Scapholunate Dissociation

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:

1. Initial injury: SL ligament torn, dynamic instability
2. Months to years: Static instability develops, SL gap widens
3. 5-10 years: SLAC Stage I (radial styloid wear)
4. 10-15 years: SLAC Stage II (radioscaphoid arthritis)
5. 15-20+ years: SLAC Stage III (capitolunate arthritis)

Not all patients progress - some compensate with secondary stabilizers (RSC ligament, scaphotrapezial ligament).

Scapholunate Interosseous Ligament (SLIL)

The SLIL is a C-shaped ligament connecting the scaphoid and lunate along their proximal articular margins.

Three Components:

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:

Intrinsic:

• Scaphotrapezial (ST) ligament
• Scaphocapitate (SC) ligament

Extrinsic:

• Radioscaphocapitate (RSC) ligament
• Dorsal intercarpal (DIC) ligament
• Dorsal radiocarpal (DRC) ligament

Clinical Relevance: Some patients maintain function despite SL injury due to competent secondary stabilizers - explains why not all injuries progress.

Carpal Kinematics Review

Normal Function:

• 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

After SL Injury:

• Scaphoid flexes further (unconstrained)
• Lunate follows triquetrum into EXTENSION
• Creates DISI (Dorsal Intercalated Segment Instability)
• Progressive widening of SL gap

Garcia-Elias Classification

This is the most useful clinical staging system:

Dynamic vs Static Instability

Dynamic Instability:

• Normal resting radiographs
• Abnormal only with stress (clenched fist)
• Secondary stabilizers still functioning
• Better prognosis
• May respond to capsulodesis

Static Instability:

• Abnormal resting radiographs
• SL gap widened without stress
• Complete ligament failure
• Secondary stabilizers failed
• Requires reconstruction

SLAC Wrist Staging

Geissler Arthroscopic Classification

History

Typical Presentation:

• Fall on outstretched hand (FOOSH)
• May have initial "sprain" that never fully recovered
• Progressive wrist pain and weakness
• Clicking or clunking with motion

Key Questions:

• Time since injury (critical for treatment planning)
• Mechanism (hyperextension + ulnar deviation)
• Previous treatment attempts
• Hand dominance and occupation
• Functional demands

Physical Examination

Inspection:

• Dorsal wrist swelling
• Scaphoid prominence (if gap wide)
• Compare to contralateral wrist

Palpation:

• 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:

1. Patient seated, forearm pronated
2. Examiner's thumb on scaphoid tubercle (palmar)
3. Other hand controls wrist
4. Start in ulnar deviation (scaphoid extended)
5. Apply dorsal pressure on scaphoid tubercle
6. Move wrist from ulnar to radial deviation
7. In radial deviation, scaphoid wants to flex - blocked by pressure
8. Positive: Painful dorsal subluxation of scaphoid
9. 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

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:

1. Draw line along palmar surface of scaphoid (or through proximal/distal poles)
2. Draw line perpendicular to lunate distal articular surface
3. Measure angle between lines
4. Normal: 30-60 degrees
5. 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

Clenched Fist View:

• Patient makes tight fist during PA X-ray
• Loads carpal joints, widens SL gap
• May reveal dynamic instability (normal static films)

Contralateral Comparison:

• Essential for interpretation
• Some bilateral laxity is normal
• Side-to-side difference more significant than absolute values

Advanced Imaging

MRI:

• 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

CT:

• Best for associated fractures
• CT arthrography useful if MRI contraindicated

Arthroscopy (Gold Standard):

• Direct visualization of SL ligament
• Allows Geissler grading
• Can treat at same sitting (debridement, repair)

Algorithm

View larger

Treatment Decision Framework

Non-Operative Management

Indications:

• Partial tears (Geissler I-II)
• Low-demand patients
• Poor surgical candidates

Treatment:

• Splinting (4-6 weeks)
• Activity modification
• Hand therapy for proprioception
• Serial clinical and radiographic follow-up

Outcomes:

• May be successful for partial tears
• Risk of progression without treatment
• Must counsel patient about natural history

Acute Surgical Repair (Less than 6 weeks)

Indications:

• Complete SL tear
• Reducible instability
• Adequate ligament tissue quality

Approach:

• Dorsal wrist incision
• Between 3rd and 4th extensor compartments
• Ligament-sparing capsulotomy

Repair Technique:

1. Debride SL interval
2. Reduce SL gap with pointed clamp
3. Repair dorsal SL with suture anchors or transosseous sutures
4. Augment with dorsal capsulodesis
5. K-wire fixation (2 SL, 1 SC)

K-wire Protocol:

• 1.1mm or 1.25mm wires
• Two through SL joint
• One scaphocapitate
• Remove at 8-12 weeks

Outcomes:

• 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

Indications:

• Chronic irreducible instability
• Failed reconstruction
• No arthritis

Technique:

