SNAC Wrist

What it is. SNAC (scaphoid nonunion advanced collapse) wrist is the predictable pattern of progressive arthritis that develops after a scaphoid nonunion that was never treated, or whose treatment failed. It shares an identical arthritis progression with SLAC (scapholunate advanced collapse) wrist and differs only in aetiology: a nonunion rather than scapholunate dissociation.

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Natural history. Untreated scaphoid nonunion leads to SNAC wrist in the vast majority of cases. In the defining study by Mack et al. (1984), degenerative change followed a predictable, time-dependent sequence: lesions confined to the scaphoid at a mean of 8.2 years, radioscaphoid arthritis at 17.0 years and generalised wrist arthritis at 31.6 years, with few non-unions remaining free of arthritis beyond 10 years.

Why the sequence matters. Understanding it is essential for counselling patients about the importance of treating a scaphoid nonunion, for staging the disease to guide surgery, and for choosing between motion-preserving and motion-eliminating procedures.

The scaphoid as a link. The scaphoid is the critical link between the proximal and distal carpal rows. It spans the radiocarpal and midcarpal joints, transfers approximately 60% of axial load from the distal row to the radius, and coordinates proximal-row flexion-extension with distal-row motion.

Why the radiolunate joint survives. The lunate keeps its spherical articulation with the lunate fossa of the radius, so the radiolunate joint stays congruent and normally loaded, with no abnormal shear or point loading. It is preserved until very late disease (Stage IV, which is rare). That preservation is the key anatomical feature that allows motion-preserving salvage in most patients.

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The fragments part company. Once the scaphoid fails to unite, the distal fragment flexes with the distal carpal row, creating a dorsal intercalated segment instability (DISI) pattern. The proximal fragment extends with the lunate, to which it remains linked, and loses its normal articulation with the distal fragment. It may develop AVN, especially after proximal pole fractures.

The load goes to the wrong joints. The dorsal lip of the flexed distal fragment comes into abnormal contact with the radial styloid, point loading the styloscaphoid articulation, while stress rises across the scaphocapitate joint. Cartilage then degenerates in a predictable sequence:

• Stage I, styloscaphoid. The cartilage wears and an osteophyte forms at the styloid tip ("beaking").
• Stage II, scaphocapitate. With the scaphoid's span function lost, the capitate migrates proximally and articulates abnormally with the scaphoid fragments, and midcarpal degeneration begins.
• Stage III, capitolunate. Further proximal migration makes the capitate-lunate articulation incongruent and its cartilage is lost. The radiolunate joint is still preserved.
• Stage IV, pancarpal (rare). Only with severe, long-standing disease is the radiolunate articulation finally involved and the spherical lunate-radius relationship lost. No motion-preserving option remains.

SNAC beside SLAC. The two share every stage and differ in the cause and the mechanism of scaphoid malposition, so the treatment algorithm is the same for both at each stage.

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SNAC staging follows the joints in the order they fail, and the stage is what decides the operation.

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The story. Wrist pain, often long-standing, with decreased grip strength and limited motion. There is a history of prior scaphoid fracture, often remote, and the patient may recall a wrist injury years earlier that was never treated.

The pain. Typically dorsoradial, worse with gripping and loading, and activity-related at first; it may become constant in advanced stages.

What it costs the patient. Push-ups and weight-bearing on the hand become difficult, as do twisting movements such as opening jars. Work capacity falls, especially in manual labourers, and sport is affected.

Examination. Inspection may show dorsal swelling from synovitis, visibly reduced motion and a possible radial deviation deformity. The radial styloid, anatomical snuffbox and scaphoid tubercle are tender, and there may be palpable crepitus with motion. Flexion, extension and radial deviation are reduced, and grip strength is typically reduced; document the preoperative range for later comparison. The Watson scaphoid shift test may be difficult to perform because of the nonunion.

Provocative tests point to the stage:

• Scaphocapitate grind test - axially load the wrist while moving it through flexion-extension. Pain or crepitus at the midcarpal level is positive and suggests Stage II or higher.
• Capitolunate stress test - direct pressure over the capitate with the wrist in neutral. Tenderness suggests Stage III involvement.
• Radiolunate grind test - positive only in Stage IV.

Differential diagnosis. Dorsoradial wrist pain with degenerative change has several mimics. The key discriminator for SNAC is an established scaphoid nonunion driving the styloid-to-pancarpal pattern with a preserved radiolunate joint.

Radiographs stage the disease (the findings for each stage are in the staging table). Three views:

• PA - overall carpal alignment and joint spaces
• Lateral - carpal height ratio and the DISI pattern
• Scaphoid views - nonunion characteristics and fragment positions

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Measurements. The scapholunate angle lies between a line along the volar scaphoid and a line through the lunate axis. It rises toward 90 degrees as the scaphoid flexes and the lunate extends, and an angle greater than 70 degrees indicates DISI. The carpal height ratio falls as the capitate migrates proximally. The radiolunate angle stays normal until Stage IV.

