| Objective:1.To explore the methods of applying finite element analysis to establish a normal spine simulation model and to verify it with animal models in other literatures,in order to determine the effectiveness of finite element analysis for spinal pathologies and to provide a research basis for subsequent studies on biomechanical alterations of the spine after degenerative spine and disc shaping.2.To explore the biomechanical changes of spinal mobility after cement application on the disc,and the effects of cement application at different doses on the adjacent endplate,nerve root stress,so as to explore the indirect decompression effect of percutaneous discoplasty.3.To explore the distribution type of vacuum phenomenon in degenerative spinal lesions,the correlation between vacuum phenomenon and vertebral disc height narrowing and MRI grading severity of vertebral disc degeneration,and whether there is a significant correlation between intraver-tebral disc vacuum phenomenon and the progression of vertebral disc degene-ration and lumbar spinal stenosis.Methods:1.L1-S1 3D geometry model buildingA 30-year-old healthy young male volunteer with no previous history of lumbar trauma and disease was selected.X-rays excluded other spinal disorders,and consecutive scans of the volunteers’ whole spine were performed(matrix 512 × 512,both layer distance and layer thickness were0.625mm).A tomography scan along the transverse aspect of the lumbar 1vertebral body with transaxial spiral CT was used to export its tomographic images in DICOM(digital imaging and communications in medicine)format and transferred to computer for saving.2.Establishment of the digital modelEstablishment of 3D models of vertebral bodies and ligaments: the data obtained from the CT scan were transferred into a computer in DICOM format,and with the aid of the software mimics11.1,a geometric model of the vertebral body and ligaments of the lumbar spine was established,and subsequently,the model was entered into Pro/E software and Pro/E software was applied for model geometry processing.Vertebral sections because of the low content of cancellous bone within the pedicles and spinous processes,we set that cancellous bone mainly existed in the vertebral bodies and then simulated a 1-mm-thick cortex on the vertebral model The ligaments were scaffolding based on the scanning data.The ANSA software was used to build the finite element mesh model of the normal lumbar spine structure in combination with ABAQUS to define the connections,boundary conditions,applied loads,etc.,and all the pre-treatment contents were completed in ANSA.3.Finite element model buildingStudies analyzed material properties of each tissue structure and implanted devices in L1-S1 spinal segments,and corresponding data from previous studies,to deduce the actual biological structure based on computational data and numerical models of the lumbar spine.The corresponding parameters of cortical bone,cancellous bone,cartilage final edition,posterior structures(vertebral arch,transverse process,spinous process),annulus fibrosus,nucleus pulposus,various ligaments(anterior and posterior longitudinal ligaments,supraspinous ligaments,interspinous ligaments,ligament flavum,ligament of joint capsule and intertrochanteric ligaments)were evaluated for relevant mechanical simulation.These ligaments were set as trusselements that were affected only by tensile loading.Based on the bioanatomical features,the ligamentous connection sites in the model were connected to the ligamentous connection points using ANSA software,and a nonlinear TRUS unit(without compression)was built to simulate the ligamentous structure considering the biomechanical structural state of the ligaments.Material properties of each tissue structure and implant device in L1-S1 spinal segments,as well as corresponding data,were analyzed in this experimental study.The computational model was established using NASA software.The models were mainly locally encrypted using tetrahedral units,hexahedral units,shell units(cortical bone),trusses units(ligaments),and partial transition units to ensure computational accuracy and computa-tional speed.Discs(containing nucleus pulposus and annulus fibrosus),as well as endplates,were integrated with augmented hourglass control for three-dimensional hexahedral subtraction c3d8 r.The hexahedral unit adopted the c3d8 r haplotype,the tetrahedral shell unit adopted the S4 R haplotype,the tetrahedral unit adopted the c3d4 unit,and the trusses unit adopted the t3d2 unit,accounting for a total of 504757 solid units,108229 shell units,165 trusses units.Hexahedral meshes were adopted because,in the same order,they have higher accuracy than triangular shell meshes than tetrahedral meshes and are less computationally expensive.A simplified integration cell is adopted because this cell type can effectively reduce the generation of cells.4.Model validity validationL4/5 ROM was calculated based on a normal control spinal model according to the aforementioned loads and restraints,and the results obtained were compared with a study by Yamamoto et al to verify model validity.5.Establishment of experimental modelDegeneration of the lumbar spine is usually manifested as a decrease in disc height,and the same model building method was used to establish 3D digital models for patients with decreased disc height and cement injection.The main established models were as follows: normal lumbar standards(baseline model,N),intervertebral disc degeneration group(L4/5 disc degeneration,taking the facet joint as the axis degeneration approximate 5 degrees,the intervertebral foramen height decreases,at the same time,the disc nucleus pulposus cavitation,degenerative height,H),intervertebral disc injection of bone cement 3 ml group H1(for L4/5 disc nucleus pulposus cavitation site,bone cement injection,1/2H increase of intervertebral height,H1=H+1/2h,H1),The intervertebral discs were infused with 5 ml of bone cement in group H2(cement was injected into the site of nucleus pulposus cavitation of L4/5discs,and the intervertebral height increased H,H2 =H+H,H2).6.Key observational indicatorsLumbar relative mobility(ROM)was expressed in terms of L4/5segmental angular displacement.Cartilage endplate,cement,nerve root stress,the maximum stress value in each motion state were observed from the stress cloud plot.Among them,neural tissue equivalent model building was performed and can be regarded as a stress sensor because the neural model is too complex,with the aim of extracting the trending effects that the variation of intervertebral foramen in various groups’ protocols brings to the neural tissue here.7.A retrospective review was performed of 85 patients with lumbar spinal stenosis presenting with vacuum