The top left x-ray of the elbow is normal. The top right x-ray of the elbow has an occult fracture of the radial head. An occult fracture is when you cannot see the break on an x-ray. How did I know the radial head bone was broken if I couldn’t see the fracture line on the x-ray????
Answer: the bottom image has the positive “fat pad sign.” This anterior and posterior fat pad sign is circled in red. The soft tissue shadow located on the front and back of the elbow bone indicates that there is blood in the elbow joint. The fat pad which normally sits adjacent to the bone is not seen on a normal x-ray.
When there is bleeding in the joint it elevates the fat pad and it looks like a sail on a sail boat. The most common cause for a sail sign is an occult fracture of the radial head. This patient was also very tender to palpation over the radial head which matched the x-ray. Thanks Carina Curnow Burns ,M.D. and Becky Fuller Olds , N.P. for the great referral and the perfect textbook radiographic image!