A small, unique category of bone exists outside of the standard skeletal framework. These specialized bones are found embedded directly within the soft tissue of a tendon or joint capsule. This unique anatomical feature functions to improve the efficiency of the musculoskeletal system.
Classification of Sesamoid Bones
The type of bone embedded within a tendon is called a sesamoid bone. The name “sesamoid” is derived from the Greek word for ‘sesame seed,’ reflecting the typically small, rounded shape of these bony structures. Unlike the bones that form the main skeleton, sesamoid bones do not articulate with other bones at a joint; rather, they exist as focal areas of ossification within the soft tissue.
These bones are classified as a type of accessory bone, meaning their presence or size can vary significantly among individuals. While every person has a consistent set of sesamoids, such as the largest one in the body, many others are considered anatomical variants, primarily located in the hands and feet. They develop in response to mechanical stress and strain on a tendon near a joint surface.
Sesamoid bones are distinct from long, short, flat, and irregular bones. Structurally, they are formed through endochondral ossification, where cartilage is replaced by bone. Their location makes them functionally distinct, as they act as mobile parts of the tendon, moving with it as the muscle contracts and relaxes.
Biomechanical Function
The primary function of a sesamoid bone is to enhance the mechanical advantage of the muscle-tendon unit. They accomplish this by acting as a spacer that holds the tendon slightly away from the center of rotation of the joint. By increasing this distance, the sesamoid bone effectively lengthens the lever arm through which the muscle’s force is applied.
This mechanism allows the muscle to generate a greater rotational force, or torque, on the bone it is trying to move. The sesamoid bone provides a smooth surface for the tendon to slide over, redirecting the line of pull and increasing the efficiency of muscle contraction.
A secondary role is to protect the tendon from excessive friction and compressive forces. By interposing the bone between the tendon and the underlying bony surface of the joint, the sesamoid absorbs the wear and tear that would otherwise degrade the tendon. This reduction in friction helps preserve the tendon’s integrity, particularly in high-stress areas like the knee and foot.
Major Anatomical Examples and Related Injuries
The most prominent and constant example of this bone type is the patella, or kneecap, which is the largest sesamoid bone in the human body. The patella is embedded within the quadriceps tendon and its role is to increase the leverage of the thigh muscles, making it easier to straighten the leg. Without the patella, the force required to extend the knee would be significantly higher, making walking and running more difficult.
Other common locations are the feet and hands, particularly the joints of the big toe and the thumb. The foot typically contains two small, pea-shaped sesamoid bones located beneath the head of the first metatarsal, at the ball of the foot. These sesamoids assist the flexor tendons of the big toe, helping to bear weight and propel the body during movement.
Because they are located in high-stress areas, sesamoid bones are susceptible to injury. A common issue is sesamoiditis, which is the inflammation of the tendons and surrounding tissues. This condition often affects athletes like dancers and runners whose activities place repetitive, high-impact stress on the forefoot. They can also fracture from an acute traumatic event or from chronic, repetitive stress. The limited blood supply to these bones can make healing a slow process.