Ceramic restorations, whether monolithic (single layer) or porcelain veneered, often chip and fracture from repeated occlusal loading. Occlusion involves the opposing tooth sliding along the cuspal incline surface with an applied biting force (off-axis loading). We hypothesize that off-axis contact–load–slide–liftoff fatigue as compared to normal axial fatigue loading produces different fracture modes and fatigue lifespan of layered ceramics. Monolithic glass plates were epoxy bonded to polycarbonate substrates as a transparent model for an all-ceramic crown on dentin. Off-axis and axial (control) cyclic loading was applied through a hard sphere in water with a mouth-motion machine. The off-axis loading is more deleterious for contact-induced occlusal surface fracture, but less harmful for flexure-induced cementation surface fracture of brittle layers than the axial loading. This is because of the tangential load component associated with the off-axis loading. Clinical relevance is discussed.
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