The fabella (Latin: little bean) is a sesamoid bone found in the head of the lateral gastrocnemius muscle. It is present in about 12-15% of the population, although there have been studies suggesting much higher rates, probably because they are frequently cartilaginous and therefore missed on plain X-rays.

Some 85% are bilateral. Its point of articulation is on the posterolateral femoral condyle and can be ossified or fibrocartilaginous. It is often mistaken for a lose body in the knee or and osteophyte.

The fabellofibular ligament attaches to it, which can be present instead of, or alongside, the arcuate ligament, aiding the stabilisation and reinforcement of the posterior knee capsule. The development of the fabella and this ligament relate to the evolutional transition to bipedal posture about 5 million years ago.

The fabella has been associated with a number of clinical problems, including common peroneal nerve compression, vascular insufficiency, osteoarthritis, and stress fracture. A link between the presence of a fabella and the development of primary osteoarthritis (OA) of the knee has been postulated, with approximately 35% of patients with primary knee OA having at least a single fabella, whilst they are present in only 15% of individuals without OA.

Symptomatic osteoarthritis of the fabella itself is exceedingly rare, with very few cases ever documented or confirmed. One such case was documented but lacked confirmation via a histological diagnosis. We confirmed a case ourselves in 2011.

References available on request.

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Author competing interests: nil relevant disclosures.

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