Dislocation of the distal radioulnar joint with concomitant fracture is an uncommon injury. Palmar dislocation of the distal ulna is an even more uncommon injury. The injury is very easy to miss in the first presentation. It would appear that the cause of misdiagnosis is to dismiss the appearance as positional/projectional.

Anatomy

"Distal ulna has convex articular surface; this articulates with the concave semicylindrical sigmoid notch of radius.The important stabilizers of the distal radio-ulnar joint include all of the separate structures composing the triangular fibrocartilage complex. Of great clinical importance is the fact that these structures blend at the fovea, thus creating the potential for instability of the distal radio-ulnar joint when the ulnar styloid process is fractured.The flat pronator quadratus muscle originates from a long, narrow strip of the volar aspect of the distal part of the ulna and has a broad insertion on the volar aspect of the radius. It acts as a secondary stabilizer of the DRUJ by providing compressive force across the joint during pronation and supination."
http://www.rcsed.ac.uk/fellows/lvanrensburg/classification/wrist/distalradioulnajoint.htm