Sesambein

Sesambein, or sesamoid bone, is a small bone embedded within a tendon or joint capsule, usually near a joint, where it alters the tendon’s angle of pull, reduces friction, and protects the tendon from wear. The patella (kneecap) is the largest and most familiar example, but many other sesamoids occur in the body, most commonly in the foot, hand, and around the knee.

In the foot, the two plantar sesamoid bones lie beneath the head of the first metatarsal within the tendons of the flexor muscles, beneath the big toe. In the hand, there are sesamoids associated with the metacarpophalangeal joints, particularly in the thumb. The wrist contains the pisiform, a small carpal bone embedded in the tendon of the flexor carpi ulnaris, sometimes categorized as a sesamoid. A variety of other small sesamoids may develop in response to stress or strain, and some individuals lack certain sesamoids entirely.

Sesamoids develop through ossification within tendons during growth and can be present from birth or acquire them later in life due to repetitive stress. They function by increasing mechanical advantage and protecting tendons from compressive forces at joints. Clinically, sesamoids can be prone to fractures, inflammation (sesamoiditis), or nonunion after injury, particularly in the big toe, knee, or thumb. Imaging typically involves X-ray, with MRI or CT providing detail on surrounding soft tissue and bone integrity. Treatment ranges from rest and physical therapy to immobilization or surgical removal or repair in persistent or severe cases.