OBJECTIVE To assess the motion of the proximal sesamoid bones (PSBs) relative to the third metacarpal bone (MC3) of equine forelimbs during physiologic midstance loads. SAMPLE 8 musculoskeletally normal forelimbs (7 right and 1 left) from 8 adult equine cadavers. PROCEDURES Each forelimb was harvested at the mid-radius level and mounted in a mate-rial testing system so the hoof could be moved in a dorsal direction while the radius and MC3 remained vertical. The PSBs were instrumented with 2 linear variable differential transformers to record movement between the 2 bones. The limb was sequentially loaded at a displacement rate of 5 mm/s from 500 N to each of 4 loads (1.8 [standing], 3.6 [walking], 4.5 [trotting], and 10.5 [galloping] kN), held at the designated load for 30 seconds while lateromedial radiographs were obtained, and then unloaded back to 500 N. The position of the PSBs relative to the transverse ridge of the MC3 condyle and angle of the metacarpophalangeal (fetlock) joint were measured on each radiograph. RESULTS The distal edge of the PSBs moved distal to the transverse ridge of the MC3 condyle at 10.5 kN (gallop) but not at lower loads. The palmar surfaces of the PSBs rotated away from each other during fetlock joint extension, and the amount of rotation increased with load. CONCLUSIONS AND CLINICAL RELEVANCE At loads consistent with a high-speed gallop, PSB translations may create an articular incongruity and abnormal bone stress distribution that contribute to focal subchondral bone lesions and PSB fracture in racehorses. (Am J Vet Res 2021;82:198–206).
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