Most Common Carpal Instability | DISI Pattern | Watson Test | SLAC Progression
- Dorsal SL ligament is strongest and most important - tears progress palmar to membranous to dorsal
- Watson scaphoid shift test - thumb on scaphoid tubercle, radial deviation produces painful clunk
- Terry Thomas sign - SL gap over 3mm on PA radiograph (named after gap-toothed comedian)
- DISI pattern - lunate extends as scaphoid flexes, SL angle over 70 degrees on lateral
- SLAC wrist inevitable if untreated - arthritis follows predictable radial styloid to capitolunate pattern
- “Always compare Watson test to contralateral side - may be positive bilaterally in lax patients
- “Acute repair (under 6 weeks) has best outcomes - direct ligament repair with K-wire fixation
- “Chronic SL injury needs reconstruction not repair - tissue too degenerated to hold sutures
- “SLAC wrist spares radiolunate joint - allows salvage with PRC or 4-corner fusion
Thumb on scaphoid tubercle, radial deviation produces painful clunk. Compare to contralateral side. Gold standard clinical test for SL instability.
SL gap over 3mm on PA radiograph. Named after gap-toothed British comedian. Also look for scaphoid ring sign (rotatory subluxation).
Normal 30-60°. Over 70° = DISI pattern. Measure on true lateral - scaphoid axis to lunate perpendicular. Capitolunate angle over 15° confirms malalignment.
Untreated = 100% arthritis. Stage I: radial styloid. Stage II: scaphoid fossa. Stage III: capitolunate. Radiolunate always spared.
- Presentation
- Recent injury, reducible
- X-ray Findings
- Dynamic widening
- Treatment
- Direct repair + K-wires
- Presentation
- Months old, reducible gap
- X-ray Findings
- Static widening, no arthritis
- Treatment
- Tendon reconstruction
- Presentation
- Fixed DISI deformity
- X-ray Findings
- Static widening, no arthritis
- Treatment
- Capsulodesis or limited fusion
- Presentation
- Radial-sided pain
- X-ray Findings
- Radial styloid arthritis
- Treatment
- Styloidectomy + denervation
- Presentation
- Diffuse wrist pain
- X-ray Findings
- Scaphoid fossa +/- CL arthritis
- Treatment
- PRC or 4-corner fusion
Overview
Definition and Importance
Scapholunate dissociation is the most common and most important form of carpal instability. It results from failure of the scapholunate interosseous ligament (SLIL), allowing abnormal motion between scaphoid and lunate with subsequent carpal malalignment.
Why It Matters:
- Most common carpal instability pattern (95%)
- If untreated, leads to predictable SLAC arthritis
- Often missed on initial presentation (occult instability)
- Treatment success highly time-dependent
- Understanding SL pathology is fundamental to wrist surgery
Epidemiology
- Incidence: Unknown; many cases undiagnosed initially
- Age: Young adults (20-40 years) most common
- Gender: Male predominance (occupational exposure)
- Associated injuries:
- Distal radius fractures (5-30% have SL injury)
- Perilunate injuries (SL always involved)
- Greater arc injuries (trans-scaphoid)
Natural History
Progression Without Treatment:
- Initial injury: SL ligament torn, dynamic instability
- Months to years: Static instability develops, SL gap widens
- 5-10 years: SLAC Stage I (radial styloid wear)
- 10-15 years: SLAC Stage II (radioscaphoid arthritis)
- 15-20+ years: SLAC Stage III (capitolunate arthritis)
Not all patients progress - some compensate with secondary stabilizers (RSC ligament, scaphotrapezial ligament).
Anatomy
Scapholunate Interosseous Ligament (SLIL)
The SLIL is a C-shaped ligament connecting the scaphoid and lunate along their proximal articular margins.
