What are the posterior fat pad and sail signs?

The posterior fat pad and sail signs indicate an elbow fracture. Ordinarily, the posterior fat pad is not visible on a lateral view, and the anterior fat pad is seen as a small crescent adjacent to the anterior surface of the supracondylar humerus. With fractures of the elbow, the hemarthrosis distends the joint capsule, making the posterior fat pad visible and lifting the anterior fat pad away from the bone, causing it to look like a sail.