X-ray of the knee of a patient with a fabella. This is a sesamoid bone, a small bone that is embedded in tendon or muscle, that is found in the gastrocnemius muscle at the back (right) of the knee joint near the end of the femur (thigh bone). It is present in around 10 to 30 per cent of people and is often asymptomatic. If it does cause symptoms it can be surgically removed.