Porcelain-veneered crowns are widely used in modern dentistry, and their fracture remains problematic, especially in all-ceramic systems. We hypothesized that substructure properties have a significant effect on the longevity of porcelain-veneered crowns. Flat porcelain/metal or porcelain/ceramic structures were cemented to dentin-like composite, and a mouth-motion cyclic load of 200 N was delivered by means of a tungsten carbide spherical indenter (r = 3.18 mm), emulating occlusal loading on crowns supported by dentin. Findings indicated that porcelain on a low-hardness gold-infiltrated alloy was vulnerable to both occlusal surface contact damage and porcelain lower surface radial fracture, while porcelain on a higher-hardness palladium-silver alloy fractured chiefly from occlusal surface damage. The advantage of a high-modulus metal substructure was less pronounced. Fracture in the porcelain/zirconia system was limited to surface damage in the veneer layer, similar to that in the porcelain/palladium-silver system. Bulk fracture, observed in veneered alumina layers, was not found for zirconia.
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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