Lunate Dislocation

Lunate dislocation is a palmar dislocation of the lunate out of the lunate fossa of the radius, with the capitate staying in line with the radius. It is the end stage (Mayfield stage IV) of the perilunate injury sequence.

It is rare and follows a high energy injury of the wrist, usually a fall from height or a road traffic collision in young adults. The lunate may lie in the carpal tunnel and can compress the median nerve and flexor tendons.

Complications are acute carpal tunnel syndrome, avascular necrosis of the lunate, flexor tendon rupture or adhesions, scapholunate instability, post-traumatic arthritis, stiffness, and complex regional pain syndrome.

Pathophysiology.

After the capitate has passed palmar to the lunate the lunate is forced volarly and rotates, usually by over 90 degrees, so its articular surface faces the palm. The volar radiolunate and short radiolunate ligaments tear, and only the volar capsule attachment may remain.

The blood supply to the lunate runs through the volar and dorsal ligament attachments, so a lunate with a single remaining attachment is at risk of avascular necrosis.

Wrist pain, swelling and fullness in the volar wrist, restricted movement and restricted finger flexion. Median nerve compression gives paraesthesia in the thumb to ring finger, thenar weakness and rising pain. Check for open wounds, pulses and capillary refill. Look for other injuries.

• Radiographs: lateral view shows the lunate rotated volarly ("spilled teacup" sign), with the capitate aligned with the radius. The posteroanterior view shows a triangular lunate ("piece of pie" sign) and overlapping carpal rows.
• CT for associated fractures (scaphoid, triquetrum, radial styloid). MRI for ligament injury and early avascular necrosis.
• Median nerve examination before and after reduction.

Differential diagnosis.

• Perilunate dislocation and scaphoid fracture.
• Scapholunate dissociation and radiocarpal dislocation.
• Barton fracture and Kienböck disease.
• Chauffeur fracture.

Emergency reduction. Reduce under regional block or general anaesthesia. Apply traction with finger traps for about 10 minutes, extend the wrist and press the lunate dorsally and distally into the fossa with the thumb, then flex the wrist. Splint in a long arm splint with the wrist neutral. Closed reduction often fails when the lunate lies in the carpal tunnel, in which case proceed to open reduction.

Definitive treatment. Open reduction, ligament repair and fixation, ideally within 7 days.

• Volar approach with carpal tunnel decompression, which allows direct reduction of the lunate. A dorsal approach repairs the scapholunate and lunotriquetral ligaments.
• Pin the scapholunate and lunotriquetral joints with 1.6 mm Kirschner wires, and repair the ligaments with suture anchors.
• Immobilise for 6 to 8 weeks, remove the wires at 8 to 10 weeks, then begin movement and strengthening.
• Chronic dislocation (over 6 weeks), failed reduction, or symptomatic avascular necrosis: proximal row carpectomy, lunate excision, or wrist fusion.

Open injury. Give intravenous co-amoxiclav 1.2 g every 8 hours (or cefazolin 2 g every 8 hours) within 1 hour of injury.

• Continue until 24 hours after wound closure for types I and II.
• For type III, continue until 72 hours or 24 hours after soft tissue cover, whichever is earlier.
• Maximum 3.6 g co-amoxiclav daily; give every 12 hours at creatinine clearance 10 to 30 mL/min; check liver tests in courses over 7 days.

For heavy contamination or soft tissue loss add gentamicin 5 mg/kg intravenously once daily for up to 72 hours. Check creatinine daily and a level before the second dose; avoid or extend the interval at eGFR under 60 mL/min/1.73 m2. With severe penicillin allergy use clindamycin 600 mg intravenously every 8 hours (maximum 2.7 g daily) with gentamicin.

Give tetanus booster 0.5 mL intramuscularly if the last dose was over 10 years ago. Add tetanus immunoglobulin 250 units intramuscularly (500 units if over 24 hours since injury or heavily contaminated) when immunisation is incomplete or unknown. Debride and stabilise within 24 hours.

Analgesia.

• Paracetamol 1 g orally every 6 hours as needed for up to 5 to 7 days (maximum 4 g daily; 3 g daily when weight is under 50 kg, with chronic alcohol use or hepatic impairment).
• Ibuprofen 400 mg orally every 8 hours with food for up to 5 days (maximum 1200 mg daily); avoid with eGFR under 30 mL/min/1.73 m2, peptic ulcer, heart failure or anticoagulant use.
• Oral morphine 5 to 10 mg every 4 hours as needed for severe pain for up to 3 days. Start 2.5 to 5 mg with age over 75 or eGFR under 30 mL/min/1.73 m2; avoid exceeding 60 mg in 24 hours without senior review. Monitor sedation and respiration and prescribe a laxative.

Operative antibiotic prophylaxis. Cefazolin 2 g intravenously (3 g when weight is 120 kg or over) within 60 minutes before incision as a single dose (repeat after 4 hours or blood loss over 1500 mL). With severe penicillin allergy, give vancomycin 15 mg/kg intravenously starting 90 to 120 minutes before incision as a single dose, with levels and creatinine checked in prolonged cases.

Monitoring. Median nerve function and perfusion hourly in the first 24 hours. Radiographs at 1 week, 2 weeks, 6 weeks, 3 months, 6 months and 12 months. Lunate density, collapse and carpal alignment at 3 to 12 months (MRI when doubtful). Finger flexion and tendon function at each visit.

Referral and escalation. Emergency referral to orthopaedics or hand surgery on diagnosis. Immediate escalation for acute carpal tunnel syndrome, failed closed reduction, open injury or vascular compromise.

Special groups.

• Older adults: lower energy injuries occur with osteoporosis, and bone health assessment follows.
• Multiple trauma: reduce and splint early, then fix once the patient is stable.
• Kidney impairment: avoid ibuprofen, reduce morphine doses and extend gentamicin intervals.