yogabook / bones / fabella (sesamoid bone)

The fabella (also: Os sesamoideum musculi gastrocnemii) is a sesamoid bone connected to the tendon of the lateral origin of the gastrocnemius on the posterior side of the lateral condyle of the femur. It thus protects the tendon from pressure exposure and increases the effective lever arm so that a greater moment is produced in the joint with the same muscle force. The fabella is sometimes incorrectly interpreted as an ossicle in X-rays, but it is actually an independent bone due to the cortical bone on all sides. A 2019 study showed an increase in the frequency of this normal variant from 11% to 37% in the 100 years from 1918 to 2018. The frequency in the population today is given as 10-30%. It is higher in Asia and increases with age. Fabella is rare in Africa. Fabella is present in both knees in 73% of cases. The predisposition to fabella appears to be genetically determined, but it presumably first develops through appropriate stress stimuli; a better diet and a higher body weight appear to contribute to this.
Rarely there are two or three smaller fabella (Fabella bipartita or tripartita). In some animals, such as dogs, cats and Old World monkeys, a fabella is present on both sides of the gastrocnemius origins; in humans it is exclusively lateral.
Depending on the perspective, the fabella can be mistaken for a bony shadow in the adductor magnus or the medial origin of the gastrocnemius and the medial ligament.
If the fabella is pronounced and the body fat percentage is low, it may be palpable.

More rarely, pain is caused by the fabella, which is often referred to as fabella syndrome (also: fabella dolorosa) and is treated either conservatively or by removing the fabella. Fractures are also possible, usually due to trauma. Splintering sometimes also occurs in connection with supination trauma or twisting trauma to the knee. In some cases, the common peroneal nerve is compressed by the fabella.