Sesamoid bone

A sesamoid bone is a special kind of bone found inside a tendon or muscle. These tiny bones get their name from the Greek word for "sesame seed" because many of them are very small, just like the tiny seeds we use to flavor bread. In the human body, the largest sesamoid bone is the patella, which is the kneecap.

Sesamoid bones help tendons move smoothly. They act like pulleys, making it easier for muscles to pull on bones and create movement. This helps our muscles work more efficiently, allowing us to jump, run, and bend with less effort. Without these small but important bones, our movements would not be as smooth or strong.

Structure

Sesamoid bones are small bones found inside tendons and muscles. They help protect and improve the movement of joints.

You can find sesamoid bones in several places in your body, such as:

• In the knee—the patella is the largest sesamoid bone and sits inside the quadriceps tendon.
• In the hand—two sesamoid bones are often found near the base of the thumb, inside tendons.
• In the wrist—the pisiform is a sesamoid bone inside a tendon.
• In the foot—usually, there are two sesamoid bones near the big toe, inside a tendon that helps you bend your toe.

Common variants

Some people have extra small bones called sesamoid bones in their bodies. One common example is under the big toe, where the sesamoid bone can sometimes be split into two parts instead of one. Another example is a tiny bone called the fabella, found behind the knee in some people, usually in about 10% to 30% of individuals. There is also a rare bone called the cyamella, located behind the knee in the tendon of a specific muscle, seen more often in other animals than in humans. These variations are just normal differences in anatomy.

Clinical significance

Sesamoid bones can sometimes cause problems, especially in the feet. Dancers may experience pain from a condition called sesamoiditis, which is inflammation of the sesamoid bones near the big toe. This happens when the tendons around these bones become irritated.

Because sesamoid bones have limited blood supply, they can be vulnerable to a serious issue known as avascular necrosis, where the bone tissue dies due to insufficient blood flow. This condition is challenging to treat.

Other animals

In horses, sesamoid bones are found at the back of the fetlock joint in both their front and back legs. These bones help the horse move smoothly.

Some animals, like giant pandas and red pandas, have special sesamoid bones that help them grasp bamboo. These bones evolved separately in each species. Elephants also have unique sesamoid bones that act like extra toes, helping them carry their heavy weight. Moles have special sesamoid bones on their front paws that help them dig.