Scapholunate Instability

Clinical Features and Evaluation

Scapholunate instability is the most common carpal instability pattern, either alone or in conjunction with another instability pattern or distal radius fracture. The most common mechanism of injury is a fall on an outstretched wrist with hyperextension, ulnar deviation, and supination of the wrist. Patients will often have pain and swelling with acute injuries. Those presenting with chronic injuries may report pain and popping with loading of the wrist. Patients may also complain of weak grip, limited motion, and point tenderness over the dorsal aspect of the scapholunate interval. Diagnosis is often delayed because the injury is thought only to be a sprained wrist. On physical examination, patients will have tenderness located over the anatomic snuffbox and dorsally over the scapholunate interval. The Watson scaphoid shift test is the provocative maneuver for scapholunate ligament injury (Fig. 40-5). The examiner places his or her thumb over the distal pole of the scaphoid and the other hand moves the wrist in an ulnar to radial direction, which elicits pain or a palpable clunk. Imaging consists of posteroan-terior and lateral radiographs of the injured wrist. Injuries may be static or dynamic. Static injuries will be seen on radiographs. On the posteroanterior wrist, the scapholunate interval may be greater than 3 mm (positive Terry Thomas sign), the scaphoid will appear shortened (positive cortical ring sign), and the lunate will be extended (Fig. 40-6). On the lateral film, the normal scapholunate angle is 30 to 60 degrees. A scapholunate angle greater than 70 degrees suggests scapholunate instability7 (Fig. 40-7). In cases of dynamic instability a load must be applied to generate abnormal findings. A clenched-fist anteroposterior radiograph of the wrist can be obtained to accentuate the scapho-lunate diastasis. Magnetic resonance imaging is a noninvasive modality that can be used to evaluate wrist ligaments, although it is thought to be technique and interpreter dependent. Arthroscopy can also be used to diagnose scapholunate injury. Geissler et al,8 devised a classification system to standardize arthroscopic observation of injury to the intercarpal ligaments. Grade I lesions involve attenuation or hemorrhage of the involved ligament. Grade II lesions involve attenuation or hemorrhage of the interosseous ligament with intercarpal step-off and a slight gap is present between carpal bones. Grade III lesions involve a step-off in carpal alignment and a probe may be passed between carpal bones. Grade IV lesions involve a stepoff in carpal alignment and there is gross instability in which the arthroscope may be passed between carpal bones. During a diagnostic arthroscopy, a positive "drive through" sign is a grade IV lesion in which the arthroscope can be passed through the scapholunate interval into the midcarpal joint.

Relevant Anatomy

The interosseous scapholunate ligament, the dorsal scapholunate ligament, and the palmar radioscaphoid ligament are involved in scapholunate instability. Isolated transection of the interosseous scapholunate ligament has been shown not to reproduce instability in cadavers. Scapholunate disassociation results from injury to the scapholunate interosseous ligament and the palmar radioscaphoid ligament.9

Treatment Options

Treatment of scapholunate instability depends on when the injury is diagnosed. Partial tears of the interosseous scapholu-nate ligament can be treated with cast immobilization for 6 to

10 weeks to allow healing. Patients with partial tears that remain symptomatic after immobilization may undergo arthroscopic debridement of the tear with some relief of symptoms. Complete tears of the scapholunate ligament should be treated surgically.

Acute Scapholunate Injuries

Injuries of the scapholunate ligament usually involve an avulsion of the ligament off the scaphoid. Closed reduction and cast immobilization are no longer used in the treatment of complete scapholunate ligament tears because there are no data supporting the success of this method. Even when an anatomic reduction could be achieved, it was rarely maintained. Closed reduction and percutaneous pinning of the scapholunate ligament have also been abandoned because of inability to maintain reduction. The preferred treatment of acute scapholunate ligament injuries is open reduction and direct repair of the scapho-lunate ligament. Through a dorsal approach, the joint is reduced and the scapholunate ligament is directly repaired using sutures through bone tunnels or suture anchors. Some authors also advocate reinforcement of the repair with dorsal capsulodesis (Blatt procedure). After open reduction and internal fixation of the scapholunate ligament, the wrist is kept in a thumb spica cast for 2 to 3 months, followed by 1 month in a short arm cast, followed by a protective splint and physical therapy.

Return to Sports

Patients may return to athletics once they have demonstrated progress in strength and range of motion of the affected extremity. One to 3 months of a supervised physical therapy program is normally required after cast removal. A protective splint should be worn while participating in athletic activities. The protective splint may be discontinued once full strength and range of motion have been obtained.

Treatment of Chronic Injuries without Degenerative Changes

Chronic instability is defined as a scapholunate ligament injury that has been present for more than 3 months. These injuries are more difficult to deal with. Over time, the scapholunate ligament becomes scarred and the edges contract so a direct repair is no longer feasible. Subacute injuries that do not have cartilage wear secondary to the injury have a joint that remains reducible, but as time progresses and fibrosis develops, the joint becomes irreducible and this influences the chosen treatment. When the scapholunate joint remains reducible, a soft-tissue reconstruction (Blatt procedure) may be performed to prevent rotatory subluxation of the scaphoid. The Blatt procedure10 involves a proximally based flap of dorsal capsule off the ulnar side of the distal radius, approximately 1 cm wide, which serves as a checkrein to volar rotation of the scaphoid. The scaphoid then is reduced and held in place by a Kirschner wire. The flap is inserted into the distal pole of the scaphoid. The flap of tissue is then secured by a pullout wire over a button on the volar surface of the wrist or with a suture anchor. The wrist is kept in a thumb spica for 2 months, followed by active range-of-motion exercises. The Kirschner wire is removed at 3 months and intercarpal motion is allowed. Other options for treatment of reducible injuries include free tendon grafts, bone-ligament-bone grafts, or other types of capsulodesis using dorsal wrist capsule.

Normal intercarpal mechanics cannot be restored when the scapholunate joint cannot be reduced; therefore, treatment of

Figure 40-5 Watson's scaphoid shift test. A and B, Clinical photographs of Watson's scaphoid shift test. C and D, Diagram of Watson's scaphoid shift test. (From Cooney WP, Linscheid RL, Dobyns JH: The Wrist. St. Louis, Mosby, 1997, p 258.)

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Figure 40-5 Watson's scaphoid shift test. A and B, Clinical photographs of Watson's scaphoid shift test. C and D, Diagram of Watson's scaphoid shift test. (From Cooney WP, Linscheid RL, Dobyns JH: The Wrist. St. Louis, Mosby, 1997, p 258.)

Figure continues

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