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.