Three Components:
- Location
- Dorsal margin
- Strength
- STRONGEST
- Composition
- True ligament (collagen fibers)
- Function
- Primary rotational stabilizer
- Location
- Volar margin
- Strength
- Second
- Composition
- True ligament
- Function
- Resists translation
- Location
- Proximal margin
- Strength
- Weakest
- Composition
- Fibrocartilage
- Function
- Minimal structural role
Biomechanical Significance:
- Dorsal SL: 260N load to failure (vs 150N palmar)
- Complete SL sectioning required for DISI pattern
- Dorsal component most important for reconstruction
Secondary Stabilizers
When SLIL fails, secondary stabilizers provide some restraint:
- Scaphotrapezial (ST) ligament
- Scaphocapitate (SC) ligament
- Radioscaphocapitate (RSC) ligament
- Dorsal intercarpal (DIC) ligament
- Dorsal radiocarpal (DRC) ligament
Some patients maintain function despite SL injury due to competent secondary stabilizers - explains why not all injuries progress.
Carpal Kinematics Review
- Scaphoid naturally tends to FLEX (oblique orientation)
- Lunate is the "intercalated segment" - no tendon attachments
- SL ligament couples scaphoid and lunate motion
- When intact, lunate follows scaphoid into flexion
- Scaphoid flexes further (unconstrained)
- Lunate follows triquetrum into EXTENSION
- Creates DISI (Dorsal Intercalated Segment Instability)
- Progressive widening of SL gap
Classification Systems
Garcia-Elias Classification
This is the most useful clinical staging system:
Partial SL Tear
- Radiographs: Normal
- Reducibility: N/A (partial tear)
- Treatment: Arthroscopic debridement, thermal shrinkage
- Prognosis: Good with conservative treatment
This stage typically responds well to non-operative management.
Dynamic vs Static Instability
- Normal resting radiographs
- Abnormal only with stress (clenched fist)
- Secondary stabilizers still functioning
- Better prognosis
- May respond to capsulodesis
- Abnormal resting radiographs
- SL gap widened without stress
- Complete ligament failure
- Secondary stabilizers failed
- Requires reconstruction
SLAC Wrist Staging
Radial Styloid Arthritis
- Radiographic Finding: Radial styloid osteophytes and sclerosis
- Arthritis Location: Radial styloid-scaphoid articulation
- Timeline: 5-10 years post-injury
- Treatment: Radial styloidectomy with denervation
Early SLAC can be managed with less invasive procedures.
Geissler Arthroscopic Classification
- Radiocarpal View
- Attenuation/hemorrhage
- Midcarpal View
- No incongruency
- Treatment Implication
- Conservative or debridement
- Radiocarpal View
- May have incongruency
- Midcarpal View
- Step-off, cannot pass probe
- Treatment Implication
- Debridement or pinning
- Radiocarpal View
- Incongruency
- Midcarpal View
- Can pass 1mm probe
- Treatment Implication
- Arthroscopic or open repair
- Radiocarpal View
- Gross instability
- Midcarpal View
- Can pass 2.7mm scope
- Treatment Implication
- Open repair essential
The topic calls SL dissociation "the most common carpal instability" but never gives the overarching nosology an examiner will expect. The Mayo (Larsen/Cooney) classification divides carpal instability into four categories, and SL dissociation is the prototype of the first:
- CID - Carpal Instability Dissociative: the abnormal motion is within a single carpal row, i.e. between bones connected by an interosseous ligament. Scapholunate dissociation (-> DISI) and lunotriquetral dissociation (-> VISI) are the two members. This is where SL dissociation sits.
- CIND - Carpal Instability Non-Dissociative: instability between rows (radiocarpal or midcarpal) with the intrinsic interosseous ligaments intact - e.g. midcarpal instability, the "catch-up clunk."
- CIC - Carpal Instability Complex (Combined): features of both dissociative and non-dissociative patterns together - e.g. perilunate dislocation (combined greater/lesser-arc instability).
- CIA - Carpal Instability Adaptive: carpal malalignment driven by an extra-carpal cause, classically a malunited distal radius (dorsal tilt) producing a compensatory midcarpal/DISI-like posture - treated by correcting the radius, not the carpus.
Exam point: when asked to classify carpal instability, lead with CID / CIND / CIC / CIA, then place SL dissociation as the commonest CID (DISI), distinguish it from LT dissociation (VISI), and remember CIA means you must look at the distal radius before blaming the ligaments.