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CT is the arbiter of union. Plain films consistently overcall union, so CT along the scaphoid axis is the standard before deciding on salvage. It shows the nonunion site (gap, cystic change, sclerosis), the position and rotation of the fragments, the integrity of the articular surface at each joint and cystic change in the carpal bones. It also gives an indirect assessment of proximal pole vascularity, shows arthritic change better than plain film, and is the planning study for salvage.

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MRI answers the vascularity question when repair is being considered: low T1 signal in the proximal pole suggests AVN. It also answers the occult-injury question, showing the ligaments (the SL ligament is often attenuated or torn), associated bone injury and other pathology, and cartilage damage, including the state of the radioscaphoid joint when the plain film is equivocal.

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Algorithm

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Non-operative treatment has a limited role. It may give temporary symptom relief but does not alter progression: untreated disease continues to advance through the stages. It is generally reserved for patients unfit for surgery, those who decline it, or as a bridge to definitive treatment, using activity modification, splinting or bracing, NSAIDs and intra-articular corticosteroid injection. Surgery is indicated for symptomatic SNAC in appropriate surgical candidates.

Stage I: the repair window

Save the scaphoid if you can. Nonunion repair with radial styloidectomy needs three things:

• A viable proximal pole on MRI
• A repairable nonunion configuration
• A patient willing to accept a prolonged recovery

The nonunion is opened and fixed with a headless compression screw, using vascularised or non-vascularised bone graft, and a 2-3 mm radial styloidectomy addresses the styloid arthritis.

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Go straight to salvage when the proximal pole has AVN, the nonunion is not reconstructable, or the patient prefers a single definitive procedure. The options are then those of Stage II/III.

Stages II and III: motion-preserving salvage

Both stages keep the radiolunate joint, so both are treated with motion-preserving salvage. What separates them is the capitate head.

Four-corner fusion (4CC, the SLAC procedure) excises the scaphoid and fuses the capitate, lunate, hamate and triquetrum. The intact radiolunate articulation is its precondition.

Proximal row carpectomy (PRC) excises the entire proximal row so that the capitate articulates with the lunate fossa.

Why the stage decides between them. A PRC creates a new joint between the head of the capitate and the lunate fossa of the radius, so it needs intact cartilage on both; damage to either contraindicates it. Stage III takes that away: capitolunate arthritis is arthritis of the capitate head, so a standard PRC would articulate a worn surface against the radius and fail. Four-corner fusion instead bypasses the diseased capitolunate joint by fusing across it, which is why it remains available at Stage III when PRC is not.

At Stage II both remain on the table, and the choice depends on surgeon and patient factors. Scaphocapitate arthritis involves the capitate's articulation with the scaphoid and may spare the proximal, lunate-facing part of the head, so inspect the capitate head intraoperatively before committing; if it is worn, convert to a four-corner fusion. Capitate resurfacing and dorsal capsular interposition have been described to extend PRC into Stage III, but they are niche solutions rather than the standard answer.

Capitolunate fusion is a smaller alternative to a full four-corner fusion: fewer surfaces to unite and less hardware, at the cost of a narrower correction. The choice between the two is decided by the state of the triquetrohamate joint.

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Stage IV: motion-eliminating options

Total wrist fusion is the gold standard. It is a complete radiocarpal fusion that includes the carpometacarpal joints, set in 10-15 degrees of extension and neutral deviation and fixed with a dorsal plate spanning radius to metacarpals. It gives definitive pain relief and a strong, stable wrist, appropriate for heavy labourers, at the price of all wrist motion and compensatory motion at other joints, which may affect function significantly.

Total wrist arthroplasty is the alternative for selected patients: the low-demand, those with bilateral disease, and those who need to preserve motion. It is less durable than fusion, has a higher complication rate, imposes strict activity restrictions and is not suitable for labourers.

Four-corner fusion

1. Approach. A dorsal longitudinal incision, developing the interval between the 3rd and 4th extensor compartments. Ligate the terminal branch of the posterior interosseous nerve and perform a dorsal capsulotomy.
2. Scaphoid excision. Excise the scaphoid completely, taking care to remove every fragment, and preserve the volar radiocarpal ligaments.
3. Cartilage preparation. Remove the cartilage from the capitate, lunate, hamate and triquetrum, initially preserving the subchondral bone plate, to create bleeding bone surfaces.
4. Bone grafting. Harvest cancellous bone from the distal radius or iliac crest, pack it into the prepared fusion surfaces and fill the scaphoid void.
5. Fixation. A circular dorsal plate (most common), spider plate, individual screws or K-wires, with compression across all fusion surfaces.
6. Closure. Close the capsule if possible, repair the extensor retinaculum, close the skin and splint in neutral.

The result is a stable construct that preserves the radiolunate articulation.

