The effects of simulated unstable scaphoid fractures on carpal motion.

The kinematics of five fresh frozen wrist specimens were studied before and after a simulated scaphoid waist fracture. To determine the change in wrist motion and fracture site characteristics associated with an unstable wrist, the relative motion of each carpal bone was determined from the movement of implanted carpal markers on biplanar radiographs obtained in neutral and the four extreme wrist positions. The kinematics of the wrist in our specimens before the osteotomy were similar to previous studies. After the osteotomy, the proximal and distal segments of the scaphoid moved independently. The distal scaphoid assumed a relatively flexed stance and displayed increased motion. The proximal scaphoid fragment and lunate assumed a relatively extended stance and displayed less motion after the osteotomy. These kinematic abnormalities produced significant interfragmentary motion that would be expected to complicate normal fracture healing. The spontaneous collapse of the two scaphoid fragments produced a dorsal angulation or "humpback" deformity that simulated the clinical situation of displaced scaphoid nonunions. The scaphoid serves an important role maintaining normal alignment of the carpal bones and producing normal wrist motion.