 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.