation desgins and different bonding substrates on the fracture resistance of Vita Enamic occlusal veneers. The result revealed that the fracture resistance of non anatomical preparation designs was significantly higher than the anatomical preparation designs. The result showed that the fracture resistance of enamel bonding substrate was significantly higher than dentin bonding substrate with enamel peripheral. Evaluation of the failure modes revealed non significant difference between the modes of failure of tested groups. |