The Effects of Impact Loading on the Equine Metacarpophalangeal Joint
Purpose: To characterize the effect of impact loading on the metacarpophalangeal (MCP) joint and compare the stresses induced by impact and static loading in the equine third metacarpal (MC3) as a first step to assessing the potential role of impact loading in the context of mechanical injury. Methods: Three analyses were used to characterize impact loading in the equine MCP joint: 1) an ex vivo analysis of vibration attenuation across the joint at a given hoof angle (strike), 2) ex vivo analysis of the contact areas between MC3 and the first phalanx (P1) and proximal sesamoid (PS) bones and 3) finite element (FE) analysis to compare the stress distributions in the distal MC3 of healthy and diseased (osteoarthritic - OA) MCP equine joints under impact and static loading. Results: Signal energy reaching MC3 was significant at 6-31% of that at the hoof. A heel-first strike produced the largest peak accelerations and highest frequencies among all strike conditions. Contact area between P1 and MC3 was well-defined and bounded by the sagittal and transverse ridges. The ratio of contact area of P1 to PS was 2.83 (P <.0001). Under FE static loading, the highest average stresses (19.38 MPa) were located in the palmar parasagittal groove in the healthy joint. The highest average stresses under impact (14.1 MPa) were located on the dorsal aspect of the medial condyle in the OA model and were greater than the static loading stresses in both models on the dorsal aspect of MC3. Conclusions: Large accelerations that occur upon impact are attenuated by the equine limb; however still carry considerable energy within the signal that could be damaging to tissue. Contact on MC3 at impact occurs primarily with P1, contrasting with midstance when both P1 and PS are equally involved. MCP FE modeling indicated that an increase in bone stiffness associated with OA may be adaptive under static loading, however increases the stress magnitude under impact loading. Stresses on the distal end of MC3 are comparable to those found during midstance and should be considered in the context of injury.