Multidirectional stability of the Graf system.

Abstract

To assess the biomechanical influences of the Graf fixation system on the spine, motion segments were tested. In normal spines, destabilized and restabilized with the Graf system conditions, the range of motion and flexibilities were found under various loading conditions. These results should explain how the Graf Fixation system affects the biomechanical response of a motion segment. Motion segments (L2-3) and (L4-5) were subjected to these loading conditions: compressive loading, flexion-extension, lateral bending, and axial rotation moments. During the loading, the main and coupled motions were measured, and flexibilities were computed. The position of the balance point in axial loading was also determined. The motion segments were tested under three conditions: intact, destabilized, and restabilized. The destabilization consisted of a bilateral total laminectomy (pedicle-to-pedicle). Restabilization was affected with the Graf stabilization system, consisting of polyester bands between pedicle screw implants. The total laminectomy significantly changed the balance point location by moving it forward. The restabilized motion segment had a balance point more similar to normal conditions. The mean compressive compliance was significantly less after application of the Graf system compared with destabilization. The range of motion for flexion-extension and axial rotation was significantly reduced for the main motion with the Graf system. For lateral bending, the main and coupled rotations were affected significantly, with lateral bending and flexion-extension motion reduced and axial rotation increased after restabilization. Restabilization decreased the flexibility of the destabilized motion segment for all of the moments. The Graf fixation system reduced the range of motion and the flexibility values in some loading conditions. These results only assess the immediate stabilization characteristics of this implant system in cadaveric material. Further research should address the fatigue characteristics.