Recognising an elbow joint effusion on lateral radiographs is an essential radiology skill. While the fluid itself is not discretely seen because it is the same density as the surrounding muscles, an effusion can be inferred by observing displacement of the anterior and / or posterior fat pads surrounding the distal humerus.
The sail sign refers to the triangular appearance of the displaced low density anterior fat pad. Normally the anterior fat pad is only seen as a straight line paralleling the distal humerus. The posterior fat pad is not normally visible on a lateral image as it is hidden within the olecranon fossa. Any visualisation of the posterior fat pad is a sign of an elbow joint effusion.
An elbow joint effusion in the setting of trauma is typically a sign of an occult fracture. In adults the occult fracture is most commonly of the radial head while in children a non-displaced supracondylar fracture should be suspected.