The elbow x-rays show a large haemarthrosis; there is elevation of both the anterior and posterior fat pads. The radiocapitellar line is disrupted as a result of radial head dislocation. There is no fracture seen in the distal humerus, radius or ulna.
The radiocapitellar line is drawn through the long axis of the radial head/neck and should bisect the capitellum in both flexion and extension of the elbow. If not, suspect a radial head dislocation, which is usually treated with a closed reduction.
Isolated radial head dislocation is rare. Hence, it is important to look for associated ulnar fracture which, if present, results in a Monteggia fracture dislocation; this injury needs operative management.