Bone · Foot

Sesamoid

Ossa sesamoidea

A sesamoid is a bone that forms inside a tendon, at the place where that tendon has to wrap around a corner. The pair named here sit under the ball of the big toe: two bones about the size of peas, buried in the tendons that run beneath the head of the first metatarsal and held apart by a small ridge on its underside, one towards the inner edge of the foot and one towards the outer. Each carries a cartilage-covered facet on its upper surface where it articulates with its own groove on the underside of the metatarsal head, but nothing sets it into the skeleton: it is slung in tendon and ligament, sliding forward and back across that groove as the toe bends. The kneecap is the same idea at a far larger scale and is the biggest sesamoid in the body, which makes it the easiest way to picture what these two are doing.