Orientation of bone mineral and its role in the anisotropic mechanical properties of bone--transverse anisotropy.

An orientation of hydroxyapatite (HAP) crystals in bovine femur mineral was investigated by means of X-ray pole figure analysis (XPFA). It was found that the c-axis of HAP generally orients parallel to the longitudinal axis of bone (bone axis) and a significant amount of c-axis was oriented in other directions, in particular, perpendicular to the bone axis. Comparing these results with those of the small angle X-ray scattering (SAXS) investigation by Matsushima et al. (Jap. J. appl. Phys. 21, 186-189, 1982) at least two types of morphology of bone mineral were found; rod like bone mineral having the c-axis of HAP crystal parallel to the longitudinal axis of the rod and that having the c-axis not parallel, in a particular case, perpendicular to its longitudinal axis. Transverse anisotropy in mechanical properties of bone was reproduced by the estimation of Young's moduli by using the structural results mainly from XPFA. It is concluded that the anisotropy in mechanical properties of bone is well explained by taking account of the non-longitudinal (off-bone) axial distribution of orientation of bone mineral.