Scapholunate Injury X-ray Signs: Widening, DISI, and SLAC Wrist

The scapholunate angle is measured on a lateral wrist radiograph as the angle between the long axis of the scaphoid and the long axis of the lunate. The normal range is 30 to 60 degrees, with a mean of approximately 47 degrees. An angle exceeding 60 to 70 degrees indicates dorsal intercalated segment instability (DISI), meaning the lunate has tilted dorsally relative to the capitate while the scaphoid has flexed palmarly after scapholunate ligament failure.

Scapholunate advanced collapse (SLAC) follows a predictable degenerative sequence. Stage I involves isolated radial styloid to distal scaphoid arthrosis. Stage II adds arthrosis between the scaphoid proximal pole and the radius. Stage III involves midcarpal arthritis between the capitate and lunate as proximal capitate migration occurs. The radiolunate joint is characteristically preserved. AI can help explain SLAC staging terms, but it cannot choose a reconstruction or wrist fusion procedure. A hand surgeon must assess the images, symptoms, and examination together. For non-SLAC degenerative joint-space narrowing, compare wrist arthritis.

Acutely, MRI shows discontinuity or high T2 signal through the dorsal or volar component of the scapholunate ligament, with associated radiocarpal joint effusion and periligamentous edema. Radiographically, a scapholunate interval wider than 3 mm on a clenched-fist PA view (Terry Thomas sign) indicates complete disruption. Chronically, the ligament is absent or replaced by fibrous tissue, the scaphoid shows rotary subluxation producing the cortical ring sign on PA view, and early SLAC arthrosis appears.