Fracture of the base of the first metacarpal bone which extends into the carpometacarpal joint. This intra-articular fracture is the most common type of fracture of the thumb. Caused by an axial force directed against the partially flexed metacarpal, such as during a fight or fall onto the thumb. In the ED they should be placed in a thumb spica splint, and referred for closed reduction and percutaneous pinning.