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.

Surface Hardening Behavior of Enamel by Masticatory Loading: Occurrence Mechanism and Antiwear Effect.

Previous studies have suggested that surface hardening occurs in human tooth enamel under certain loading conditions. However, the occurrence mechanism and significance remain unclear. In this study, the surface hardening behavior of enamel under masticatory loading was studied in vitro using impact treatment and the nanoindentation/scratch technique to identify the mechanism and antiwear effect. The fundamental block of enamel is made of hydroxyapatite (HAP) nanofibers, which consist of fine nanoparticles held together by protein. These fibers respond to masticatory loading in two ways: bending deflection at low loads and fragmentation at high loads. When the contact pressure exceeds the bonding strength between the nanoparticles, the HAP fibers split into fine nanoparticles and then form a surface layer consisting of tightly packed nanoparticles. This results in surface hardening dominated by an increased hardness and elastic modulus. The maximum degree and depth of surface hardening were determined as approximately 60% and 100 nm, respectively. With the occurrence of surface hardening, the wear resistance of the enamel is enhanced, which is manifested by a reduced friction coefficient and wear volume. In summary, the surface hardening of enamel induced by masticatory loading is a result of HAP nanoparticle rearrangement as a response of the enamel hierarchical structure to high chewing loads. It is adaptive overload protection derived from the enamel hierarchical structure and plays a critical role in resisting excessive wear induced by high chewing loads.

Research of the role of microstructure in the wear mechanism of canine and bovine enamel.

The relationship between the microstructure and tribological behavior of mammalian tooth enamel has not been fully understood. In this paper, the microstructure, mechanical properties, and tribological behavior of canine (carnivore) and bovine (herbivore) enamel are studied using scanning electronic microscopy and nano-indentation/scratch technique, aiming to reveal the contribution of enamel microstructure to its mechanical and tribological properties. Canine enamel has a microstructure of hard keyhole-like rods embedded in soft inter-rod enamel, and its surface exhibits high resistance against both micro-crack initiation and crack-induced delamination during friction and wear process. Bovine enamel with the microstructure consisting of the hydroxyapatite (HAP) nano-fibers in decussation has higher surface hardness and better capabilities of resisting wear and encumbering crack propagation, as compared to canine enamel. In sum, the soft inter-rod enamel in the canine enamel contributes to high load tolerance and then protects enamel surface from brittle damage, while the staggered arrangement of HAP nano-fibers benefits hard bovine enamel in crack propagation resistance and then help resist wear and fatigue. The findings suggest that there exists a self-adaptation in enamel microstructure and tribological performance of mammals with their feeding habits, which will promote and assist the bionic design of high-performance materials.