In Vitro Motion of the Proximal Sesamoid Bones under Physiological Midstance Loads

Introduction: A subchondral osteopenic lesion has been associated with proximal sesamoid bone (PSB) fracture in racehorses, but the genesis of this lesion is unclear. We hypothesized race-speed loads would elicit an articular surface incongruity between the PSBs and third metacarpal (MC3) condyle that could promote lesion formation.

Materials and Methods: Eight unilateral cadaver forelimbs were instrumented to measure PSB separation and loaded to standing, walk, trot, and gallop mid-stance loads (1.8–10.5 kN). Position of PSBs relative to the MC3 condyle and transverse ridge (TR) and metacarpophalangeal (MCP) joint angle were measured from lateromedial radiographs. Measures of PSB position relative to MC3 were correlated with loads.

Results: MCP and MC3-PSB joint angles were positively correlated to load (r ² = 0.98 and 0.92; p < 0.001). The PSBs translated distally with increasing load (r ² = 0.73, p < 0.001) and moved past the TR at the gallop load (10.5 kN). The PSBs separated and rotated with increasing load.

Discussion/Conclusion: The results demonstrate that at a race-speed gallop the PSBs move beyond the MC3 TR, and thus beyond their normal articulation with the palmar aspect of the MC3 condyle. This articular incongruity could influence development of the subchondral lesions by increasing PSB focal stresses. Further, PSB abaxial displacement and rotation may contribute to axial avulsion PSB fractures.

Acknowledgment: Funded by the Students Training in Advanced Research Program, School of Veterinary Medicine, University of California at Davis and the Center for Equine Health with funds provided by the State of California satellite wagering fund and contributions by private donors. Richard Evans provided statistical analysis for this project.