The fabella is an accessory ossicle located inside the gastrocnemius lateral head tendon on the posterior side of the knee, in about 25% of people. It can thus serve as a surrogate radio-opaque marker of the posterior border of the knee's synovium.
In approximately 25% of people, the fabella is a small bone embedded within the gastrocnemius tendon on the back of the knee. This accessory bone can be used to identify the posterior edge of the knee joint's lining in radiographic images.