A sesamoid bone is a bone embedded within a tendon.
Sesamoids are found in locations where a tendon passes over a joint, such as the hand, knee, and foot. Functionally, they act to protect the tendon and to increase its mechanical effect. The presence of the sesamoid bone holds the tendon slightly farther away from the center of the joint and thus increases its moment arm.
Sesamoid bones also prevent the tendon from flattening into the joint as tension increases and therefore also maintain a more consistent moment arm through a variety of possible tendon loads. This differs from menisci, which are made of cartilage and rather act to disperse the weight of the body on joints and reduce friction during movement.
Sesamoid bone: Sagittal section of right knee-joint
Sesamoid bones can be found on joints throughout the body, including:
- In the knee — The patella, within the quadriceps tendon, is a sesamoid bone.
- In the hand — Two sesamoid bones are commonly found in the distal portions of the first metacarpal bone, within the tendons of adductor pollicis and the flexor pollicis brevis. There is also commonly a sesamoid bone in distal portions of the second metacarpal bone. The pisiform of the wrist is a sesamoid bone (within the tendon of flexor carpi ulnaris).
- In the foot — The first metatarsal bone usually has two sesamoid bones at its connection to the big toe, both within the tendon of flexor hallucis brevis. In some people, only a single sesamoid is found on the first MTP.
