Graded microstructure and mechanical properties of human crown dentin

The mineralized tissue of intertubular dentin is a collagen-mineral composite with considerable local variations of mechanical properties. Area scans of human coronal dentin were made by complementary methods to investigate correlations between local mechanical properties and the density, size, and crystallinity of the mineral particles. Scanning images from the same specimen were collected with Fourier-transform infrared microspectroscopy in reflectance mode (FTIR-RM), small angle X-ray scattering (SAXS), quantitative backscattered electron imaging (qBEI), and Nanoindentation in an atomic force microscope. The mineral content of dentin was found to decrease and the thickness of mineral crystals to increase towards the dentin-enamel junction (DEJ). Hardness and elastic modulus both decreased towards the DEJ. In a correlation analysis, the mineral content and, even more, the thickness of mineral crystals were found as the best predictors of hardness. The dentin layer close to the DEJ corresponds to a local minimum in hardness and elastic modulus, a configuration known to be an effective obstacle for crack propagation. Hence, the observed variations of mechanical and structural properties in an area between 0 and 1.5 mm below the DEJ define crown dentin as a gradient material optimized for its mechanical function.