The sail sign on an elbow radiograph describes the elevation of the anterior fat pad to create a silhouette similar to a billowing spinnaker sail from a boat. It indicates the presence of an elbow joint effusion.
The anterior fat pad is usually concealed within the coronoid fossa or seen paralleling the anterior humeral line. When there is a joint effusion, the anterior fat pad (which is intra-articular, but extra-synovial) becomes elevated.
Elevation of the anterior fat pad usually heralds the presence of an intra-articular fracture. In adults, this is usually a radial head fracture whereas in children, the commonest cause of a raised elbow fat pad is a supracondylar fracture.
Where a fat pad is raised and no fracture is demonstrated, an occult fracture should be suspected.
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