SLAC & SNAC Wrist

SLAC Wrist

Stages:

  • Stage 1: Radial styloid - scaphoid OA
  • Stage 2: Whole Scaphoid fossa OA
  • Stage 3: Mid carpal OA (Lunocapitate & Scaphocapitate OA)
    • Capitate drives into scapholunate interval
  • Stage 4: Pan Carpal Arthritis

SNAC Wrist

  • Same stages except Stage 4 is not usually part of it
  • Key Difference:
    • In SNAC, proximal scaphoid pole remains attached to lunate by SL ligament
    • This spares the area from OA
    • In SLAC, SL ligament incompetence allows OA to develop between scaphoid and lunate
    • Capitate drives into this area

Common Features in Both

  • Dissociative carpal instability (proximal row)
  • DISI (Dorsal Intercalated Segment Instability) caused by:
    • Bony uncoupling in SNAC (flexed at fracture)
    • Soft tissue disruption in SLAC (whole scaphoid flexed)
  • SL Angle >60° (normal is 40-60°)
  • Lunate fossa of radius only affected in most severe cases
    • Usually spared but more likely affected in SLAC
    • Due to capitate driving into the gap, displacing lunate ulnarly

Aetiology

SNAC

  • Traumatic non-union +/- AVN

SLAC

  • Usually SL ligament trauma
  • May be caused by pseudogout (pyrophosphate deposition disease)
    • Often bilateral if pseudogout
  • No evidence that SL repair prevents SLAC
  • Even without SLAC, SL rupture may cause persistent pain

Management

Non-Surgical Management

  • No long-term studies on non-operative treatment
  • Natural history not fully understood
  • Splinting, activity modification, and analgesia may suffice for some

Surgical Treatment

Goals:

  • Relieve pain
  • Maintain motion (in early stages)
  • Restore function

Broad Surgical Options:

  1. Wrist denervation
  2. Radial Styloidectomy (+/- distal scaphoid excision in SNAC)
  3. Limited Lunocapitate fusion (+/- excision of triquetrum)
  4. 4-corner fusion (+ excision of un-united proximal scaphoid)
  5. Proximal Row Carpectomy (PRC)
  6. Total Wrist Fusion

Surgical Procedures

Wrist Denervation

  • AIN & PIN denervation to eliminate articular branches
  • Unpredictable results
  • No change to long-term mechanics
  • Safe option as it does not burn surgical bridges

Radial Styloidectomy +/- Distal Scaphoid Excision (SNAC)

  • Useful in early stages (esp. Stage 1)
  • SNAC: Un-united distal pole excised to reduce pain
  • Problem: Pain may continue, and collapse may progress
  • May be a good temporizing measure

Isolated Lunocapitate Fusion

  • Scaphoid & triquetrum excision may improve union rates
  • Increases Radiolunate contact pressure
  • Outcomes equivalent to 4CF

4-Corner Fusion (4CF)

  • Lunate, capitate, triquetrum, and hamate fusion
  • +/- Scaphoid excision
  • Fixation: K-wires or plates
Problems with 4CF
  • Non-union risk
  • Broken metalware
  • Tendon irritation
  • Continued pain & restricted motion
Key Surgical Tips for 4CF Success
  • Use bone graft (from distal radius)
  • If using spider plate, use 2 screws per bone
  • Avoid burrs/power instruments
  • Lunate position at fusion does not affect motion

Proximal Row Carpectomy (PRC)

  • Contraindicated if proximal capitate pole is severely arthritic
    • Will cause continued pain
  • Advantages:
    • No metalware
    • No need for fusion
    • Faster rehab
  • Disadvantages:
    • Slightly diminished grip strength

Choosing Between 4CF and PRC

  • 4CF preferred in younger, high-demand patients
  • PRC better for older or lower-demand patients
  • Surgeon preference plays a large role as both have good reported outcomes

Total Wrist Fusion

  • Most predictable results
  • Treatment of choice for Stage 4 disease (or Stage 3 if preferred)
  • Disadvantages:
    • Stiff wrist
    • Hardware complications
    • Non-union (especially of 3rd CMCJ)

Treatment Algorithm

Stage 1

  • Neurectomy
  • Radial styloidectomy

Stage 2

  • Lunocapitate fusion + scaphoid excision
  • PRC
  • 4-corner fusion + scaphoid excision

Stage 3

  • 4-corner fusion + scaphoid excision

Stage 4

  • Total wrist fusion

Summary

  • No clear evidence on natural history of SLAC/SNAC symptoms
  • Treatment is focused on symptom relief
  • Goal is to preserve motion if possible
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