On a joint x-ray, the fat pad sign, also known as the sail sign, suggests an occult fracture. It is caused by displacement of the fat pad around the joint. Both anterior and posterior fat pad signs exist, and both can be found on the same x-ray.
In children, a posterior fat pad sign suggests a condylar fracture of the humerus. The fat pad sign only occurs after an intra-articular fracture.
The fat pad sign is invaluable in assessing for the presence of an intra-articular fracture of the joint. An anterior fat pad is often normal. However, a posterior fat pad seen on a lateral x-ray of the joint is always abnormal. the patient will be unable to flex their joint and will require orthopaedic input.
Read the attached Radiological fat pads for more detailed information and images