Carpal Instability

Biomechanics & Anatomy

  • The carpus is comprised of 2 rows:
    • Distal row is virtually fixed
    • Proximal row is an intercalated segment
  • No muscle attachments
  • Movements are dependent on the action of the wrist joint, the connections between the bones, and movements of the distal row
  • The scaphoid spans both rows

Carpal Stability

Intrinsic Ligaments

  • Short, stout ligaments between the bones of the same row
  • SLL strongest dorsally
  • LTL strongest volarly but less discrepancy between volar & dorsal

Extrinsic Ligaments

  • Volar are the strongest ligaments in the wrist
  • Especially Radio-scapho-capitate ligament
  • Space of Poirier is a deficient area in volar capsulo-ligamentous tissue
    • Allows dislocation of the lunate

Kinematics

  • 2/3 flexion occurs at the midcarpal joint
  • 2/3 extension occurs at the radiocarpal joint
  • Radial deviation causes scaphoid flexion
  • Ulnar deviation causes scaphoid extension

3 Theories Regarding Kinematics

  • Row Theory:
    • Distal row fixed, proximal is an intercalated segment
  • Column Theory – Carpus is 3 columns:
    • Radial (scaphoid) – mobile column as scaphoid spans both rows
    • Central (capitate & lunate) – flexion & extension
    • Ulna Column (triquetrum & hamate) – responsible for rotation
  • Oval Theory:
    • Compromise of the two theories

Instability Patterns

  • All injury patterns may be lesser or greater arc:
    • Greater arc = through bones
    • Lesser arc = through soft tissues
  • Gillula’s lines (below) used to assess XRs to look for instability patterns

Carpal Instability Dissociative (CID)

  • Between bones of the same row – bony or soft tissue
  • Intercalated instability refers to CID of the proximal row
  • DISI & VISI patterns

Carpal Instability Non-Dissociative

  • Instability is between rows
  • Less common and difficult to diagnose
  • Mid-carpal instability is the most common pattern

Carpal Instability Complex (CIC)

  • Involves a combination of CID & CIND
  • May be fractures or ligamentous instability

Carpal Instability Adaptive (CIA)

  • Compensatory instability pattern seen because of extra-carpal problems
    • Most commonly a dorsal malunited distal radius fracture leads to DISI

Mayfield Classification of Peri-lunate Dislocation

  • This is the most common pattern of Complex carpal instability

Stages:

  1. Stage 1: Uncoupling of scaphoid and lunate
  2. Stage 2: Stage 1 plus uncoupling of Capitate and lunate
  3. Stage 3: Stage 1 & 2 plus uncoupling of Lunate and Triquetrum
  4. Stage 4: Lunate dislocation (volar usually)
  • There is debate on whether lunate and perilunate dislocations are the same
    • In terms of management, they are the same thing

Management of Specific Carpal Instability Patterns

Scapholunate Instability (DISI)

Acute SLL Injury (< 4 weeks)

  • Primary repair with suture anchors and K-wires
  • Open repair is reported up to 6 months after injury
  • Results likely to deteriorate

Chronic SLL Injury

  • Brunelli & Modified Brunelli Reconstruction:
    • Involves split FCR tunneling through scaphoid and lunate
    • Good results in terms of pain relief and radiographic correction
    • At the expense of wrist stiffness
    • No proven reduction in late arthritis
  • Blatt Capsulodesis:
    • Less commonly done
    • Involves suture of dorsal capsule to scaphoid to stop it from flexing
  • Partial Wrist Fusion:
    • Corrective STT fusion, for instance
    • Less commonly performed
    • Technical difficulty and loss of motion

Lunotriquetral Injury

  • Rare in isolation
  • Often picked up late

Acute

  • Direct repair of LT ligament with anchors and wires

Chronic (more often)

  • Lunotriquetral fusion
  • FCU tenodesis – stops triquetrum extending relative to lunate

Perilunate Injuries

  • Emergent Closed Reduction usually possible:
    • Hyperdorsiflexion and thumbing lunate back in
    • Hyperdorsiflexion and pushing carpus volar to hinge on capitate

Definitive Treatment

  • Always required
  • Dorsal Approach
    • Anchors to repair ligaments
    • K-wires in Christmas tree pattern to maintain carpal bone alignment
    • Wires out at 8 weeks
    • Plaster for 12 weeks
    • Concurrent carpal tunnel decompression
    • Volar or dorsal approach to fix distal radius/styloid fractures

Complications

  • Median nerve injury
  • Wrist stiffness
  • Wrist arthrosis
  • CRPS
  • Proximal pole scaphoid AVN if trans-scaphoid fracture present
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