The scaphoid is stabilized by the scapholunate ligament (directly) and lunotriquetral ligament (indirectly). Disruption of either of these ligaments leads to a pattern of instability that, left untreated, leads to altered mechanics of the wrist and ultimately debilitating arthritis and collapse. Although arthroscopy remains the gold standard for diagnosis of these injuries, plain films and advanced imaging are useful adjuncts. In the acute setting, conservative treatment may be attempted, but recalcitrant cases require surgical stabilization. Salvage procedures are also available for those patients who fail initial stabilization or present with late degeneration.
Key points
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The dorsal aspect of the scapholunate ligament is the strongest of its 3 subregions.
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The common mechanism of injury to the scapholunate and lunotriquetral ligaments is extension, ulnar deviation, and intercarpal supination.
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Although scapholunate widening on a posteroanterior view of the wrist indicates scapholunate ligament injury, this is not a sensitive finding, and continued complaints of pain and/or instability should warrant further work-up.
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Arthroscopic debridement is an appropriate treatment of partial tears.
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Repair should be attempted for acute tears.
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Reconstruction can be attempted for chronic tears that remain reducible.
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Salvage options can be offered for irreducible injuries or in cases with degenerative changes.
Introduction
Injuries to the scapholunate and lunotriquetral ligaments can have severe deleterious effects. The scaphoid acts as a connecting rod between the proximal and distal rows. The scaphoid is tethered directly by the scapholunate ligament and indirectly by the lunotriquetral ligament. Disruption of these stabilizing ligaments leads to abnormal mechanics of the carpal joints. Both the quality of reduction and the timing of definitive management affect outcomes following dissociative carpal instability. This article explains the anatomy, biomechanics, mechanism of injury, studies, and treatment algorithms involved in caring for patients with dissociative carpal instability.
Introduction
Injuries to the scapholunate and lunotriquetral ligaments can have severe deleterious effects. The scaphoid acts as a connecting rod between the proximal and distal rows. The scaphoid is tethered directly by the scapholunate ligament and indirectly by the lunotriquetral ligament. Disruption of these stabilizing ligaments leads to abnormal mechanics of the carpal joints. Both the quality of reduction and the timing of definitive management affect outcomes following dissociative carpal instability. This article explains the anatomy, biomechanics, mechanism of injury, studies, and treatment algorithms involved in caring for patients with dissociative carpal instability.
Anatomy
The carpal bones are divided into 2 U-shaped rows: the proximal row containing the scaphoid, lunate, triquetrum, and pisiform; and the distal row, which is composed of the trapezium, trapezoid, capitate, and hamate. Both intrinsic and extrinsic ligaments connect the two rows. These ligaments are generally characterized as either palmar or dorsal and are often described as thickenings of the joint capsule.
Of the intrinsic ligaments of the wrist, the most important for stability are the scapholunate and lunotriquetral interosseous ligaments. The scapholunate ligament is a C-shaped ligament composed of dorsal, central, and palmar subregions. These three subregions attach at their respective articular margins of the scaphoid and lunate.
The dorsal subregion of the scapholunate ligament consists of transversely oriented collagen fibers. This dorsal aspect is the strongest and thickest of the three subregions and provides the greatest contribution to the stability of the scapholunate articulation. The central subregion is thinner than the dorsal component and is obliquely oriented, in contrast with the transverse fibers of the dorsal component. The central subregion is not considered a true ligament but more a fibrocartilaginous structure. It merges with the dorsal aspect of the scapholunate ligament, but is separated from the volar subregion by the volar radioscapholunate ligament (ligament of Testut). The volar region of the scapholunate ligament is a thin (often less than 1 mm thick) layer composed of obliquely oriented fibers. These fibers are confluent with the radioscapholunate ligament proximally and are sometimes seen to be interconnected with the radioscaphocapitate ligament distally. A sharp division exists palmarly between the volar region and the long radiolunate ligament. The volar region is thought to function as a ligament, but does not confer as much stability to the scapholunate ligament as does the dorsal region.
Additional stabilizers to the scapholunate joint exist on the palmar and dorsal sides of the wrist. The scaphotrapeziotrapezoid and scaphocapitate ligaments lie on the dorsal aspect and provide resistance against the tendency of the scaphoid to palmar flex. The radioscaphocapitate and short and long radiolunate ligaments lend stability on the palmar side of the wrist.
The lunotriquetral ligament is also composed of 3 subregions, 2 true ligaments on the dorsal and palmar aspects and a fibrocartilaginous central segment, similar to the scapholunate ligament. The relative strengths of the three subregions are opposite to those of the scapholunate ligament, with the volar aspect being the thickest and the greatest contributor to lunotriquetral stability.
