Bennett Fracture

Bennett fracture is an intra-articular fracture dislocation of the base of the first metacarpal. A small volar ulnar fragment stays attached to the trapezium by the anterior oblique (beak) ligament, while the metacarpal shaft is pulled radially, dorsally and proximally. It is the most common fracture of the base of the thumb metacarpal.

The mechanism is an axial load on a partially flexed thumb, such as a punch or a fall on the thumb. It affects young men most often.

Complications are carpometacarpal subluxation, post-traumatic arthritis, malunion, loss of grip and pinch strength, adductor contracture of the first web, and complex regional pain syndrome.

Pathophysiology.

The abductor pollicis longus pulls the shaft proximally and dorsally, and the adductor pollicis pulls it toward the palm. The beak ligament holds the volar ulnar fragment in place, so the joint subluxes and cannot hold a reduction in a cast.

Stability and outcome depend on joint congruity. An articular step of over 1 to 2 mm is associated with arthritis.

Pain, swelling and bruising at the base of the thumb, with a prominent metacarpal base, loss of thumb abduction and weak pinch. There is tenderness at the carpometacarpal joint, and a deformity with the thumb in adduction. Check capillary refill and sensation over the radial digital nerve. Look for associated injuries to the scaphoid and distal radius.

• Radiographs: true lateral and posteroanterior views, and a Robert view (the forearm hyperpronated, with the thumb dorsum on the cassette), which shows the carpometacarpal joint without overlap.
• Measure the size of the volar ulnar fragment as a percentage of the articular surface, the articular step, and the subluxation of the shaft.
• CT when the joint surface or fragment size is unclear.

Differential diagnosis.

• Rolando fracture and extra-articular metacarpal base fracture.
• Carpometacarpal dislocation and scaphoid fracture.
• Trapezium fracture and ulnar collateral ligament injury of the thumb.
• De Quervain tenosynovitis.

Treatment is nearly always operative because the fracture is unstable.

• Closed reduction with longitudinal traction, abduction and pronation of the metacarpal, with pressure over the base. Check with fluoroscopy.
• If the fragment is under 25% of the articular surface and a congruent reduction is achieved: closed reduction and percutaneous pinning with two 1.2 to 1.6 mm Kirschner wires, from the metacarpal into the trapezium or the second metacarpal.
• If the fragment is 25% or more, or there is a step of over 1 to 2 mm after closed reduction: open reduction with fixation by lag screws (1.5 to 2.0 mm) or Kirschner wires through a Wagner volar radial approach.
• Immobilise in a thumb spica for 4 to 6 weeks. Remove wires at 4 to 6 weeks, then start movement and grip exercises. Heavy use at 3 months.
• Late arthritis: trapeziectomy with ligament reconstruction, or carpometacarpal fusion.

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.

Monitoring. Radiographs at 1 week and 2 weeks to detect loss of reduction, at 4 to 6 weeks before wire removal, and at 3 months. Pin sites at each visit. Thumb range, pinch and grip at 3 months. Carpometacarpal arthritis at 1 year and later.

Referral and escalation. Refer every Bennett fracture to orthopaedics or hand surgery within 24 to 48 hours. Same day referral for open injury, neurovascular compromise, or irreducible dislocation.

Special groups.

• Children: physeal injuries of the thumb metacarpal base need specialist assessment.
• Older adults: osteoporotic fractures need rigid fixation.
• Kidney impairment: avoid ibuprofen and reduce morphine doses.