Scapholunate Dissociation

Scapholunate dissociation is the most common and most important form of carpal instability, the most common carpal instability pattern (95%). It results from failure of the scapholunate interosseous ligament (SLIL), which allows abnormal motion between the scaphoid and lunate and, with it, carpal malalignment.

Why it matters. The injury is often missed at first presentation, because the instability is occult, and the success of treatment is highly time-dependent. An untreated static dissociation follows a predictable pattern of arthritis, the SLAC wrist. Understanding SL pathology is fundamental to wrist surgery.

Who. The true incidence is unknown, since many cases go undiagnosed at first. Young adults aged 20-40 are the commonest group, with a male predominance put down to occupational exposure.

Associated injuries. Look for the SL ligament whenever one of these is present:

• Distal radius fracture - 5-30% have an SL injury
• Perilunate injury - the SL ligament is always involved
• Greater arc (trans-scaphoid) injury

Natural history. The torn ligament starts as dynamic instability. In the wrists that progress, static instability develops over months to years and the SL gap widens, and the arthritis that follows runs in the SLAC sequence staged below. Not every patient progresses: the rate varies with activity level and with the competence of the secondary stabilisers.

The ligament. The SLIL is C-shaped and joins the scaphoid and lunate along their proximal articular margins. Its three regions differ in structure and in strength, and the dorsal region is the one that matters.

The yield strengths come from Berger's study of 24 cadaver wrists (PMID 10509273). The dorsal standard deviation is nearly half its mean, so the rank order is secure but the gap between any two individual wrists is not. Whether examining or reconstructing, focus on the dorsal component: repair or reconstruction must address it.

Secondary stabilisers. When the SLIL fails, other ligaments give some restraint. Where they are competent, a patient may keep function despite the SL injury, which is why not every injury progresses.

• Intrinsic - scaphotrapezial (ST) and scaphocapitate (SC) ligaments
• Extrinsic - radioscaphocapitate (RSC), dorsal intercarpal (DIC) and dorsal radiocarpal (DRC) ligaments

Normal kinematics. The scaphoid, set obliquely, naturally tends to flex. The lunate is the intercalated segment, with no tendon attachments; while the SL ligament is intact it couples the two bones, and the lunate follows the scaphoid into flexion.

After the ligament fails. The unconstrained scaphoid flexes further while the lunate follows the triquetrum into extension. That is dorsal intercalated segment instability (DISI), and the SL gap widens progressively. Complete SL sectioning is required to produce the DISI pattern.

Garcia-Elias staging is the most useful clinical staging system. Each stage carries its treatment:

Dynamic instability. The resting radiographs are normal and the widening shows only under stress, such as a clenched fist; the secondary stabilisers are still functioning. The prognosis is better, and it may respond to capsulodesis.

Static instability. The SL gap is widened on resting films, without stress. The ligament has failed completely and the secondary stabilisers have failed with it, and the wrist requires reconstruction.

The SLAC wrist. Untreated static dissociation degenerates in a predictable order, and the stage sets the treatment:

The timelines are convention: the series that named the pattern could not tell how often or how fast an untreated dissociation progresses.

The radiolunate joint is always spared. The lunate keeps a congruent articulation with the radius, and no abnormal load is transmitted across the radiolunate joint. That preservation is the key to salvage surgery: a proximal row carpectomy relies on the intact lunate fossa for the capitate to articulate with.

Geissler arthroscopic grading describes the tear as seen through the arthroscope:

History. The typical story is a fall on the outstretched hand, with the wrist in hyperextension and ulnar deviation. There may have been an initial "sprain" that never fully recovered, followed by progressive wrist pain and weakness, with clicking or clunking on movement. The questions that shape treatment:

• Time since injury - critical for treatment planning
• Previous treatment attempts
• Hand dominance and occupation
• Functional demands

Inspection and palpation. Look for dorsal wrist swelling and, if the gap is wide, a scaphoid prominence, and compare with the other wrist. The SL interval lies 1cm distal to Lister's tubercle, in line with the third metacarpal. It is tender over the dorsal SL ligament, and a step-off between scaphoid and lunate may be felt.

