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