Finite Element Analysis Of Posterior Single Segment Fusion With Adjacent Segment Decompression And Double Segment Fusion In L4-S1 Segment

Objective L4-5 and L5-S1 segments are the most frequent sites of lumbar degenerative diseases.Posterior lumbar interbody fusion is the main surgical method to treat the disease.When L4-S1 double-segment spinal stenosis,the selection of fusion segments is still controversial.This study uses the finite element method to establish a normal L3-S1 segment model,and simulates the PLIF surgery to establish a posterior single-segment fusion(L4-5)+adjacent segment decompression(L5-S1)and double-segment fusion(L4-5、L5-S1)finite element model.Through the finite element analysis of the biomechanical stress changes of the small joint,intervertebral disc,endplate,nail bar and fusion cage after the two operations,the mechanical basis is provided for reducing the degeneration of the adjacent segment,reducing the failure of internal fixation and selecting the operation mode reasonably.Methods A healthy adult volunteer was selected to eliminate spine related diseases.Thin layer CT scanning was carried out on the L3-S1 segment.The obtained image data was imported into the 3D reconstruction software MICs 20.0 in DICOM format,and the 3D contour model of L3-S1 was obtained.After being exported in STL format,the 3D model was read into the Geomagic software,and the 3D model was polished,grid and other reverse engineering reconstruction was carried out,and finally,the solid 3D L3-S1 model was generated.Use the functions of lofting,stretching,resection and other functions in Solid Works to make intervertebral discs and assemble with the vertebral body to form a complete L3-S1 model.At the same time,use Solid Works software to build a three-dimensional model of Cage and nail rod.Three-dimensional models of screws and Cage were inserted into the L3-S1 model using Solid Works software according to the clinical operation.Three groups of models were established: normal L3-S1 lumbosacral model(group A),PLIF model of posterior single-segment fusion(L4-5)+ adjacent segment decompression(L5-S1)(group B)and PLIF model of posterior two-segment fusion(L4-5,L5-S1)(group C).Finally,each group ofmodels was constrained and loaded by ANSYS software to observe the stability of the three groups of models in six motion states,including forward flexion,backward extension,left and right lateral flexion and left and right rotation.By analyzing the stress of small joints and intervertebral disc,the influence of two PLIF models on adjacent segment degeneration was obtained.At the same time,the stress changes and distribution laws of interbody fusion cage,internal fixation device and end plate were measured and analyzed.Results In this experiment,the normal lumbar 3-sacral 1 segment finite element model was established,the model is effective.On this basis,the finite element model of posterior single and double segment interbody fusion was established and studied.The number of tetrahedron elements and nodes of three models are as follows:102448 and 193834 in group A,205880 and 365780 in group B,253172 and 433851 in group C.Under the six motion states of applying load,the activity of model L3-4 and L4-5 in group B and C was significantly lower than that of the normal model,the most significant is group C,while the activity of L5-S1 in group C was significantly lower than that of the normal model,the activity of L5-S1 in group B was not significantly higher than that of the normal model.In group B and C,the stress of the small joints in the adjacent segment(L3-4)of the head side of the model was significantly increased,especially in flexion,but there was no significant difference between group B and group C.The stress of the small joints in the decompression and non fusion segment(L5-S1)of group B was higher than that of the normal group A during forward flexion and left-hand movement.Intervertebral disc stress in adjacent segments(L3-4)of group B and C increased,group C was larger than group B,and intervertebral disc stress of decompression unfused segment(L5-S1)in group B increased slightly.Compared with the model of group A,the stress of the upper endplates of group B and group C increased significantly.The stress of group B and C models was mainly concentrated at the junction of the lower screw tail and the rod of the entire nail rod system.The internal fixation stress was greater during forward flexion,rotation and right flexion,group C was larger than group B,and the stress was the largest when flexing forward,located in the S1 screw and L5-S1 segmental fusion device of the group C model.Conclusion Both of them can achieve good stability.In both PLIF models,the stress of the facet joint and the intervertebral disc in the adjacent segment(L3-4)of the head sidewas increased,especially in the facet joint.There was little difference between the two PLIF models,suggesting that PLIF surgery may accelerate the degeneration of the facet joint and the intervertebral disc in the adjacent segment.In the PLIF model of single segment fusion +adjacent segment decompression,the facet joint of decompression and non fusion segment(L5-S1)only increased during flexion and left rotation,There was no significant increase in the stress of L5-S1 intervertebral disc,it is suggested that the preservation of bilateral facet joint is very important for the stability of the decompression segment and the reduction of ASD.In both PLIF models,the S1 screw and the L5-S1 segmental fusion device in the two-segment fusion model have greater stress,and fusion failure is prone to occur here.