Radiology, Vol 127, 113-121, Copyright © 1978 by Radiological Society of North America
The fabella sign: fabella displacement in synovial effusion and popliteal fossa masses. Normal and abnormal fabello-femoral and fabello- tibial distances
AC Friedman and TP Naidich
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.
