Mechanism of carpal injuries.

The mechanism responsible for carpal injuries has been elusive. Direct results of the inability to identify the mechanism include a confusing system of classification and a lack of a rational plan of treatment for the many fractures and fracture-dislocations that occur about the carpus. The mechanism of injury determined experimentally was 3-dimensional, including extension, ulnar deviation, and intercarpal supination. The resultant spatial vector in conjunction with the magnitude and duration of loading determined the combination of injuries produced. Carpal dislocations resulted from a force vector that emphasized ulnar deviation and intercarpal supination. Scaphoid fractures were produced by a vector that emphasized extension and were fractured by the dorsal rim of the radius. The degrees of carpal instability produced experimentally were divided into four stages, depending upon the amount of ligamentous damage and joint instability. Stage I (scapholunate instability) was the most stable and Stage IV (lunate dislocation) was least stable. Scaphoid fractures started on the palmar surface and propagated dorsally. Type I fractures had an intact dorsal soft-tissue hinge and fracture stability could be created by flexion. Types II and III scaphoid fractures were associated with significant perilunar instability (PLI) and were classified as fracture-dislocations.