Biomechanical study of anterior spinal instrumentation configurations

The biomechanical impact of the surgical instrumentation configuration for spine surgery is hard to evaluate by the surgeons in pre-operative situation. This study was performed to evaluate different configurations of the anterior instrumentation of the spine, with simulated post-operative conditions, to recommend configurations to the surgeons. Four biomechanical parameters of the anterior instrumentation with simulated post-operative conditions have been studied. They were the screw diameter (5.5-7.5 mm) and its angle (0 degrees - 22.5 degrees), the bone grip of the screw (mono-bi cortical) and the amount of instrumented levels (5-8). Eight configurations were tested using an experimental plan with instrumented synthetic spinal models. A follower load was applied and the models were loaded in flexion, torsion and lateral bending. At 5 Nm, average final stiffness was greater in flexion (0.92 Nm/degrees) than in lateral bending (0.56 Nm/degrees) and than in torsion (0.26 Nm/degrees). The screw angle was the parameter influencing the most the final stiffness and the coupling behaviors. It has a significant effect (p < or = 0.05) on increasing the final stiffness for a 22.5 degrees screw angle in flexion and for a coronal screw angle (0 degrees) in lateral bending. The bi-cortical bone grip of the screw significantly increased the initial stiffness in flexion and lateral bending. Mathematical models representing the behavior of an instrumented spinal model have been used to identify optimal instrumentation configurations. A variation of the angle of the screw from 22.5 degrees to 0 degrees gave a global final stiffness diminution of 13% and a global coupling diminution of 40%. The screw angle was the most important parameter affecting the stiffness and the coupling of the instrumented spine with simulated post-operative conditions. Information about the effect of four different biomechanical parameters will be helpful in preoperative situations to guide surgeons in their clinical choices.