Effects of radioulnar instability on the radiocarpal joint: a biomechanical study

Five fresh cadaver upper extremities were studied with use of a static positioning frame, pressure-sensitive film and a microcomputer-based videodigitizing system to assess the effect of increasing radioulnar instability on the load distribution within the proximal carpal joint. Three stages of radioulnar instability were studied: (1) an avulsion fracture at the base of the ulna styloid; (2) an avulsion fracture at the base of the ulna styloid plus disruption of the dorsal portion of the distal radioulnar joint capsule; and (3) an avulsion fracture at the base of the ulna styloid, disruption of the dorsal portion of the distal radioulnar joint capsule, and disruption of the radioulnar interosseous membrane. All stages of radioulnar instability demonstrated a decrease in the lunate contact area in positions with the forearm in supination. In stage 3 instability there was also less lunate contact area in positions with the forearm in neutral pronation/supination. In stage 3 instability the lunate high pressure area centroid was abnormally palmar in all positions and the scaphoid high pressure area centroid was abnormally palmar in positions with the forearm in pronation or supination.