The two proximal sesamoid bones are located just proximal to fetlock joint on the palmar aspect and embedded within the suspensory ligament. The sesamoid bones provide stability to the suspensory apparatus as it courses around the back of the fetlock joint, and they function to prevent extreme overextension of the fetlock joint when the horse is performing.
The proximal sesamoid bones are roughly triangular in shape, with the proximal most portion acting as a point of insertion for the suspensory ligament, and the base acting as a point of origin for the distal sesamoidean ligaments. The intersesamoidean ligament is a dense ligament that firmly secures the proximal sesamoid bones together along their axial aspect.
The dorsal surface is the articular surface, it is concave and articulates with distal end of the cannon bone. The flexor surface (palmar surface) is marked by a smooth groove covered by a layer of cartilage (scutum proximale) for the flexor tendons. The surface for interosseous muscle is the axial or abaxial (medial or lateral in horses) surface for attachment of interosseus muscle.
The proximal sesamoid bones are two elongated, slightly curved small bony structures located on the palmar surface of each of the four main metacarpophalangeal joints, in the tendons of insertion of the interosseous muscles. They articulate primarily with the head of each metacarpal bone and secondarily with the palmar tubercles of the base of each proximal phalanx. Their truncated distal ends articulate by small facet with the palmar tubercles of the corresponding proximal phalanges. On digit I, there is a single proximal sesamoid bone.