Low-frequency ultrasonic velocity measurements in human calcaneal trabecular bone

Ultrasonic velocities were measured in three orthogonal directions for 17 cubes (2 x 2 x 2 cm approximately) of defatted calcaneal trabecular bone using a novel pulse transmission method with 37 kHz transducers. Since the wavelength was greater than the cross-sectional dimensions of the specimens, it was assumed that bar wave propagation was occurring and this allowed Young's modulus to be derived from velocity and apparent density. Velocity varied from 1585 +/- 104 m s-1 in axis 1 (proximo-distal) to 947 +/- 131 m s-1 in axis 3 (medio-lateral). Thus, the velocities measured in axis 3 were considerably lower than those typically seen in clinical measurements of the calcaneus. The derived Young's moduli ranged from 834 +/- 248 MPa (axis 1) to 299 +/- 98 MPa (axis 3), and were comparable in magnitude to some previously published data from mechanical testing. The results suggest that velocities measured with this technique do indeed correspond to bar velocities, and consequently that low-frequency ultrasound can be used to directly predict the mechanical properties of trabecular bone specimens in vitro. On the other hand, given the marked differences in geometry, wavelengths used and the presence of fat, it raises questions about the validity of applying the bar wave equation in the context of higher-frequency velocity measurements in the intact calcaneus in vivo, despite the fact that this has been found to be useful by a number of previous workers. This may require a review of the clinical implications of high-frequency ultrasound data.