Imaging description

Fat planes are often present on radiographs but may be displaced or obliterated by soft tissue swelling and hemorrhage after acute trauma. Several of these fat pads have been described [1], but by far the most useful are the intracapsular fat pads of the elbow and suprapatellar fat pads of the knee.

In the absence of an effusion, the posterior fat pad of the elbow usually rests within the olecranon fossa, hidden from view by the humeral condyles on the lateral radiograph of the elbow in 90 degrees of flexion (Figure 82.1). If the elbow is extended, the posterior fat pad may be seen even in the absence of an effusion [2, 3]. There are also two normal anterior fat pads, which lie along the anterior aspect of the distal humerus; one in the coronoid fossa, and the second in the radial fossa. The anterior humeral fat pads are normally visible in adults without fractures. On the lateral elbow radiograph, they are superimposed and appear flat or triangular in shape (Figure 82.2).

Any process that distends the joint capsule of the elbow will elevate the fat pads. Identification of the posterior fat pad on a technically adequate radiograph virtually assures the presence of effusion [4]. Displacement of the anterior fat pad creates a sail shape, though this may be difficult to see if the paired fat pads are no longer superimposed [5]. Following trauma, the most likely reason for an effusion is an intracapsular fracture (Figures 82.3–82.5), most commonly a radial head fracture in adults (Figure 82.3) [5], and a supracondylar fracture in children [6]. However, 6–29% of children with an isolated posterior fat pad sign will not have an intracapsular fracture on follow-up imaging and clinical assessment [6].