Carpal instability represents about 7% to 10% of all reported injuries to the carpal bones. The majority of carpal instability is centered around the lunate, given its location in the middle of the wrist. Injuries progress sequentially depending on the severity, from scapholunate instability to lunate dislocation. Perilunate dislocations and lunate dislocations are high-energy wrist injuries involving falls from height, motor vehicle collisions, or athletic injuries. While not common, these injuries can result in a multitude of complications. Despite the great force typically required, up to 25% of these injuries are missed clinically and radiographically. Perilunate and lunate dislocations with an associated fracture are twice as prevalent as those without fracture.
The lunate is a crescent-shaped carpal bone which articulates with the distal radius proximally, and the capitate distally. It is located between the scaphoid radially and the triquetrum ulnarly in the proximal row of carpal bones. The scapholunate, capitolunate, and lunotriquetral intrinsic ligaments all stabilize the lunates position in the wrist. There are no muscular of tendinous insertions onto the lunate. The proximal carpal row is relatively unstable when compared to the distal forearm and distal carpal row, resulting in a higher propensity for injury.
Carpal instability is frequently the result of trauma. These injuries frequently result in a fall on an outstretched hand, during which an axial force is directed on the carpals with the wrist in hyperextension. Injury progresses around the lunate in a clockwise semicircular pattern depending on the severity of the injury. The Mayfield classification involves 4 stages describing the sequence of ligamentous and carpal disruptions caused by wrist hyperextension, ulnar deviation, and intercarpal supination reliably reproduced on cadavers. The scapholunate ligament is disrupted in the first stage, resulting in instability between the scaphoid and lunate. In the second stage, there is disruption of the capitolunate articulation resulting in possible dislocation of the capitate. In the third stage, there is disruption of the lunotriquetral articulation resulting in the lunate dislocating dorsally. In the fourth and final stage, there is a failure of the dorsal radiocarpal ligament, and this results in volar dislocation of the lunate into the carpal tunnel.
Wrist fracture-dislocations are common. Isolated dislocations are uncommon.
Because of the wide range of ligamentous injuries, clinical findings very significantly. The patient typically presents with a history of wrist trauma. Pain may be reported localized in the middle of the wrist, although pain can be reported to be more widely depending on the severity of the injury. On a normal physical examination, the lunate can be palpated distal to the radius and in line with the middle finger. With flexion of the wrist, the lunate becomes more prominent.
Physical findings associated with scapholunate ligament injury can be subtle. The scapholunate ligament is the most commonly injured ligament of the wrist. Patients may report localized pain and swelling to the radial side of the wrist. They may also report a snapping or clicking sensation with wrist deviation. Joint instability should be assessed using the scaphoid shift test. Patients may also report pain with hyperextension.
In perilunate and lunate dislocations, patients present with generalized pain and swelling to the wrist. Pain is typically worsened with wrist range of motion. Unlike many other joint dislocations, gross deformity is not typically present in carpal dislocations. There may be a possible fullness of the carpal tunnel appreciated. Median nerve injury is common if the lunate dislocates volarly into the carpal tunnel. This can result in reduce sensation in the thumb, index, and middle fingers, and radial half of the ring finger. Decreased strength of thumb flexion, opposition, and abduction may also be seen with median nerve injuries. Acute carpal tunnel syndrome has been reported in up to 46% of dislocations.
An x-ray series of the wrist including anteroposterior (AP), lateral, and oblique views are necessary to assess carpal instability. Although carpal instability injuries are frequently missed, the diagnosis should be able to be made using these plain films.
In normal wrist radiographs, the distance between the ulnar border of the scaphoid and the radial border of the lunate is less than 3 millimeters. In scapholunate dissociation, posteroanterior radiographs may demonstrate a widened scapholunate space, commonly referred to as the "Terry Thomas" sign. If the injury is not seen on routine wrist films, the abnormal gap seen may be reproduced by obtaining a grip compression view. When scapholunate ligament instability occurs, the scaphoid rotates to a more transverse position. This results in an increased scapholunate angle to greater than 60 degrees. The scaphoid becomes shortened and develops a dense ring-like image around its distal edge, referred to as the "signet-ring sign."
For perilunate dislocations, the lateral view will reveal the lunate in its correct position aligned with the distal radius, but the capitate will be dorsally dislocated. A perilunate dislocation may obscure associated carpal bone fractures of the scaphoid and lunate.
In the case of lunate dislocations, the lateral x-ray shows the lunate displaced volarly but, the distal radius, carpus, and metacarpals are otherwise in normal alignment. This is frequently referred to as the “spilled teacup” sign. On the posteroanterior view, a lunate dislocation will have a characteristic triangular appearance due to the rotation of the lunate in a volar direction. This is referred to as the “piece of pie sign."
Patients with suspected scapholunate dissociation should be placed in a thumb spica, radial gutter, or short arm volar splint. Referral to an orthopedist or hand surgeon is necessary, as closed reduction with percutaneous pinning or open reduction is required for treatment. The patient is placed in a short-arm cast postoperatively, and k-wires are removed in 8 to 10 weeks. Patients are unable to perform heavy lifting using the wrist for 4 to 6 months post-injury.
Immediate reduction is essential when a lunate or perilunate dislocation is present, to relieve pressure on the median nerve and prevent further cartilage damage. These injuries require immediate orthopedic or hand surgery consultation for reduction and stabilization and frequently require arthroscopically guided reduction. Closed reduction is achieved using finger traps, with elbow at ninety-degrees of flexion. The hand is held in traction, while dorsal dislocations are reduced by wrist extension, traction, and finally wrist flexion. A sugar tong splint is then applied and followed up with surgical repair. There is frequently injury to all perilunate ligaments, requiring surgical stabilization. Without intervention, progressive instability may occur. A short arm cast is placed post-operatively for at least 6 weeks duration.
The evaluation of patients with acute wrist pain is broad. Patients with acute carpal instability frequently have associated carpal bone fracture. Scaphoid fractures are the most commonly fractured carpal bone. Other fractures, including the most common fracture of the wrist, the distal radius fracture, should be the clinician's differential diagnosis. Wrist sprains, frequently involving only the extrinsic ligaments, can be diagnosed after more serious injuries have been carefully excluded.
Scapholunate dissociation is the most common cause of degenerative arthritis of the wrist. Without early recognition and treatment, there may be a proximal migration of the capitate between the scaphoid and lunate. This results in a degenerative disease known as SLAC wrist (scapholunate advanced collapse).
Wrist osteoarthritis is a common long-term complication after perilunate and lunate injuries. Other complications include chronic carpal instability, rupture of tendons, delayed union, nonunion, malunion, median nerve compression, complex regional pain syndrome, and avascular necrosis of the lunate.