The fabella sign: fabella displacement in synovial effusion and popliteal fossa masses. Normal and abnormal fabello-femoral and fabello-tibial distances.

Posterior displacement of the fabella is a reliable sign of synovial effusion or mass which may be applied even when inflammation or post-traumatic edema obscures the extrasynovial fat lines of the knee joint. The fabella normally moves posteriorly as the knee is flexed, but correlation of fabella position with degree of flexion defined ranges of normal and abnormal fabella position which are useful in diagnosis. If all lateral views are obtained with the knee flexed over a standard angle bolster, fabella position is standardized and fabella displacement readily detected without need for measurement. With experience, fabella displacement is readily apparent in non-standard positions as well. Normal fabella position does not rule out small effusion or effusion which is confined to the suprapatellar pouch because the knee is extended.