Mason classification of radial head/neck fractures. A, Type I, nondisplaced. B, Type II, displaced partial articular fracture of ≥2mm displacement or ≥30% of the articular surface. C, Type III, comminuted fracture. D, A type IV injury, described by Johnson  in 1962, indicates an associated ipsilateral ulnohumeral dislocation. Image from Terrible Triad Injury of the Elbow: Current Concepts, JAAOS March 2009 vol. 17 no. 3 137-151 .