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