Clinical Assessment
History
- Fall on outstretched hand (FOOSH)
- May have initial "sprain" that never fully recovered
- Progressive wrist pain and weakness
- Clicking or clunking with motion
- Time since injury (critical for treatment planning)
- Mechanism (hyperextension + ulnar deviation)
- Previous treatment attempts
- Hand dominance and occupation
- Functional demands
Physical Examination
- Dorsal wrist swelling
- Scaphoid prominence (if gap wide)
- Compare to contralateral wrist
- SL interval: 1cm distal to Lister's tubercle, in line with 3rd metacarpal
- May feel step-off between scaphoid and lunate
- Tenderness over dorsal SL ligament
Special Tests
Watson Scaphoid Shift Test (Primary Test):
Technique:
- Patient seated, forearm pronated
- Examiner's thumb on scaphoid tubercle (palmar)
- Other hand controls wrist
- Start in ulnar deviation (scaphoid extended)
- Apply dorsal pressure on scaphoid tubercle
- Move wrist from ulnar to radial deviation
- In radial deviation, scaphoid wants to flex - blocked by pressure
- Positive: Painful dorsal subluxation of scaphoid
- Release pressure: Painful clunk as scaphoid reduces
Interpretation:
- Compare to opposite side (physiologic laxity common)
- Positive = pain + clunk (apprehension is also significant)
- Negative Watson doesn't rule out SL injury
Other Tests:
- Scaphoid compression test: Axial load on scaphoid causes pain
- SL ballottement: Stabilize lunate, translate scaphoid D/P
- Finger extension test: Pain with resisted finger extension
Grip Strength
- Compare to contralateral side
- Often 50-70% of normal
- Important functional measure
- Tracks with treatment success
Differential Diagnosis
- Distinguishing Features
- Positive Watson test, dorsal SL tenderness, FOOSH mechanism
- Key Investigation
- SL gap over 3mm on PA, SL angle over 70 degrees on lateral
- Distinguishing Features
- Anatomical snuffbox and scaphoid tubercle tenderness, axial thumb pain
- Key Investigation
- Scaphoid-view radiographs, MRI/CT if occult
- Distinguishing Features
- Ulnar-sided pain, positive Reagan ballottement/shuck test
- Key Investigation
- Volar tilt of lunate (VISI), broken Gilula arc ulnarly
- Distinguishing Features
- High-energy injury, gross swelling, median nerve symptoms
- Key Investigation
- Loss of carpal arcs, 'piece of pie'/'spilled teacup' on lateral
- Distinguishing Features
- Ulnar-sided pain, positive fovea sign, painful supination/pronation
- Key Investigation
- MR arthrography or wrist arthroscopy
- Distinguishing Features
- Soft dorsal swelling, transilluminates, no instability on Watson
- Key Investigation
- Ultrasound or MRI
- Distinguishing Features
- Older patient, insidious pain, crepitus rather than clunk
- Key Investigation
- Radial styloid and radioscaphoid joint-space changes
Investigations
Plain Radiographs
Standard Views:
- PA in neutral rotation (forearm pronated)
- True lateral (radius and ulna superimposed)
- Clenched fist PA (stress view)
- PA in radial and ulnar deviation
PA View Findings:
Terry Thomas Sign (David Letterman Sign):
- SL gap greater than 3mm indicates SL ligament injury
- Named after British comedian with gap-toothed smile
- Compare to contralateral wrist
Scaphoid Ring Sign:
- Flexed scaphoid appears foreshortened
- Distal pole projects as a ring/cortical outline
- Indicates scaphoid malrotation
Gilula's Arcs:
- Arc I: Proximal carpal row proximal surfaces
- Arc II: Proximal carpal row distal surfaces
- Arc III: Hamate and capitate proximal surfaces
- Disruption indicates carpal malalignment
Lateral View Findings:
SL Angle Measurement:
- Draw line along palmar surface of scaphoid (or through proximal/distal poles)
- Draw line perpendicular to lunate distal articular surface
- Measure angle between lines
- Normal: 30-60 degrees
- Greater than 70 degrees = DISI pattern
Capitolunate Angle:
- Line through capitate long axis
- Compare to lunate perpendicular
- Normal: Less than 15 degrees
