Most people are familiar with the largest of these structures, the kneecap. Sesamoid bones are a category of bone that is not included in the standard count of 206 bones, because their number varies widely from one person to the next.
Defining Sesamoid Bones
A sesamoid bone is an anatomical structure embedded within a tendon or, less commonly, within a muscle. Unlike the majority of bones in the skeleton, sesamoids are unique because they do not articulate with other bones at a joint. Instead, they are suspended entirely within the soft tissue structure of the tendon.
The Fixed Count Versus The Variable Count
Determining the exact total number of sesamoid bones in the body is impossible because their presence is highly individual and often asymmetrical. A fixed, consistent count is typically limited to the two patellae, one in each knee, which are present in every human being. Beyond these two, all other sesamoid bones are considered accessory or variable, meaning they may or may not develop in a given person. The sheer number of these variable bones leads to a wide range in total count among individuals.
While some sources commonly cite a total of 20 to 25 sesamoid bones, studies have reported finding as many as 42 in a single adult skeleton. The development of these accessory bones is thought to be influenced by both genetic factors and mechanical stress placed on tendons. Locations that experience high levels of strain, friction, or pressure are the most likely places for a sesamoid bone to form over time. The highest variability in the count is found in the hands and feet.
For instance, the thumb joint almost always has two sesamoid bones, but the presence of similar small bones in the index and little fingers is far less consistent. The small size and varied ossification of these structures mean that a person’s total count can shift throughout their lifetime. The number is not considered fixed because the formation of these bones is a response to dynamic forces, making them an example of the skeleton adapting to an individual’s unique movement patterns.
Primary Locations and Functions
The existence of sesamoid bones is directly tied to their mechanical function, which primarily involves increasing the efficiency of muscle action. These bones act as a pulley, which keeps the tendon slightly farther away from the center of the joint. This increased distance, known as the moment arm, allows the associated muscle to generate greater force for the same amount of effort. They also provide a smooth surface for tendons to glide over, thereby reducing friction and protecting the soft tissue from excessive wear and tear.
The Patella
The patella, or kneecap, is the largest and most well-known example, and it perfectly illustrates the concept of leverage. It is embedded within the tendon of the powerful quadriceps muscle group. The patella increases the mechanical advantage of the quadriceps, allowing the muscle to extend the knee with significantly greater power than it could without the bone. This increased leverage is especially important for activities like running, jumping, and climbing stairs.
Sesamoids of the Foot
In the foot, two small sesamoid bones are consistently found beneath the head of the first metatarsal bone, which is the joint at the base of the big toe. These tiny bones are embedded within the tendon of the flexor hallucis brevis muscle. They play a significant role in weight bearing and walking mechanics, helping to absorb pressure and facilitate the act of pushing off the ground. The sesamoids of the big toe protect the tendons from flattening when pressure is applied, ensuring that the toe can flex effectively during movement.
Sesamoids of the Hand
Similarly, in the hand, two small sesamoids are nearly always present on the palm side of the metacarpophalangeal joint of the thumb. These bones are located within the tendons that control thumb movement and are integral to gripping and pinching motions. They help to stabilize the joint and increase the power of the short muscles that allow the thumb to oppose the fingers. The sesamoids in the hand contribute directly to the dexterity and strength required for fine motor tasks.