Stresses In The Vertebrae After Pedicle Screw Removal: A Finite Element Study

Objective:(1)To establish the limiting element model of the lumbar vertebral body of the spine(a.lumbar 2-4 model;b.lumbar 3 pedicle screw removed vertebral body and lumbar 4 normal vertebral body model)for clinical study.(2)To explore the ultimate stress of lumbar 3 and lumbar 4 vertebral bodies and the stress distribution in each region of the vertebral body removed by pedicle screws in different directions and under different load intensities.Methods:(1)An 18-year-old male patient(1.5 years ago for lumbar spine fracture with posterior lumbar percutaneous internal fixation)was selected,imaging data showed good fracture healing,bone density examination suggested normal bone volume,signed informed consent after internal fixation removal,CT plain scan(T12-L5)was performed,image raw data(DICOM format)was obtained,and image data were imported into DICOM format MIMICS software.The L2-L4 vertebral body cortical bone threshold was selected and the L2-L4 vertebral body hollow shell model was constructed,after which the model was imported into Geomagic for smooth and perfect repair,Soliderworks for assembly and material assignment.The contact relationship between the nail tract,cortical bone,cancellous bone,and endplate was set as a binding relationship,and the face-to-face contact between the fibrous ring,nucleus pulposus,and endplate was defined as a face-to-face contact,and the friction coefficient was set to 0.1.The paravertebral ligament was simulated with a nonlinear material spring.The unitary simulation was performed by applying mechanical loads using Abaqus to finally form a complete finite element model.A single lumbar 3(pedicle screw removal)and a single lumbar 4(normal vertebral body)finite element model were constructed in the same way and tested for ultimate stresses.(2)The test was divided into three groups,group Ⅰ: fixing the lower surface of L4 vertebra,applying a load of 500 N of 2/3 of human body weight uniformly on the upper surface of L2 vertebra axially,and at the same time applying a bending moment of 7.5N-m on this basis to simulate five motion states of vertebra axial compression,forward flexion,back extension,lateral flexion and rotation,etc.Group Ⅱ:applying a load of 600 N uniformly on the upper surface of L2 vertebra,and at the same time applying a bending moment of 7.5N-m on this basis Group Ⅱ: A load of 600 N was applied uniformly on the upper surface of L2 vertebra,and a bending moment of 7.5N-m was also applied on this basis to simulate five kinds of motion states such as axial compression,forward flexion,back extension,lateral flexion and rotation of the vertebra,Group Ⅲ: A load of 700 N was applied uniformly on the upper surface of L2 vertebra,and a bending moment of 7.5N-m was also applied on this basis to simulate five kinds of motion states such as axial compression,forward flexion,back extension,lateral flexion and rotation of the vertebra.The L2 vertebrae were fixed and divided into 7 zones by nail tract: left nail tract lateral zone(A zone),left nail tract zone(B zone),anterolateral zone(C zone),middle zone(D zone),posterior zone(E zone),right nail tract zone(F zone),and right nail tract lateral zone(G zone).The stress values of each of the 7 zones in the three groups were compared.Results:(1)The ultimate load of lumbar 3 vertebral body cortical bone was 6.53 MPa,the ultimate load of cancellous bone was 7.16 MPa,and the ultimate load of nail tract was 28.00 MPa;the ultimate load of lumbar 4 vertebral body cortical bone was 6.32 MPa,and the ultimate load of cancellous bone was 16.35 MPa.(2)When the axial load was applied,the pressure value of the posterior lateral zone(E zone)increased with the increase of the load value.(2)When the axial load was applied,the pressure value in the posterior region(E region)increased the most with the increase of the load value;under the same load,the bilateral nail path regions(B and F regions)were the main pressure bearing regions.When the forward bending load is applied,the increase of pressure value in the posterior side area(E area)is the largest with the increase of load value;under the same load condition,the bilateral nail path area(B and F area)is the main pressure bearing area.When post-extension load is applied,the pressure increase in the posterior lateral area(E area)is the largest as the load value increases;under the same load,the bilateral pegging area(B and F area)is the main pressure-bearing area.When the lateral flexural stress load is applied,the pressure in the middle zone(D area)increases the most as the load value increases;under the same load,the left side nail path area(B area)is the main pressure-bearing area.When the rotating stress load is applied,the pressure increase in the middle zone(D area)is the largest as the load value increases;under the same load,the bilateral nail path area(B and F area)is the main pressure-bearing area.Regardless of the direction of the applied load,the nail path area is the main pressure bearing point;under the same load posterior extension,the nail path area has the largest force,and the nail path area has the smallest force in forward flexion;with the increase in load,the middle area and the posterior area are the areas with the largest increase in pressure value.Conclusion: 1.The mechanical strength of the pedicle nail pathway after pedicle screw removal is sufficient to support the axial load in the physiological state of the patient,and the presence of the pedicle nail pathway increases the axial load resistance of the vertebral body,so the patient can move down early after the pedicle screw removal.2.According to the results of finite element analysis,the nail pathway area is the main stress point regardless of the direction of the applied load,and the nail pathway area is the most stressed in posterior extension and the least stressed in anterior flexion.Therefore,patients with internal fixation should do less posterior extension movement.