Scapholunate Ligament Instability

The scapholunate ligament is the intrinsic ligament that binds the scaphoid and lunate. Because the scaphoid bridges the proximal and distal carpal rows, it is not surprising that the scapholunate ligament also has a greater propensity for injury. It is the most common ligament injury of the wrist. Injury most often is from a fall on the outstretched hand with impact on the thenar eminence. These individuals will complain of pain and swelling on the radial side of the wrist and sometimes a clicking sensation with wrist movement. Examination reveals localized tenderness on the dorsum of the wrist in the area immediately distal to Lister's tubercle. Ballottement of the scaphoid may also produce pain in this area.

This injury is often referenced to the various radiographic appearances it may take. There are three different radiographic signs that may occur separately or in combination with one another. Scapholunate dissociation is a widening of the scapholunate joint space on the PA projection of more than 3 mm ( Fig 26.2-4). This has been called the "Terry Thomas" sign, named after a British comedian with notable dental diastema between his upper front incisors. If it is not apparent on routine views, a grip-compression stress view or motion study may be necessary to demonstrate the abnormal gap. These maneuvers are particularly helpful in identifying an incomplete tear of the ligament. Rotatory subluxation of the scaphoid is another abnormality that often accompanies scapholunate dissociation. A torn scapholunate ligament can cause the scaphoid to tilt more palmar and increase the scapholunate angle to greater than 60 degrees on the lateral view (normal angle is 30 to 60 degrees). On the PA view, the scaphoid tilts toward the observer. The scaphoid appears shorter as it is viewed more on its end and the circular cortex of the bone becomes visible like a ring (cortical ring sign; see Fig.. .262.z4). A third possible radiographic abnormality is a carpal instability pattern, Dorsal intercalated segment instability (DISI). The normal flexed posture of the scaphoid produces a flexion torque on the lunate that is counterbalanced by an extension torque from the triquetrum. When the scapholunate ligament is torn, this balance is disrupted. The lunate tilts dorsal from the unopposed extension torque from the triquetrum, while the scaphoid tilts more palmar (rotatory subluxation of the scaphoid) because it has lost support from the lunate. The dorsal tilt of the lunate also causes a slight flexion tilt of the capitate. In the lateral view, the normal colinear arrangement of the axes of the radius, lunate, and capitate are replaced by a characteristic zigzag pattern. Both the scapholunate and capitolunate angles are increased. The concept of the proximal carpal row being the middle-link or "intercalated segment" in this system, combined with the lunate's pathological dorsal tilt and zigzag pattern, is how this abnormality came to be named dorsal intercalated segment instability, DISI (see Fig.. ...262:.3B).

FIG. 262-4. Scapholunate dissociation with accompanying rotatory subluxation of the scaphoid. The scaphoid and lunate are separated by a gap of more than 3 mm (arrowhead) and the scaphoid appears shorter from rotation with a dense ring (cortical ring sign, arrow).

These injuries are treated acutely with a radial gutter splint or posterior mold. Appropriate orthopedic referral is necessary because these injuries require either closed reduction and pinning or, more often, open reduction and surgical repair of the ligament. Early severe degenerative arthritis is a possible sequelae if left untreated.

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