A sesamoid bone is embedded within a tendon or, less commonly, a muscle. Its name originates from the Greek word for “sesame seed,” alluding to their typically small size. Unlike most bones that connect at joints, sesamoid bones connect to muscles through their encompassing tendons. These bones often develop in response to mechanical strain or can be normal anatomical variations.
Defining Features and Common Locations
Sesamoid bones are small and nodular, functioning within soft tissues. They protect the tendons they are embedded in, particularly where tendons glide over bony surfaces near joints. The human body contains numerous sesamoid bones.
The largest sesamoid bone is the patella, or kneecap, situated within the quadriceps tendon at the knee. In the foot, two small sesamoid bones are found beneath the head of the first metatarsal bone, under the big toe joint. These are embedded within the flexor hallucis brevis tendon.
In the hand, sesamoid bones are located near the thumb, two bones at the base of the thumb’s metacarpophalangeal joint. A smaller sesamoid is also found at the interphalangeal joint of the thumb. The pisiform bone in the wrist is also a sesamoid bone.
The Purpose of Sesamoid Bones
Sesamoid bones enhance the biomechanics of movement. They function like pulleys, providing a smooth surface for tendons to slide over. This mechanism increases the leverage of muscles and tendons, improving the efficiency and force generation of movement. For example, the patella boosts the quadriceps muscle’s ability to extend the lower leg by approximately 40%.
These bones also protect by reducing friction and stress on tendons as they move across joints. By redistributing forces, sesamoid bones alleviate tension within muscles and tendons, protecting them from excessive wear and injury. This protective and leverage-enhancing role allows for a greater range of motion and increased weight-bearing capacity in areas like the foot and knee.
When Sesamoid Bones Cause Problems
Despite their protective functions, sesamoid bones can be susceptible to problems, particularly from repetitive stress. One common condition is sesamoiditis, which involves inflammation of the sesamoid bones and surrounding tendons, occurring in the ball of the foot under the big toe. Symptoms include pain, tenderness, swelling, or redness in the affected area. This condition affects individuals who engage in activities putting significant pressure on the balls of their feet, such as runners and dancers.
Sesamoid bones can also experience fractures, either from acute trauma or chronic, repetitive stress, stress fractures. A sesamoid fracture in the foot can cause sharp or aching pain under the big toe, accompanied by swelling. Unlike sesamoiditis, which develops gradually, a fracture presents with immediate pain following an injury.
Treatment for both conditions begins with conservative measures, including rest, ice, anti-inflammatory medications, and supportive footwear or orthotics to relieve pressure. Limiting movement through taping or bracing may be recommended.