Radial Head Fracture Imaging Signs: Fat Pad Sign and Mason Classification

The Mason classification categorizes radial head fractures into four types. Type I fractures are non-displaced or minimally displaced (less than 2 mm) with no mechanical block; these are treated non-operatively with early mobilization. Type II fractures involve displacement of greater than 2 mm or greater than 30 degrees of angulation and may require fixation if a mechanical block to forearm rotation exists. Type III fractures are comminuted involving the entire radial head, typically requiring radial head replacement rather than fixation. Type IV fractures, added by Johnston, are any radial head fracture associated with an elbow dislocation, significantly increasing instability and complexity of management.

A posterior (sail sign) fat pad on lateral elbow radiograph in the setting of trauma indicates a joint effusion and a presumed occult fracture, most commonly a radial head fracture, even when the fracture line itself is not visible. The anterior fat pad, when elevated and sail-shaped rather than lying flat, also supports an effusion. If radiographs are equivocal and a fracture is suspected clinically, CT provides superior fracture characterization including fragment size, displacement, and comminution, and is essential for surgical planning. MRI is useful when associated ligament injuries or chondral damage need assessment.

Yes. Radial head fractures frequently occur alongside other elbow and forearm injuries. The terrible triad injury combines elbow dislocation, radial head fracture, and coronoid fracture, creating multiplanar instability requiring comprehensive surgical reconstruction. An Essex-Lopresti injury involves a radial head fracture with disruption of the interosseous membrane and distal radioulnar joint, leading to proximal radial migration if the radial head is excised without replacement. Medial collateral ligament tears often accompany radial head fractures and must be assessed under fluoroscopic valgus stress testing.