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Radial Head Fractures:
Radial head fractures can often be difficult to visualize on plain films especialing Mason Type 1 fractures (see prior pearl on classification system) which are nondisplaced. Often the only sign of a fracture will be a posterior fat pad sign which is always considered to be pathologic. The posterior fat pad lies outside the synovium of the elbow joint and is normally hidden in the fossa of the distal humerus preventing it from being seen on lateral films of a normal elbow. Trauma to the elbow that results in a intraarticular fracture (typically a radial head fracture) produces an intra-articular hemorrhage that distends the synovium and displaces the fat out of the fossa, producing the typical triangular radiolucent shadow posterior to the distal end of the humerus.
References
Elbow Fat Pad Sign in Atlas of Signs in Musculoskeletal Radiology
A. Gentili MD, M. Beller MD, S. Masih MD, L.L. Seeger MD
http://www.gentili.net/signs/11.htm