Lunate & Perilunate Dislocations
The Lunate is the "Keystone" of the carpus. Dislocations are severe, high-energy injuries (e.g., fall from height) that disrupt the delicate ligamentous balance of the wrist.
[Image of Lunate Dislocation X-ray]1. Classification (Mayfield Stages)
Progressive ligament failure causes a spectrum of injury:
- Perilunate Dislocation (Stage 3): The lunate stays in place, but the rest of the carpus dislocates dorsally.
- Lunate Dislocation (Stage 4): The lunate is pushed out volarly (anteriorly) into the Carpal Tunnel, while the rest of the carpus returns to neutral.
2. Clinical Features
- Median Nerve Compression: Acute Carpal Tunnel Syndrome is very common because the dislocated lunate fills the tunnel.
- X-Ray Signs:
- AP View: "Piece of Pie" sign (Lunate looks triangular).
- Lateral View: "Spilled Teacup" sign (Lunate tilts volar).
3. Management
- Emergency Reduction: Required to relieve nerve pressure.
- Surgery (Gold Standard): Open Reduction and Internal Fixation (ORIF) with ligament repair is almost always needed to prevent chronic instability (VISI/DISI).
25 Practice MCQs
Q1. In Lunate Dislocation, the lunate usually displaces:
Answer: A). It rotates forward into the carpal tunnel.
Q2. The "Spilled Teacup" sign is seen on which X-ray view?
Answer: B). The lunate cup tilts forward, looking like a spilled cup.
Q3. Which nerve is immediately at risk?
Answer: A). The lunate compresses it within the carpal tunnel.
Q4. Kienbock's Disease refers to:
Answer: A). Can occur after trauma or spontaneously.
Q5. The "Piece of Pie" sign on AP X-ray indicates:
Answer: A). The normal quadrilateral shape turns triangular due to rotation.
Q6. Which ligament is the primary stabilizer of the Lunate?
Answer: A). Disruption leads to instability.
Q7. In Perilunate dislocation, the lunate:
Answer: A). The rest of the carpus dislocates around it.
Q8. Mayfield classification describes:
Answer: A). Stage 4 is full lunate dislocation.
Q9. Terry Thomas sign (gap > 3mm) indicates damage to:
Answer: A). Often associated with lunate injuries.
Q10. Conservative treatment (Casting alone) for lunate dislocation has:
Answer: A). Surgery is preferred to repair ligaments.
Q11. What is VISI (Volar Intercalated Segment Instability)?
Answer: A). A chronic instability pattern.
Q12. What is DISI (Dorsal Intercalated Segment Instability)?
Answer: A). Common after scaphoid fracture or SL dissociation.
Q13. Negative Ulnar Variance is a risk factor for:
Answer: A). Short ulna increases shear force on the lunate.
Q14. Typical mechanism of injury:
Answer: A). Similar to scaphoid/Colles but much higher force.
Q15. Murphy's Sign is:
Answer: A). Seen in advanced Lunate AVN/collapse.
Q16. Post-reduction casting usually lasts:
Answer: A). Ligaments heal slowly.
Q17. Is SLAC (Scapholunate Advanced Collapse) wrist a complication?
Answer: A). The most common pattern of wrist arthritis.
Q18. The Space of Poirier is:
Answer: A). The lunate escapes through this weakness.
Q19. Which surgical approach is often used?
Answer: A). To repair ligaments on both sides and release the carpal tunnel.
Q20. Blood supply to the lunate is:
Answer: A). Dislocation can strip the blood supply.
Q21. "Lichman Classification" is used for:
Answer: A). Stages of AVN progression.
Q22. Is Grip Strength permanently reduced?
Answer: A). These are devastating injuries to wrist mechanics.
Q23. Early rehab focuses on:
Answer: A). Wrist is kept immobile for weeks.
Q24. Trans-scaphoid Perilunate Dislocation implies:
Answer: A). The force travels through the bone instead of the SL ligament.
Q25. Proximal Row Carpectomy is a salvage for:
Answer: A). Removes Scaphoid, Lunate, Triquetrum.
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