Dislocation of the distal radioulnar joint (DRUJ) without concomitant fracture is an uncommon injury. DRUJ subluxation is very easy to miss in the first presentation. It would appear that a common cause of misdiagnosis is to dismiss the appearance as positional/projectional.

Anatomy

"Distal ulnaulna 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

A study of the role of the interosseous membrane in the stability of the DRUJ reported the following

- widening of RU joint on AP view;
- fracture (or non union) at base of ulnar styloid;
- significant shortening of the radius;
- obvious dislocation on the lateral view;
- it is essential that the lateral view be taken w/ proper technique so that the radial styloid process overlies the proximal pole of the scaphoid, lunate, and triquetrum;
- when proper positioning is ensured, dorsal or volar subluxation is noted by the relative position of the ulna above or below the radius;

http://www.wheelessonline.com/ortho/radial_ulnar_joint_instability

Case 1
This 45 year old lady presented to the Emergency Department after falling onto her wrist.

There is overlap of the distal radio-ulnar joint. There is also a moderate degree of positive ulnar variance.	The position of the ulna on this oblique wrist view is unusual.	The ulna appears to be subluxed in a volar direction. The wrist is minimally dorsally rotated off lateral as evident by the position of the pisiform. Is this sufficient to account for the position of the ulna?

Case 2
This 38 year old male presented to the Emergency Department with a right forearm injury after a pile of wood fell onto his arm.

This is a mixed AP/lateral view of the forearm- the elbow is AP and the wrist is lateral. This is not ideal positoning for forearm imaging, but is considered acceptable when the patient is unable to adopt true AP/Lateral positioning. There is a fracture of the ulna and the distal ulna appears to be displaced dorsally raising the possibility of a DRUJ subluxation. Unfortunate marker placement.	There is a butterfly fracture of the mid/distal third of the ulna. There is positive ulnar variance.
The wrist appears to be in a lateral position. The ulna is subluxed dorsally. For this position of the ulna to be positional/projectional, you would expect the wrist to be in an oblique position. There is possibly obliteration of the pronator quadratus.	This is a normal lateral wrist (different patient) for comparison.
There is positive ulnar variance. There is a suggestion of a subtle fracture of the distal ulna (arrowed).	Normal PA wrist for comparison (different patient). Note the normal ulnar variance.

Isolated subluxation/dislocation of the DRUJ is an easily missed diagnosis. If the radiographer has identified the possibility of a DRUJ injury, appropriate supplementary views, including a PA and/or lateral view of the contra-lateral wrist, may be useful.


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