Fabella

The fabella (Latin for little bean) is a small sesamoid bone found in some mammals embedded in the tendon of the lateral head of the gastrocnemius muscle behind the lateral condyle of the femur. It is a variant of normal anatomy and present in humans in 10% to 30% of individuals. Rarely, there are 2 or 3 of these bones (fabella bi- or tripartita). It can be mistaken for a loose body or osteophyte. On radiographs it appears as a small bony shadow behind the knee joint. Intra-articular fluid in the knee may sometimes cause displacement of the fabella what is called fabella sign.

Plain film lateral with fabella indicated (arrow)

MRI Sagittal T1 with fabella indicated by arrow; normal low signal cortex and high signal marrow

In osteoarthritis of the knee, the fabella may undergo cartilaginous fibrillation and erosion and bone proliferation. Flattening and sclerosis may be observed radiographically on the anterior surface. An enlarged, malpositioned or even normal fabella may lead to entrapment or peroneal nerve injury. Other pathologic conditions involving the fabella include fractures, dislocations and, in younger patients, chondromalacia.