The scapholunate interosseous ligament (SLIL) is the most important intrinsic stabilizer of the proximal carpal row, connecting the scaphoid and lunate bones. The C-shaped ligament has three components: dorsal (thickest, most important biomechanically — primary stabilizer), proximal (membranous, often absent in middle-aged), and palmar (volar, prevents palmar gapping). Disruption of the dorsal SLIL is the critical event leading to instability. The SLIL works synergistically with extrinsic ligaments (radioscaphocapitate, dorsal radiocarpal, dorsal intercarpal) to maintain proximal row alignment.
Mechanism: classic fall on outstretched hand (FOOSH) with wrist extended, ulnar deviated, and forearm pronated, causing intercarpal supination through the proximal row. May coexist with distal radius fracture (perilunate dislocation in extreme injury). Spectrum from acute (less than 6 weeks) to chronic (more than 6 months). Geissler arthroscopic classification: Grade I (attenuation, no gap), Grade II (partial tear with gap or step-off but probe doesn't pass through), Grade III (complete tear with probe passing between scaphoid and lunate from radiocarpal joint, no gap on fluoroscopy), Grade IV (complete tear with palpable widening 'drive-through sign', gap visible on stress fluoroscopy).
Diagnosis: clinical findings include radial wrist pain over scapholunate interval (palpable just distal to Lister tubercle), positive Watson test (pressure on scaphoid distal pole during ulnar to radial deviation produces dorsal subluxation and pain — pathognomonic when positive), reduced grip strength, and clicking. Imaging starts with PA, lateral, and clenched-fist (stress) wrist radiographs. Findings: SL gap over 3 mm on PA (Terry Thomas sign), scaphoid ring sign (foreshortened cortex), dorsal scaphoid tilt with cortical ring on lateral, increased SL angle (greater than 60°, normal 30-60°), DISI deformity (lunate extension on lateral, capitolunate angle greater than 30°). MRI (3-Tesla, dedicated wrist coil, MR arthrography ideal) detects partial tears. Wrist arthroscopy is gold standard, allowing direct visualization, dynamic stability testing, and treatment. Untreated chronic complete SLIL tears progress through static dissociation, DISI deformity, scaphoid degenerative change at radial styloid, then radioscaphoid arthritis (SLAC stage I), then capitolunate arthritis (SLAC stage II), and total midcarpal arthritis (SLAC stage III). Treatment depends on acuity, completeness, reducibility, and degenerative changes. Acute tears (less than 6 weeks): primary arthroscopic or open repair with suture anchors, capsulodesis (Blatt or modified). Subacute (6 weeks-6 months) reducible: reconstruction with tendon (modified Brunelli using FCR strip, three-ligament tenodesis using palmaris longus). Chronic non-reducible without arthritis: ligamentoplasty, RASL procedure, or salvage. SLAC arthritis: proximal row carpectomy, four-corner fusion (scaphoid excision with capitolunate, capito-hamate, hamate-triquetrum fusion), or wrist arthrodesis. Outcomes correlate with timing — acute repair best, chronic with degenerative changes worst.