A simple model of crack propagation in dental restorations

Although natural teeth often exhibit microcracks, they rarely demonstrate bulk fracture. However, conventional fullcrown restorations periodically exhibit failure due to fracture. Presented here is evaluation of a simple model of crack propagation that estimates crack growth during cyclic loading. A finite element model of a premolar tooth provides the tensile stresses adjacent to cusp loading. If the crack propagation rates for natural teeth, porcelain-fused-to-metal crowns and composite crowns are compared with the wear rates of their respective materials as determined in an artificial mouth, it is evident that the low wear rate of composites may predispose them to fracture. Natural teeth disperse occlusal stresses throughout the dentin so that the effect of high occlusal stress is minimized. Porcelain tends to wear the opposing dentition, which reduces areas of high occlusal stress. Composite, however, demonstrates crack propagation rates higher than those of either natural teeth or porcelain. This, in addition to its low wear rate, might predispose the material to fracture. This model should be used only as a qualitative indicator of fracture tendency. The high calculated crack propagation rates in composites may explain the observed clinical failures and microchipping at the area of occlusal contact, as noted in SEM analysis.