Pathway of ultrasound waves in the equine third metacarpal bone

The velocity of ultrasound waves through bone has been used widely as a non-invasive method for assessing bone quality. Accurate measurement of velocity depends on accurate assessment of the distance travelled by the sound wave. It has been argued that the sonic pathway is deflected around the marrow cavity and so does not follow a straight line through the bone; therefore, correction factors have been developed to account for the extra distance travelled. This hypothesis was examined in vitro using sections from the equine third metacarpal bone. Two 1 MHz transducers used with the transmitting transducer energized by a 600 V electrical spike generator produced a 0.1 microsecond pulse width and the received signal was recorded on a delayed time-base oscilloscope, from which the velocity was calculated. Two distinct peaks were apparent in the received signal, corresponding to a direct cortical transmission wave and a direct medullary transmission wave. This observation was confirmed quantitatively using models of the third metacarpal made from homogeneous materials that allow accurate determination of the transit times of each component of the signal. Perspex was used to mimic cortical bone, with water as the mimic for the contents of the medullary canal; these materials were chosen because they have transmission velocities similar to the materials they were mimicking. The results confirmed that the pathway went straight through the bone with a time lag in the medullary wave due to the slower transmission velocity of the marrow. To ensure that the cortical wave is always received, transducers larger than the medullary width should always be used.