The proximal carpal row has no tendinous insertion, and is thus often termed the intercalated segment. Three sets of tendons cross the proximal row: the extrinsic flexors and extensors of the fingers; the flexors and extensors of the wrist; and the abductor pollicis longus and the extensor pollicis brevis, which course around the radial styloid. Movement between the carpal bones is negligible. Because there are no muscular attachments onto the proximal carpal row, the movement of the proximal row carpal bones is determined by their ligamentous attachments and the mechanical forces of the tendons that cross the wrist.
Biomechanics
Because the proximal carpal row has no tendinous insertion, the forces that act on its proximal and distal articular surfaces dictate the forces that act on the intercalated segment. The tendons that cross the proximal row exert a compressive force that is resisted by the connecting-rod action of the scaphoid. Instability of the scaphoid leads to an alteration of the articulation between the proximal and distal rows.
During radial deviation of the normal wrist, the trapezoid and trapezium exert a volarly directed force on the distal pole of the scaphoid. This force causes the scaphoid to flex about its waist; this flexion force is then transmitted to the lunate via the scapholunate ligament and to the trapezium via the lunotriquetral ligament. The proximal row radially deviates as 1 unit.
Ulnar deviation of the normal wrist causes the hamate to project a dorsally directed force on the triquetrum. As the triquetrum is rotated dorsally by the hamate, a competent lunotriquetral ligament imparts an extension moment on the lunate, and indirectly on the scaphoid via the scapholunate ligament. It is through this mechanism that ulnar deviation causes extension of the wrist.
The lunate can be thought of as existing in a balanced suspension between the scaphoid and the triquetrum. The scaphoid has a flexion bias, and through the scapholunate ligament exerts a flexion moment on the lunate. However, the triquetrum has an extension bias, and exerts an extension moment on the lunate through the lunotriquetral ligament. In a balanced proximal carpal row, the lunate remains centered on the distal radius without tilting into flexion or extension. The tendons that cross the wrist exert a compressive force across the carpus through the centrally located capitate at the capitolunate articulation.
In the uninjured wrist, the lunate is held in a tightly coupled balance between the scaphoid and the triquetrum. However, injury to either the scapholunate ligament or the lunotriquetral ligament causes respective extension or flexion of the lunate. With disruption of the scapholunate ligament, a gap opens between the scaphoid and the lunate, into which the capitate eventually collapses. The scaphoid, now free of its tether to the lunate, assumes a position of flexion as the capitate comes to occupy the distal space between the lunate and the scaphoid. The lunate, no longer balanced by the flexion moment from the scaphoid, is rotated into extension by the triquetrum. The lunate angles dorsally, producing the pattern known as dorsal intercalated segment instability (DISI).
Unlike injury to the scapholunate ligament, isolated injury of the lunotriquetral ligament is rarely sufficient to allow the flexion moment of the scaphoid to palmar flex the lunate. However, if the dorsal radiocarpal ligament is also injured, the tethering effect on the dorsum of the lunate is lost and the scaphoid and lunate angle into flexion while the capitate begins to migrate between the lunate and the triquetrum. Thus, injury to both the lunotriquetral and dorsal radiocarpal ligaments leads to volar intercalated segment instability (VISI).
Mechanism of injury and classification
Injury to the scapholunate or lunotriquetral ligaments is most commonly caused by a fall on an outstretched hand. Other common causes include sports injuries and high-energy trauma such as motor vehicle collisions. Mayfield and colleagues were able to reproduce scapholunate injuries via direct impact to the hypothenar region with the wrist in extension, ulnar deviation, and intercarpal supination. These types of trauma can lead to a spectrum of injuries, including injury to the ligaments of the wrist, fracture of the distal radius, and even fracture of one or more carpal bones depending on the angle of the carpus when the impact occurs.
Dissociative carpal instability is often directly caused by hyperextension of the wrist. Mayfield and colleagues proposed that a continuum of injuries occur about the wrist with extension, ulnar deviation, and carpal supination. They described a 4-part circular progression of injury, which they termed progressive perilunar instability (PLI) as follows ( Fig. 1 ):
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Stage I: dorsal migration of the proximal pole of the scaphoid with resulting injury to the scapholunate ligament.
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Stage II: further extension, ulnar deviation, and supination of the carpus leads to progression of the force through the space of Poirier.
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Stage III: progression of these forces causes the triquetrum to translate away from the lunate with resultant injury to the lunotriquetral ligament.
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Stage IV: disruption of the dorsal radiocarpal ligament allows the lunate to rotate on its palmar ligamentous hinge and dislocate with resultant articulation of the capitate into the lunate facet of the distal radius.
