Discussion Author(s): Grant Lattin
Dislocations of bones in the wrist usually occur following a fall on an outstretched hand (FOOSH), accounting for 10% of carpal injuries. Dislocation of any of the bones can occur but the lunate is most commonly dislocated. These dislocations are divided into lunate, perilunate, and midcarpal dislocations. These designations are based on the position of the lunate or adjacent carpals in reference to the radius. Disruption of the ligaments attached to the lunate leads to these different types of dislocation. Severity of a dislocation is staged on a scale of I-IV with I being the least severe and IV corresponding to the worst.
Staging of Wrist Dislocations:
Stage I: Scapholunate ligament failure. Leads to scapholunate dissociation or rotary scaphoid subluxation. Radiographically, >3mm between scaphoid and lunate on PA view.
Stage II: Capitolunate ligament failure. Leads to capitate dislocation (perilunate dislocation); dorsal dislocation on lateral view. Radiographically, Lunate still in proper position in relation to radius on lateral view.
Stage III: Lunotriquetral ligament failure. Leads to midcarpal dislocation. Radiographically, volar lunate shift on PA view in relation to radius with dorsal capitate displacement.
Stage IV: Dorsal radiocarpal ligament failure. Leads to complete lunate dislocation.
Although this case presented as a lunate dislocation it should be pointed out that perilunate dislocations are 2-3 times more likely than a lunate dislocation and are usually associated with a transverse fracture of the scaphoid.
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