Sesamoid bones of foot

At a glance

English

Sesamoid bones of foot

Latin (TA2)

Ossa sesamoidea pedis

System

Bone

Region

Foot

Side

Paired, left and right

About the sesamoid bones of foot

Sesamoid bones are small nodules of bone that form within a tendon where it passes over a joint. In the foot the constant pair lies under the head of the first metatarsal, at the joint of the big toe with the foot, beneath the ball of the foot. Each sits within one of the two tendons of flexor hallucis brevis: the medial, or tibial, sesamoid in the inner tendon and the lateral, or fibular, sesamoid in the outer one. A few people have only one. Smaller sesamoids are sometimes found under the other metatarsal heads or in the interphalangeal joint of the big toe.

The two bones rest in shallow grooves on the underside of the first metatarsal head, separated by a low ridge, and are bound to each other and to the base of the proximal phalanx by the thick plantar plate of the joint. Their upper surfaces are covered in cartilage and glide against the metatarsal head. The tendon of flexor hallucis longus runs between them, protected in the channel they form. Abductor hallucis joins the inner sesamoid and adductor hallucis the outer one. The medial sesamoid is usually the larger and takes more of the load.

Like the patella at the knee, the sesamoids raise the flexor tendons away from the joint, giving them a better lever to press the big toe down, and they spread the pressure that falls on the forefoot as the heel lifts off in walking and running. They take a large share of body weight at push-off.

This loading makes them a source of pain in the forefoot. Sesamoiditis, inflammation around the bones, is common in dancers and runners. They can also suffer stress fractures. The medial sesamoid often ossifies from two centres and remains in two pieces, a bipartite sesamoid, which on an X-ray can look like a fracture; comparing both feet helps tell them apart. In bunion deformity the sesamoids drift outwards as the metatarsal head moves inwards.

Function and supply

Function

Lift the short flexor tendons off the first metatarsal head to improve their lever on the big toe, and share the weight carried by the ball of the foot.

Nerve supply

Not innervated as a muscle is; the plantar digital nerves of the big toe, from the medial plantar nerve, run beside them and can be irritated in sesamoid pain.

Blood supply

Small branches of the medial plantar artery and the first plantar metatarsal artery, entering mainly from the back and sides.