- Greater than 15 degrees = malalignment
Stress Radiographs
- Patient makes tight fist during PA X-ray
- Loads carpal joints, widens SL gap
- May reveal dynamic instability (normal static films)
- Essential for interpretation
- Some bilateral laxity is normal
- Side-to-side difference more significant than absolute values
Advanced Imaging
- Can visualise the SL ligament directly
- Diagnostic performance is field-strength dependent: in a systematic review and meta-analysis of 24 studies (1902 examinations, SLIL injury prevalence approximately 33%), pooled sensitivity was 45.7% for 1.5T MRI, 75.7% for 3.0T MRI, and 82.1% for MR arthrography, with specificity 80.5%, 97.1%, and 92.8% respectively (Hafezi-Nejad et al., 2016)
- Less reliable for partial tears; MR arthrography and 3.0T imaging are preferred when MRI is used
- Best for associated fractures
- CT arthrography useful if MRI contraindicated
- Direct visualization of SL ligament
- Allows Geissler grading
- Can treat at same sitting (debridement, repair)
Management Algorithm
Treatment Decision Framework
Direct Repair Strategy
- Complete SL tear
- Injury within 6 weeks (ideally under 3 weeks)
- Reducible instability
- Good ligament tissue quality
- Open dorsal approach
- Bone anchor repair of SL ligament
- Dorsal capsulodesis augmentation
- K-wire fixation (2 SL, 1 SC) for 8-12 weeks
- 80-90% good to excellent results
- Motion 80-90% of normal
- Grip strength 70-80% of normal
Early surgical repair offers the best chance of recovery.
Non-Operative Management
- Partial tears (Geissler I-II)
- Low-demand patients
- Poor surgical candidates
- Splinting (4-6 weeks)
- Activity modification
- Hand therapy for proprioception
- Serial clinical and radiographic follow-up
- May be successful for partial tears
- Risk of progression without treatment
- Must counsel patient about natural history
Acute Surgical Repair (Less than 6 weeks)
- Complete SL tear
- Reducible instability
- Adequate ligament tissue quality
- Dorsal wrist incision
- Between 3rd and 4th extensor compartments
- Ligament-sparing capsulotomy
- Debride SL interval
- Reduce SL gap with pointed clamp
- Repair dorsal SL with suture anchors or transosseous sutures
- Augment with dorsal capsulodesis
- K-wire fixation (2 SL, 1 SC)
- 1.1mm or 1.25mm wires
- Two through SL joint
- One scaphocapitate
- Remove at 8-12 weeks
- 80-90% good/excellent results
- Motion 80-90% of normal
- Grip strength 70-80% of normal
Chronic SL Instability - Soft Tissue Reconstruction
Indications:
- Chronic injury (greater than 6 weeks)
- Reducible instability
- No arthritis
Options:
Dorsal Capsulodesis (Blatt):
- Radially-based capsular flap
- Secured to dorsal scaphoid
- Good for dynamic instability
- Less reliable for static
Modified Brunelli Reconstruction:
- FCR strip (half tendon)
- Through scaphoid tunnel
- Secured to lunate
- Combined with capsulodesis
SLAM Procedure (Scapholunate Axis Method):
- Adds volar limb to Brunelli
- More anatomic reconstruction
- Higher complexity
Bone-Ligament-Bone Graft:
- Free bone-SL-bone from cadaver
- Technical challenges
- Limited availability
SL Arthrodesis
- Chronic irreducible instability
- Failed reconstruction
- No arthritis
- Decorticate SL articular surfaces
- Bone graft
- Headless screws or K-wires
- High nonunion rate (30-50%)
- Motion reduced ~50%
- Consider alternatives first
Salvage Procedures (SLAC Wrist)
- Remove scaphoid, lunate, triquetrum
- Capitate articulates with lunate fossa
- Prerequisites: Intact capitate head, intact lunate fossa
- 50-60% motion preserved
- Remove scaphoid
- Fuse capitate, lunate, hamate, triquetrum
- Better grip strength than PRC
- 40-50% motion preserved
- PRC: Lower-demand, faster recovery
- 4CF: Manual workers, better power
The chronic-instability options above (capsulodesis, Brunelli/three-ligament tenodesis, SLAM, arthrodesis) omit a named technique an examiner may raise: RASL - Reduction and Association of the Scaphoid and Lunate.
