Fabella sign

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

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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.^[2]

References

1. ^ Hall F (1978). "The fabella sign and radiologic assessment of knee joint effusion". Radiology 129 (2): 541-2. 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. PMID 635171.