fabella

The fabella is a small sesamoid bone embedded within the tendon of the lateral head of the gastrocnemius muscle at the posterolateral aspect of the knee. It is one of several accessory bones that can occur in the human knee and is variable in presence among individuals.

Anatomy and variation: The fabella typically lies near the lateral femoral condyle behind the knee joint. It may articulate with the femur or remain nonarticular. The bone is connected to surrounding structures by ligaments such as the fabellofemoral and fabellopatellar ligaments, and a fabellofibular ligament may link it to the fibula. Ossification of the fabella occurs in late childhood or early adulthood.

Prevalence and presentation: The fabella occurs in a minority of people, with its presence varying widely by population and individual; it can be unilateral or bilateral when present. On radiographs, it appears as a small round or oval bone behind the knee, typically near the posterior aspect of the lateral femoral condyle.

Clinical significance: Most fabellae are asymptomatic and discovered incidentally. They can, however, cause posterolateral knee pain or discomfort if enlarged, after trauma such as a fracture, or in association with knee osteoarthritis—an occurrence sometimes referred to as fabella syndrome. Diagnostic imaging includes X-ray, CT, or MRI. Treatment is generally conservative; persistent symptoms may be alleviated by surgical removal of the fabella (fabellectomy).

Etymology: The name fabella derives from Latin fabella, meaning a small bean.