Normal wrist kinematics and the analysis of the effect of various dynamic external fixators for treatment of distal radius fractures.

The purpose of this study was to evaluate the effect that external fixation devices used for distal radius fractures have on normal carpal kinematics, using high speed video data acquisition. Cadaver forearms were stabilized, allowing free excursion of the wrist for passive range of motion. Synergistic muscle tension was simulated by looping the extensor and flexor pairs of the wrist. Global wrist flexion and extension, intercarpal angles, and the instantaneous screw axis (ISA) were studied. These parameters allowed a quantification of normal carpal kinematics and comparison to the kinematics of the carpal bones with external fixators attached. This study shows that normal carpal kinematics during wrist flexion and extension do not have an ISA fixed in or limited to the capitate. In addition, the ISA data provide evidence that translational motion is a real and measurable component of normal carpal motion. This would change the understanding of carpal kinematics in previous studies that suggested that the center of rotation was fixed in the capitate. The proximal carpal (radiolunate) joint contributes more motion (has a greater rotation angle) in normal global wrist flexion and the midcarpal (capitolunate) joint contributes more motion in normal global wrist extension. In addition, the global range of motion and intercarpal joint angle of the wrist are limited with the application of any of the external fixators tested. None of the external fixators allowed normal ISA vector surfaces during wrist motion.