In vitro assessment of the motion of equine proximal sesamoid bones relative to the third metacarpal bone under physiologic midstance loads.

The research examines the movement of two specific bones, the proximal sesamoid bones (PSBs) and third metacarpal bone (MC3), in the forelimbs of horses under different load conditions typically experienced during various activities such as standing, walking, trotting, and galloping.

Research Method

• The study used eight forelimbs from equine cadavers, all considered musculoskeletally normal.
• Each forelimb was secured at the mid-radius level in a testing system, which allowed for the hoof to move in a dorsal (upwards) direction while keeping the radius and MC3 bones vertical.
• To track the movement between the PSBs and the MC3, the PSBs were equipped with two linear variable differential transformers.
• The forelimb was loaded at a consistent rate of displacement from 500 N to each of the four load conditions which were 1.8kN (standing), 3.6kN (walking), 4.5kN (trotting), and 10.5kN (galloping). Each load was held for 30 seconds while radiographs were obtained before unloading back to 500N.
• The radiographs were then used to measure the position of the PSBs in relation to the third metacarpal bone (MC3) and the angle of the metacarpophalangeal (fetlock) joint.

Research Findings

• The results indicated that the distal edge of the PSBs moved distal to the transverse ridge of the MC3 condyle only when the load reached the level associated with galloping (10.5 kN) but not at lower loads.
• The research also found that the lower surfaces of the PSBs rotated away from each other as the fetlock joint extended, with the degree of rotation increasing proportionally with the load.

Conclusions

• The study concluded that at high-speed gallop loads, the movement and ultimately translations of the PSBs could potentially lead to an articular incongruity and abnormal bone stress distribution.
• This pattern of bone stress and movement could contribute to the development of focal subchondral bone lesions and PSB fractures in racehorses, which are common injuries encountered in this species.