- Concept: after reducing the SL gap, a headless compression screw is placed across the scapholunate interval (scaphoid into lunate). Rather than a true bony fusion, the aim is to hold the reduction while a stable fibrous union/pseudarthrosis forms across the interval, allowing some preserved scapholunate motion.
- Indication: chronic reducible SL instability without arthritis - an alternative to soft-tissue reconstruction; cannula/cannulated headless screws (and more recently a continuous-compression implant) are used.
- The catch - know the complications: RASL has a notable rate of screw loosening/breakage, scaphoid and lunate osteolysis/cyst formation around the screw, loss of reduction and the need for hardware removal, which is why many surgeons prefer tendon-based reconstruction (three-ligament tenodesis) for chronic reducible instability. It is examinable precisely as a "named option with a high reoperation/loosening profile."
Exam point: RASL = reduce the SL gap and hold it with a headless screw across the SL joint aiming for a stable fibrous union; it is an option for chronic reducible instability but carries screw-loosening, osteolysis and reduction-loss complications, so it has largely been superseded by tendon reconstruction in many units.
Surgical Technique
Indication: Acute tear (under 6 weeks), reducible
- Supine, arm table
- Regional or general anesthesia
- Upper arm tourniquet
- Longitudinal incision over Lister's tubercle (4-5cm)
- Retract EPL radially
- Incise extensor retinaculum between 3rd and 4th compartments
- Ligament-sparing capsulotomy (preserve DRC, DIC ligaments)
- Expose SL interval
- Debride fibrous tissue from SL interval
- Assess dorsal ligament quality
- Reduce SL gap with pointed reduction forceps
- Place bone anchors (1.4-2.0mm) in scaphoid and lunate
- Pass sutures through dorsal ligament remnants
- Tie sutures with interval reduced and wrist neutral
- Raise radially-based capsular flap
- Advance capsule to dorsal scaphoid (Blatt capsulodesis)
- Insert K-wires: 2 across SL joint, 1 scaphoid to capitate
- Confirm reduction and wire position with fluoroscopy
Meticulous technique is essential for optimal outcomes.
- 1.1-1.25mm diameter wires
- Remove at 8-12 weeks
- May bury or leave proud
- Repair extensor retinaculum
- Skin closure
- Volar splint with wrist neutral
Careful surgical technique and adequate immobilization are key to successful repair.
Complications
Complications of SL Injury
- Predictable if untreated
- Stage I: 5-10 years
- Stage II: 10-15 years
- Stage III: 15-20+ years
- Grip weakness
- Activity limitation
- Work capacity impairment
Surgical Complications
- Infection (1-2%)
- Stiffness (most common)
- CRPS
- Nerve injury (superficial radial, PIN)
- Recurrent instability (20-40%)
- K-wire complications (migration, infection, breakage)
- Donor site morbidity (FCR harvest)
- Nonunion (30-50% for SL fusion)
- Hardware prominence
- Adjacent joint degeneration
- PRC: Radiocapitate arthritis (long-term)
- 4CF: Nonunion, hardware failure
Prognostic Factors
- Acute injury (less than 6 weeks)
- Reducible instability
- Good tissue quality
- Isolated SL injury
- Non-smoker
- Chronic injury
- Irreducible deformity
- Associated fractures
- Heavy manual occupation
- Smoking, diabetes
Postoperative Care
Acute Repair Protocol
- Volar splint or short arm cast
- Wrist in neutral position
- Thumb free for motion
- K-wires in situ
- Office procedure under local anesthesia
- Confirm healing on X-ray before removal
- Check for maintained reduction
- Hand therapy referral after wire removal
- Gentle active ROM exercises
