A Finite Element Study of Mechanical Behaviour of Instrumented Lumbar Spine with Three Different Longitudinal Rods Designs: Sliding Articulated, Flexible and Rigid

Spinal fusion is the gold standard treatment for severe pathologies in lumbar spine. However, fusion can yield adverse effects, such as Adjacent Segment Degeneration (ASD). Alternatively, non-fusion techniques were proposed. Although infrequent, mechanical complications were reported such as screw loosening and screw breakages. This work aims to use Finite Element Model (FEM) to investigate implants with different rod designs: articulated sliding, flexible and rigid (fusion). A L3-S1 validated spine model was used. Surface contact elements modeled connector-rod interface for the sliding implant. Beam elements modeled the rod with different Young modulus between the rigid and the flexible implant. Rigid to flexible Young’s modulus ratio was 10000. The instrumented segment was L4-L5. S1 was rigidly fixed. Imposed rotations were applied to L3 vertebra considering physiologic L3-S1 ROM. These values were the same for each modeled configuration: intact, injured, injured with sliding implant, injured with rigid implant and injured with flexible implant. Screw and discs stress and Range of Motion (ROM) were obtained for all configurations and different loads. Modeling of the sliding implant was validated by comparing the instrumented model ROM with experimental in-vitro corridor on 6 cadaveric segments. Bending moment in the sliding rod was significantly smaller than others configurations. Flexible implant decreased peak stress compared to fusion in adjacent segments. They did not reduce axial rotation ROM.