Fabella sign

The fabella sign is displacement of the fabella that is seen in cases of synovial effusion and popliteal fossa masses. [cite journal |author=Hall F |title=The fabella sign and radiologic assessment of knee joint effusion |journal=Radiology |volume=129 |issue=2 |pages=541–2 |year=1978 |pmid=704877]

The fabella is a sesamoid bone 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 x-ray 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. [cite journal |author=Friedman A, Naidich T |title=The fabella sign: fabella displacement in synovial effusion and popliteal fossa masses. Normal and abnormal fabello-femoral and fabello-tibial distances |journal=Radiology |volume=127 |issue=1 |pages=113–21 |year=1978 |pmid=635171]

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