- Avoid forceful gripping initially
- Progressive strengthening from 12 weeks
- Gradual return to sports/work
- May need activity modification
- Continue strengthening program
Reconstruction Protocol
- Cast or splint immobilization
- K-wires protect tenodesis
- Longer healing time than acute repair
- Confirm no recurrent widening
- May see some gap recurrence (expected)
- Progressive ROM and strengthening
- Accept some residual weakness
- Functional goals, not normal anatomy
Salvage Procedure Protocols
- Early motion at 2 weeks (key advantage)
- No internal fixation to protect
- Rapid functional recovery
- Extended immobilization until fusion (8-12 weeks)
- Serial X-rays to confirm union
- Later motion but better strength
Outcomes/Prognosis
Acute Repair Outcomes
- 80-90% good to excellent results
- Motion: 80-90% of contralateral
- Grip strength: 70-80% of contralateral
- Pain: Significant improvement in 85%
- 10-20% recurrent instability
- Risk factors: Delayed repair (over 3 weeks), poor tissue quality, non-compliance
Chronic Reconstruction Outcomes
- 60-70% satisfactory results
- Some gap recurrence common (but may be asymptomatic)
- Motion: 70-80% of normal
- Grip: 60-70% of normal
- Pain relief in 70-80%
- High nonunion rate: 30-50%
- Motion loss: approximately 50%
- Only 50-60% satisfactory outcomes
- Not recommended as first-line
Salvage Outcomes
- Pain relief: 80-90%
- Motion: 50-60% preserved (40-50 degrees flexion-extension)
- Grip: 70-80% of contralateral
- Satisfaction: 80-85%
- Long-term concern: Radiocapitate arthritis (10-20% at 10+ years)
- Pain relief: 85-90%
- Motion: 40-50% preserved
- Grip: 80-90% of contralateral (better than PRC)
- Satisfaction: 80-90%
- Nonunion: 5-15%
- Hardware issues: 10-20% require removal
SLAC Progression Without Treatment
Natural History:
- Stage I (radial styloid): 5-10 years post-injury
- Stage II (radioscaphoid): 10-15 years
- Stage III (capitolunate): 15-20+ years
- Essentially 100% progression over time
- Variable rate depending on activity level and secondary stabilizer competence
Prognostic Factors
- Acute presentation (under 3 weeks)
- Younger age (under 40)
- Reducible instability
- Good tissue quality
- Non-smoker
- Compliant patient
- Chronic injury (over 12 weeks)
- Irreducible deformity
- Associated fractures
- Heavy manual labor
- Smoking
- Poor compliance
Guidelines, Registries & Global Practice
Global Epidemiology
Scapholunate dissociation is the most common pattern of carpal instability, yet its true incidence is unknown because many injuries are occult or misdiagnosed as a simple wrist sprain. The best population-anchored data come from its strong association with distal radius fractures: in Geissler's arthroscopic series of 60 displaced intra-articular distal radius fractures, SLIL tears were present in 32% and any intracarpal soft-tissue injury in 68% (Geissler et al., 1996). Across diagnostic imaging cohorts, the prevalence of SLIL injury among patients investigated for wrist pain is approximately 33% (median, interquartile range 25-42%) (Hafezi-Nejad et al., 2016). Typical patients are young, working-age adults sustaining a fall on the outstretched hand with the wrist in extension, ulnar deviation and intercarpal supination.
Guideline and Society Guidance (Side-by-Side)
No single body publishes a disease-specific clinical practice guideline for scapholunate dissociation; management is governed by consensus from hand-surgery societies and the primary literature rather than by graded national guidelines. The table summarises the orientation of the major bodies.
