Elastic modulus of dental enamel: effect of enamel prism orientation and mineral content

Abstract

Nanomechanical properties of mineralized tissues are determined by microstructural tissue organization and relative composition of organic, mineral, and water phases. We combine nanoindentation and quantitative backscattered electron (qBSE) imaging to understand how these factors influence material properties at the ultrastructural and tissue level. Developmental stages of equine tooth enamel provide a good situation for studying the effects of enamel prism orientation and mineralization. Equine cheek teeth continue to form new enamel whilst mature tissue is being worn away. Hence we can study graded mineral content that increases with enamel maturity. Hunter-Schreger bands (HSBs) containing enamel prisms of contrasting orientations allow investigation of the effect of orientation on modulus. An equine second mandibular molar (2-year-old) was embedded in poly-methylmethacrylate, sectioned longitudinally, polished, carbon coated, imaged in qBSE, and subjected to nanoindentation testing. Elastic modulus increases significantly during maturation as enamel protein matrix degradation makes space for increased mineral content with closer proximity of enamel crystals, finally reaching values exceeding 80 GPa for little further increase in mineral content. Indentations spanning HSBs in immature, poorly mineralized enamel yielded modulus values that varied with prism orientation in a sinusoidal pattern where modulus varies by as much as a factor of three. Nanoindentation helps to elucidate how the composition and ultrastructural organization of enamel contribute to mechanical properties.