|Differential diagnosis||synovial effusion|
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. On a lateral radiograph of the knee, an increase in the distance from the fabella to the femur or to the tibia can be suggestive of fluid or of a mass within the synovial fossa. This is of particular use in radiographic detection of knee effusions, as the cause for the effusion may obscure the subcutaneous planes on x-ray that can also be used to determine presence of effusion or effusion size.
- Hall F (1978). "The fabella sign and radiologic assessment of knee joint effusion". Radiology 129 (2): 541–2. doi:10.1148/129.2.541a. PMID 704877.
- "The fabella sign: fabella displacement in synovial effusion and popliteal fossa masses. Normal and abnormal fabello-femoral and fabello-tibial distances". Radiology 127 (1): 113–21. 1978. doi:10.1148/127.1.113. PMID 635171.