- Nature of guidance
- No disease-specific CPG; ASSH educational consensus favours early diagnosis, acute primary repair under 6 weeks, reconstruction for chronic reducible injury, salvage for SLAC
- Evidence base
- Expert consensus / Level III-IV cohorts
- Nature of guidance
- No NICE guideline for SL injury; BSSH education and UK practice mirror the stage-based algorithm; arthroscopic assessment widely used
- Evidence base
- Expert consensus / Level III-IV
- Nature of guidance
- Endorse Garcia-Elias prognosis-based staging and three-ligament tenodesis for chronic reducible instability
- Evidence base
- Level IV technique series
- Nature of guidance
- Technique-level recommendations for arthroscopic and open repair within the broader carpal instability literature
- Evidence base
- Expert consensus
Registry Evidence
There is no dedicated arthroplasty-style registry for scapholunate dissociation, as the index procedures are soft-tissue repair, reconstruction, limited fusion and motion-sparing salvage rather than joint replacement. The highest-level comparative evidence for end-stage (SLAC) management is therefore systematic-review level: Mulford et al. pooled 52 studies and found similar grip, pain relief and patient-reported outcomes between proximal row carpectomy and four-corner fusion, with PRC giving better motion but more frequent (usually asymptomatic) radiocarpal arthritis (Mulford et al., 2009).
Practice Variation
- Diagnosis: where 3.0T MRI or MR arthrography is available, advanced imaging is used to characterise partial tears; in resource-limited or rural settings, clinical examination, plain and stress radiographs, and diagnostic arthroscopy remain the mainstay given the poor sensitivity of 1.5T MRI (Hafezi-Nejad et al., 2016).
- Acute repair: the threshold for arthroscopic versus open repair varies by unit and surgeon volume; the agreed principle internationally is that the best outcomes follow repair within the acute window.
- Chronic reconstruction: tendon-based reconstructions (three-ligament tenodesis and its variants) predominate in Europe and increasingly worldwide; dorsal capsulodesis is reserved for dynamic instability.
- Salvage: choice between PRC and four-corner fusion is driven by occupational demand, age and capitate-head status rather than by registry mandate.
- Occupational injury and rehabilitation: because typical patients are working-age adults and the injury often arises at work, structured functional capacity assessment, hand therapy and a planned return-to-work programme with workplace modification are central to managing manual workers.
Memory Aids
DPM - DORSAL PREDOMINANT for MOTION controlSL Ligament Structure
Hook:The dorsal SL ligament is like the main cable of a suspension bridge - when it snaps, the whole structure fails — Exam Tip: When examining or reconstructing, focus on the dorsal component - repair/reconstruction must address this portion
R-S-C (Radial-Scaphoid-Capitate) - Spares the LunateSLAC Wrist Progression
Hook:'Row Straight Cars' - R, S, C stages. The lunate fossa remains pristine because it never bears abnormal load — Exam Tip: SLAC spares radiolunate joint - this is why PRC works (capitate articulates with preserved lunate fossa)
P-U-R-CWatson Test Technique
Hook:'Push Until Really Clunks' - apply pressure, move ulnar to radial, feel the painful clunk — Exam Tip: Always compare to opposite side - many people have physiologic laxity. A positive test requires pain, not just motion
MCQ Practice Points
Q: Which portion of the scapholunate ligament is the strongest and most important to reconstruct? A: Dorsal portion - The dorsal SL ligament has tensile strength of 260N vs 150N for palmar portion and is the primary rotational stabilizer.
Q: What is the diagnostic threshold for Terry Thomas sign on PA radiograph? A: SL gap greater than 3mm - Named after the gap-toothed British comedian. Also look for scaphoid ring sign indicating rotatory subluxation.
Q: What SL angle on lateral radiograph confirms DISI pattern? A: Greater than 70 degrees - Normal SL angle is 30-60 degrees. DISI occurs as scaphoid flexes while uncoupled lunate extends.
Q: What constitutes a positive Watson scaphoid shift test? A: Painful clunk with dorsal subluxation - Thumb pressure on scaphoid tubercle during ulnar-to-radial deviation blocks scaphoid flexion. Must compare to contralateral side.
Q: What is the optimal timing window for direct SL ligament repair? A: Less than 6 weeks (ideally under 3 weeks) - Acute repair achieves 80-90% good results. Chronic injuries require reconstruction as tissue degenerates.
Q: What is the predictable pattern of SLAC wrist arthritis and which joint is always spared? A: Radial styloid → Radioscaphoid → Capitolunate. Radiolunate joint ALWAYS spared - This preservation allows PRC and 4-corner fusion salvage procedures.
