Sesamoid Bones

Sesamoids are small bones embedded inside tendons where they pass joints. The most important are the two pea-sized sesamoids under the big toe's metatarsal head, which act as bearings for push-off; others occur under the thumb and occasionally elsewhere. The kneecap is the body's largest sesamoid. A bipartite (two-piece) sesamoid is a common normal variant.

Key facts

Sesamoids develop inside tendons, not as part of the main skeleton.
The two under the big toe are called the tibial and fibular sesamoids.
They protect the flexor tendon and improve its leverage.
They bear weight directly — they have their own small joint surfaces.
Extra sesamoids are found in many people and are usually incidental.

What can go wrong

Inflammation of the big-toe sesamoids (sesamoiditis) causes pain under the ball of the big toe in runners and dancers, and they can fracture under load or degenerate. Because a two-piece sesamoid looks like a fracture on X-ray, the smooth edges of the variant are the distinguishing clue.

Related procedures & tests

X-ray (general)

Related symptoms

Foot pain

Related muscles

Frequently asked questions

Why do dancers injure their sesamoids?

Working on demi-pointe loads the ball of the big toe with several times body weight through two pea-sized bones — exactly the overuse pattern that inflames or stress-fractures them.

Is a two-piece sesamoid a fracture?

Usually not: a bipartite sesamoid is a developmental variant with smooth, rounded edges and often present on both feet. A fracture has sharp, irregular edges and a matching history of injury.

Educational information only. This atlas explains normal anatomy and describes conditions in general terms; it is not medical advice and does not replace assessment by a qualified clinician. If you have symptoms that worry you, seek medical care.