Is scapholunate dissociation the same as lunate collapse?

Scapholunate Dissociation vs. Lunate Collapse: Distinct Wrist Pathologies

No, scapholunate dissociation and lunate collapse are two distinct wrist pathologies with different anatomical characteristics, pathophysiology, and imaging findings.

Scapholunate Dissociation (SLD)

Scapholunate dissociation is characterized by:

• Disruption of the scapholunate interosseous ligament complex
• Widening of the gap between the scaphoid and lunate bones (≥4 mm on radiographs) 1
• Rotatory subluxation of the scaphoid 2
• Dorsal tilt of the lunate >10° suggesting dorsal intercalated segmental instability 1
• Scapholunate angle >70° (typically around 100°) 2

SLD is the most common dissociative carpal instability and the most frequent cause of wrist osteoarthritis, defined as scapholunate advanced collapse (SLAC) wrist 3.

Lunate Collapse (Kienböck's Disease)

Lunate collapse, also known as Kienböck's disease, is characterized by:

• Avascular necrosis of the lunate bone
• Progressive collapse of the lunate bone structure
• No widening of the scapholunate interval (unlike SLD)
• Normal scapholunate ligament integrity (initially)

According to the American College of Radiology guidelines, MRI without contrast is the most appropriate imaging study for suspected Kienböck's disease, as it can detect early bone marrow changes consistent with osteonecrosis 1.

Key Differences in Imaging

1. Radiographic findings:

   • SLD: Widened scapholunate gap (≥4 mm), abnormal scapholunate angle (>70°), rotatory subluxation of scaphoid 1, 2
   • Kienböck's: Sclerosis, fragmentation, and eventual collapse of the lunate bone 1

2. Advanced imaging:

   • For SLD: CT arthrography has nearly 100% sensitivity, specificity, and accuracy for detecting scapholunate ligament tears 1
   • For Kienböck's: Non-contrast MRI is usually appropriate for diagnosis 1

Clinical Implications and Management

The distinction between these conditions is critical as they require different management approaches:

• SLD management: When chronic (>6 weeks post-injury), ligament reconstruction or tenodesis techniques may be indicated if the instability is still reducible 3, 4

• Kienböck's management: Focuses on preserving lunate vascularity and preventing further collapse

Important Considerations

1. Bilateral radiographic SLD is more common than previously thought (80% in one study), often asymptomatic, and doesn't inevitably lead to degenerative arthritis 5

2. While SLD typically leads to radioscaphoid arthritis with preservation of the radiolunate joint (classic SLAC pattern), some cases show radiolunate arthritis without radioscaphoid involvement 6

3. Early diagnosis and treatment of SLD is crucial to prevent progressive carpal malalignment and eventual arthritis 4

4. CT arthrography has the highest sensitivity and specificity for detecting scapholunate ligament tears compared to MRI or MR arthrography 1

In summary, these are distinct pathologies affecting different anatomical structures in the wrist, with different imaging findings and treatment approaches, despite both potentially leading to wrist pain and dysfunction.