Clinical Decision Scenarios
Practise clinical reasoning and management decisions out loud
“A 28-year-old rock climber falls while bouldering, landing on an outstretched hand. He presents at 10 days with dorsal wrist pain and weakness. On examination, Watson test is markedly positive with pain and clunk. PA X-ray shows 4mm SL gap and scaphoid ring sign.”
“A 42-year-old carpenter presents with 2-year history of wrist pain after an injury. He has grip weakness limiting his work. Examination shows positive Watson test. X-rays show 5mm SL gap, SL angle 85 degrees, but no arthritis.”
“A 55-year-old woman presents with progressive wrist pain over 5 years. X-rays show widened SL gap, sclerosis at the radial styloid and radioscaphoid joint, but the capitolunate joint appears preserved. How do you manage this?”
Key Anatomy
- Most common carpal instability (95%)
- Dorsal SL ligament = strongest (260N vs 150N palmar)
- Complete SL sectioning + secondary stabilizer failure = DISI
- Lunate is intercalated segment (follows triquetrum into extension)
Radiographic Findings
- Terry Thomas sign = SL gap greater than 3mm
- Scaphoid ring sign = flexed scaphoid on PA
- Normal SL angle 30-60°, DISI greater than 70°
- Capitolunate angle greater than 15° confirms carpal collapse
Clinical Tests
- Watson test = painful clunk with dorsal subluxation
- Compare to contralateral side (physiologic laxity common)
- Positive test requires PAIN, not just motion
- Finger extension test may cause pain (resisted extension)
Treatment Algorithm
- Acute (under 6 weeks) = direct repair + K-wires
- Chronic reducible = Modified Brunelli/SLAM
- Chronic irreducible = Limited fusion or salvage
- SLAC Stage II/III = PRC or 4-CF salvage
SLAC Progression
- Stage I = radial styloid (5-10 years)
- Stage II = radioscaphoid (10-15 years)
- Stage III = capitolunate (15-20+ years)
- Radiolunate ALWAYS spared = allows PRC/4CF
Key Outcomes
- Acute repair = 80-90% good results
- Chronic reconstruction = 60-70% satisfactory
- SL arthrodesis = 30-50% nonunion rate
- PRC = faster recovery, 4CF = better grip
Evidence Base
Gross and Histologic Anatomy of the SLIL
- Serial dissection of 37 cadaver wrists showed the SLIL is consistently divisible into three regions. The dorsal region is thick with short, transversely oriented true collagen fibres; the proximal region is principally fibrocartilage; the palmar region is thin with obliquely oriented fascicles.
Biomechanical Stabilisers of Scaphoid and Lunate
- In a wrist simulator, the scapholunate interosseous ligament was the primary stabiliser between scaphoid and lunate, with the radioscaphocapitate and scaphotrapezial ligaments acting as secondary restraints. Repetitive cyclic motion after sectioning produced further kinematic deterioration.
The SLAC Wrist - Pattern of Degenerative Arthritis
- Review of 4000 wrist radiographs identified a predictable pattern of degenerative arthritis: the most common pattern (57% of 210 degenerative cases) was radioscaphoid (radial styloid then scaphoid fossa) progressing to capitolunate, with the radiolunate joint consistently spared.
SLIL Tears with Distal Radius Fractures (Geissler)
- Among 60 displaced intra-articular distal radius fractures assessed arthroscopically, 68% had associated soft-tissue injuries including SLIL tears in 19 (32%); injuries clustered with fractures involving the lunate facet. This series underpins the Geissler arthroscopic grading of interosseous ligament injury.
Three-Ligament Tenodesis for SL Dissociation
- Described a prognosis-based treatment algorithm and the three-ligament tenodesis technique for non-repairable complete SLIL rupture with reducible malalignment and no osteoarthritis, reporting promising results with this soft-tissue reconstruction.
PRC vs Four-Corner Fusion - Systematic Review
- Systematic review of 52 studies of PRC versus four-corner fusion for SLAC/SNAC wrists found similar grip strength, pain relief and subjective outcomes. PRC offered better range of movement and avoided fusion-specific complications, but carried a significantly higher rate of subsequent (usually asymptomatic) radiocarpal osteoarthritis.