Gradient structural anisotropy of dental enamel is optimized for enhanced mechanical behaviour

Exceptional mechanical performance of dental enamel in the harsh environment of oral cavity can be preserved on decade timescale, which is a unique property in comparison with functional nanomaterials. Hierarchic architecture of enamel, based on site-specific structural organization of apatite nanocrystals has a key role in this durability.In the present study, a novel SEM imaging based method is presented for obtaining quantitative information on enamel prism orientation in sound primary dental enamel. This missing puzzle of quantification of the hierarchical enamel structure, along with spatial mechanical and chemical mapping, shed light on the optimum anisotropic gradient behaviour of elastic modulus of dental enamel.Specifically, orientation and composition dependent contributions in both the spatially changing hardness and elastic modulus were separated. Anisotropy of the enamel's modulus was predicted and verified by the spatial variation of average prism orientation. Based on our results we conclude that the anisotropy of modulus for the bulk enamel arises from the elastic gradient in direction normal to the enamel external surface combined with the nearly constant value of modulus in the perpendicular cross section. This behaviour results in high surface strength and additionally can be responsible to the superior durability of human enamel.