Comparison of Range of Motion in a Degenerative Spondylolisthesis Spine Finite Element Analysis Model: Cortico-Pedicular Posterior Fixation Versus Traditional Pedicle Screw Fixation

Introduction: A novel cortico-pedicular posterior fixation device (CPPF) has been FDA-approved and provides cortical fixation of the lamina, pedicle, and facet joint. The objective of this study is to characterize the difference in biomechanical stability between CPPF and traditional pedicle screw fixation (PSS) in a grade 1 degenerative spondylolisthesis finite element analysis (FEA) model.

Methods: A validated L1-S1 osteoligamentous spinal finite element model was created with grade 1 degenerative spondylolisthesis at L4-5 with 5mm of translation. The model was simulated with a central laminectomy at L4-5 and two different posterior fixation types. Group 1 received standard bilateral pedicle screw fixation with 6.5mm diameter screws and 5.5mm diameter rods, whereas Group 2 received bilateral CPPF with 222N of compression around the facet joints. An intact model was also used as a non-operative control group for range of motion (ROM). All models were simulated for flexion, extension, lateral bending (LB), and torsion using a maximum moment of 7.5Nm. Outputs recorded were the ROM, tensile forces in CPPF, and bone stress profile in CPPF simulations. Mechanical properties were assigned to physiologic tissues per published literature. Spinal weight and muscle stabilization of a 90kg male was applied through 414N follower load.

Results: Groups 1 and 2 provided similar stabilization of the index level with an average of 73±14% reduction in ROM across all planes, compared to the non-operative group. Furthermore, the maximum ROM across all instrumented models were within 0.4 degrees. In all simulations, the maximum tensile force in CPPF was 49% of the device yield load and the maximum bone strain was 66% of bone yield strain.

Conclusion : Both CPPF and pedicle screw fixation techniques provided similar reduction in index level spinal element motion. In CPPF simulations at maximum loading conditions, the tensile forces within CPPF and surrounding bony anatomy remained below the respective yield loads.