SNAC Wrist

SNAC (Scaphoid Nonunion Advanced Collapse) wrist represents the predictable pattern of progressive arthritis that develops following untreated or failed treatment of scaphoid nonunion. It shares an identical arthritis progression pattern with SLAC (Scapholunate Advanced Collapse) wrist, differing only in the underlying etiology.

The natural history of 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. Understanding this progression is essential for:

• Counseling patients regarding the importance of scaphoid nonunion treatment
• Staging the disease to guide appropriate surgical management
• Selecting motion-preserving versus motion-eliminating procedures

Normal Carpal Kinematics

The scaphoid serves as a critical link between the proximal and distal carpal rows:

• Span function: Bridges radiocarpal and midcarpal joints
• Load transmission: Transfers approximately 60% of axial load from distal row to radius
• Motion coupling: Coordinates proximal row flexion-extension with distal row motion

Consequences of Scaphoid Nonunion

When scaphoid nonunion occurs:

Distal Fragment Behavior:

• Flexes with the distal carpal row
• Creates dorsal intercalated segment instability (DISI) pattern
• Causes abnormal contact between dorsal lip of distal fragment and radial styloid

Proximal Fragment Behavior:

• Extends with the lunate (remains linked)
• May develop AVN (especially proximal pole fractures)
• Loses normal articulation with distal fragment

Altered Load Transmission:

• Abnormal point loading at styloscaphoid articulation
• Increased stress across scaphocapitate joint
• Progressive cartilage degeneration in predictable sequence

Why Radiolunate Joint is Preserved

The radiolunate joint remains congruent and normally loaded because:

• Lunate maintains spherical articulation with lunate fossa of radius
• No abnormal shear or point loading across this joint
• Preserved until very late stage disease (Stage IV - rare)

This preservation of the radiolunate articulation is the key anatomic feature that allows motion-preserving salvage procedures in SNAC wrist.

Mechanism of Progressive Collapse

The progression from scaphoid nonunion to advanced collapse follows a predictable sequence:

Stage 1: Styloscaphoid Arthritis

1. Nonunion allows distal fragment to flex
2. Dorsal lip of distal fragment contacts radial styloid
3. Abnormal point loading causes cartilage wear
4. Osteophyte formation at styloid tip ("beaking")

Stage 2: Scaphocapitate Arthritis

1. Loss of scaphoid span function
2. Capitate migrates proximally
3. Abnormal articulation between capitate and scaphoid fragments
4. Midcarpal joint degeneration begins

Stage 3: Capitolunate Arthritis

1. Further proximal migration of capitate
2. Capitate-lunate articulation becomes incongruent
3. Articular cartilage loss at capitolunate joint
4. Radiolunate joint still preserved

Stage 4: Pancarpal Arthritis (Rare)

1. Occurs only with severe long-standing disease
2. Finally involves radiolunate articulation
3. Loss of spherical lunate-radius relationship
4. No motion-preserving options possible

Comparison: SNAC vs SLAC

SNAC Staging System

History

Presenting Complaints:

• Wrist pain, often long-standing
• Decreased grip strength
• Limited wrist motion
• History of prior scaphoid fracture (often remote)
• May recall wrist injury years prior that was never treated

Pain Characteristics:

• Activity-related initially
• May become constant in advanced stages
• Dorsoradial location typically
• Worse with gripping and loading

Functional Limitations:

• Difficulty with push-ups or weight-bearing on hand
• Problems with twisting motions (opening jars)
• Reduced work capacity (especially manual laborers)
• Sports participation affected

Physical Examination

Inspection:

• Dorsal wrist swelling (synovitis)
• Reduced visible wrist motion
• Possible radial deviation deformity

Palpation:

• Tenderness over radial styloid
• Anatomical snuffbox tenderness
• Scaphoid tubercle tenderness
• May have palpable crepitus with motion

Range of Motion:

• Reduced flexion and extension
• Reduced radial deviation
• Document pre-operative ROM for comparison

Special Tests:

• Watson scaphoid shift test: May be difficult due to nonunion
• Grip strength: Typically reduced
• Radiolunate grind test: Positive only in Stage IV

Provocative Testing

Scaphocapitate Grind Test:

• Axially load wrist while moving through flexion-extension
• Positive: Pain or crepitus at midcarpal level
• Suggests Stage II or higher

Capitolunate Stress Test:

• Direct pressure over capitate with wrist in neutral
• Positive: Tenderness suggests Stage III involvement

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.

