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

'Center for Craniofacial Molecular Biology, University of Southern California School of Dentistry, CSA 103, 2250 Alcazar Street, Los Angeles, CA 90033, School of Dentistry, UCLA, Los Angeles, CA 90095, snwhite{at}ucla.edu

W. Luo

'Center for Craniofacial Molecular Biology, University of Southern California School of Dentistry, CSA 103, 2250 Alcazar Street, Los Angeles, CA 90033

M.L. Paine

'Center for Craniofacial Molecular Biology, University of Southern California School of Dentistry, CSA 103, 2250 Alcazar Street, Los Angeles, CA 90033

H. Fong

Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195

M. Sarikaya

Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195

M.L. Snead

'Center for Craniofacial Molecular Biology, University of Southern California School of Dentistry, CSA 103, 2250 Alcazar Street, Los Angeles, CA 90033

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.

Key Words: tooth • enamel • microstructure • organization • toughness.

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Journal of Dental Research, Vol. 80, No. 1, 321-326 (2001)
DOI: 10.1177/00220345010800010501


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