The radial head fracture (a structure within the synovial compartment) leads to an increase in volume in the synovial cavity via hemarthrosis, causing displacement of the anterior and posterior fat pads that sit within the fossae of the distal humerus. 

This is the fat pad sign due to a joint effusion observed on many abnormal elbow radiographs. A great indicator of occult fractures (in the context of trauma) or other inflammatory processes.