Biological Organization of Hydroxyapatite Crystallites into a Fibrous Continuum Toughens and Controls Anisotropy in Human Enamel

• Published in: Journal of dental research Vol. 80; no. 1; pp. 321 - 326
• Main Authors: White, S.N., Luo, W., Paine, M.L., Fong, H., Sarikaya, M., Snead, M.L.
• Format: Journal Article
• Language: English
• Published: Los Angeles, CA SAGE Publications 01.01.2001; SAGE PUBLICATIONS, INC
• Subjects: Anisotropy; Biomechanical Phenomena; Crystallization; Dental Enamel - anatomy & histology; Dental Enamel - chemistry; Dental Stress Analysis; Dentistry; Durapatite - chemistry; Elasticity; Hardness; Humans; Microscopy, Electron, Scanning; Models, Structural; enamel; microstructure; toughness; tooth; organization
• ISSN: 0022-0345; 1544-0591
• DOI: 10.1177/00220345010800010501

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 frequent fracture is not observed, we measured enamel organization using histology, imaging, and fracture mechanics modalities, and compared enamel with crystalline hydroxyapatite (Hap), its major component. Enamel was approximately three times tougher than geologic Hap, demonstrating the critical importance of biological manufacturing. Only modest levels of enamel anisotropy were discerned; rather, our measurements suggest that enamel is a composite ceramic with the crystallites oriented in a complex three-dimensional continuum. 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 model.