Project 9 is the Biomechanics of Dentin. The mechanical properties of dentin are of great importance in all aspects of preventive and restorative dentistry. In spite of this importance, the mechanical properties are still ill-determined: there is a three-fold uncertainty in the Young?s modulus, and two-fold discrepancies in the reported measures of strength are not uncommon. Also, surprisingly few studies have adopted a fracture mechanics approach to dentin, even though a critical review of the dental literature would suggest that flaws control the mechanical properties of dentin, and that a fracture mechanics approach is better suited than a strength of materials approach. The proposed research seeks to resolve the contradictions surrounding the mechanical properties of dentin by firmly establishing the elastic properties, and by extending the theories of composite fracture mechanics to dentin failure. The proposed research uses the techniques of resonant ultrasound microscopy, lateral atomic force microscopy, fracture mechanics, high-resolution synchrontron x-ray tomographic microscopy, and finite-element modeling. Resonant ultrasound spectroscopy, which has never been applied to mineralized tissues, will allow unambiguous measurement of all of the elastic constants of normal and altered forms of dentin for the first time. With these elastic properties accurately determined, the experimental validity of linear elastic fracture mechanics will be established, and the orientation and site dependence of the fracture toughness will be measured. These data will be combined with high-resolution, three dimensional images of the tooth to provide a physically valid finite-element model for studying the effects of dentin pathologies and restorative procedures on tooth strength.
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