Effect of range of motion (ROM) for pedicle-screw fixation on lumbar spine with rigid and semi-rigid rod materials: A finite element study

This study aims to understand the biomechanical effect of pedicle-screw fixation with different implant materials on lumbar spine. FE method is used to develop three-dimensional model of lumbar spine (L1 -S). Three level pedicle-screw-rod is attached to the model with two different rods: Stainless Steel (SS) and poly-ether-ether-ketone (PEEK). The implanted models along with intact models are simulated for different physiological conditions such as axial rotation, lateral bending and flexion-extension under +/- 6, 8 and 10 Nm moment with 500 N compressive loads. The spinal biomechanics found sensitive to the elastic modulus of the rod material. In case of pedicle screw fixation with SS, the ROM is very less (1.3 degrees for extension and 2.88 degrees for flexion) as compared to natural model. For PEEK rod, ROM is increased 3 to 4 times for axial rotation, 4 to 7 times for lateral bending and 1.8 to 2 times for flexion-extension. SS rods share more load and generate less stress on vertebrae compared and PEEK rods. For pedicle screw fixation, the foramen height does not vary significantly with variation of load and rod materials. SS rods resist the ROM on the implanted region affecting the kinematics of the spine undesirably. The ROM may be improved to some extent by using lower stiffness rod materials like PEEK. However pedicle screw fixation device prevents reducing inter-vertebral foramen height and foramen area, without affected by material stiffness.