Carpal instability

Scapholunate Dissociation

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Definition

Scapholunate dissociation is the most common carpal ligamentous injury, characterized by instability between the scaphoid and lunate, leading to dysfunction of wrist biomechanics.

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Epidemiology

• Most frequently encountered ligamentous injury of the wrist.

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Anatomy

• Scapholunate Ligament:
  • C-shaped structure with three components:
    • Dorsal: Strongest. Mcq ❓️
    • Proximal.
    • Volar.

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Etiology

1. Traumatic:
   • Axial loading with wrist extension and ulnar deviation.
   • Associated with conditions like Kienböck’s disease.
2. Degenerative:
   • Common in rheumatoid arthritis (RA).

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Pathophysiology

• Scaphoid: Flexes.
• Lunate: Extends → Dorsal Intercalated Segment Instability (DISI).
• Progression to Scapholunate Advanced Collapse (SLAC) if untreated.

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Clinical Presentation

• History: Trauma or repetitive stress injury.
• Symptoms:
  • Pain: Dorsal and radial wrist pain, aggravated by loading, extension, and radial deviation.
  • Weak grip strength.
  • Snapping or clicking sensations.
• Examination Findings:
  • Localized tenderness distal to the Lister tubercle (scapholunate interval).
  • Positive Watson Test: Pain and clicking during scaphoid manipulation.
  • Negative Lunotriquetral Shuck Test (Reagan test).

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Imaging

X-rays:

• Views:
  • AP, Lateral, Radial/Ulnar deviation, Flexion/Extension, Clenched fist.
• Key Findings: Mcq ❓️ شفوى
  • Scapholunate gap >3 mm (Terry Thomas Sign).
  • Scaphoid (Ring Sign): Due to scaphoid flexion.
  • Humpback deformity of the scaphoid.
  • Scapholunate Angle: >70° (Normal: 30–60°).
  • Radiolunate Angle: >15° (Normal: 0°).
  • Capitolunate Angle: >15° (Normal: 0°).

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MRI:

• Detects scapholunate ligament tears.

Arthrography:

• Dye flow between radiocarpal and midcarpal spaces indicates ligament injury.

Arthroscopy:

• Gold standard for diagnosis.

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Differential Diagnosis

• Other causes of wrist pain:
  • Lunotriquetral instability.
  • Kienböck’s disease.
  • TFCC injuries.

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Management

Acute Injury (<6 weeks)

1. Closed Reduction + Percutaneous Pinning → Immobilization with a cast (6 weeks).
2. Open Reduction via dorsal approach + Scapholunate ligament repair→ Pinning.
3. Arthroscopic Reduction + Percutaneous Pinning.

Chronic Injury (>6 weeks)

1. Open Repair:
   • Indicated for reducible injuries <18 months.
   • Techniques:
     • Dorsal capsulodesis (Blatt procedure).
     • FCR tendon transfer (Brunelli procedure).
     • ECRB tenodesis.
     • Bone-ligament-bone graft reconstruction using extensor retinaculum.
2. Salvage Procedures:
   • Partial wrist fusion:
     • Scaphotrapeziotrapezoid.
     • Scapholunocapitate fusion.
   • Total wrist fusion for irreducible injuries or advanced SLAC.

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