Proximal row carpectomy

1. Approach. The same dorsal approach as for 4CC, which gives good visualisation of the entire proximal row.
2. Carpal excision. Excise the scaphoid first (easiest access), then the lunate, then the triquetrum, removing the proximal row completely.
3. Cartilage assessment. The critical step: inspect the capitate head and lunate fossa. If the cartilage is damaged, convert to 4CC or consider fusion; if it is intact, proceed.
4. Reduction. Let the capitate reduce into the lunate fossa, check for smooth, painless motion and assess stability.
5. Closure. Repair the capsule, consider a temporary K-wire if the construct is unstable, and start the early motion protocol.

Scaphoidectomy with midcarpal tenodesis

A tendon slip is used in place of a fusion after the scaphoid is removed. The tenodesis substitutes for the excised scaphoid without committing the patient to a fusion; the flexor carpi radialis and extensor carpi radialis brevis variants both aim to hold the capitate in the lunate fossa and prevent ulnar translation once the scaphoid has been removed.

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Four-corner fusion

Nonunion (5-10%) is the most significant complication. Smoking, inadequate fixation and poor bone quality are the risk factors, and it is managed by revision with bone graft and improved fixation. Fixation choice matters: in Vance's series (see the evidence below), first-generation circular dorsal plates had a 26% nonunion rate and a 48% major complication rate, against 6% with traditional wires, staples or screws.

Hardware problems include prominent hardware irritating the extensor tendons, screw loosening and plate failure, and the hardware may need removal after union. A circular plate must sit below the level of the articular surface: if it is proud it impinges on the dorsal rim of the radius, a recognised cause of persistent dorsal pain and hardware removal.

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Stiffness. A range of motion less than expected usually follows prolonged immobilisation, and therapy may improve it.

The DRUJ. Pre-existing DRUJ arthritis may become symptomatic; consider a concurrent Darrach or DRUJ fusion.

The preserved joint. The radiolunate joint may develop arthritis in the long term and may require conversion to total wrist fusion. Hardware prominence and progression of the preserved joint are the commonest reasons these wrists come back.

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Proximal row carpectomy

Progressive arthritis at the radiocapitate articulation raises concern for long-term durability and may require conversion to fusion. PRC does not "burn bridges": fusion remains available later.

Carpal instability. Capitate subluxation is rare and the construct is usually stable.

Weakness. Some patients are disappointed with their grip strength.

Either operation

• Infection - the standard surgical risk
• Nerve injury - superficial radial nerve, PIN
• Stiffness - expected; counsel patients preoperatively
• Complex regional pain syndrome - rare but devastating

After 4CC the wrist is protected until serial radiographs show fusion. After PRC active wrist motion begins from week 2, and recovery is typically faster than after 4CC.

Without treatment pain and function decline progressively, grip strength deteriorates, and some wrists eventually reach pancarpal arthritis (the generalised arthritis of Mack's series in the Overview).

After Stage I repair union rates are 70-90% with proper selection. If union is achieved, SNAC progression is prevented and the wrist returns to near-normal function, with good long-term results.

Global Epidemiology

• Scaphoid fracture incidence: A validated Swedish nationwide registry of 34,377 patients reported a true incidence of 22 per 100,000 person-years after correcting a 41% false-positive diagnosis rate (Swärd 2019).
• Nonunion as the substrate for SNAC: In the same registry, nonunion risk was higher in men than women (3% versus 1%), mirroring the male predominance seen in SNAC populations.
• Time course: Mack's natural-history series (1984) remains the reference: radioscaphoid arthritis at a mean of approximately 17 years and pancarpal arthritis at approximately 31 years, with displacement and carpal instability accelerating collapse.

Guidance and Society Positions, Side by Side

There is no high-level prospective randomised trial comparing four-corner fusion (4CC) and proximal row carpectomy (PRC); guidance is therefore consensus- and cohort-based rather than from formal guideline committees. Positions converge internationally:

Registry and Comparative Evidence

• No dedicated arthroplasty registry captures SNAC salvage, because 4CC and PRC are not implant-arthroplasty procedures (no AOANJRR/NJR/AJRR implant survival data apply). Total wrist arthroplasty for Stage IV is captured in some registries but represents a small, low-demand subset.
• The strongest comparative synthesis is Mulford 2009 (systematic review of 52 studies): comparable grip, pain relief and subjective outcomes; PRC tended to give better motion but more radiographic osteoarthritis (mostly asymptomatic).
• Age is the key practice modifier: DiDonna 2004 showed all PRC failures occurred in patients aged 35 or younger, so many surgeons favour 4CC (or attempted reconstruction) in younger, higher-demand patients.

Practice Variation

• High-resource settings offer the full ladder (vascularised grafting for Stage I, 4CC, PRC, total wrist arthroplasty for selected Stage IV).
• Limited-resource settings more often default to PRC (technically simpler, no implant cost, faster rehabilitation) or total wrist arthrodesis for advanced disease.
• In high-income settings, both 4CC and PRC are routine at hand surgery units; sports- and work-related scaphoid injuries in young men are the dominant feeder population, consistent with the registry epidemiology above.

SNAC and SLAC remain core fellowship topics, examined on pattern recognition, staging and the 4CC-versus-PRC decision.