Anisotropy of longitudinal wave velocity in spherically shaped bovine cortical bone

Quantitative ultrasound (QUS) is a good method for measuring elastic properties of bone in vivo, because the obtained ultrasound wave properties directly reflect the longitudinal elasticity. Bone tissue is composed of minerals like hydroxyapatite (HAp) and collagen matrix. HAp crystallites orientation is thus one parameter of bone quality. In this study, we made spherically shaped specimens from anterior part of the cortical bone of three bovine femurs. By using these specimens, we experimentally investigate the anisotropy of ultrasonic wave velocity and the HAp crystallites orientation in axial-radial, axial-tangential and radial-tangential planes in detail. Longitudinal ultrasonic wave propagation was investigated by using a conventional ultrasonic pulse system. Wave velocity anisotropy was clear in each plane. The direction of the fastest wave velocity showed a small tilt from the bone axis in axial-radial and axial-tangential plane. Moreover, X-ray pole figure measurement indicated that there were also small tilts of the HAp crystallites orientation from the axial direction. The tilts were similar with the tilts of fastest velocity direction. There were actually good correlation between velocity and HAp crystallites orientation in axial-radial and tangential plane. But there was no correlation in radial-tangential plane.