The hierarchical nature of mammalian tooth enamel resilience, and a novel measure of decussational strength in human molars.

The resilience of mammalian tooth enamel to fatigue and fracture stems from its anisotropic structure, which is manifest at various hierarchical levels from the nanoscale to the macroscale. At the nanoscale level, the alignment trajectories of thin hydroxyapatite nanocrystals and the organic matrix serve to convey anisotropy and deformability. Various prism packing configurations provide additional structural heterogeneity at the microscale level, while at the mesoscale, the angles at which parazonal and diazonal prisms decussate furnish yet more structural incongruity. At the macroscale level, Hunter-Schreger band (HSB) packing patterns appear to conform to masticatory loading mechanics. This study undertakes a quantitative evaluation of the strength of enamel decussation at the mesoscale in human permanent molars by measuring the degree to which prism pitch angle deviates in adjacent diazones and parazones using scanning electron microscopy. While HSB packing tends to be denser in "functional" cusps, decussational strength tends to be greater in "guiding" cusps, and whereas HSB packing tends to be denser in maxillary molars, decussational strength tends to be greater in mandibular molars. The general lack of concordance between measures of decussation at the mesoscale and macroscale levels may suggest that decussational strength may serve to compensate for lower HSB packing densities in human permanent molars.