Fracture in enamel is anisotropic with respect ot the orientation of the enamel rods, with Wf for fracture parallel to the rods being 0.3 X 10(2) J/meter2 but on the order of 2.0 X 10(2) J/meter2 for fracture perpendicular to the rods. Fractographs of enamel showed that the enamel rods behaved as integral units during controlled fracture. A model was proposed to explain the fracture properties of enamel involving the assumption that the hydroxyapatite crystals did not fracture but that their orientation determined the fracture properties. Dentin was also anisotropic, with fracture parallel to the dentinal tubules being the strong direction - Wf was 5.5 X 10(2) J/meter2 for fracture parallel to the tubules, but 2.7 X 10(2) J/meter2 for fracture perpendicular to the tubules. Wf for dentin was of the same magnitude as the high strain rate Wf found for bone (2.0 X 10(2) J/meter2 [ref 3]). The data obtained in this investigation indicated that dentin and enamel are brittle substances and that forces occasionally imposed during mastication or bruxism probably are capable of initiating fracture when the normal anatomy of the tooth has been altered by cavity preparation. As consequence, the design of cavity preparations should include considerations to reduce stress concentrations and thus improve the practical fracture strength of tooth structure.

SEM fractography and work-of-fracture techniques were used to investigate the fracture properties of human enamel and dentin as a function of the temperature of an aqueous environment. Both enamel and dentin were found to be anisotropic with respect to mechanical failure but were found to have little variation in fracture properties with respect to temperatures from 0 degree to 70 degrees C. Fractographs of a naturally fractured tooth were consistent with experimental findings. The results of this research indicate that natural fracturing of teeth probably cannot be explained on the basis of a reduced work of fracture resulting from chewing hot or cold foods. It was also shown that if dentin surfaces can be kept below 100 degrees C during cavity preparation, the restored tooth will not be weakened by an irreversible reduction of its work of fracture.