Scapholunate Ligament Injury

Scapholunate (SL) ligament injury is the most common cause of carpal instability, usually from a fall on the outstretched hand (FOOSH) with the wrist extended, and is often dismissed as a wrist sprain.

A complete tear with failure of the secondary stabilisers lets the scaphoid flex and the lunate extend, producing dorsal intercalated segment instability (DISI). Untreated, this can progress over years to scapholunate advanced collapse (SLAC) arthritis, although not every wrist does.

The SL ligament is C-shaped with 3 parts: dorsal (the strongest and main stabiliser), proximal (membranous and weakest) and volar. Secondary stabilisers include the dorsal intercarpal, scaphotrapezial and radioscaphocapitate ligaments. A tear of the SL ligament alone can stay stable; when the secondary stabilisers also fail, the bones separate.

The injury is described as a spectrum:

• •Predynamic: symptoms with normal radiographs, including stress views
• •Dynamic: the SL gap widens only on stress views
• •Static: the gap is widened at rest, and the lateral view may show DISI
• •SLAC wrist: arthritis that starts at the radial styloid, spreads across the radioscaphoid joint and then reaches the capitolunate joint; the radiolunate joint is spared

How far a tear progresses varies: some never progress, while others reach static instability and arthritis.