• Decorticate SL articular surfaces
• Bone graft
• Headless screws or K-wires

Outcomes:

• High nonunion rate (30-50%)
• Motion reduced ~50%
• Consider alternatives first

Salvage Procedures (SLAC Wrist)

Proximal Row Carpectomy (PRC):

• Remove scaphoid, lunate, triquetrum
• Capitate articulates with lunate fossa
• Prerequisites: Intact capitate head, intact lunate fossa
• 50-60% motion preserved

Scaphoid Excision + Four-Corner Fusion (SLAC procedure):

• Remove scaphoid
• Fuse capitate, lunate, hamate, triquetrum
• Better grip strength than PRC
• 40-50% motion preserved

Selection:

• PRC: Lower-demand, faster recovery
• 4CF: Manual workers, better power

Complications of SL Injury

SLAC Wrist Progression:

• Predictable if untreated
• Stage I: 5-10 years
• Stage II: 10-15 years
• Stage III: 15-20+ years

Chronic Pain and Disability:

• Grip weakness
• Activity limitation
• Work capacity impairment

Surgical Complications

General:

• Infection (1-2%)
• Stiffness (most common)
• CRPS
• Nerve injury (superficial radial, PIN)

Repair/Reconstruction Specific:

• Recurrent instability (20-40%)
• K-wire complications (migration, infection, breakage)
• Donor site morbidity (FCR harvest)

Arthrodesis Specific:

• Nonunion (30-50% for SL fusion)
• Hardware prominence
• Adjacent joint degeneration

Salvage Procedure Specific:

• PRC: Radiocapitate arthritis (long-term)
• 4CF: Nonunion, hardware failure

Prognostic Factors

Better Outcomes:

• Acute injury (less than 6 weeks)
• Reducible instability
• Good tissue quality
• Isolated SL injury
• Non-smoker

Worse Outcomes:

• Chronic injury
• Irreducible deformity
• Associated fractures
• Heavy manual occupation
• Smoking, diabetes

Acute Repair Protocol

Immobilization (0-8 weeks):

• Volar splint or short arm cast
• Wrist in neutral position
• Thumb free for motion
• K-wires in situ

K-wire Removal (8-12 weeks):

• Office procedure under local anesthesia
• Confirm healing on X-ray before removal
• Check for maintained reduction

Rehabilitation (8-16 weeks):

• Hand therapy referral after wire removal
• Gentle active ROM exercises
• Avoid forceful gripping initially
• Progressive strengthening from 12 weeks

Full Activity (4-6 months):

• Gradual return to sports/work
• May need activity modification
• Continue strengthening program

Reconstruction Protocol

Extended Immobilization (0-10 weeks):

• Cast or splint immobilization
• K-wires protect tenodesis
• Longer healing time than acute repair

Wire Removal (10-12 weeks):

• Confirm no recurrent widening
• May see some gap recurrence (expected)

Rehabilitation (12-20 weeks):

• Progressive ROM and strengthening
• Accept some residual weakness
• Functional goals, not normal anatomy

Salvage Procedure Protocols

PRC:

• Early motion at 2 weeks (key advantage)
• No internal fixation to protect
• Rapid functional recovery

4-Corner Fusion:

• Extended immobilization until fusion (8-12 weeks)
• Serial X-rays to confirm union
• Later motion but better strength

Acute Repair Outcomes

Success Rates:

• 80-90% good to excellent results
• Motion: 80-90% of contralateral
• Grip strength: 70-80% of contralateral
• Pain: Significant improvement in 85%

Failure Rate:

• 10-20% recurrent instability
• Risk factors: Delayed repair (over 3 weeks), poor tissue quality, non-compliance

Chronic Reconstruction Outcomes

Soft Tissue Reconstruction:

• 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%

SL Arthrodesis:

• High nonunion rate: 30-50%
• Motion loss: approximately 50%
• Only 50-60% satisfactory outcomes
• Not recommended as first-line

Salvage Outcomes

PRC:

• 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)

4-Corner Fusion:

• 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

Better Prognosis:

• Acute presentation (under 3 weeks)
• Younger age (under 40)
• Reducible instability
• Good tissue quality
• Non-smoker
• Compliant patient

Worse Prognosis:

• Chronic injury (over 12 weeks)
• Irreducible deformity
• Associated fractures
• Heavy manual labor
• Smoking
• Poor compliance

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.

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.

Australian Context

In Australia, scapholunate injuries frequently present through workers' compensation and require careful documentation and functional capacity assessment. Diagnostic and therapeutic wrist arthroscopy is widely available in metropolitan centres, whereas access to 3.0T MRI and MR arthrography may be limited in rural and regional areas, increasing reliance on clinical examination, stress radiographs and arthroscopy. Hand therapy is integral to outcomes and is well resourced in metropolitan settings; return-to-work planning and workplace modification are important for manual workers.