CARPAL DISLOCATIONS

Wrist dislocations are a continuum of perilunate injuries which range from disassociation to dislocation. The usual mechanism is a fall on an outstretched hand. The four successive stages of injury progress from radial to ulnar side and indicate increased carpal instability. The lateral view of the wrist is the optimal view to diagnose wrist dislocations using lines drawn through the radius which, in a normal wrist, should intersect the lunate and capitate. The AP view is also helpful in assessing three arcs which follow the proximal and distal surfaces of the scaphoid, lunate and triquetrum as well as the proximal surfaces of the hamate and capitate. Disruption of any one of these arcs is indicative of carpal disruption.
Scapholunate dissociation (stage 1) is rupture of the scapholunate ligament with greater than a 3 mm gap between the lunate and scaphoid (Terry-Thomas sign or David Letterman sign). A ring sign on the AP view may be seen secondary to rotary subluxation of the scaphoid.
Perilunate dislocation (stage 2) is 2-3 times more common than lunate dislocation. The capitate dislocates dorsally and the lunate maintains a normal articulation with the radius. Seventy-five percent of cases are accompanied by transcaphoid fractures. Triquetrum, capitate and ulnar styloid fractures can also be seen. Although the lunate may appear triangular in shape on the AP view of the wrist (normally rhomboid in shape), this is not pathognomonic for dislocation as it may appear triangular in normal flexion/extension as the lunate tilts.
Midcarpal dislocation (stage 3) is due to rupture of the triquetral ligaments. The lunate dislocates volarly and the capitate and carpus dislocate dorsally.
Lunate dislocation (stage 4) occurs when the lunate dislocates and rotates volarly and the capitate remains aligned with the radius. If undiagnosed, this can lead to median nerve impairment from compression.