Boxer's knuckle--dorsal capsular rupture of the metacarpophalangeal joint of a finger.

An injury to the dorsal aspect of the metacarpophalangeal joint of a finger usually damages the sagittal fibers of the extensor tendon mechanism. Rupture of the underlying joint capsule, a far more serious injury, has received scant attention in the medical literature. The problem is often unrecognized because the extensor tendon may remain in its normal midline position. Eight dorsal capsular ruptures are described in six patients who had closed direct trauma to either the knuckle of their index or long fingers. Most of the injuries occurred in professional boxers.