Scapho-Lunate Instability

Scapholunate (SL) instability is the most common form of carpal instability, resulting from disruption of the scapholunate ligament complex. The scapholunate ligament (SLL) is the primary intrinsic stabiliser between the scaphoid and lunate — when disrupted, the scaphoid and lunate dissociate, producing a predictable pattern of carpal malalignment. If untreated, progressive carpal collapse occurs in a characteristic pattern (SLAC wrist — scapholunate advanced collapse), leading to wrist arthritis. Early diagnosis and appropriate management are essential to prevent this progression.

• The SLL is a C-shaped ligament consisting of three components: dorsal (the strongest and most important — the primary restraint to SL dissociation; thick, transverse fibres), proximal (fibrocartilaginous — weakest component), and volar (oblique fibres — secondary restraint); injuries to the dorsal component are the most critical and most likely to produce symptomatic instability
• Normal SL kinematics: the scaphoid and lunate normally move in synchrony; in wrist flexion, both flex; in extension, both extend; the SLL prevents independent motion; when the SLL is torn, the scaphoid follows the trapezoid (which pulls it into flexion) while the lunate follows the triquetrum (which extends it via the lunotriquetral ligament) — producing the DISI (dorsal intercalated segment instability) deformity: the lunate tilts dorsally and the scaphoid rotates into flexion
• Mechanism: falls on an outstretched hand (FOOSH) with the wrist extended and ulnar deviated; high-energy trauma (perilunate dislocations involve complete SL disruption); progressive attritional tears in inflammatory arthropathy or calcium pyrophosphate disease (pseudogout)

• History: dorsoradial wrist pain following a FOOSH injury; clicking or clunking with wrist movements; weakness of grip; may be a chronic presentation (months to years after an injury that was initially dismissed as a `sprain`); in chronic cases, progressive wrist pain with activities and reduction in grip strength
• Watson scaphoid shift test (scaphoid shift test): the most widely used clinical test for SL instability; the examiner places their thumb on the palmar scaphoid tubercle and the fingers on the dorsum of the wrist; the wrist is moved from ulnar to radial deviation (during which the scaphoid normally flexes); the examiner`s thumb prevents the scaphoid from flexing; in a positive test, the scaphoid subluxes dorsally over the dorsal lip of the radius as the wrist moves to radial deviation, producing a palpable and/or audible clunk and dorsal pain; a positive Watson test has a sensitivity of approximately 69% and specificity of approximately 66% — not highly specific but the most useful clinical test; must compare with the contralateral wrist as false positives occur in hypermobile patients
• SL ballottement test (Reagan test): the scaphoid and lunate are gripped separately and translated relative to each other; pain or laxity = positive; useful for assessing lunotriquetral instability simultaneously
• Point tenderness: dorsal wrist, just distal to Lister`s tubercle, over the SL interval — the most sensitive sign; deep dorsal wrist pain at this location after a FOOSH injury should always raise suspicion of SL injury

• Plain radiographs — key findings in SL instability: AP (PA) view — SL gap (>3 mm = Terry-Thomas sign — named after the gap-toothed comedian; normal SL gap <2 mm); cortical ring sign (the rotated, flexed scaphoid projects as a ring on the AP view as its distal pole overlaps its body); DISI deformity on lateral view — the lunate tilts dorsally (lunate dorsal tilt >10–15°); scapholunate angle on lateral view — normal 30–60°; SL angle >70° = DISI (abnormal); the lateral view is essential for assessing the SL angle and DISI pattern
• Stress radiographs: AP view in grip compression (the patient grips a dynamometer or clenches a fist) — increases the SL gap in dynamic instability; useful when standard films are equivocal
• MRI: high-resolution MRI (3T) or MRI arthrogram (gadolinium injected into the radiocarpal joint) to visualise the SLL; MRI arthrogram has higher sensitivity than standard MRI for partial and complete tears; sensitivity approximately 70–90% for complete dorsal SLL tears; useful for pre-operative planning and for cases where clinical and plain radiograph findings are inconclusive
• Wrist arthroscopy: the gold standard for diagnosis and grading (Geissler); simultaneously diagnostic and therapeutic; identifies partial vs complete tears, associated chondral damage, and concurrent ligament injuries

