Carpal dislocations: pathomechanics and progressive perilunar instability.

The pathomechanics, ligamentous damage, and degree of carpal instability in perilunate and lunate dislocations were analyzed by experimentally loading 32 cadaver wrists to failure. Thirteen perilunate and two lunate dislocations were produced. The mechanism of injury was extension, ulnar deviation, and intercarpal supination. These dislocations occurred in a sequential fashion due to progressive and specific ligamentous disruptions and were classified according to the degree of perilunar instability (PLI). Stage I perilunar instability (scapholunate diastasis) had the least degree of carpal instability. Lunate dislocations (stage IV PLI) had the highest degree of carpal instability. Radial styloid fractures were produced in seven as a result of avulsion. Scaphoid rotation was created in eight and was due to rupture of the radioscaphoid and scapholunate ligaments. Reduction was accomplished by reversing the mechanism of injury--that is, intercarpal pronation, radial deviation, and palmar flexion. Stress roentgenograms employing longitudinal carpal compression in radial and ulnar deviation were helpful in determining the degree of associated carpal instability.