Imaging (Fat Pads)

Fig 1: Normal lateral x-ray of the elbow

Fig 2: Displaced anterior fat pad (yellow ring) in association with a posterior fat pad (red ring)

On a normal lateral x-ray of the elbow (Fig 1), an anterior fat pad is visible due to fat in the joint capsule. It is never normal to see a posterior fat pad because this is hidden in the intercondylar fossa.

Fig 2 shows a displaced anterior fat pad in association with a posterior fat pad. This is known as the sail sign because of its resemblance to the sails of a boat. In the setting of acute trauma it represents blood in the joint. In the non-trauma setting effusion may be due to an inflammatory cause. Note that if the fracture is extra-articular, then there may not be a joint effusion and therefore the fad pad sign will be absent.

Examine the radial head closely in these injuries as there is often a subtle fracture. Presence of a posterior fat pad has been associated with a 75% rate of occult fracture [1].

Learning bite

The anterior humeral line, radio-capitellar line and the presence or absence of fat pads can aid x-ray interpretation.