INTRODUCTION

Carpal dislocations—recognized or unrecognized, acute or chronic—frequently result in long-term disability and pain. These injuries are rare and most often occur in young males as a result of high-energy trauma such as motor vehicle accidents, falls from a height, or industry-related trauma. Up to 25% of carpal dislocations are missed as a result of associated, and potentially distracting, injuries or inadequate radiographic examination. In addition to diagnostic challenges, these injuries can be difficult to treat.

For the purposes of this chapter, we concentrate only on radiocarpal and intercarpal dislocations.

ANATOMY

The anatomy of the carpus is complex and has been the focus of numerous studies. The carpus consists of eight carpal bones arranged in a proximal and a distal row. Wrist motion occurs simultaneously at the intercarpal and radiocarpal joints. Motion is initiated by the extrinsic musculature as it acts on the distal carpal row. The proximal row is thought to be an “intercalated segment” during wrist motion, which moves in accordance with the vector of the distal carpal row. The stability and intercarpal motions of the wrist depend on muscular balance, articular congruence, and ligamentous integrity.

The extrinsic ligament system links the carpus to the bones of the forearm. The volar extrinsic wrist ligaments are the strongest. The volar radial ligaments consist of the radioscaphocapitate, the long radiolunate, the radioscapholunate (ligament of Testut), and the short radiolunate ( Fig. 43-1 ). The radioscaphocapitate forms the radial limb of the arcuate ligament and acts as a fulcrum for scaphoid palmarflexion. In addition, it is the primary restraint to ulnar translocation. Both the long and short radiolunate ligaments act as a tether for lunate displacement, the latter being the more significant of the two. The radioscapholunate ligament, also referred to as the ligament of Testut, has been shown histologically to be composed primarily of small vessels from the radiocarpal arch and terminal nerve branches of the anterior interosseous nerve. It is thought that this “ligament” is not a ligament at all, but rather a neurovascular conduit to the scapholunate ligament.

The volar wrist ligaments . C, capitate; H, hamate; L, lunate; P, pisiform; R, radius; S, scaphoid; T, triquetrum; Td, trapezoid; Tm, trapezium; U, ulna.

The volar ulnar ligaments consist of the ulnotriquetral, ulnocapitate, and ulnolunate. The ulnotriquetral and ulnocapitate ligaments converge with the radioscaphocapitate ligament to form the arcuate ligament complex. The arcuate ligament forms an inverted V with the apex centered on the capitate. Just proximal to this apex is the space of Poirier, an area of capsular weakening devoid of overlying ligament through which the lunate dislocates in a perilunate injury pattern.

There are two dorsal extrinsic ligaments: the dorsal radiocarpal and the dorsal intercarpal ( Fig. 43-2 ). The former extends from Lister’s tubercle on the radius to insert on the triquetrum. The dorsal intercarpal ligament runs from the triquetrum and attaches on the dorsal aspects of both the scaphoid and the trapezoid.

The dorsal wrist ligaments . C, capitate; LT, lunotriquetral; S, scaphoid; T, triquetrum.

The carpal bones are interconnected by a system of intrinsic intercarpal ligaments ( Fig. 43-3 ). Because the proximal carpal row has no tendinous insertions, the intrinsic ligaments provide the stability and coordination of the proximal row as it moves in relation to the distal row. The scapholunate ligament and the lunotriquetral ligaments adjoin their respective named carpal bones. Each ligament consists of three components: volar, dorsal, and a proximal fibrocartilaginous membrane. The dorsal portion of the scapholunate ligament is the strongest portion of the ligament. Conversely, the volar segment of the lunotriquetral ligament is the strongest. The lunotriquetral ligament prevents ulnar translation of the triquetrum. Three intrinsic ligaments exist in the distal carpal row: the trapeziotrapezoid, trapeziocapitate, and capitohamate.

The intercarpal ligaments . C, capitate; H, hamate; L, lunate; P, pisiform; S, scaphoid; Td, trapezoid; Tm, trapezium.

In addition, the scaphoid is linked to the distal carpal row by the scaphocapitate and scaphotrapeziotrapezoid (STT) ligaments via the scaphoid tuberosity. These ligaments are important in scaphoid stabilization.

PHYSICAL EXAMINATION

Radiographic Findings

Although carpal dislocations may be obvious on either plain radiographs or computed tomography scan ( Fig. 43-4 A–C), more subtle findings may be appreciated on examination of initial imaging studies.

