Sesamoid bones are small, independent bones embedded within a tendon or, less commonly, a muscle. They are unique because they do not connect to other bones at a joint through ligaments, but instead are suspended within soft tissue. Their name, derived from the Greek word for “sesame seed,” reflects the typical small size of most of these bones. Sesamoids can form in response to strain or may be present as a normal anatomical variation.
Major Sesamoid Bone Locations
The patella, commonly known as the kneecap, is the largest and most widely recognized sesamoid bone in the human body. It is located at the front of the knee joint, specifically embedded within the quadriceps tendon. This tendon attaches the quadriceps muscles in the thigh to the tibia, or shin bone.
In the foot, two prominent sesamoid bones are consistently found beneath the big toe’s metatarsophalangeal joint. These small, oval-shaped bones lie within the tendons of the flexor hallucis brevis muscle, one on the medial side and the other on the lateral side. In the hand, two sesamoid bones are typically located in the thumb. They are near the metacarpophalangeal joint, embedded within the tendons of the adductor pollicis and flexor pollicis brevis muscles.
Less Common Sesamoid Bone Locations
Beyond the major sites, sesamoid bones can appear in other body areas and often vary among individuals. In the hand, beyond the two sesamoids of the thumb, others may be present less frequently. For instance, a sesamoid bone can sometimes be found near the interphalangeal joint of the thumb, or in the metacarpophalangeal joints of the index and little fingers.
In the foot, additional sesamoid bones can occasionally be found under the metatarsophalangeal joints of the other toes. Other less common sesamoids include the pisiform bone in the wrist, embedded in the flexor carpi ulnaris tendon, and the fabella, a small bone sometimes found in the gastrocnemius muscle tendon behind the knee. The cyamella, located within the popliteus muscle tendon, and the os peroneum, within the peroneus longus tendon, are also examples of less common sesamoid variants.
The Role of Sesamoid Bones
Sesamoid bones play a biomechanical role by acting as a pulley or lever for the tendons they are embedded within. This arrangement enhances the mechanical advantage of the associated muscles, allowing them to exert greater force with less effort. For example, the patella significantly increases the leverage of the quadriceps tendon on the femur, improving the efficiency of knee extension.
Sesamoids also function to reduce friction and protect tendons from excessive wear and tear as they glide over joints. The patella, for instance, provides a smooth surface for the quadriceps tendon to move across the knee joint, preventing direct contact between the tendon and the femur. Similarly, the sesamoid bones under the big toe help distribute pressure during weight-bearing activities, protecting the flexor hallucis brevis tendon from stress as the foot pushes off the ground.