Stiffness and energy dissipation across the superficial and deeper third metacarpal subchondral bone in Thoroughbred racehorses under high-rate compression.

Subchondral bone injury due to high magnitude and repetition of compressive loading is common in humans and athletic animals such as Thoroughbred racehorses. Repeated loading of the joint surface may alter the subchondral bone microstructure and initiate microdamage in the bone adjacent to the articular cartilage. Understanding the relationship between microdamage, microstructure and mechanical properties of the subchondral bone adjacent to the articular cartilage is, therefore, essential in understanding the mechanism of subchondral bone injury. In this study, we used high-resolution µCT scanning, a digital image-based strain measurement technique, and mechanical testing to evaluate the three-dimensional pre-existing microcracks, bone volume fraction (BVF) and bone mineral density (BMD), and mechanical properties (stiffness and hysteresis) of subchondral bone (n = 10) from the distopalmar aspect of the third metacarpal (MC3) condyles of Thoroughbred racehorses under high-rate compression. We specifically compared the properties of two regions of interest in the subchondral bone: the 2 mm superficial subchondral bone (SSB) and its underlying 2 mm deep subchondral bone (DSB). The DSB region was 3.0 ± 1.2 times stiffer than its overlying SSB, yet it dissipated much less energy compared to the SSB. There was no correlation between structural properties (BVF and BMD) and mechanical properties (stiffness and energy loss), except for BMD and energy loss in SSB. The lower stiffness of the most superficial subchondral bone in the distal metacarpal condyles may protect the overlying cartilage and the underlying subchondral bone from damage under the high-rate compression experienced during galloping. However, repeated high-rate loading over time has the potential to inhibit bone turnover and induce bone fatigue, consistent with the high prevalence of subchondral bone injury and fractures in athletic humans and racehorses.