Bennett's Fracture and other Thumb Injuries

Synonyms for Bennett's fracture: Bennett's fracture-dislocation, Bennett's fracture-subluxation, carpometacarpal fracture-subluxation of thumb, Bennett fracture (USA).

This is an oblique fracture of the base of the first (thumb) metacarpal with associated subluxation and fracture of the carpometacarpal articular surfaces. It is the commonest fracture to affect the thumb and is a serious injury; failure to treat it quickly and appropriately can lead to much disability through disruption of the pinching and opposition functions of the thumb.

Mechanism of Injury

Bennett's fracture usually occurs due to an axial blow on a partially flexed first metacarpal, such as would occur during delivery of a punch with a clenched fist.¹ The volar fracture fragment on the ulnar aspect of the metacarpal base is held firmly in place by the volar anterior oblique ligament, whilst the traction of the abductor pollicis longus muscle tendon pulls the distal metacarpal fragment (containing most of the articular surface) proximally, radially and dorsally.¹

Presentation

There is acute, severe pain and swelling at the base of the thumb with grossly reduced movement at the 1st carpometacarpal joint. Instability at the carpometacarpal joint may be noted with gentle stressing of the thumb metacarpal.

Radiology

• Take true AP and lateral views to fully appreciate the presence of fracture and degree of displacement.
• There is a classical oblique fracture line at the base of the first metacarpal with a triangular fragment at the ulnar base of the metacarpal.
• The distal metacarpal portion will be displaced to a variable degree. Highly displaced fractures, those with significant crushing or impaction at the carpometacarpal joint, and those with shearing or impaction injury to radial side of articular surface of the trapezium carry a worse prognosis and are inevitably going to need operative intervention.

Management^1,2

• Fractures with <1–2 mm of disruption of articular congruity and minimal displacement may be treated by closed reduction and, if the reduction is maintained, placement in a thumb spica cast for 6 weeks or so.
• Reduction is achieved through longitudinal traction on the thumb with metacarpal extension, pronation and abduction.
• However, most cases will require surgical intervention, either through closed reduction and percutaneous placement of K-wires, or open reduction and internal fixation using K-wires and/or screws.
• After operative intervention, splinting in a thumb spica cast for 2–6 weeks is necessary, depending on the degree of stability obtained during surgery.
• Subsequent immobilisation in a thermoplastic splint is used to allow gradual mobilisation until healing is complete.

Complications

• Osteoarthritis and loss of mobility of thumb carpometacarpal joint, as a result of osteocartilaginous injury, even if joint congruity and fragment reduction is achieved.
• Loss of motion at carpometacarpal joint due to prolonged immobilisation.
• Loss of reduction of fracture with recurrent joint subluxation/instability.
• Postoperative infection of carpometacarpal joint/osteomyelitis.
• Dorsal sensory branches of radial nerve injury causing anaesthesia.

Rolando's fracture^3,4

• This is a 3-way fracture of the base of the trapeziometacarpal joint.
• There is fracture of the metacarpal as in Bennett's fracture and a large dorsal fragment, resulting in a comminuted Y- or T-shaped intra-articular fracture at the base of the thumb metacarpal.
• It is a relatively uncommon fracture but carries a worse prognosis than Bennett's fracture.
• AP and lateral x-rays need to be taken to identify it and assess the degree of displacement.
• Highly comminuted ± minimally displaced fractures may be treated by thumb spica cast for 3–4 weeks with gradual mobilisation.
• More likely to require operative intervention and open reduction with internal fixation with K-wires ± screws.

Metacarpal head/shaft fractures

These are relatively rare and caused by direct impact to the mid-thumb. May require closed reduction and fixation with K-wires if significant displacement/angulation, but often managed conservatively with thumb spica splinting.

Fractures of thumb phalanges

May occur as:

• Extra-articular tuft fractures due to crush injury (e.g. hammer blow), often with associated damage to the nail.
• Intra-articular tendon avulsion injury.

Usually treated conservatively, depending on degree of injury/displacement; nail may need trephination if significant subungual haematoma. Tendon avulsion injuries may require reconstructive surgery.

Thumb dislocations⁵

• May affect the interphalangeal joint (thumb has only 2 phalanges) or metacarpophalangeal joint.
• Metacarpophalangeal dislocation tends to occur due to hyperextension injury and usually dorsal dislocation.
• Often associated injury to sesamoid bones and disruption of collateral ligaments.
• Reduced by flexion of metacarpal to relax intrinsic muscles and traction applied to complete reduction, but can be irreducible.
• If no disruption of ligaments and reducible dislocation, then conservative splinting of the joint in slight flexion for 2–3 weeks.
• Open reduction and fixation/repair for irreducible fractures or those with radial or ulnar instability due to collateral ligament damage; fractured sesamoid bones are sewn together with sutures through the volar plate.
• Interphalangeal dislocations are less common and usually displaced dorsally. May be irreducible due to trapping of the ruptured palmar plate.

Mallet thumb⁶

• Avulsion of the extensor tendon causing thumb stuck in moderate flexion.
• May be associated with laceration causing tendon disruption.
• Can be treated by conservative splinting but may require open tendon repair or fixation of avulsed bone.

Gamekeeper's thumb (Skier's/ski-pole thumb)⁷

• This is due to injury to ulnar collateral ligament of the metacarpophalangeal joint due to a sharp outward and hyperextending force on the thumb.
• Gamekeepers suffered chronic injury due to breaking of rabbits' necks between thumb, fingers and ground.
• Nowadays much more common injury in skiers who fall against the ski-pole/strap or ground while the thumb is abducted.
• The thumb is hyperextended and laterally deviated with swelling and bruising over the joint.
• Splint and apply ice for first aid whilst transferring to receive medical attention.
• Refer to hand specialist for decision on need for surgery, depending on degree of disruption of ligament and associated damage.

Complications of traumatic thumb injuries

• Post-traumatic arthritis
• Bony malunion
• Chronic ligamentous damage and instability
• Vascular injury
• Neurological injury