On crack growth in molar teeth from contact on the inclined occlusal surface

Extracted human molar teeth are indented by hard balls laid at the central fossa, sectioned, and their interior examined for damage. Contact on the fissured enamel coat generally occurs on three distinct spots. The main forms of damage are radial cracks growing from the DEJ to the occlusal surface and median radial and cylindrical cracks growing from a contact spot to the DEJ. For large balls failure by edge chipping near a cusp apex may occur. The median cracks tend to run unstably to the DEJ upon reaching the middle part of the enamel coat. The corresponding load, P-FM, and the load needed to initiate radial cracks at the DEJ, P-FR, are taken to signal crown failure. The mean values of P-FM and P-FR are on the order of 1000 N. A conical bilayer model defined by thickness d, inclination angle theta, failure stress sigma(F) and toughness K-C of the enamel coat is developed to assess crown failure. The analytical predictions for PFR and P-FM agree well with the tests. The results indicate that enamel thickness is so designed as to ensure that P-FR and P-FM just exceed the maximum bite force under normal conditions while the choice of theta seems to reflect a compromise between needs to resist crown failure and break hard food particles. Both P-FR and P-FM are greatly reduced with reducing d, which points to the danger posed by tooth wear. The analytical expressions for P-FR and P-FM may also apply to other multi-cusp mammalian or prosthetic molar crowns. Cone cracking, suppressed in the anisotropic tooth enamel, may be an important failure mode in prosthetic crowns. (C) 2014 Elsevier Ltd. All rights reserved.