Fabella sign

The fabella sign is displacement of the fabella that is seen in cases of synovial effusion and popliteal fossa masses.[1]

Fabella sign
Differential diagnosissynovial 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.[2]

References
  1. 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.
  2. Friedman A, Naidich T (1978). "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. doi:10.1148/127.1.113. PMID 635171.
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