Boxer's Fracture

Boxer's fracture is a fracture of the neck of the fifth metacarpal, usually from punching a hard object with a closed fist. The term is also used for the fourth metacarpal neck. It is the most common metacarpal fracture, with a peak in young men aged 10 to 29 years.

The distal fragment is angulated palmarly because the interosseous muscles pull the head into flexion. The lateral view shows the apex dorsal angulation.

The carpometacarpal joint of the little finger moves about 25 to 30 degrees, so the fifth metacarpal neck tolerates more angulation than the others. Many centres accept 40 to 60 degrees (some accept up to 70) in a cooperative adult when there is no rotational deformity. The index and middle metacarpals tolerate only 10 to 20 degrees.

Complications are malrotation (scissoring of the finger on making a fist), loss of the knuckle contour, extensor lag and pseudoclawing, malunion, and infection after a "fight bite" when the knuckle skin is broken by a tooth.

Pathophysiology.

An axial load through the clenched fist exceeds the strength of the metacarpal neck, a weak point of the thin cortex. The head flexes under interosseous pull, and the shaft apex presses dorsally.

A wound over the metacarpophalangeal joint after a punch is a human bite until proved otherwise, because the tooth penetrates the joint capsule and extensor tendon, and oral flora cause septic arthritis and osteomyelitis.

Swelling, bruising and tenderness over the dorsal ulnar hand, loss of the fifth knuckle prominence, and pain with movement. Check finger rotation by asking the patient to flex the fingers slowly: all fingers should point toward the scaphoid tubercle, and overlap (scissoring) means malrotation.

Inspect the knuckle for a small wound over the metacarpophalangeal joint (fight bite) and for extensor tendon function. Check sensation of the ulnar digital nerves and capillary refill.

• Radiographs of the hand: posteroanterior, lateral and oblique views. Measure the neck angle on the true lateral view (the angle between the shaft axis and the head axis).
• Look for intra-articular extension, shortening (over 5 mm) and multiple metacarpal fractures.
• Rotation is judged clinically, not by radiograph.
• Radiographs for a retained tooth fragment or air in the joint after a wound.

Differential diagnosis.

• Metacarpal shaft and base fractures and metacarpophalangeal dislocation.
• Carpometacarpal fracture dislocation of the fifth ray.
• Extensor tendon injury and sagittal band rupture.
• Fight bite with septic arthritis.

Closed fractures.

• Reduce when angulation exceeds the acceptable limit or the head is prominent in the palm. Under haematoma or ulnar nerve block, use the Jahss manoeuvre: flex the metacarpophalangeal and proximal interphalangeal joints to 90 degrees, push the proximal phalanx dorsally to lift the head, and counter with volar pressure on the metacarpal shaft.
• Functional treatment is as effective as splinting for most: buddy strapping to the ring finger with early movement from 1 to 2 weeks.
• Splint option: ulnar gutter splint with the metacarpophalangeal joints at 70 to 90 degrees of flexion and the interphalangeal joints free, for 3 to 4 weeks.
• Operative indications: malrotation, angulation over the acceptable limit after reduction, shortening over 5 mm, multiple metacarpal fractures, open fracture, and intra-articular extension.
• Techniques: percutaneous Kirschner wires (1.6 mm, intramedullary or transverse), intramedullary headless screw, or a mini plate with screws.

Fight bite (open wound over the knuckle). Do not close the wound. Irrigate and explore in theatre for capsule penetration, with extensor tendon repair and debridement.

Intravenous co-amoxiclav 1.2 g every 8 hours when infected or after joint penetration (maximum 3.6 g daily; every 12 hours at creatinine clearance 10 to 30 mL/min; avoid with a history of co-amoxiclav cholestatic jaundice). Change to oral co-amoxiclav 500/125 mg every 8 hours to complete 5 to 7 days for uncomplicated wounds (longer when joint or bone is involved).

With penicillin allergy use doxycycline 100 mg orally twice daily with metronidazole 400 mg orally every 8 hours for the same duration (check liver function and avoid alcohol with metronidazole). Give a tetanus booster 0.5 mL intramuscularly if the last dose was over 10 years ago, with tetanus immunoglobulin 250 units intramuscularly (500 units if over 24 hours since injury or heavily contaminated) when immunisation is incomplete or unknown.

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 after reduction to detect loss of position, and at 4 to 6 weeks for union. Finger rotation and grip at each visit. Review fight bites at 24 to 48 hours for spread of infection. Hand therapy for stiffness at 3 to 6 weeks.

Referral and escalation. Refer malrotation, angulation over the acceptable limit, multiple fractures, intra-articular fractures and open fractures to orthopaedics or hand surgery within 24 to 48 hours. Same day referral for any wound over a knuckle after a punch, a swollen hot joint, spreading cellulitis, or tendon injury.

Special groups.

• Children: fractures near the physis follow remodelling rules, and surgery is needed for malrotation.
• Older adults: osteoporotic metacarpal fractures usually heal with buddy strapping.
• Kidney impairment: avoid ibuprofen and reduce morphine doses.
• Diabetes: lower threshold for admission and intravenous antibiotics after a bite.