Heterogeneous hardening of enamel surface by occlusal loading: Effect of nanofiber orientation

Human tooth enamel is composed of enamel rods and surrounding inter-rod enamel. As the fundamental block of enamel, hydroxyapatite (HAP) nanofibers are mostly longitudinally aligned in the rods but inclined in the inter-rod enamel. The surface hardening of enamel by occlusal loading is reportedly a result of hydroxyapatite nanofiber fragmentation and rearrangement and plays an important role in the anti-wear performance of enamel, but little is known about the effect of HAP nanofiber orientation on enamel surface hardening. In this study, the occlusal loading-induced surface hardening behaviors of enamel at different zones (rod and inter-rod) and different orientations (occlusal and axial) were investigated in vitro using impact treatment and a nanoindentation technique, aiming to reveal the effect of nanofiber orientation on enamel surface hardening. It was found that surface hardening by occlusal loading occurs in the rod and inter-rod areas, but the former shows a greater hardening degree than the latter, leading to an increase in the mechanical heterogeneity of enamel surface. Under the same loading condition, the HAP nanofibers in the inter-rod enamel are more likely to tilt into transverse nanofibers than those in the rods. Compared with longitudinal nanofibers, transverse nanofibers fragment into more transverse crystal particles, but the transverse particles impair the compactness of the hardening layer and decrease its hardening degree. In sum, inherent non-uniform nanofiber orientation endows the enamel with the ability to undergo heterogeneous surface hardening upon occlusal loading, which is critical for providing and maintaining its surface mechanical heterogeneity. These findings extend the understanding of the relationship between microstructure and mechanical properties of dental enamel and provide valuable insights into the bionic design of engineering materials.