The skeletal system is composed of bones that typically connect to one another at joints, but a unique group of structures exists outside this standard arrangement. A sesamoid bone is a small, rounded bone that is commonly found embedded within a tendon or, less frequently, a muscle, near a joint surface. The name itself is derived from the Arabic word for “sesamum,” or sesame, reflecting the small, seed-like shape of many of these bones. This classification makes them unique because they do not articulate with other bones in the way most of the skeleton does.
What Defines a Sesamoid Bone
Sesamoid bones are distinct because they develop through ossification directly within a soft-tissue structure like a tendon. They function as a type of biological pulley, providing a smooth surface for the tendon to slide over. This mechanical action increases the angle of the tendon as it passes over a joint, which enhances the leverage of the muscle. By increasing this mechanical advantage, the muscle can generate greater force to move the joint. Sesamoids also serve a protective role by redistributing forces and reducing friction, shielding the tendon from excessive wear and compression where it passes over a bony prominence.
The Main Examples of Sesamoid Bones
The Patella
The most prominent sesamoid bone is the patella, commonly known as the kneecap. Situated within the tendon of the quadriceps muscle, the patella is the largest of the sesamoid bones. It is necessary for proper lower extremity function, especially in the final degrees of knee extension.
Foot and Hand Sesamoids
Beyond the knee, smaller sesamoid bones are consistently found in the foot, specifically under the joint of the big toe, or hallux. Typically two sesamoids are embedded in the tendon of the flexor hallucis brevis muscle, positioned beneath the head of the first metatarsal bone. These small bones are critical for absorbing the weight and stress placed on the forefoot during walking and running.
The hands also contain predictable sesamoid bones, most notably in the thumb, or pollex. Two sesamoids are commonly located at the metacarpophalangeal joint, aiding in complex movements and gripping strength. The pisiform bone in the wrist is also classified as a sesamoid, developing within the tendon of the flexor carpi ulnaris.
Accessory and Variable Sesamoid Bones
While some sesamoids are present in almost everyone, the body can contain up to 42 sesamoid bones, with many being small and variable in their occurrence. These accessory sesamoids often develop in response to localized mechanical stress and strain on tendons. They are frequently found in the hands and feet beyond the main examples.
One well-known variable sesamoid is the fabella, a small bone sometimes found embedded in the tendon of the lateral head of the gastrocnemius muscle behind the knee. It is present in approximately 10 to 30% of individuals. The os peroneum is another variable sesamoid, located within the tendon of the peroneus longus muscle in the foot.
The main clinical relevance of these smaller bones is that they can be misidentified on X-rays as fractures because they may be naturally bipartite, meaning they are composed of two or more parts. They can also be a source of pain through inflammation, a condition known as sesamoiditis, which most commonly affects the sesamoids under the big toe.