Biomechanical assessment of a novel pedicle screw system using finite element analysis

Introduction:
The biomechanical behavior of lumbar spine instrumentation is critical in understanding its efficacy and durability in clinical practice. In this study, we aim to compare the biomechanics of the lumbar spine instrumented with single-level posterior rod and screw systems employing two distinct screw designs: paddle screw versus conventional screw system.
Method:
A fully cadaveric-validated 3D ligamentous model of the lumbopelvic spine served as the foundation for our comparative biomechanical analysis . To simulate instrumentation, the intact spine was modified at the L4L5 level, employing either paddle screws or standard pedicle screws (SPS). The implants were composed of Ti-6AL-4V. Fixation at the S1 ensured consistency across loading scenarios. Loading conditions included a 400-N compressive load combined with a 10 N.m pure bending moment at the level of L1, replicating physiological motions of flexion-extension, lateral bending and axial rotation. We extracted data across various scenarios, focusing on the segmental range of motion at both implanted and adjacent levels.
Result:
In the flexion of L4L5, the applied force ranged from -29.2 to 29.3 N in the paddle screw, while it ranged from -25 to 25 N in the PS system. Similarly, the extension of L4L5 ranged from -3.1 to 2.6 N in the paddle and ranged from -4.5 to 3.9 N in the SPS system. In terms of stress distributions on the screw, stress concentrations decreased in several cases in the paddle design compared to the SPS systems.
Conclusion:
The paddle screw enhanced the range of motion overall in both the upper adjacent segment (L3L4) and the lower adjacent segment (L5S1) compared to the conventional SPS system. The stability of the aimed segment was increased by 33% on average with the paddle screw compared to conventional PS. Increasing the stability of the host segment decreases the possibility of non-union and the rate of fusion failure .