Mayfield and colleagues’ model has often been cited in the literature as an explanation of the progression of scapholunate injury through perilunate dislocation, lunotriquetral disruption, and eventually to lunate dislocation. However, this model does not explain an isolated lunotriquetral ligament injury without concomitant scapholunate injury.
A mechanism opposite to that of Mayfield and colleagues was recently proposed to explain ulnar-sided injuries of the wrist. Shin and colleagues investigated isolated lunotriquetral injuries to the ulnar side of the wrist caused by fall, twisting, or sports. Subsequent cadaveric testing led to a description of a 3-stage mechanism for ulnar-sided injury to the wrist leading to dorsal perilunate dislocation. The model is as follows:
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Stage I: disruption of the lunotriquetral ligament
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Stage II: stage I plus disruption of the ulnolunate, ulnotriquetral, ulnocapitate ligaments, and dorsal scaphotriquetral and radiotriquetral ligaments
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Stage III: stage II plus disruption of the scapholunate and radioscapholunate ligaments with potential dorsal perilunate dislocation
In the setting of a chronic scapholunate ligament injury, arthritic changes may occur and the deformity of the capitate, lunate, and scaphoid may become static. The abnormal position of these bones can lead to wear of the articular cartilage and the surrounding articular surfaces, a condition known as scapholunate advanced collapse (SLAC). As with most forms of osteoarthritis, SLAC wrist develops in stages:
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Stage I: arthritic change of the radial styloid
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Stage II: arthritic change of the scaphoid facet of the distal radius
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Stage III: arthritis of the articulation between the capitate and the lunate
The radiolunate articulation typically does not develop arthritic change in the SLAC wrist because of the normal location of the lunate within the lunate facet of the distal radius.
Presentation
Most patients with acute dissociative carpal instability give a history of a specific injury to the wrist, whether it is a fall on an outstretched hand, a twisting or pulling motion with immediate pain, injury from a direct blow, or some combination of injuries. In these cases, it is useful if the patient can recall the position of the wrist at the time of injury to determine the force vector that caused the injury. These patients typically complain of pain, swelling, or instability with lower stage injuries, progressing to an inability to use the hand with perilunate or lunate dislocations. Carpal tunnel syndrome must be evaluated, because patients with lunate dislocations have increased pressure within the carpal canal and may develop acute median nerve compression. The neurovascular status of the hand must also carefully be documented because compartment syndrome of the hand may develop.
Although acute injuries do occur, many patients present weeks to months after the initial insult, believing that they had merely sprained their wrists but without complete resolution of pain. Patients who present with subacute or chronic injuries may report pain with decreased grip strength. They may complain of weakness of the wrist, catching, clunking, or episodes of giving way. The physician must be aware of the possibility of dissociative carpal injury in even the mildest of complaints.
Examination
A patient with suspected carpal instability requires a thorough history and careful examination. The physician should palpate the wrist in an attempt to localize point tenderness. In the acute setting, swelling may make this portion of the examination difficult. It is important to determine whether the injury to the wrist is primarily radial, ulnar, or both. Pain elicited by radial or ulnar deviation of the wrist should promote suspicion of injury to the respective side of the wrist. Tenderness over the lunotriquetral or scapholunate joint indicates injury to the underlying ligament ( Figs. 2 and 3 ). Pain with subjective or palpated clicking or popping may aid in diagnosis of ligamentous injury.
The Watson shift maneuver (or scaphoid shift test) was first described in 1988. It is useful in determining the presence or absence of scapholunate ligament injury. In order to perform this test, the examiner sits across from the patient with a table in between. The physician grasps the radial side of the injured wrist with his same hand (eg, right hand grasps right wrist). The thumb is placed over the palmar prominence of the scaphoid while the fingers provide counterpressure on the dorsum of the wrist proximal to the carpal row. The examiner’s other hand provides ulnar to radial deviation by grasping the metacarpals ( Figs. 4 and 5 ). As the wrist moves beyond neutral and into radial deviation, the dorsally directed pressure on the palmar aspect of the scaphoid may cause a subluxation. This subluxation may cause pain or apprehension in the patient, or the examiner may feel the scaphoid sublux or even dislocate out of the scaphoid fossa of the radius. The quality of wrist motion (ie, smooth or with grinding) is also noted. The pressure on the distal pole of the scaphoid is released and a clunk may be realized as the scaphoid relocates back to within the scaphoid fossa of the radius. The contralateral wrist must also be examined in order to assess for ligamentous laxity or hypermobility that may lead to a false-positive Watson shift maneuver.
