Scaphoid Lunate Advanced Collapse (SLAC) - Hand

Introduction

A condition of progressive instability causing advanced arthritis of radiocarpal and midcarpal joints
- describes the specific pattern of degenerative arthritis seen in chronic dissociation between the scaphoid and lunate
Pathoanatomy
- chronic SL ligament injury creates a DISI deformity
  - scaphoid is flexed and lunate is extended as scapholunate ligament no longer restrains this articulation
    - scapholunate angle > 70 degrees
    - lunate extended > 10 degrees past neutral
- resultant scaphoid flexion and lunate extension creates
  - abnormal distribution of forces across midcarpal and radiocarpal joints
  - malalignment of concentric joint surfaces
- initially affects the radioscaphoid joint and progresses to capitolunate joint

Classification

Watson classification
- describes predictable progression of degenerative changes from the radial styloid to the entire scaphoid facet and finally to the unstable capitolunate joint, as the capitate subluxates dorsally on the lunate
- key finding is that the radiolunate joint is spared, unlike other forms of wrist arthritis, since there remains a concentric articulation between the lunate and the spheroid lunate fossa of the distal radius

Watson Stages
Stage I	Arthritis between scaphoid and radial styloid
Stage II	Arthritis between scaphoid and entire scaphoid facet of the radius
Stage III	Arthritis between capitate and lunate
note: radiolunate joint spared

While original Watson classification describes preservation of radiolunate joint in all stages of SLAC wrist, subsequent description by other surgeons of "stage IV" pancarpal arthritis observed in rare cases where radiolunate joint is affected
- validity of "stage IV" changes in SLAC wrist remains controversial and presence pancarpal arthritis should alert the clinician of a different etiology of wrist arthritis

Presentation

Symptoms
- difficulty bearing weight across wrist
- patients localize pain in region of scapholunate interval
- progressive weakness of affected hand
- wrist stiffness
Physical exam
- tenderness directly over scapholunate ligament dorsally
- decreased wrist ROM
- weakness of grip strength
- Watson scaphoid shift test
  - patients may have positive Watson scaphoid shift test early in the process,
  - will not be positive in more advanced cases as arthritic changes stabilize the scaphoid
  - technique
    - with firm pressure over the palmar tuberosity of the scaphoid, wrist is moved from ulnar to radial deviation
    - positive test seen in patients with scapholunate ligament injury or patients with ligamentous laxity, where the scaphoid is no longer constrained proximally and subluxates out of the scaphoid fossa resulting in pain
    - when pressure removed from the scaphoid, the scaphoid relocates back into the scaphoid fossa, and typical snapping or clicking occurs
    - must compare to contralateral side

Evaluation

Radiographs
- obtain standard PA and lateral radiographs
  - PA radiograph will reveal greater than 3mm diastasis between the scaphoid and lunate
    - Stage I SLAC wrist
      - PA radiograph shows radial styloid beaking, sclerosis and joint space narrowing between scaphoid and radial styloid
    - Stage II SLAC wrist
      - PA radiograph shows sclerosis and joint space narrowing between scaphoid and the entire scaphoid fossa of distal radius
    - Stage III SLAC wrist
      - PA radiograph shows sclerosis and joint space narrowing between the lunate and capitate, and the capitate will eventually migrate proximally into the space created by the scapholunate dissociation
  - lateral radiograph
    - will reveal DISI deformity and subluxation of capitate dorsally onto lunate
- stress radiographs unnecessary
MRI
- unnecessary for staging, but will show
  - thinning of articular surfaces of the proximal scaphoid
  - scaphoid facet of distal radius and capitatolunate joint with synovitis in radiocarpal and midcarpal joints

Treatment

Nonoperative
- NSAIDs, wrist splinting, and possible corticosteroid injections
  - indications
    - mild disease
Operative
- radial styloidectomy and scaphoid stabilization
  - indications
    - Stage I
  - technique
    - prevents impingement between proximal scaphoid and radial styloid
    - may be performed open or arthroscopically via 1,2 portal for instrumentation
- PIN and AIN denervation
  - indications
    - Stage I
  - technique
    - since posterior and anterior interosseous nerve only provide proprioception and sensation to wrist capsule at their most distal branches, they can be safely dennervated to provide pain relief
    - can be used in combination with below procedures for Stage II or III
- proximal row carpectomy
  - indications
    - Stage II
    - contraindicated if there is an incompetent radioscaphocapitate ligament
    - contraindicated with caputolunate arthritis (Stage III) because capitate articulates with lunate fossa of the distal radius
  - technique
    - excising entire proximal row of carpal bones (scaphoid, lunate and triquetrum) while preserving radioscaphocapitate ligament (to prevent ulnar subluxation after proximal row carpectomy)
  - outcomes
    - provides relative preservation of strength and motion
- scaphoid excision and four corner fusion
  - indications
    - Stage II or III
  - technique
    - also provides relative preservation of strength and motion
    - wrist motion occurs through the preserved articulation between lunate and distal radius (lunate fossa)
  - outcomes
    - similar long term clinical results between scaphoid excision/ four corner fusion and proximal row carpectomy
- wrist fusion
  - indications
    - Stage III
    - any form of pancarpal arthritis
  - outcomes
    - wrist fusion gives best pain relief and good grip strength at the cost of wrist motion