A sesamoid bone is a small, rounded bone embedded within a tendon or muscle, typically found near joints. Unlike most bones, sesamoids are unique because they connect to muscles via tendons. Named for their resemblance to sesame seeds, these bones develop within tendons where they cross long bones, often forming in response to mechanical stress.
Key Locations
Sesamoid bones are present in several key areas of the human body, with the most prominent examples found in the foot, hand, and knee.
In the foot, two sesamoid bones are consistently located beneath the head of the first metatarsal bone, which connects to the big toe. These are embedded within the flexor hallucis brevis tendon on the foot’s plantar surface, with one located medially (tibial sesamoid) and the other laterally (fibular sesamoid).
In the hand, sesamoid bones are commonly found at the metacarpophalangeal (MCP) joint of the thumb. Two sesamoids are present here, situated within the tendons of the adductor pollicis and flexor pollicis brevis muscles. The pisiform, another sesamoid bone, is located in the wrist, embedded within the flexor carpi ulnaris tendon. Less commonly, sesamoids may also appear at the MCP joints of the index and little fingers.
The largest sesamoid bone in the body is the patella, commonly known as the kneecap. It is situated anterior to the knee joint, embedded within the quadriceps femoris tendon. While these are the most consistent locations, sesamoid bones can occasionally develop in other areas where tendons experience significant stress, such as the fabella or cyamella.
Purpose of Sesamoid Bones
Sesamoid bones serve several functions due to their unique placement within tendons. They act as natural pulleys, changing the angle at which a tendon pulls on a bone. This increases the mechanical advantage and leverage of associated muscles, making movements more efficient and powerful. For instance, the patella significantly enhances the quadriceps muscle’s ability to extend the knee.
These bones also reduce friction and protect tendons from excessive wear as they glide over bony surfaces. By acting as a spacer, they prevent direct contact and compression between the tendon and the underlying bone. In weight-bearing areas like the foot, sesamoids help distribute pressure across joints, absorbing forces during activities such as walking, running, and jumping.
Common Conditions Affecting Sesamoids
Due to their location and function, sesamoid bones are susceptible to several specific conditions.
Sesamoiditis refers to the inflammation of the sesamoid bones and surrounding tendons, particularly common in the foot. This condition often results from repetitive stress or overuse, frequently affecting dancers, runners, and athletes. Symptoms include a dull, aching pain under the big toe joint, which can worsen with activity or specific footwear, along with localized swelling and tenderness.
Sesamoid bones are also prone to fractures, which can be either acute (from direct trauma) or chronic (from repetitive stress). Fractures in the foot’s sesamoids can lead to immediate pain and swelling in acute cases, or longstanding, activity-aggravated pain in chronic stress fractures.
Another condition is osteonecrosis, also known as avascular necrosis, which involves the loss of blood supply to the bone, potentially leading to tissue death and bone collapse.
In the knee, the patella can be involved in conditions such as patellofemoral pain syndrome (PFPS), sometimes called “runner’s knee.” This condition causes pain around or under the kneecap, often due to overuse, muscle imbalances, or issues with how the kneecap tracks in its groove.