| Objective: Using Three-dimensional finite element method(3D-FEM)to compare the effects of pedicle cortical bone screw and traditional pedicle screw on inserted and adjacent segments in lumber vertebra,and to investigate the biomechanics characteristics of those two screws.Materials and Methods: We selected a normal and healthy volunteer who was scanned by CT as the research object and established the L3-S1 lumbar spine three-dimensional finite element model which was accurate and detailed.According to the clinical technology,we inserted TT and CBT screws into the vertebral of L4/5 segment and replaced the material of intervertebral disc with cancellous bone and the cage in order to simulate the surgery of lumbar interbody fusion.The intact model established in this manner was compared with the relevant data in the literature and the biomechanical test results to prove the validity of the finite element model.Then we simulated the physiological motion states in six different directions: flexion,extension,left flexion,right flexion,left rotation and right rotation in order to compare the stress,distribution of different trajectory screws on the facet joint process,endplate,intervertebral disc and vertebral of the inserted and adjacent segments.We also need to compare the range of motion(ROM)of each segment,and then analyzed the impact of the two kinds of trajectory screw on the inserted segment and adjacent segments.Results:(1)The established 3D-FEM model of L3-S1 segments was similar to the realistic lumbar.A uniformly distributed 400 N surface load was applied on the upper surface and 7.5 N ·m of moment was simultaneously applied on the upper surface of the vertebral body.Simulating six motion states in different directions: flexion,extension,left flexion,right flexion,left rotation,and right rotation to compare the difference between two kinds of trajectory screw.(2)In the six motion states,the maximum stress of CBT screw system in the L4/5 segment is larger than that of the TT,and it is almost twice the stress of the TT screw.(3)The maximum stress of the CBT screw model in the facet joint of the L4/5 segment,the inferior endplate of L4 and the superior endplate of L5 is smaller than that of the TT screw,but the maximum stress of the facet joints of L3/4、 L5/S1,the superior endplate of L3、S1,the inferior endplate of L4、L5,are larger than TT in the adjacent segments.(4)In the intervertebral disc of L3/4 and vertebral body of L3 and S1,there is no significant difference in the maximum stress between the two kinds of screws.And the maximum stress of the CBT screw in the vertebral body of L4 and L5,and the adjacent segments of L5/S1 disc is larger than that of the TT.(5)The range of motion of the CBT screw in the L4/5 segment is smaller than that of the TT screw,although in the segment of L3/4 and L5/S1 it is larger than TT.Conclusion: The biomechanical analysis of the 3D-FEM model of L3-S1 segments showed that in the six different physiological motion states,both of the CBT and TT screws can fix the motion unit well.However,they also have an impact on the intervertebral discs,vertebral bodies,facet joints,endplates and ROM of the adjacent segments.Compared with the TT screw,the CBT screw can suffer more stress to protect the facet joints and endplates in the inserted segment from degeneration,it can also reduce the range of motion improve the stability better in the inserted segment.But in the adjacent segments,comparing with TT screw,CBT screw has no advantage in the impact on facet joints,endplates,ROM,intervertebral discs,etc.The maximum stress of the CBT screw in some structures is similar to that of the TT screw,and in several structures the stress is even larger than TT. |
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