Carpal and carpometacarpal dislocations are serious wrist injuries usually occurring from hyperextension. Patients complain of decreased range of motion, pain, swelling, and ecchymosis. Lunate dislocation can occur in a palmer or dorsal position with the lunate displaced relative to the other carpals (“spilled teacup sign”). The normal lunoradial relationship is disrupted, and the median nerve is commonly involved. If the lunoradial articulation is intact and the other carpal bones are dislocated relative to the lunate, it is termed a perilunate dislocation.
Another potentially serious injury is scapholunate dissociation, often mistakenly diagnosed as a sprained wrist. Although exam may be unremarkable except for pain, an AP radiograph reveals a widening of the scapholunate joint space (Terry Thomas sign). A space of ≥ 3 to 4 mm should prompt suspicion of scapholunate ligament disruption. The lateral radiograph may reveal an increase of the scapholunate angle to greater than 60 to 65 degrees (normal 45-50 degrees). All these injuries may present with concomitant fractures of the carpal bones or distal forearm.
Lunate Dislocation. This photograph demonstrates swelling associated with a volar lunate dislocation. (Photo contributor: Cathleen M. Vossler, MD.)
Carpometacarpal dislocations of the index and long metacarpals are fortunately rare since functional loss is often marked. Thumb, ring, and small finger carpometacarpal dislocations are more common and are frequently missed injuries.
Management and Disposition
Initial management includes adequate radiographic evaluation followed by ice, elevation, and splinting. Referral to a hand specialist is essential for adequate reduction and long-term care.
A true lateral wrist radiograph best demonstrates a lunate dislocation by exhibiting the usual cup-shaped lunate bone as lying on its side and displaced either dorsally or palmerly.
On lateral wrist radiographs, the metacarpal, capitate, lunate, and radius should all align within a line drawn through their long axes. If this is not found, some element of dislocation, subluxation, or ligamentous instability likely exists.
Carpometacarpal dislocations are frequently difficult to reduce and require open reduction and fixation in approximately 50% of cases.
Trauma to the lunate can lead to Keinbock’s disease (avascular necrosis of the lunate), which causes chronic stiffness and pain. Other medical conditions such as lupus and sickle cell disease can predispose patients to developing this condition.
Perilunate Dislocation. The force from a fall on an outstretched hand disrupted the lunate-capitate articulation; the capitate and other carpal bones were driven posteriorly with respect to the lunate. (Photo contributor: Alan B. Storrow, MD.)
Lunate Dislocation. (A) PA wrist view shows disruption of the carpal arcs. (B) Lateral reveals volar displacement of the lunate. The radius, capitate, and third ...