Effect of Age-Induced Transparency on the Mechanical Properties of Human Dentin
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Effect of Age-Induced Transparency on the Mechanical Properties of Human Dentin Ravi K. Nalla1, John H. Kinney2, John A. Pople3, Thomas M. Breunig4, Antoni P. Tomsia1 and Robert O. Ritchie1 1 Lawrence Berkeley National Laboratory, Berkeley, CA 94720, U.S.A. 2 Lawrence Livermore National Laboratory, Livermore, CA 94550, U.S.A. 3 Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, Menlo Park, CA 94025, U.S.A. 4 Coating Place, Inc., Verona, WI 53593, U.S.A. ABSTRACT Most non-traumatic fractures occur in teeth that have been treated, for example restored or endodontically repaired. It is therefore essential to evaluate the structure and mechanical properties of altered forms of dentin. One such altered dentin is transparent (sclerotic) dentin, which forms gradually with aging. Accordingly, in the present study, we seek to study differences in the structure, i.e., dentinal mineral concentration, mineral crystallite size, and the mechanical properties, i.e., elastic moduli, fracture toughness and fatigue behavior, of normal and transparent root dentin. The mineral concentration, measured by x-ray computed tomography, was found to be significantly higher in transparent dentin, with the majority of the increase being due to the closure of the tubule lumens. Crystallite size, as measured by small angle x-ray scattering, appeared to be slightly reduced in transparent dentin, although the difference was not statistically significant. The elastic properties remained unchanged, although transparent dentin showed almost no yield/post-yield behavior. The fracture toughness was lowered by roughly 20%, while the fatigue resistance was deleteriously affected at high stress levels. These results are discussed in terms of the altered microstructure of transparent dentin.
INTRODUCTION Dentin, the major structural component of the tooth, is composed of a hydrated organic matrix consisting mostly of collagen and an inorganic carbonated apatite phase. Prominent features in the microstructure of dentin are tubules that radiate outward from the pulp to the dentin-enamel junction in coronal dentin, and from the pulp canal to the cementum in the root. In normal human dentin, the tubules are lined with a highly mineralized cuff of peritubular dentin [1]. Although the primary function of dentin is mechanical, it has only been in the last few years that its mechanical properties - the elastic properties, fracture resistance and cyclic fatigue behavior - have begun to be understood in terms of its hierarchical microstructure [2-7]. All these studies were conducted on healthy dentin; this has been important from the perspective of understanding the fundamental properties of dentin as its microstructure is sufficiently complicated without adding additional uncertainties from mineral variations (e.g., caries) or other age- or disease-related alterations. Nevertheless, many fractures occur in teeth that have been treated, e.g., restored or endodontically repaired. It is therefore essential to extend these previous s
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