Extra-synovial intra-articular fat pads are usually concealed within olecranon and coronoid fossae. A smooth fat hump may be be seen anteriorly in normal elbows. A sail sign is evidence for a joint effusion, displacing the anterior fat pad anteriorly “like a billowing spinnaker“- and often heralds an intra-articular fracture: typically a radial head fracture in adults. In fact, even when one cannot be seen, in the setting of trauma a sail sign should be equated to an occult fracture, and the elbow treated as such.
For the x-ray of the same case, please visit Radiopaedia.org here.
Credit: Dr Frank Gaillard