The Watson scaphoid shift test is the primary clinical test, and the gold standard for SL instability.:

1. Patient seated, forearm pronated
2. Examiner's thumb on the scaphoid tubercle (palmar)
3. Other hand controls the wrist
4. Start in ulnar deviation (scaphoid extended)
5. Apply dorsal pressure on the scaphoid tubercle
6. Move the wrist from ulnar to radial deviation
7. In radial deviation the scaphoid wants to flex, and the pressure blocks it
8. Positive: painful dorsal subluxation of the scaphoid
9. Release the pressure: a painful clunk as the scaphoid reduces

Reading the Watson test. A positive test requires pain with the clunk, not just motion, and apprehension is also significant. Physiological laxity is common, so always compare the opposite side. A negative test does not rule out an SL injury.

Other tests.

• Scaphoid compression test - axial load on the scaphoid causes pain
• SL ballottement - stabilise the lunate and translate the scaphoid dorsally and palmarly
• Finger extension test - pain with resisted finger extension

Grip strength. Measure it against the other side: it is often 50-70% of normal. It is an important functional measure and tracks with the success of treatment.

Radiographs. Four views, and the other wrist for comparison:

• PA in neutral rotation (forearm pronated)
• True lateral - radius and ulna superimposed
• Clenched-fist PA - the stress view
• PA in radial and ulnar deviation

View larger

The Terry Thomas sign (the David Letterman sign) is an SL gap greater than 3mm on the PA, and indicates SL ligament injury. It is named after the gap-toothed British comedian; compare with the contralateral wrist.

The scaphoid ring sign. The flexed scaphoid appears foreshortened on the PA and its distal pole projects as a ring, a cortical outline. It indicates scaphoid malrotation, the rotatory subluxation of the scaphoid.

Gilula's arcs. Disruption of any arc indicates carpal malalignment:

• Arc I - proximal surfaces of the proximal carpal row
• Arc II - distal surfaces of the proximal carpal row
• Arc III - proximal surfaces of the hamate and capitate

The SL angle is measured on the true lateral, from the scaphoid axis to the lunate perpendicular. Normal is 30-60°; greater than 70° is the DISI pattern.

1. Draw a line along the palmar surface of the scaphoid (or through its proximal and distal poles)
2. Draw a line perpendicular to the lunate distal articular surface
3. Measure the angle between them

The capitolunate angle runs between the long axis of the capitate and the lunate perpendicular. Normal is under 15°; greater than 15° confirms malalignment.

Stress and comparison views. A clenched fist during the PA loads the carpal joints and widens the SL gap, and may reveal dynamic instability when the static films are normal. Contralateral comparison is essential for interpretation: some bilateral laxity is normal, and the side-to-side difference is more significant than the absolute values.

MRI can visualise the SL ligament directly, but its diagnostic performance depends on field strength. 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). It is less reliable for partial tears, and MR arthrography and 3.0T imaging are preferred when MRI is used.

CT is best for associated fractures; CT arthrography is useful if MRI is contraindicated.

Arthroscopy is the gold standard. It visualises the SL ligament directly, allows Geissler grading, and can treat the injury at the same sitting by debridement or repair.

Algorithm

View larger

The decision. Time since injury, whether the malalignment reduces and whether there is arthritis set the treatment; the table puts them side by side.

Non-operative management. It suits partial tears (Geissler I-II), low-demand patients and poor surgical candidates. Partial tears have a good prognosis with conservative treatment and typically respond well to it, but there is a risk of progression without treatment, and the patient must be counselled about the natural history.

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

Acute repair, within 6 weeks. A complete tear that is reducible, with good ligament tissue, is repaired directly, and early surgical repair offers the best chance of recovery. The window is 6 weeks, ideally under 3. Through an open dorsal approach the dorsal SL ligament is repaired with bone anchors or transosseous sutures, augmented with a dorsal capsulodesis, and held with K-wires (two SL, one SC) for 8-12 weeks. The steps are under Surgical Technique.

