● The fabella can also be fibrocartilaginous in nature and is occasionally found in the medial head of the gastrocnemius. The fabella articulates with its respective (medial or lateral) femoral condyle.
● It is an accessory bone, an anatomical variation present in 39% of humans.
● Rarely, there are two or three of these bones (fabella bi- or tripartita).
● It can be mistaken for a loose body or osteophyte.
● The word fabella is a Latin diminutive of faba 'bean'.
● It is more common in men than women, older individuals compared to younger, and there is high regional variation, with fabellae being most common in people living in Asia and Oceania and least common in people living in North America and Africa.
● Bilateral cases (one per knee) are more common than unilateral ones (one per individual), and within individual cases, fabellae are equally likely to be present in right or left knees.
● The fabella can lead to posterolateral knee pain either due to cartilage softening (chondromalacia fabellae) or other osteoarthritic changes on its articular surface.
● The most relevant differential diagnosis is the cyamella which is a rare sesamoid bone that exists as a normal variant within the popliteus tendon, characteristically located at the posterolateral aspect of the distal femur in the popliteal groove.
■ A cyamella (sometimes called popliteal fabella, fabella distalis, sesamoideum genu inferius laterale).
■ Cyamella is best seen on the AP view of plain radiograph as opposed to the fabella, which is located in the lateral head of the gastrocnemius and best appreciated on the lateral view.
■ It is round/ovoid, sometimes flattened and generally smaller than fabella.
■ Three locations have been described :
▪︎ Lateral to lateral femoral condyle, near popliteal sulcus (origin of popliteus tendon).
▪︎ Lateral to tibial condyle, near tibial plateau and fibular head (close to popliteofibular ligament).
▪︎ Between tibia and fibular head, posteromedial to tibiofibular joint (distal popliteus).