Biological organization of hydroxyapatite crystallites into a fibrous continuum toughens and controls anisotropy in human enamel

Enamel forms the outer surface of teeth, which are of complex shape and are loaded in a multitude of ways during function. Enamel has previously been assumed to be formed from discrete rods and to be markedly aniostropic, but marked anisotropy might be expected to lead to frequent fracture. Since fr equent fracture is not observed, we measured enamel organization using hist ology, imaging, and fracture mechanics modalities, and compared enamel with crystalline hydroxyapatite (Hap), its major component. Enamel was approxim ately three times tougher than geologic Hap, demonstrating the critical imp ortance of biological manufacturing. Only modest levels of enamel anisotrop y were discerned; rather, our measurements suggest that enamel is a composi te ceramic with the crystallites oriented in a complex three-dimensional co ntinuum. Geologic apatite crystals are much harder than enamel, suggesting that inclusion of biological contaminants, such as protein, influences the properties of enamel. Based on our findings, we propose a new structural mo del.