sesamoid

Sesamoid bones are small bones embedded within tendons or within a joint capsule, usually near a joint where a tendon passes over a bone. They typically resemble sesame seeds in size and shape. The primary functions of sesamoids are to protect tendons from compressive forces, reduce friction, and increase the tendon's mechanical advantage by altering its angle of pull. They can be fully ossified bones or consist of cartilaginous or fibrous tissue, and their presence and number vary among individuals.

Most people have a few notable examples. The largest sesamoid is the patella, embedded in the quadriceps tendon at the knee. In the wrist, the pisiform bone is sometimes described as a sesamoid within the tendon of the flexor carpi ulnaris. In the knee, a small sesamoid called the fabella may be present in the lateral head of the gastrocnemius tendon behind the knee. In the forefoot, two plantar sesamoids lie under the head of the first metatarsal at the big toe joint; additional sesamoids can occur in other joints but are less common.

Sesamoid injuries are typically caused by repetitive stress or direct trauma and may present as pain at a joint with tenderness under a tendon. Common conditions include sesamoiditis in the foot and stress fractures of the sesamoid bones. Diagnosis often relies on X-ray imaging, with MRI or CT providing additional detail when needed. Treatment ranges from rest, activity modification, padding or orthotics, and physical therapy to surgical removal (sesamoidectomy) in refractory cases, though this can affect joint function.