Method: Clinically failed dental restorations, including bilayer zirconia crowns, galvano-ceramic crowns and PFM crowns, were collected and characterized by failure analysis to define the crack paths and to disclose the fracture origins. A general practice for fractography failure analysis of advanced ceramics was applied, examined by an optical microscope and field emission SEM.
Result: Four types of fracture features were defined in veneer porcelains, being worn, Hertzian cone crack, chipping, and delamination, respectively. The wear occurred on the porcelain surface in all four kinds of restorations. The Hertzian cone crack, that can be ascribed to fatigue damage under repetitive compressive bite load, resulted in porcelain chipping-off associated with thicker porcelains or porcelain delamination associated with thinner porcelains. The initial formation of micro-cracks on the core/porcelain interface initiated by a momentary high tensile stress, whereas its extension can be accelerated both under tensile and compressive stresses.
Conclusion: Porcelain fracture and delamination occurs when tensile stress concentration at porcelain interface due to the distortion of core material. This kind of core distortion happens more easily when metallic materials with lower elastic modulus or ceramic materials with lower elastic modulus but high fracture strength are used to make cores. The fracture features of veneer porcelain on bilayered restorations are related to the core materials and the ratio of porcelain/core thickness.