Inherent stiffness of three different posterior transpedicular fixation instrumentation. A mechanical comparative study

Instrumented lumbar fusion with posterior pedicle screw systems enhances fusion rate. Although rigid pedicle-screw instrumentation increases fusion rate, recently many adverse effects have been reported in lumbar spinal fusion augmented with rigid instrumentation. This study compares static, non-destructive mechanical “in vitro” properties of three different systems [segmental contouring system (SCS), Claris and Twinflex], to investigate the inherent stiffness of these devices. Forty-eight identical lumbar synthetic models, each including three vertebral bodies and intervertebral discs, were instrumented with each of the three instrumentations Twinflex, Claris and SCS. Constructs with instrumented vertebrae as well as non-instrumented models were used in this study. The constructs underwent non-destructive static torsional and compressive-bending loading. Rigid instrumentation provided the highest torsional and compression stiffness. Torsional stiffness differed significantly (P < 0.001) among constructs (SCS > Claris > Twinflex). Compressive stiffness also demonstrated statistically significant (P < 0.05) differences between different constructs (SCS > Claris, SCS > Twinflex). Both Claris and Twinflex provided similar compressive stiffness. In torsion, all three instrumentations equally reduced at 80% the amount of relative torsional displacement between adjacent instrumented vertebrae (P < 0.001). In compression, Claris and Twinflex similarly reduced at 89% the angle of sagittal wedging between two adjacent vertebrae. SCS reduced the angle of vertebral sagittal wedging between adjacent vertebrae at 94%, which was significantly greater than that of Claris and Twinflex (P < 0.001). Conclusions The present comparative mechanical study disclosed that the inherent stiffness of the SCS is the highest, of the Claris higher than that of the Twinflex. The significance of these immediate findings to enhance fusion, material failure rate and adjacent non-instrumented segment degeneration cannot be elucidated from this study. Further long-term clinical and roentgenographic studies are needed to show the effects of instrumentation stiffness on the adjacent segment degeneration.