The Sesamoid Bones

In the 17th century, anatomists discovered a knob-like projection at the end of the incus bone in the ear. They called it the lenticular process of the incus. For more than a century, scientists debated whether the lenticular process is a separate bone, but studies show it is connected to the incus by a bony stub called a pedicle. What makes this projection a sesamoid bone is that it is a part of a chain of bones that acts as an acoustic vibration lever; the lenticular process is the short arm of the lever.

Located in the mid-neck, between the lower jaw and the shoulder girdle, the hyoid is classified as a sesamoid bone even though it does not articulate with any other bones. Apart from the cervical vertebrae, this horseshoe-shaped bone is the only bone in that anterior region of the neck. It is attached to five areas: the skull’s base, the mandible, the tongue, the larynx, and the scapular girdle. It is a stabilizer vital for a variety of everyday functions including speech and swallowing.

The pisiform is a sesamoid bone in the wrist. Located on the upper section of the carpal bones below the smallest finger, the pea-shaped structure resides in the flexor carpi ulnaris or FCU that enables flexion and adduction of the hand. The pisiform acts as a pulley for the FCU and the abductor digiti minimi muscles as well as tendons and ligaments. Along with the hamate, it forms a fibro-osseous structure called the Guyon canal, a pathway for the ulnar nerve and artery.

A few metacarpal sesamoid bones are located in the joints of certain fingers. Their number depends on a variety of factors, including gender and ethnicity. Studies show nearly all people have a sesamoid bone in the thumb, but only sixty percent have them in the index and small fingers. Between 13 and 18 percent have one in the interphalageal joint, and fewer than five percent have one in the middle and ring fingers.

The patella or kneecap is a circular bone with a downward apex that is the starting point of the patellar ligament. It is the largest sesamoid bone in the body, a necessary factor as it supports the body’s weight during multiple regular activities. The kneecap is embedded in the tendon of the quadriceps, helping the muscle to increase its pull on the tibia. The bone allows smooth movement between tibia and femur, and acts as a shield, protecting both from direct fracture.

A shallow groove in the femur is a path for the popliteal tendon that supports the popliteus muscle behind the knee joint. The cyamella sesamoid bone in this tendon is often a source of medical discussion. While it is common in certain mammals, the cyamella is extremely rare in humans, and when it is present, it is usually closer to the tibia. Its purpose is not known, but its presence may cause knee pain.

The head of the gastrocnemius muscle, the leg triceps, is the usual location of the fabella sesamoid bone. Just like the cyamella, the presence of this bone is rare; it appears in only ten to 30 percent of humans, according to medical literature. Because of its location at the back of the knee joint, it may be responsible for redirecting knee joint forces. While some are not be aware of their fabella, others experience pain or irritation.

The first metatarsal bone is strong enough to support the body’s weight. The head of this bone is large and grooved, making it perfect for gliding easily across tissues. The bone increases the leverage of the flexor hallucis brevis and longus muscles that flex the big toe.

Sesamoids tend to be bear a lot of the body’s weight, and nowhere is that more evident than in the foot. Inflammation in the bone, sesamoiditis, affects runners and ballet dancers because they tend to balance a lot of their weight on the balls of their feet. Symptoms include pain when bending or straightening the big toe and intensifying pain the ball of the foot. To diagnose the injury, doctors use X-rays and physical examination. Most of the time, surgery is unnecessary and individuals just require rest and over-the-counter medication.

Avascular necrosis, AVN or osteonecrosis, happens when a lack of blood supply to bones occurs following an injury. This condition typically happens at specific spots, specifically sesamoid bones. It is rare and damage to the surrounding tissue usually precedes avascular necrosis, as this damage cuts off blood to that bone. AVN-affected sesamoid bones weaken until they collapse or die. The pain can be overlooked, but if an injured individual continues to ignore the symptoms, their mobility will become more and more limited over time.