• Non-operative: partial tears (Geissler Grade I–II) without instability on stress X-ray; splinting for 6–12 weeks; activity modification; physiotherapy; steroid injection for symptom relief; return to sport with protective splint
• Acute complete SLL tear (within 6 weeks of injury, ligament reparable): open or arthroscopic-assisted repair of the dorsal SLL; the dorsal ligament is repaired using suture anchors at the scaphoid and lunate; the repair is augmented with temporary SL K-wire fixation (3 K-wires across the SL joint) for 8–12 weeks to protect the repair; concurrent dorsal capsulodesis (reinforcing the dorsal capsule) is often added; repair within 6 weeks gives the best results — the ligament ends are identifiable and viable; healing rate approximately 80–90% in acute repairs
• Subacute/chronic complete SLL tear (6 weeks to 6 months, ligament attenuated but wrist reducible): direct repair is less reliable; options — dorsal capsulodesis (Blatt capsulodesis — a distally based dorsal capsular flap is used to restrain scaphoid flexion); RASL procedure (reduction and association of the scaphoid and lunate — a headless compression screw is placed across the SL joint to hold the reduction); these are salvage procedures with moderate long-term outcomes
• Chronic SLL tear with DISI (irreducible or fixed deformity, no OA): ligament reconstruction is required; the SLIL (scapholunate interosseous ligament) cannot be directly repaired; tendon graft reconstruction — the Brunelli procedure (FCR tendon graft) and its modifications (Van Den Abbeele — 3-ligament tenodesis); the graft is threaded through the scaphoid, over the dorsal lunate, and anchored to the dorsal radius, recreating the dorsal SLL and dorsal radiocarpal ligament; moderate long-term results; progression to SLAC wrist arthritis is reduced but not eliminated
• SLAC wrist (established arthritis): proximal row carpectomy (PRC — removes the scaphoid, lunate, and triquetrum; the capitate articulates with the radial fossa; good range of motion preserved but strength reduced); four-corner fusion (scaphoidectomy + fusion of the lunate, capitate, hamate, and triquetrum — preserves wrist motion through the radiolunate joint); total wrist arthrodesis (for severe pan-carpal OA or high-demand patients)

• DISI deformity — key radiological landmarks: on the lateral wrist X-ray, the normal alignment of radius-lunate-capitate is colinear; in DISI, the lunate tilts dorsally (the dorsal lip of the lunate points posteriorly — lunate dorsiflexion); the scapholunate angle increases (>70°) because the scaphoid rotates into flexion while the lunate extends; the capitolunate angle also increases; the DISI pattern is the radiological hallmark of SL ligament disruption; VISI (volar intercalated segment instability — lunate tilts volarly) is associated with lunotriquetral ligament disruption
• Perilunate dislocations and SL disruption: high-energy perilunate dislocations (the Mayfield sequence — Stage I involves SL disruption) always involve complete SLL tear; in a trans-scaphoid perilunate dislocation (the scaphoid fractures rather than the SL ligament tearing), the SL ligament may be partially intact; in both cases, urgent closed or open reduction and stabilisation is required; the SL ligament injury must be addressed at the time of fracture fixation in trans-scaphoid perilunate dislocations; unreduced perilunate dislocations are orthopaedic emergencies (risk of median nerve compression and carpal vascular compromise)
• Four-corner fusion vs PRC: four-corner fusion (scaphoidectomy + lunate-triquetrum-capitate-hamate fusion) preserves wrist motion through the preserved radiolunate joint (which is protected from OA in SLAC wrist until Stage III); PRC removes the proximal row and allows the capitate to articulate with the radial lunate fossa — simple, reliable, preserves 50–70% of normal wrist motion; the choice depends on the presence or absence of capitolunate OA (Stage III SLAC) — if Stage III, PRC is contraindicated because the capitate articular surface is already OA; four-corner fusion can be performed up to Stage III if the radiolunate joint is preserved

• SL instability: most common carpal instability; dorsal SLL is the primary restraint; DISI deformity — lunate tilts dorsally, scaphoid flexes; Terry-Thomas sign (SL gap >3 mm on AP X-ray)
• Watson scaphoid shift test: thumb on palmar scaphoid tubercle; wrist moved from ulnar to radial deviation; positive = dorsal scaphoid clunk + pain; sensitivity ~69%, specificity ~66%
• SL angle on lateral X-ray: normal 30–60°; >70° = DISI (SL instability); capitolunate angle: normal <30°; >30° = abnormal
• Cortical ring sign: flexed rotated scaphoid projects as a ring on AP X-ray; sign of SL dissociation
• Geissler Grade IV = drive-through sign (2.7 mm arthroscope passes from midcarpal to radiocarpal through SL interval); complete disruption; open repair/reconstruction mandatory
• Acute repair (within 6 weeks): suture anchors + K-wire fixation; 80–90% success; best results with early treatment
• Chronic DISI: tendon graft reconstruction (Brunelli / Van Den Abbeele 3-ligament tenodesis); moderate results; SLAC wrist progression reduced but not eliminated
• SLAC wrist: Stage I (radial styloid OA) → II (radioscaphoid OA) → III (+capitolunate OA); Stage III = PRC contraindicated (capitate OA); four-corner fusion indicated for Stage II–III
• PRC: removes proximal row; capitate on radial fossa; 50–70% motion preserved; contraindicated if Stage III SLAC (capitolunate OA)
• Perilunate dislocation: Mayfield Stage I = SL disruption; orthopaedic emergency; urgent reduction + fixation; SL repair concurrent with fracture management