History:
A 13 y/o boy developed pain over his left elbow after a fall to the ground.

Questions:

1. What are the findings?
2. What is the diagnosis?

Answers:

1. Anteroposterior (AP) and lateral radiography of the elbow:
   l Prominent anterior and posterior fat-pad signs.
   l Minimal displaced step off defect and a radiolucent line over the distal humerus.
   l There is slight posterior angulation of the distal humerus .. 
2. Supracondylar fracture of the distal humerus

Discussion
The fat-pad sign at the elbow indicating an elbow effusion. Distension of the elbow capsule by fluid lifts up the anterior and posterior fat pads at the superior aspect of the elbow joint, displacing them from their normal positions and rendering them visible radiographically on the lateral view. A small portion of the anterior fat pad normally is visible as a thin triangular radiolucency with a short base facing the elbow joint; the posterior fat pad is contained fully within the olecranon fossa and is not visible. In the setting of acute trauma, the fat-pad sing is indicative of fracture, but an effusion from any cause may result in a fat-pad sign. More than 90 % of children or adolescents with posterior fat-pad sigh will have a demonstrable fracture, and the absence of the fat-pad sign in this age groups virtually excludes an intraarticular fracture, unless the injury is so severe that there is disruption of the elbow joint capsule. The supracondylar fracture comprises 60% of fractures around the elbow in children. Radial head and neck fracture, common in adult, generally are not seen in children. Supracondylar fractures occur with hyperextension of the elbow, usually from a fall. The fracture extends transversely across the distal humerus through the coronoid and olecranon fossa, above the level of the condyles. The distal fragment is angulated posteriorly, so that the anterior humeral line passes anterior to the capitellum. A posterior fat-pad sign almost always is present. The fracture usually is complete, but greenstick fracture, torus fractures, or plastic bowing are possible. The typical treatment is closed reduction with casting.

Reference
1. S. P. Bohrer. The fat pad sign following elbow trauma: its usefulness and reliability in suspecting "invisible" fracture. Clin Radiol 1970; 21: 90-94