Bennett's fracture and other thumb injuries

Identifying and treating thumb injuries is extremely important. Thumb function constitutes 50% of hand function as a whole.

What is Bennett's fracture?^{1 2}

A Bennett's fracture is an intra-articular fracture of the base of the first metacarpal with resultant dislocation of the first carpometacarpal joint. The fracture is unstable and inadequate treatment leads to osteoarthritis, weakness and loss of function of the first carpometacarpal joint. It is the most common fracture to affect the thumb and is a serious injury.

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.

• 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.

Bennett's fracture symptoms

• Acute, severe pain and swelling at the base of the thumb, with grossly reduced movement at the first carpometacarpal joint.

• Instability at the carpometacarpal joint may be noted with gentle stressing of the thumb metacarpal.

Radiology

• Posteroanterior, lateral and oblique views should be taken in the assessment of thumb fractures and injuries.

• 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. This fragment remains attached to the trapezium and there is proximal displacement of the metacarpal.

Bennett's fracture treatment

• Small avulsion fractures with minimal articular disruption and minimal instability may be treated by closed reduction and, if the reduction is maintained, placement in a thumb spica cast for six 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 of the thumb carpometacarpal joint as a result of osteocartilaginous injury, even if joint congruity and fragment reduction are achieved.

• Reduced movement at the carpometacarpal joint due to prolonged immobilisation.

• Recurrent joint subluxation/instability.

• Postoperative infection of skin/osteomyelitis.

• Dorsal sensory branches of radial nerve injury causing anaesthesia.

Other thumb injuries and fractures

Rolando's fracture^{3 4}

• This is a 3-part fracture at the base of the thumb metacarpal.

• There is the same fracture of the metacarpal as in Bennett's fracture but, as well as this, there is a large dorsal fragment, resulting in a comminuted Y- or T-shaped intra-articular fracture at the base of the thumb metacarpal. Essentially the three fragments of the fracture are: metacarpal shaft, dorsal metacarpal base and volar metacarpal base. However, Rolando's fracture has come to be used to describe all comminuted thumb metacarpal base fractures.

• It is a relatively uncommon fracture but carries a worse prognosis than Bennett's fracture.

• The mechanism of injury is a significant axial load that splits and crushes the metacarpal articular surface.

• Presentation can be with a swollen, tender, visibly deformed thumb base.

• Anteroposterior and lateral X-rays may not show the full extent of comminution and so a Robert view (a hyperpronated view of the thumb base) or CT scanning may be needed.

• Highly comminuted fractures are sometimes treated conservatively in a thumb spica cast for 3-4 weeks with gradual mobilisation after that time but external fixation and bone grafting may also be used.

• Reduction of large volar and dorsal fragments and reconstruction of the articular surface may be carried out using open reduction and internal fixation with K-wires/screws.

• Complications can include infection, nerve damage and joint stiffness.

Metacarpal head/shaft fractures⁵

• These are relatively rare and caused by direct impact to the mid-thumb.

• They may require closed reduction and fixation with K-wires if there is significant displacement/angulation but are often managed conservatively with thumb spica splinting.

Fractures of thumb phalanges⁶

• Proximal phalanx fractures are fractures of the phalangeal head and shaft.

• Distal phalanx fractures may occur as:
  

  • Extra-articular tuft fractures due to crush injury (eg, a hammer blow), often with associated damage to the nail.

  • Intra-articular tendon avulsion injury.

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

Thumb dislocations⁷

• These may affect the interphalangeal joint (the thumb has only two phalanges) or the metacarpophalangeal (MCP) joint.

• MCP dislocation:
  

  • This tends to occur due to hyperextension injury and usually dorsal dislocation.

  • There is often associated injury to the sesamoid bones and disruption of collateral ligaments.

  • They are reduced by flexion of the metacarpal to relax the intrinsic muscles and then traction being applied to complete reduction (but they can be irreducible).

  • If there is no disruption of ligaments and the dislocation is reducible then the joint is usually splinted in slight flexion for 2-3 weeks.

  • Open reduction and fixation/repair is performed 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 dislocation:
  

  • This is less common - usually displaced dorsally.

  • They may be irreducible due to trapping of the ruptured palmar plate.

Mallet thumb⁸

• There is avulsion of the extensor tendon causing the thumb to be stuck in moderate flexion. Bony fragments may be present.

• It usually occurs due to forceful flexion of the interphalangeal joint.

• An open mallet thumb can be caused by laceration causing tendon disruption.

• Closed mallet thumb can be treated by conservative splinting but may require open tendon repair or fixation of avulsed bone.⁹

Gamekeeper's thumb (skier's/ski-pole thumb)^{6 10}

• This is injury to the ulnar collateral ligament (UCL) of the MCP joint (on the medial side of the thumb) due to forced abduction of the MCP.

• The ligament can be torn partially or fully and there can be an associated avulsion fracture of the volar base of the proximal phalanx.

• Gamekeepers suffered chronic injury due to wringing the necks of game between their thumb and index finger.

• Nowadays this is a much more common injury in skiers who fall against the ski-pole/strap or ground while the thumb is abducted, and so it is seen as an acute injury. It is the most common upper limb injury in skiers. It may also be seen in sportspeople who play with balls (eg, basketball, netball) and in those who hold sticks (eg, hockey, lacrosse).

• The thumb is hyperextended and laterally deviated with swelling and bruising over the joint. Pain is experienced over the ulnar side of the MCP joint. Grasp and pinching ability may be reduced.

• Adequate treatment is important so that pinch grip can be maintained.

• Splint and apply ice for first aid whilst transferring to receive medical attention. X-ray should be performed to exclude associated fracture if there is tenderness over the UCL.

• If a fracture has been excluded, evaluate the stability of the ligament by applying abducting pressure to one side of the UCL while applying counterpressure to the other side. Compare to the uninjured hand.

• Referral to a hand specialist may be necessary for a decision on the need for surgery, depending on the degree of disruption of ligament and associated damage.

• If there is joint instability and a tender mass palpable, there may be a Stener lesion present. In this condition, the proximal end of the UCL is trapped outside the adductor aponeurosis of the thumb. Surgery by a hand surgeon is required.

• If there is just incomplete ligamental injury and the joint is stable, treatment can be immobilisation in a thumb spica splint for 4-6 weeks. Otherwise, surgical repair is usually indicated.

• Complications include osteoarthritis of the MCP joint. However, correct, prompt treatment carries a good prognosis with regards to return to normal function.

Complications of traumatic thumb injuries

• Post-traumatic arthritis.

• Bony malunion.

• Chronic ligamentous damage and instability.

• Vascular injury.

• Neurological injury.