Three-dimensional Finite Element Analysis Of Lumbar Pedicle-screw-based Dynamic Fixation Device

Spinal fusion surgery is considered as one of the standard treatment for degenerative lumbar disc disease (DDD) and spinal stenosis,and achieves a better fusion rate and early clinical results. With the gradually in-depth understanding of adjacent segment degeneration (ASD) after spinal fusion, the dynamic stabilization concept is proposed, domestic and foreign scholars have conducted studies of various dynamic fixation.Objectives1. To establish and validate a three-dimensional finite element model of L3-L5 lumbar spine bi-segment and fixation system.2. With three-dimensional finite element method, to analyse the stress distribution of pedicle screws and rods of Bioflex system under different loading states, and the effect of Bioflex system on adjacent segmental intervertebral disc stresses (IDSs) and facet joint pressures (FJPs) by comparing with decompression model and rigid fixation model with pedicle screw system.Materials and Methods1. The simulation model was created by Mimics 10.01 based on CT imaging data (DIOCM) of L3-L5 segments of a healthy male volunteer, and the finite element model was formed by Abaqus6.10. ROMs of L3/4 segment were investigated after being subjected to moments of 10Nm and preload of 400N. For validation of the model, the results were compared with literatures.2. The CAD models of Bioflex dynamic stabilization system and titanium rigid fixation system were established with SolidWorks 2008 SP0.0 .3. Based on the intact finite element model,new models were generated by simulating decompression by fenestration, dynamic fixation with Bioflex system and rigid fixation with pedicle screw system.Every model was loaded with pure moments of 10Nm in three main anatomic planes and with axial preload of 400N to simulate five lumbar movement states including stand, anteflexion, extension, right lateral bending and left rotation. Pedicle screws and rods'Von Mises stresses of Bioflex as well as IDSs and FJPs of L3/4 segment were investigated.Results1. Established the three-dimensional simulation model and the finite element models of lumbar(L3-L5)motion segments successfully.The models had excellent modality similarity,and precisely reproduce the L3-L5 segmental anatomy shape.2. Established the geometric models of Bioflex system and titanium rigid fixation system, which are conducive to assemble and modify the properties.3. With finite element analysis, the stress distributions of Bioflex system at different parts in different movement states were as follows: the stresses of spring rods in vertical compression state are much smaller than those in flexion, extension, lateral bending and rotation states; the maximum stresses emerged at the rotation state of which the stresses concentrated in the spring coil section of the left spring rod and its junction, especially the lower junction, with straight rod section. Within the remaining four states, the stresses can be evenly distributed in the bilateral or unilateral coil region, without visible stresses concentration. In five movement states, pedicle screws have different degrees of stresses concentration at the junctions with vertebral bodies, whereas much smaller than the maximum stresses of the spring rods.4. The effect on the adjacent segment(L3/4):Comparing with intact finite element model, IDSs of decompression model and Bioflex model had no significant difference (P> 0.05); IDSs of rigid fixation model in flexion, extension, lateral bending states significantly increased (P <0.05), more than those of decompression model and the Bioflex fixation model. Comparing with intact finite element model, FJPs of decompression model were not significantly different, those of Bioflex fixation model increased slightly and those of rigid fixation model increased significantly.Conclusions1. The finite element model was validated and could be used on biomechanics test.2. Being dynamic fixed with Bioflex, IDSs of the adjacent segment weren't influenced significantly, and FJPs of adjacent segment were increased slightly, which could theoretically slow the degeneration of adjacent segments.3. Bioflex fixation system induces no evident abnormality changes to the load on lumbar spine motion segments, in theory, could avoid the complications caused by significant load changes after rigid fixation of lumbar spine.