Plain Radiographs

Standard Views:

• PA view: Overall carpal alignment, joint spaces
• Lateral view: Carpal height ratio, DISI pattern
• Scaphoid views: Nonunion characteristics, fragment positions

Key X-ray Findings by Stage:

Measurements:

• Scapholunate angle: Greater than 70 degrees indicates DISI
• Carpal height ratio: Decreased with proximal migration
• Radiolunate angle: Normal until Stage IV

CT Scan

Indications:

• Assess nonunion characteristics
• Evaluate proximal pole vascularity (indirect)
• Pre-operative planning for salvage procedures
• Better visualization of arthritic changes

Findings:

• Nonunion site: Gap, cystic changes, sclerosis
• Fragment positions and rotation
• Articular surface integrity at each joint
• Cystic changes in carpal bones

MRI

Indications:

• Proximal pole vascularity assessment (if considering repair)
• Evaluate ligamentous structures
• Assess for associated pathology

Key Findings:

• Proximal pole signal: Low T1 signal suggests AVN
• Cartilage assessment: Can visualize articular damage
• SL ligament: Often attenuated or torn

Algorithm

View larger

Non-operative Treatment

Limited Role in SNAC:

• May provide temporary symptom relief
• Does not alter disease progression
• Generally used only for:
  • Patients unfit for surgery
  • Patients declining surgery
  • Bridge to definitive treatment

Options:

• Activity modification
• Splinting/bracing
• NSAIDs
• Intra-articular corticosteroid injections

Management Algorithm

4CC vs PRC Comparison

Four-Corner Fusion Complications

Nonunion (5-10%)

• Most significant complication
• Risk factors: Smoking, inadequate fixation, poor bone quality
• Management: Revision with bone graft, improved fixation

Hardware Problems

• Prominent hardware causing extensor tendon irritation
• Screw loosening
• Plate failure
• May require hardware removal after union

Stiffness

• ROM less than expected
• Usually from prolonged immobilization
• Therapy may improve

DRUJ Issues

• Pre-existing DRUJ arthritis may become symptomatic
• Consider concurrent Darrach or DRUJ fusion

Radiocarpal Arthritis Progression

• Radiolunate joint may develop arthritis long-term
• May require conversion to total wrist fusion

Proximal Row Carpectomy Complications

Progressive Arthritis

• Cartilage wear at radiocapitate articulation
• Concern for long-term durability
• May require conversion to fusion

Carpal Instability

• Capitate subluxation rare
• Usually stable construct

Weakness

• Grip strength typically 60-80% of normal
• Some patients disappointed with strength

Conversion to Fusion

• May be needed if progressive arthritis develops
• PRC does not "burn bridges" for future fusion

General Complications

Infection: Standard surgical infection risk Nerve injury: Superficial radial nerve, PIN Stiffness: Expected - counsel patients pre-operatively Complex regional pain syndrome: Rare but devastating

Four-Corner Fusion Protocol

Week 0-2:

• Bulky dressing and splint
• Strict elevation
• Finger range of motion exercises

Week 2-6:

• Short arm cast or removable splint
• Continue finger motion
• No loading

Week 6-12:

• Serial radiographs to assess fusion
• Begin wrist ROM when fusion evident
• Gradual strengthening

Week 12+:

• Progressive loading as tolerated
• Full activities when fully fused (typically 3-4 months)
• Grip strengthening program

Proximal Row Carpectomy Protocol

Week 0-2:

• Bulky dressing and splint
• Elevation
• Finger motion

Week 2-6:

• Begin active wrist ROM
• Splint for comfort between exercises
• No loading

Week 6-12:

• Progressive strengthening
• Discontinue splint
• Gradual return to activities

Week 12+:

• Full activities as tolerated
• Typically faster recovery than 4CC

Natural History Without Treatment

• Few nonunions remain free of arthritis beyond 10 years (Mack 1984); mean time to radioscaphoid arthritis approximately 17 years
• Progressive pain and functional decline
• Eventual pancarpal arthritis in some (mean approximately 31 years in Mack's series)
• Grip strength deterioration

Outcomes After Surgical Treatment

Stage I - Nonunion Repair + Styloidectomy:

• Union rates 70-90% with proper selection
• If union achieved, good long-term function
• Prevents SNAC progression

Stage II/III - 4CC:

• Pain relief: 80-90%
• ROM: 50% of normal (30-40 degrees F/E arc)
• Grip: 60-80% of normal
• Patient satisfaction: 75-85%
• Revision rate: 5-10% (mostly nonunion)

Stage II/III - PRC:

• Pain relief: 80-90%
• ROM: 50% of normal
• Grip: 60-80% of normal
• Patient satisfaction: 75-85%
• 10-year survival: 90%+ in most series

Stage IV - Total Wrist Fusion:

• Pain relief: 90%+
• ROM: 0 degrees (complete fusion)
• Grip: Often improves from pre-operative
• High satisfaction for appropriate patients

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.
• Australian context: 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 FRACS/FRCS (Tr & Orth) topics, examined on pattern recognition, staging and the 4CC-versus-PRC decision.