Quantitative characterization of indentation initiated crack growth in bovine enamel.

In this in vitro study, indentation cracks in orthogonal directions and different areas on bovine enamel occlusal surface were analyzed by quantitative characterization of crack number, length and displacement, aiming to reveal the correlation between the microstructure and crack growth behavior of bovine enamel. Results showed that the cracks induced by indenting on the enamel occlusal surface tend to initiate at the rod/inter-rod boundaries. The rod/inter-rod hydroxyapatite (HAP) nanofibers and associated decussation cause a preferential extension of the cracks along the rod/inter-rod interface by inducing crack deflection, bifurcation, and bridging. In addition, the inter-rod nano-structure, consisting of orderly assembled HAP nanofibers, leads to an additional toughening mechanism of zig-zag cracking to hinder crack growth within the narrow inter-rod region. In summary, the unique microstructural architecture of bovine enamel, especially the decussation within rod/inter-rod nanofibers, plays an important role in functionally guiding cracks on bovine enamel occlusal surface to grow along the interface between rod and inter-rod rather than across the inter-rod enamel. The anisotropic crack growth behavior helps prevent bovine enamel from substantial fracture and chipping induced wear. These findings extend the understanding of the toughening mechanisms of mammalian enamel.