Chronic reducible instability, no arthritis. A chronic injury, over 6 weeks, needs reconstruction rather than repair: the tissue is too degenerated to hold sutures. Soft-tissue reconstruction can restore acceptable function. The options:

• Dorsal capsulodesis (Blatt) - a radially based capsular flap secured to the dorsal scaphoid; good for dynamic instability, less reliable for static
• Modified Brunelli - a strip of half the FCR tendon passed through a scaphoid tunnel and secured to the lunate, combined with capsulodesis
• SLAM (scapholunate axis method) - adds a volar limb to the Brunelli; more anatomical, at higher complexity
• Bone-ligament-bone graft - a free bone-SL-bone graft from cadaver; technically challenging and of limited availability

Chronic irreducible instability, no arthritis. A fixed DISI deformity that will not reduce, or a failed reconstruction, leaves fusion. SL arthrodesis decorticates the SL articular surfaces, adds bone graft and fixes them with headless screws or K-wires. It has a 30-50% nonunion rate, reduces motion by approximately half and is not recommended first-line; consider reconstruction alternatives first, and STT or SC fusion as alternatives to it. Counsel about motion loss. Limited fusion is a last resort before salvage.

Salvage for the SLAC wrist. Indicated for SLAC stage II or III, significant arthritis, or failed prior reconstruction; stage I is treated by styloidectomy (SLAC table above).

• Proximal row carpectomy (PRC) - remove the scaphoid, lunate and triquetrum so that the capitate articulates with the lunate fossa. Requires intact capitate head cartilage, intact lunate fossa cartilage and no inflammatory arthritis. For lower-demand patients, with faster recovery
• Scaphoid excision and four-corner fusion (the SLAC procedure) - remove the scaphoid and fuse the capitate, lunate, hamate and triquetrum. For manual workers who need power
• Total wrist fusion - for end-stage disease; pain relief at the cost of complete motion loss

Motion-sparing salvage gives good pain relief, with reduced but functional motion that allows a return to modified activities.

Choosing between PRC and four-corner fusion. The outcome figures below favour four-corner fusion for grip and PRC for motion and speed of recovery. The systematic review of 52 studies by Mulford and colleagues found grip strength, pain relief and subjective outcomes similar between the two; PRC gave better range of movement and avoided fusion-specific complications, at the price of more subsequent, usually asymptomatic, radiocarpal osteoarthritis. Patient demand and the state of the capitate head should guide the choice.

Of the injury. The wrist that progresses develops SLAC arthritis in the sequence staged above, with chronic pain and disability: grip weakness, activity limitation and impaired work capacity.

Of surgery in general. Stiffness is the most common.

• Infection (1-2%)
• CRPS
• Nerve injury - superficial radial nerve, PIN

Of repair and reconstruction. Recurrent instability in 20-40%, K-wire complications (migration, infection, breakage) and donor-site morbidity from the FCR harvest.

Of arthrodesis. Nonunion (30-50% for SL fusion), hardware prominence and adjacent joint degeneration.

Of salvage. Radiocapitate arthritis in the long term after PRC; nonunion, hardware failure and hardware prominence after four-corner fusion.

After acute repair. The wrist is immobilised for the first 8 weeks in a volar splint or short arm cast, wrist neutral, thumb free, K-wires in situ.

• 8-12 weeks - K-wire removal as an office procedure under local anaesthesia, after confirming healing and maintained reduction on X-ray
• 8-16 weeks - hand therapy referral after wire removal; gentle active range of motion, avoiding forceful gripping at first; progressive strengthening from 12 weeks
• 4-6 months - gradual return to sport and work, which may need modification, with continued strengthening

After reconstruction. Healing takes longer than after acute repair. The wrist is immobilised in a cast or splint for 10 weeks with the K-wires protecting the tenodesis, and the wires come out at 10-12 weeks. Check for recurrent widening at removal, although some gap recurrence is expected. Rehabilitation from 12 to 20 weeks is progressive range of motion and strengthening, aimed at functional goals rather than normal anatomy, accepting some residual weakness.

After PRC. Early motion is the key advantage: there is no internal fixation to protect, and functional recovery is rapid.

• Volar splint for 2 weeks; sutures out at 2 weeks
• Active range of motion immediately after the splint comes off
• Progressive strengthening from 6 weeks
• Full activity at 3-4 months

After four-corner fusion. Immobilisation in a cast or splint for 8-12 weeks, until serial X-rays confirm fusion. Range-of-motion exercises start once the fusion is solid and strengthening at 4-6 months: later motion than PRC, but better strength.

Why acute repairs fail. Delayed repair (over 3 weeks), poor tissue quality and non-compliance are the risk factors for recurrent instability.

Prognostic factors.

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.
• 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.