Anisotropy of longitudinal wave velocity and hydroxyapatite orientation in bovine cortical bone

The anisotropy of longitudinal wave velocity and the hydroxyapatite (HAp) crystallite orientation in bovine cortical bone were experimentally investigated in detail. A ring-shaped cortical bone sample was obtained from the midshaft of a 32-month-old bovine femur. Four cylindrical specimens (diameter 11 mm) were taken from the ring-shaped cortical bone specimen along the radial direction. Velocity was measured by a conventional ultrasonic pulse system, by rotating the specimen in the axial-tangential plane. The direction of the fastest wave velocity observed was inclined 5-10° from the axial direction. The HAp crystallite orientation in the specimen was also investigated by X-ray diffraction pole-figure analysis. We observed a strong intensive peak of (0002), indicating HAp crystallite c-axis alignment along the bone axis. However, the pole-figure also revealed a small tilt in the HAp crystallite orientation from the axial direction. Comparing the velocity and X-ray diffraction peak at each angle of the axial-tangential plane, a significant correlation between the velocity and the HAp orientation was observed.