Biomechanical stability of five pedicle screw fixation systems in a human lumbar spine instability model.

In this study, the three-dimensional biomechanical stabilizing capabilities of five pedicle screw fixation systems and facet screw fixation were determined. These systems were the Ace device without and with transverse wiring, AO device, CD device, Steffee plate, Wiltse device with single and double rods, and facet screw fixation. All systems were applied to the L5S1 level in a human in vitro spine rendered unstable by transection of the posterior ligaments and transverse holes drilled through the intervertebral disc. There were no statistically significant differences in the biomechanical stability provided by any of the pedicle screw devices, where stability was defined as the average stiffness from the load-displacement curve. All devices restored motion to less than 50% of intact levels under flexion moments. In extension, all devices, except the facet screw method, restored motion to below intact levels. In lateral bending, all devices restricted motion to less than 50% of intact motion. Under axial torque, the CD device provided the least motion while the AO device did not restore motion to the intact level.