Review Question - QID 217881

Scapholunate interosseous ligament (SLIL) injuries are often missed on initial presentation and can result in a dorsal intercalated segment instability (DISI) deformity where the lunate extends relative to the scaphoid (scapholunate angle >70°).

DISI is the most commonly encountered form of wrist instability and is often the result of chronic SLIL injuries. When the SLIL is disrupted, the scaphoid and lunate are able to move independently of one another through the wrist range of motion. The scaphoid will move in continuity with the distal carpal row and flexes from the pull of the strong radioscaphocapitate (RSC) ligament. In contrast, the lunate and triquetrum move together as a proximal carpal row unit and extend. This combination of scaphoid flexion and lunate extension produces the classic DISI deformity with an increased scapholunate angle (>70°). This instability leads to the characteristic early-onset arthritic pattern termed scapholunate advanced collapse (SLAC) wrist.

Walsh et al. performed a review of SLIL injuries and described its anatomic features, biomechanical properties, and treatment algorithms. They report that arthroscopy is considered the gold standard for the complete evaluation of SLIL injuries. They note that procedures such as carpal fusion or capsulodesis can limit excessive scaphoid motion, promote wrist stability, and potentially prevent arthritis.

Bednar et al. performed a review of the different carpal ligamentous injuries and classifications of carpal instability. They report that acute injuries (4-6 weeks since injury) may undergo arthroscopic evaluation and open reduction with ligamentous repair. Chronic injuries (older than 6 weeks) without degenerative changes may undergo ligamentous reconstruction with capsulodesis versus intercarpal arthrodesis. They conclude reconstructive efforts such as radiocarpal arthrodesis, midcarpal arthrodesis, or other salvage procedures should be considered in cases with advanced degenerative changes.

Figure A shows AP imaging of the patient’s bilateral wrists. There is notable scapholunate gapping (Terry Thomas Sign) on the patient’s left wrist compared to his right wrist, indicative of a SLIL injury. Figure B is a lateral radiographic image of the patient’s left wrist, showing a notably extended lunate relative to the long axis of the scaphoid (increased scapholunate angle). Illustration A displays how to calculate the scapholunate angle on lateral radiographic imaging. Normal scapholunate angles are between 30 and 70 degrees.

Incorrect Answers:
Answer 1: A fused lunate and triquetrum describes the most commonly encountered carpal coalition pattern. Multiple coalitions in the same wrist may be syndromic (Holt-Oram syndrome, arthrogryposis, etc.), however many isolated coalitions are asymptomatic and found incidentally on radiographic imaging.
Answer 3: Increased volar flexion of the lunate on lateral radiographs describes a volar intercalated segment instability (VISI) deformity. This may be an incidental finding in patients with ligamentous laxity or more commonly in the setting of lunotriquetral ligament injuries. The scapholunate angle would be <30°.
Answer 4: Ulnar subluxation of the carpus relative to the distal radius describes a global extrinsic ligamentous instability pattern. The main ligamentous restraint to ulnar translation of the carpus is the radioscaphocapitate (RSC) ligament, which may be traumatically injured in radiocarpal dislocations or iatrogenically during proximal row carpectomy procedures.
Answer 5: Volar subluxation of the carpus relative to the distal radius describes an injury to the short radiolunate ligament. In the setting of distal radius fractures, failure to address the critical corner of the distal radius (volar-ulnar corner of the articular segment) where the short radiolunate ligament attaches can result in volar translation of the carpus with devastating complications.