Mechanism and Patterns of Injury

Perilunate injuries are a spectrum of carpal injuries that are the result of high-energy trauma. The mechanism of injury is typically hyperextension, ulnar deviation, and intercarpal supination of the wrist. Indirect forces typically cause perilunate injuries when the point of rotation is distal to the radius and there is more stress on the carpal bones. In very young patients, such a force typically results in a physeal injury to the distal radius, and in older patients with osteopenic bone, a distal radius fracture typically occurs through the soft bone of the distal radius. This probably explains why perilunate injuries are typically seen in young patients with good bone quality who are involved in high-energy activities.

Mayfield^3,6,7 described a spectrum of progressive perilunar instability that has greatly enhanced understanding of these injuries. Perilunate injuries are divided into four stages based on the concept of a sequential pattern of traumatic intercarpal wrist instability.

All perilunate injuries except the very rare “reverse perilunate” begin with injuries on the radial side of the wrist and progress ulnarward. The first stage in lesser arc injuries is scapholunate dissociation followed by lunocapitate dislocation with lunotriquetral disruption and finally lunate dislocation from the radius. A lesser arc injury is a purely ligamentous injury that typically manifests with dorsal dislocation of the capitate from the lunate and rarely with lunate dislocation toward the carpal tunnel.

Mayfield also noted that carpal fractures may occur in perilunate injuries in lieu of ligament ruptures. In these cases, the force is concentrated on the carpus away from the lunate as it progresses from radial to ulnar, which results in the carpal bone fractures. The injuries that result are called greater arc injuries because the route of injury includes a path more remote from the lunate than lesser arc injuries (Fig. 43-1).

FIGURE 43-1 Greater and lesser arc injuries of the wrist. Lesser arc injuries result in a disruption of ligaments that bind the carpus to the lunate. The path of injury passes close to the body of the lunate and results in a purely ligamentous injury to the wrist. Greater arc injuries occur following a force that disrupts the carpus in a path further away from the lunate. Greater arc injuries may often include carpal bone fractures and avulsion fractures of the radial and ulnar styloids in a variety of combinations.

Progressive perilunar instability as described by Mayfield has four stages that follow a common sequence after exposure of the wrist to forceful hyperextension with a strong supination torque (Fig. 43-2). Stage I injuries result in scapholunate dissociation.^6,7 Hyperextension of the scaphoid places a strong torque on the lunate and its ligaments. The extrinsic palmar and dorsal ligaments tightly stabilize the lunate, and the force is concentrated on the scapholunate ligament, which ultimately begins to tear from palmar to dorsal. If the force is strong enough, the scapholunate ligament completely ruptures, completing stage I.

FIGURE 43-2 The four stages of perilunar carpal instability. Stage I: As the distal carpal row is forced into extension (arrows), the scaphotrapeziocapitate ligaments (1) pull the scaphoid into extension, opening the space of Poirier (asterisk). The lunate is constrained by the short radiolunate ligament (2), and the hyperextension force of the scaphoid results in concentration of energy to the scapholunate ligament. A scapholunate ligament rupture or scaphoid fracture may occur. The scapholunate ligament usually ruptures from palmar to dorsal with complete dissociation noted when the dorsal portion of the scapholunate ligament fails (3). Stage II: Midcarpal dislocation may occur with the capitate usually moving dorsal to the lunate. The midcarpal translation is limited by the integrity of the radioscaphocapitate ligament (4). Stage III: If hyperextension and intercarpal supination progresses, the ulnar carpal ligaments (5) and the lunotriquetral interosseous ligament (6) may fail. Stage IV: In end-stage injury, additional hyperextension forces the capitate to push the lunate palmarly through the torn volar capsule and the space of Poirier. The lunate may dislocate into the carpal tunnel hinged on the intact short radiolunate ligament (2).

In stage II injuries, there is sufficient force to dislocate the midcarpal joint with the capitate typically dislocating dorsal to the lunate. Midcarpal dislocation requires additional injury to the extrinsic ligaments, including the scaphocapitate and radioscaphocapitate. The space of Poirier, which is a deficiency in the palmar wrist capsule between radial and ulnar capsular ligaments, is opened by the progressive injury with a palmar capsular rent that exposes the midpalmar joint. In stage III injuries, the force continues in an ulnar direction with rupture of the lunotriquetral ligament and may include additional capsular injuries to the ulnolunate ligaments.

