#522 Biomechanical Investigation of a Novel Posterior Neutral Stabilization Spinal System: A Finite Element Approach
Poster Presented by: S. Vadapalli
P. Maxy (1)
J. Prevost (2)
S. Vadapalli (2)
(1) Medtronic Spine & Biologics, Tolochenaz, Switzerland
(2) Medtronic Spine & Biologics, Memphis, TN, USA
Introduction: Published clinical evidence suggests that adjacent segment degeneration is a consequence of spinal fusion. A Posterior Neutral Stabilization (PNS) Rod consisting of a PEEK component with a lower stiffness section is proposed to reduce supraphysiological stresses and motion introduced at the level adjacent to a more rigid fixation. The study's objective was to evaluate the behavior of lumbar segments implanted with the PNS Rod in comparison to more rigid standard fusion treatment options and to assess the effects of the PNS rod on the segment adjacent to the fusion.
Methods: A validated 3-D L1-S1 FEA model was used. A destabilized spine was simulated by introducing a severe L4/5 destabilization and slight L3/4 destabilization by decreasing the mechanical characteristics of the disc's nucleus and annulus. Three specific implants were considered:
a) Ø5.5mm Titanium Rods at L4/5,
b) Homogeneous PEEK Rods at L4/5,
c) PNS Rods at L3/5 with its C-shape sections positioned at L3/4 for stabilizing the adjacent level above the fusion (Fig. 1).
Displacements were applied in Flexion/Extension, Torsion and Lateral Bending in order to match the global motion of the L1-S1 segment from a previously loaded intact model.
Results: Flexion/Extension results for the level adjacent to the fusion (L3/4) were normalized relative to intact spine condition (Table 1). At the fused segment (L4/5), all implants had the same stabilizing effect and reduced the angular motion by ~80% compared to the destabilized spine. The more rigid rods induced increased motion at the adjacent segment (L3/4) while the PNS rod reduced the motion.Also, the more rigid rods increased the tension in the disc's fibers and facet capsules at the adjacent segment, while the PNS Rod reduced tensions at the level but lead to higher tensions in the non instrumented segments compared to single-level constructs. For example, the more rigid and PNS rods induced 25% and 81% higher disc's fiber tensions at the L2/3 segment in Flexion/Extension, respectively.
Conclusion: Fusion alters the kinematics in the adjacent segment by redistributing the range of motion and leads to increase tension in the disc's fibers and facet capsules. The PNS system, due to its C-shape configuration, resulted in reducing the ROM and tension at the adjacent level above the fusion in comparison with more rigid fixation systems and creates a transitional zone between the fused and remaining mobile lumbar segments.
Picture 1: FEA model and the three implants being investigated.
Table 1: Range of Motion (ROM), Tension in the Disc's Fibers and Tension in the Facet Capsules for the adjacent level above the fusion (L3/4).