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Helpful clues in the evaluation of elbow trauma
Fat pads: The fat pad sign can be seen with any joint effusion (infection, inflammation) but in the setting of trauma, effusions are indicative of fractures about the elbow (even if no fracture line can be identified).
There are two fat pads within the elbow. Normally, on a true lateral radiograph only the anterior fat pad is seen as a small triangular radiolucent shadow anterior to the distal humeral diaphysis. The posterior fat pad is ordinarily not visualized on a lateral radiograph because it is tucked away within the olecranon fossa.
Normal lateral view: http://nypemergency.org/images/ElbowNormal.jpg
With fractures, the joint becomes distended with blood. The anterior fat pad becomes displaced superiorly and outward from the humerus giving the so called "sail sign." Similarly, the posterior fat pad gets displaced out of the olecranon fossa and becomes visible on the lateral radiograph.
Anterior (sail) and posterior fat signs: http://nypemergency.org/images/Elbowsfatpadarrow.jpg
References
http://nypemergency.org