Bennett's fracture-subluxation

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This describes a fracture at the base of the thumb metacarpal which extends into the carpometacarpal joint. It is caused by a longitudinal compression force to the thumb, often by punching, and should be suspected in any sprained thumb sustained in such activity.

The thumb appears short. The injury is unstable because:

  • the fracture line is oblique
  • the proximal fragment consists of a small triangular segment attached to the medial part of the trapezium
  • the distal fragment has subluxated proximally under the pull of the extensor pollicis brevis tendon that attaches to it