Perilunate and lunate dislocations represent the final stages of midcarpal ligament disruption. Much like the scapholunate and triquetrolunate injuries, these injuries are also the result of forceful dorsiflexion and impact on the outstretched hand, but usually with much greater force, such as a fall from a height or impact from a motor vehicle accident. The injury can begin on either side of the lunate, but typically starts on the radial side, tearing the scapholunate ligament. It progresses around the lunate in a semicircular fashion, tearing the volar ligament arcade at the radiocapitate ligament. Remember that the extrinsic ligaments form two strong volar arcades with an inherently weak area between them that widens with dorsiflexion of the wrist. This space of Poirier lies at the junction of the lunate and capitate. The space of Poirier is opened further as additional loading disrupts the triquetrolunate ligament. Besides ligament disruption, any number of the surrounding carpal bones can fracture along an arc around the lunate (see Fig..262-5). If a sufficient force is applied, the ligaments and carpal bones surrounding the lunate are stripped away. The capitate is displaced posterior to the lunate, producing a perilunate dislocation. If the capitate rebounds with sufficient force, it can push the lunate off the radius and into the palm, creating a lunate dislocation. These injuries are all part of a continuous spectrum of ligamentous disruption.
On clinical examination, there is often generalized swelling, pain, and tenderness. However, a gross deformity, typical of many other joint dislocations, is often absent and can be misleading. Radiographic interpretation is the key to diagnosis. The perilunate dislocation is best appreciated on the lateral view. The linear arrangement of the three Cs sign is disrupted with the capitate, represented by the third C, displaced posterior to the lunate. The lunate still maintains its contact with the radius. The scapholunate and capitolunate angles are increased. On the PA view, the three smooth arcs are disrupted and the capitolunate joint space is obliterated as the bones overlap one another. The scapholunate and triquetrolunate joint space may either be increased because of torn ligaments, or obliterated by rotation of fractured carpal fragments. The scaphoid will appear shortened from rotatory subluxation or fracture. A perilunate dislocation may, unfortunately, overshadow any associated carpal bone fracture. The scaphoid and capitate are most often involved, so it is prudent to carefully inspect these bones for fractures. These fractures are designated by adding the prefix "trans" to the carpal's name; e.g., transscaphoid perilunate dislocation (see Figi..ii262:6A, Fig. 262-66).
FIG. 262-6. Transscaphoid perilunate dislocation. A. PA view shows subtle overlap of the lunate and capitate. A displaced scaphoid fracture is present (arrows). B. Lateral view shows the lunate (L) maintaining contact with the radius while the capitate (C) is positioned posterior to the lunate. (With permission from Chin HW, Visotsky J: Ligamentous wrist injuries. Em Med Clin North Am 11:3, 1993.)
A lunate dislocation has many similar and several distinct features when compared to a perilunate dislocation. On the lateral view, it also disrupts the three Cs sign. The lunate (represented by the middle C) is pushed off the radius and into the palm. This has been called the "spilled teacup" sign because it resembles a cup spilling in the direction of the palm. The capitate may rebound back and even rest on the radius. On the PA view, the lunate has a triangular shape ("piece-of-pie" sign) that is pathognomonic for lunate dislocation (see Fig 2.6.2.-7A, f ig.: 262-7B). The signs of ligament disruption and the associated carpal fractures described with perilunate injuries may also be present.
FIG. 262-7. Lunate dislocation. A. PA view reveals pathognomonic triangular shape of lunate (piece of pie sign, arrow). B. Lateral view shows lunate tilting into the palm (spilled teacup sign, arrow) and the capitate positioned posterior to the lunate (arrowhead). Note the associated scaphoid and triquetrum fractures. (With permission from Chin HW, Visotsky J: Ligamentous wrist injuries. Em Med Clin North Am 11:3, 1993.)
All patients with perilunate or lunate dislocations require emergency orthopedic consultation. Treatment is determined by the extent of injury. Closed reduction and long-arm splint immobilization is appropriate for reducible dislocations. Open, unstable, and irreducible dislocations require open reduction and internal fixation, with repair of the ligaments and fractures. Some clinicians operate on all perilunate and lunate dislocations. The complications are development of carpal instability patterns that lead to early degenerative arthritis, delayed union, malunion, nonunion, avascular necrosis, and, occasionally, median nerve compression from the volar dislocation of the lunate into the carpal tunnel.
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