A, Radiograph showing a perilunate dislocation, Mayfield Stage III. B and C , Lateral and sagittal computed tomography (CT) scans demonstrating the dorsal perilunate dislocation.

Scapholunate instability manifests as a gap of 3 mm or more in the scapholunate interval on the anteroposterior radiograph. In addition, a cortical ring sign is often evident on the scaphoid as the scaphoid flexes and the beam catches the tubercle on end. The lunate extends as the scaphoid flexes. On the lateral radiograph, a scapholunate angle of more than 70 degrees (normal is 47 degrees) is often evident in patients with scapholunate instability ( Fig. 43-5 ).

Radiographic measurements in dorsal and volar lunate instability .

In lunotriquetral instability, the scaphoid and the lunate are in palmarflexion. The cortical ring sign of the scaphoid is evident on the anteroposterior view, and the lunate has a triangular appearance. The lunate is palmarflexed on the lateral view. The scapholunate angle is less than 30 degrees. There is often no clear widening of the lunotriquetral interval (see Fig. 43-5 ).

Ulnar translocation of the carpus can be detected on anteroposterior radiographs. Translocation is measured by the distance between the center of the head of the capitate and a line drawn down the longitudinal axis of the ulna. This distance is divided by the length of the third metacarpal. The normal value is 0.30 + 0.03. A value of less than 0.30 is indicative of ulnar translocation ( Fig. 43-6 ).

Measuring ulnar translocation of the carpus on an anteroposterior radiograph .

PATHOMECHANICS OF INJURY AND INJURY CLASSIFICATION

Injuries to the carpus have been classified by Cooney and associates into four groups: lesser arc injuries (perilunate dislocations), greater arc injuries (transcarpal perilunate dislocations), axial dislocations, and isolated carpal bone dislocations. Isolated carpal bone dislocations have been extensively described in the literature and so are not included here.

Greater and Lesser Arc Injuries

The landmark work of Mayfield and associates illustrated the spectrum of progressive perilunate instability. The injury occurs when an extended ulnarly deviated and supinated carpus is axially loaded. In clinical situations, this corresponds to a fall onto an extended, pronated forearm. Force transmission occurs in a circular pattern about the carpus, resulting in the spectrum of perilunate instability. The classification by Mayfield and associates is the most commonly used to describe the sequential progression of injury ( Fig. 43-7 ).

Progressive perilunate instability . The four stages of progressive perilunate dislocation are I, scapholunate; II, capitolunate; III, perilunate; and IV, lunate.

Stage I

The volar radiocarpal ligaments, scapholunate (SL) ligament, and scaphotrapeziotrapezoid ligament are taut in maximum extension. With an injury mechanism of forceful extension, ulnar deviation, and supination, the scaphoid extends, tightening the scapholunate ligament and the scaphotrapeziotrapezoid ligament. The short radiolunate and long radiolunate ligaments are taut, holding the lunate reduced and resisting the torque transferred from the extended scaphoid via the scapholunate ligament. With continued force transmission through an extended scaphoid, one of two things will occur: either the scapholunate ligament will tear in a volar to dorsal direction, or a scaphoid waist fracture will occur. The ligamentous or bony disruption thereby opens the space of Poirier.

Stage II

The scaphoid is now separated from the proximal row via either fracture or ligament disruption. As the force transmission continues through an extended supinated carpus, the scaphoid and distal row are forced into further extension, and the capitate dislocates dorsally. The entire distal row and radial portion on the proximal row dislocate dorsally. An intact radioscaphocapitate ligament limits the dorsal displacement of the capitate.

Stage III

As the force transmission continues, it is transferred to the lunotriquetral interval via the ulnar limb of the arcuate ligament. With enough force, the lunotriquetral ligament will tear, most often in a volar-to-dorsal direction. Alternately, a triquetral avulsion fracture may result. After rupture of the lunotriquetral ligament, the extension-supination vector is transmitted to the long radiolunate ligament, resulting in failure of this ligament as well. The short radiolunate and ulnolunate ligaments are left as the sole stabilizing structures of the lunate. These forces produce a dorsal perilunate dislocation.

Stage IV

With continuation of the force, the capitate is pulled proximally and volarly, both by the intact radial limb of the radioscaphocapitate ligament and by muscle contraction. The dorsal radiocarpal ligament is disrupted and the capitate pushes the lunate volarly. The intact short radiolunate and ulnolunate ligaments act as a hinge, and the lunate dislocates into the carpal tunnel through the space of Poirier. This clinical scenario is referred to as a lunate dislocation.