In stage IV injuries, the lunate is forced volarly to dislocate into the carpal tunnel through the opened space of Poirier. Isolated volar dislocations of the lunate are the end stage of the spectrum of progressive perilunar instability. Lunate dislocations are divided into three types. Type 1 lunate dislocations result in a lunate that has rotated palmarly up to 90 degrees. Type 2 lunate dislocations produce a lunate with more than 90 degrees of palmar rotation as it is hinged on the short radiolunate ligament. Type 3 injuries result in dislocation of the lunate into the carpal tunnel. Reverse perilunar instabilities result from hyperpronation of the wrist and may be the mechanism for rare perilunate injuries with palmar dislocation of the midcarpal joint.³

In greater arc injuries, the variable pattern of fracture might include avulsion of the radial styloid or fracture of the scaphoid, capitate, triquetrum, or ulnar styloid. The most common pattern observed in greater arc injuries is the trans-scaphoid perilunate fracture-dislocation. The trans-scaphoid variant is observed in 60% of perilunate wrist injuries.^1,3,6 The scaphoid fracture is through the waist in 60% to 72% of cases.^1,3 The scaphoid may fracture at any level, however, including the proximal pole.

Fracture of the capitate may occur and be part of the “scaphocapitate” syndrome.^3,6,8–10 This injury consists of a variation of a greater arc perilunate fracture-dislocation, in which the scaphoid and the capitate are fractured. The capitate fracture typically occurs proximally at the proximal neck of the bone. Complete rotation of the capitate fragment is typical with the proximal fragment often rotated 90 to 180 degrees. The mechanism of the fracture seems to involve a direct impact of the capitate against the dorsal lip of the radius with the wrist in a hyperextended and ulnarly deviated position. Rotation of the proximal capitate fragment is a result of the return of the distal fragment to its neutral position after dislocation (Fig. 43-3).

FIGURE 43-3 Scaphocapitate syndrome. Capitate fractures may occur with fracture of the scaphoid in greater arc perilunate injuries. The capitate fracture is usually through the neck and may occur after extreme wrist extension (I). The final wrist position after injury may result in the capitate rotated up to 180 degrees with the articular surface facing cancellous bone (II and III).

Greater and lesser arc injuries usually manifest with ligament ruptures on the ulnar side of the lunate, including complete rupture of the lunotriquetral ligament. Greater arc perilunate injuries may manifest with fractures of the triquetrum. Triquetral fractures may occur in any plane, but the most common pattern is a sagittal split in the body of the triquetrum or an avulsion fracture of the proximal pole.

Fractures of the radial or ulnar styloids also may be observed in greater arc perilunate injuries. These are typically smaller avulsion fractures, but may be of significant size and include the origins of major wrist stabilizers, such as the triangular fibrocartilage complex or radioscaphocapitate ligaments.

Presentation and Diagnosis

Patients who present with a perilunate fracture-dislocation have obvious pain and swelling of the hand and wrist. A full musculoskeletal examination and screening for concomitant axial or appendicular skeleton injuries is important because these represent high-energy injuries, and the patient may have sustained additional trauma.

Patients may present with symptoms of median or ulnar nerve compression. The incidence of acute carpal tunnel syndrome manifesting with perilunate injury ranges from 16% to 46%.¹¹ Most patients have resolution of nerve compression after closed reduction, but persistent nerve dysfunction after closed reduction may require urgent surgical decompression. Close attention should be paid to the patients’ neurovascular status, and the status of the median nerve in particular should be carefully established before and after reduction.

A standard four-view wrist series should be diagnostic of perilunate injuries and exhibit typical features. Most importantly, the dislocation of the midcarpal joint is best appreciated on the lateral view where lunocapitate dissociation can be easily visualized. The position of the lunate is best determined on the lateral view, which may reveal malrotation or dislocation of the lunate (Fig. 43-4).

FIGURE 43-4 A, The lateral injury film reveals dislocation of the midcarpal joint with dorsal dislocation of the capitate from the lunate. B, Midcarpal dislocation accompanied with palmar rotation of the lunate. C, Palmar dislocation of the lunate (Mayfield stage 4).

In perilunate injury, one also may appreciate on the anteroposterior view that there are radiographic breaks in the smooth arcs between the carpal rows described by Gilula.¹² These arcs are named after Gilula, the radiologist who described the concept of the greater and lesser arc injuries. Additionally, the carpus appears foreshortened with loss of carpal height. Malrotation of the lunate produces a “triangular-shaped” lunate that overlaps the capitate on the anteroposterior radiograph (Fig. 43-5