Collateral ligaments of the distal sesamoid bone in the digit of Equus: re-evaluating midstance function.

This research examines the function of the collateral ligaments of the distal sesamoid bone (CLDS) in horse forelimbs during weight bearing, and it challenges the previous understanding that these ligaments prevent flexion of the proximal interphalangeal joint (PIPJ) during midstride. Instead, the study suggests that the CLDS primarily function to limit the extension of the distal interphalangeal joint.

Understanding the Objective of the Research

• The study aimed to understand the actual function of the collateral ligaments of the distal sesamoid bone (CLDS) in a horse’s forelimbs. These ligaments are vital for the stability of the digit (toe) and are thought to play a significant role in preventing certain foot pathologies.
• The researchers originally hypothesized that these ligaments prevent flexion (bending) of the proximal interphalangeal joint (PIPJ), which is the first joint of a horse’s toe, during the stages of movement where the horse’s foot is in contact with the ground (midstance).

Methodology of the Research

• To test this hypothesis, the researchers conducted an experiment on forelimb specimens from three mixed-breed horses. They applied weight to these limbs in a controlled setting, once with the CLDS intact and once after they had been cut (transected).
• The amount of rotation in the PIPJ and the metacarpophalangeal joint (MCPJ, commonly known as the fetlock) under maximum load was measured and compared between the intact and transected trials.

Findings of the Research

• The results refuted the initial hypothesis. The researchers found that the PIPJ did not change its angle under maximum load, whether the CLDS was intact or transected.
• Instead, they found that after the CLDS was transected, the digits of the horses extended less at the fetlock joint, indicating that it’s the CLDS that limits this extension.
• Additionally, the research suggested that without the CLDS, there is an increased rotation of the distal phalanx (the bone at the tip of the digit), resulting in the observed motion of the proximal and middle phalanx.

Implications of the Research

• These findings contradict previous theory about the function of the CLDS in the forelimbs of horses. It indicates that rather than preventing flexion of the PIPJ at midstance, these ligaments instead stabilize and limit extension of the distal interphalangeal joint.
• Understanding the primary function of these ligaments could potentially provide a clearer understanding of possible biomechanical implications of pastern diseases and ligament injuries in horses, and may be valuable in designing effective therapeutic treatments or preventative measures.