Anisotropic fracture in bovine root and coronal dentin.

OBJECTIVES: This purpose of this study was to address the fundamental question of how the fracture properties of dentin are related to its composite structure.
METHODS: Strain concentration tests and impression-induced damage tests were designed to compare bovine root dentin with coronal dentin, and to understand the role of individual structural elements in the fracture of dentin.
RESULTS: Absent in peritubular dentin, root dentin is insensitive to cracks and exhibits higher fracture resistance than coronal dentin that has a typical brittle fracture behavior along the peritubular dentin. Fracture analysis and impression damage experiments found that root dentin is highly anisotropic in fracture behavior. Cracking is predominantly controlled by the organization of collagen fibrils, with the incremental lines being the weakest planes. In coronal dentin, highly mineralized peritubular dentin that intersects with the incremental lines creates additional weak orientations that compete with the incremental lines and thus greatly decrease the degree of fracture anisotropy. SIGNIFICANCE: This study demonstrated that dentin is by no means homogeneous in terms of fracture properties. Location and orientation (especially in terms of incremental lines) should be taken into account when examining tooth failure both in laboratory and in clinical studies.