Three-Dimensional Anisotropy of Ultrasonic Wave Velocity in Bovine Cortical Bone: Effects of Hydroxyapatite Crystallites Orientation and Microstructure

The three-dimensional anisotropy of longitudinal wave velocity and the hydroxyapatite (HAp) crystallites orientation in bovine cortical bone were experimentally investigated in detail. Bovine cortical bone has two typical microstructures, plexiform and haversian. Two spherical specimens (diameter: 9 mm) were obtained from the anterior (plexiform) and posterior (haversian) parts of a 30-month-old bovine femur. The three-dimensional anisotropy of longitudinal wave velocity was measured using a conventional ultrasonic pulse system by rotating the spherical specimen in the axial--tangential (A--T), axial--radial (A--R), and radial--tangential (R--T) planes. The velocity clearly changed depending on the propagation direction in all the planes. In the A--T and A--R planes, the direction of the highest velocity was slightly inclined from the bone axis direction. Moreover, the results from the X-ray pole figure analysis indicated that there were small tilts in the HAp crystallites orientation. The tilts were similar to those of the highest velocity direction and there were good correlations between velocity and HAp crystallites orientation. However, a comparatively low correlation was found in the posterior part, which shows the stronger effects of bone microstructure. On the other hand, in the R--T plane, where small HAp crystallites oriented, a weak velocity anisotropy was found owing to the bone microstructure. Ultrasonic wave velocities depended on both HAp crystallites orientation and microstructure. The degrees of contribution of these factors varied owing to the position and propagation direction.