Rationale: The metacarpophalangeal (MCP) joint of racehorses is especially susceptible to a wide range of injuries which can be not only career ending, but life ending as well. Chronic cyclical loading can lead to abnormal subchondral bone (SCB) adaptation resulting in sclerotic or necrotic bone. Results from previous studies suggest that contact between the bones in a joint may play an important role in SCB adaptation. The objective of this study, therefore, was to quantitatively evaluate the change in contact area under two loads and subjectively compare the contact area with the SCB density pattern in the intact equine MCP joint.

Methods: Computed tomographic scans of the MCP joint were performed for 6 equine forelimbs and the data was analyzed using a custom written analysis program that allows for density measurements and 3D image rendering. The hoof and humerus of each limb were mounted to the materials testing system. The limb was then loaded to 150 degrees (representing stance) and 120 degrees (representing stance phase of gallop) extension in the MCP and the joint was stained with safranin-O and toluidine blue respectively via an intra-articular injection. After testing, the joint was disarticulated and the surface area was digitized and rendered. Percentage contact area at each angle was compared using a paired t-test. The 3D computer model revealing contact area was subjectively compared to the 3D CT rendering revealing the SCB density pattern.

Results: There was a significant increase in the contact area at the higher load. It was also found that areas of contact subjectively appeared to have higher density on CT scans.

Conclusions and Relevance: Contact area increased with increasing load suggesting that areas not normally loaded may experience a high degree of stress during impact loading. In addition, areas consistently in contact being associated with increased SCB density supports the idea that the SCB adapts to the load applied to it. Quantifying the association between contact area and SCB density may provide insight into specific loading patterns that may predispose some horses to injury.

This study has been published:

Easton KL, Kawcak CE. Evaluation of increased subchondral bone density in areas of contact in the metacarpophalangeal joint during joint loading in horses. Am J Vet Res 2007;68:816-821.