Structure-Property Relations and Crack Resistance at the Bovine Dentin-Enamel Junction

The present report is a study of the fracture behavior of the dentin-enamel complex, involving enamel, dentin, and the dentin-enamel junction (DEJ), that combines experimental design, computational finite element analysis, and fractography. Seven chevron-notched short-bar bovine DEJ specimens were utilized in this study. The general plane of the DEJ was approximately perpendicular to the fracture plane. All specimens were stored at 37°C and 100% relative humidity for 24 h prior to being tested. A fracture test set-up was designed for application of tensile load on the DEJ specimens to initiate a crack at the vertex of the chevron in the enamel, across the DEJ zone and into the bulk dentin. During fracture testing, a water chamber was used to avoid dehydration of the specimen. The results showed that the lower boundary value of the fracture toughness of the DEJ perpendicular to its own plane was 3.38 ± 0.40 MN/m^1.5 and 988.42 ± 231.39 J/m², in terms of K_IC and G_IC, respectively. In addition, there was an extensive plastic deformation (83 ± 12%) collateral to the fracture process at the DEJ zone. The fractography revealed that the deviation of the crack path involved an area which was approximately 50-100 μm deep. The parallel-oriented coarse collagen bundles with diameters of 1-5 μm at the DEJ zone may play a significant role in resisting the enamel crack. This reflects the fact, that in the intact tooth, the multiple full thickness cracks commonly found in enamel do not typically cause total failure of the tooth by crack extension into the dentin.