DISLOCATION OF CARPUS
The strong volar radiocarpal ligament between the lunate, radius, and distal row of the carpal bones provides strong support for the volar aspect of the carpus; ligament support is weaker on the dorsal side. In addition, the ligament attachments from the radius to the proximal carpal row are much stronger than the attachments from the proximal carpal row to the distal carpal row. This disparity in the support between the two carpal rows and the lack of a significant lunocapitate support make the carpus particularly susceptible to dislocation and chronic instability (see Plate 3-28).
Carpal instability results from hyperextension of the wrist, as in a fall on the outstretched hand. The amount and direction of the force determine the degree of resulting instability around the lunate. The first stage, and most minor degree, of perilunate instability is the tearing of the ligament between the scaphoid and the lunate, followed by disruption of the radioscaphoid ligament. These injuries produce a scapholunate diastasis. In the second stage, with further dorsiflexion, the radiocapitate ligament ruptures, leading to dislocation of the lunate. In the third stage of injury, the radiotriquetral ligament ruptures, resulting in perilunate dislocation associated with lunotriquetral instability. In the final stage, the hand and distal row of the carpus supinate on the triquetrum, tearing the dorsal radiotriquetral ligament and causing the capitate to push the unstable lunate volarly; these events result in a volar dislocation. The signs and symptoms of a volar dislocation of the lunate include pain and swelling in the wrist. Paresthesia and dysesthesia of the median nerve are quite common associated problems.
With lunate and perilunate dislocations, the antero- posterior radiograph often shows the lunate as wedged, or pie shaped, rather than four sided. On the lateral radiograph, the lunate appears rotated out of its articulation with the head of the capitate and pointing volarly; sometimes, the lunate is completely dislocated volarly.
Initial treatment of lunate and perilunate dislocations includes a thorough neurovascular examination followed by closed reduction of the dislocation. The reduction can be performed using regional or general anesthesia. Traction is applied by placing the fingers in fingertraps and hanging a 10- to 20-lb counterweight from the upper arm. An anteroposterior radiograph should be taken of the wrist in traction to determine the degree of ligament damage and to identify any associated osteochondral fractures.
After allowing the wrist to remain distracted for 10 to 15 minutes, the examiner places his or her thumb on the volar aspect of the wrist over the dislocated lunate. The injured wrist is gradually flexed volarly and pronated while thumb pressure is applied over the lunate to reduce it. If adequate closed reduction is obtained, the wrist is splinted in anticipation of definitive treatment.
Post-traumatic carpal instability is now recognized as a common complication of these injuries, and many orthopedic surgeons prefer open reduction of lunate and perilunate dislocations and stabilization of carpus injuries with wires or screws. ORIF of a lunate dislocation has several advantages. The procedure achieves and maintains anatomic reduction of the fragments and allows repair of the torn ligaments at the same time. Also, the wrist joint is debrided of any loose osteochondral fragments.
Carpus dislocations may also involve fractures of the scaphoid, triquetrum, and capitate, as well as the styloid process of the radius. In these injuries, the dislocations cause fractures rather than ligament ruptures. The best way to ensure adequate alignment and reduce the risk of late wrist instability is anatomic reduction and rigid internal fixation.
Prompt recognition and treatment of carpus instability can restore satisfactory hand and wrist function. However, decreased range of motion and early degenerative arthritis are still common complications, particularly after severe injuries.