Explain the role of the proximal sesamoid bones in the stay mechanism.

• A. They serve as anchors for the distal sesamoidean ligaments and as attachment points for the suspensory ligament, stabilizing the fetlock
• B. They are part of the hock
• C. They are muscles that extend the fetlock
• D. They are joints that connect phalanges

Answer: (B)

The proximal sesamoid bones function as a pulley-like fulcrum in the fetlock stay mechanism. The suspensory ligament and the distal sesamoidean ligaments wrap around these bones, so when the limb bears weight, tension in these ligaments is redirected around the proximal sesamoids. This arrangement converts tensile forces into a stabilizing pull that keeps the fetlock in partial extension with little muscular effort. In other words, the proximal sesamoids are key to the passive lock that allows the horse to stand quietly by maintaining fetlock extension. They are not part of the hock; they belong to the fetlock region.