On the structure-property relationship of sound and hypomineralized enamel.

Developmental defects in dental enamel pose significant clinical challenges which have highlighted our limited understanding of the structure and properties of this tissue. In this study, we first investigated the contact-size dependence of the physical properties of sound and hypomineralized enamel, and then examined the microstructure to establish a structural basis for their differing properties. Depth-sensing indentation tests were carried out over a wide range of peak loads in a direction perpendicular to the enamel prisms. Hypomineralized enamel demonstrated stronger penetration dependence for measured hardness and elastic modulus than sound enamel. The microstructure of sound and hypomineralized enamel was observed using field emission scanning electron microscopy and transmission electron microscopy with support of a focused ion beam milling system. Images of sound enamel showed barely distinguishable sheath regions with minimal organic presence. In contrast, hypomineralized enamel showed thicker sheath structures surrounding the prisms and higher levels of organic content within both the prisms and the sheath regions. It is argued that the higher organic content within prism structure was responsible for an initial lower hardness and elastic modulus of hypomineralized enamel under low-load indentation. As the indentation depth increased, the thicker organic-rich sheath regions played a more important role in reducing the mechanical properties of the hypomineralized enamel. On the basis of Spears finite element model [Spears IR. A three-dimensional finite element model of prismatic enamel: a re-appraisal of the data on the Young's modulus of enamel. J Dental Res 1997; 76(10):1690-97], elastic moduli of sound and hypomineralized enamel were predicted, which matched experimental results.