MULTIDIRECTIONAL STABILITY OF THE GRAF SYSTEM

Study Design. To assess the biomechanical influences of the Graf fixat ion system on the spine, motion segments were tested. In normal spines , destabilized and restabilized with the Graf system conditions, the r ange of motion and flexibilities were found under various loading cond itions. Objectives. These results should explain how the Graf Fixation system affects the biomechanical response of a motion segment. Method s. Motion segments (L2-3) and (L4-5) were subjected to these loading c onditions: compressive loading, flexion-extension, lateral bending, an d axial rotation moments. During the loading, the main and coupled mot ions were measured, and flexibilities were computed. The position of t he balance point in axial loading was also determined. The motion segm ents were tested under three conditions: intact, destabilized, and res tabilized. The destabilization consisted of a bilateral total laminect omy (pedicle to-pedicle). Restabilization was affected with the Graf s tabilization system, consisting of polyester bands between pedicle scr ew implants. Results. The total laminectomy significantly changed the balance point location by moving it forward. The restabilized motion s egment had a balance point more similar to normal conditions. The mean compressive compliance was significantly less after application of th e Graf system compared with destabilization. The range of motion for f lexion-extension and axial rotation was significantly reduced for the main motion with the Graf system. For lateral bending, the main and co upled rotations were affected significantly, with lateral bending and flexion-extension motion reduced and axial rotation increased after re stabilization. Restabilization decreased the flexibility of the destab ilized motion segment for all of the moments. Conclusions. The Graf fi xation system reduced the range of motion and the flexibility values i n some loading conditions. These results only assesss the immediate st abilization characteristics of this implant system in cadaveric materi al. Further research should address the fatigue characteristics.