The CMC joints of fingers 2 through 5 are small plane synovial joints between the metacarpal bases and the distal carpal row. The second and third CMC joints are essentially immobile, forming the fixed central column of the hand that does not move during grip. The fourth and fifth CMC joints allow 15 to 20 degrees of motion that permits the palm to cup around cylindrical objects and allows the hypothenar eminence to deepen during small object grip.
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Fracture-dislocations of the CMC joints (excluding the thumb) occur from axial metacarpal loading, typically a punch, and mostly involve the fourth and fifth joints where the metacarpal bases articulate with the hamate. They are often missed because they may reduce spontaneously, so true lateral and oblique radiographs or CT are needed. The reverse Bennett fracture is an intra-articular fracture of the fifth metacarpal base in which extensor carpi ulnaris pulls the shaft dorsally and proximally. These injuries are distinct from the more common boxer's fracture of the fifth metacarpal neck. Failure to diagnose and stabilise CMC fracture-dislocations produces chronic instability and pain.
Axial loading disruption of the ring and little finger CMC joints producing dorsal displacement of the metacarpal bases; it may reduce spontaneously but needs pinning or open fixation if unstable.
Intra-articular fracture-subluxation of the fifth metacarpal base with the shaft pulled proximally by extensor carpi ulnaris, managed with percutaneous pinning or open fixation.
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