phenomenon,evaluating five discs in each patient,CT to assess the presence,shape and location of vacuum phenomenon,and digital plain radiographs to assess the disc height index(DHI).MR assesses the grading of disc degeneration,graded from grade I to V according to pfirman-s.To grade the degree of lumbar spinal canal stenosis,advanced stages of IVD degeneration,grades IV and V,were utilized.The grading was based on the morphological appearance of the dural sac on T2-weighted axial imaging,specifically the cerebrospinal fluid/root ratio,and ranged from A to D.Severe stenosis was defined as grades C and D.Results:1.Verification of model validity of the finite element model:the finite element model segmental mobility was 7.52° in flexion,5.45° in extension,4.034° in lateral flexion,and 3.105° in unilateral rotation.The ROM of L4/5segments of the normal spine finite element model in the present study under different motion states was consistent with data in the literature,which proves that the finite element model obtained in this study has validity under certain conditions.2.Relative activity of lumbar spine under different working conditions:the ROM of L4/5 segment of each model under different working conditions are shown,and the ROM values of all the motion states of each finite element model are ordered as H2,H1,H,and N from small to large.Among them,ROM of H2 was the closest to H1 and ROM of H2 was the smallest among the three groups of surgical models.The degrees of freedom of rotation in all directions decreased in the present situation of degeneration: the more flexion and extension were about 95% of the normal benchmark,the more flexion and extension were about 88% of the normal benchmark,and the right and left flexion was about 89% of the normal benchmark.Freedom of rotation in all directions after cement infusion into H1 was significantly reduced: the forward flexion extension was about 62% of the normal benchmark,the left-right rotation was about 47% of the normal benchmark,and the left-right flexion was about 50% of the normal benchmark.The freedom of rotation in all directions after cement injection into H2 was significantly reduced: the forward flexion extension was about 63% of the normal benchmark,the left-right rotation was about 45% of the normal benchmark,and the right and left flexion was about 49% of the normal benchmark Normal group(baseline).3.Cartilage endplate,cement,nerve root stress peaks: the cartilage endplate,cement,nerve root stress peaks of each finite element model are shown,cartilage endplate stress peaks: the normal group(baseline,N),after degeneration changes little,after injection of cement,the cartilage endplate stress increased significantly.In both cement data sets,the H1 model had the greatest endplate stress during extension,left-right rotation,and left-right scoliosis at 293%,299%,202%,240%,and 315% of the baseline,respectively.Cartilage endplate stress was the greatest during flexion in the H2 model with a relative ratio of 460%,and the cartilage endplate stress in the other directions was slightly reduced in the H2 model compared with the H1 model.After degeneration in each finite element model due to the decreased intervertebral height and compression on the surrounding tissues,the nerve root stress of group H was the largest,the nerve root stress of group H2 was the smallest,flexion was 90% of the normal benchmark,extension was 44%,left bending was 25% of the normal benchmark,right bending was 56% of the normal benchmark,left rotation was 56% of the normal benchmark,right rotation was 51% of the normal benchmark,and the differences of cement stress were not significant.Nerve tissue showed obvious tendency of compression compared with the normal model when rotating in all directions,and the stress of nerve root was reduced after injecting cement.The greater the amount of cement,the higher the intervertebral height,and the trend of nerve root stress decreased.The peak of cement stress in the two groups changed little,and the trend was not significant.4.The results showed that DHI was significantly higher in the vacuum phenomenon negative group than in the insular group(P<0.01),but no significant differences were found for linear and spot types.Among the vacuum phenomenon positive discs,DHI was highest in point vacuum phenomenon,lower in linear vacuum phenomenon,and lowest in island vacuum phenomenon.Furthermore,DHI was significantly lower in the insula group than in the other groups(P<0.01).The study found a significant association between the shape of the vacuum phenomenon and MRI grade of disc degeneration.Additionally,the distribution of intradiscal vacuum phenomenon was significantly correlated with MRI grade(Fisher’s exact test,P<0.0001).Of all discs analyzed by MRI,42.8%(182)were in advanced stages of degeneration(Pfirrmann grades IV and V).Vacuum type and spinal stenosis classification showed that the proportions of grade C and D in the island group were 15.79% and 65.79%,that of grade C and D in the linear group were15.05% and 54.84%,that of grade C and D in the dotted group were 17.58%and 37.50%,and that of grade C and D in the VP negative group were 18.29%and 14.63%,respectively.The type of vacuum phenomenon and classification of spinal stenosis generally showed a trend that the more obvious the vacuum phenomenon of intervertebral disc was,the more serious the classification of spinal stenosis was.Correlation between type and distribution of vacuum phenomena and lumbar spinal stenosis revealed a significant correlation between shape type of vacuum phenomena and MRI grading of spinal stenosis(P<0.001).Conclusions:1.The difference between L4/5 segment mobility in different movement states of finite element model of normal spine and the literature is consistent,and there is no significant difference when the prediction results of the model are compared with the available measurements under simple loading conditions,which considers the model valid.2.The mobility of lumbar vertebrae was reduced after disc degeneration,and the mobility of surgical segments was further reduced after discoplasty,but the amount of cement injection had no obvious effect on spinal mobility,and the cartilage end plate stress became smaller with the increase of cement amount.3.The nerve root stress increased after disc degeneration,and decreased after the height of bone cement injected into the intervertebral space was restored,the effect of indirect decompression was significant,and the nerve root stress decreased after the dose of bone cement injected increased.4.The study found that vacuum phenomenon is commonly observed in degenerative spondylopathies.Additionally,the type and distribution of vacuum phenomenon within the disc showed significant correlation with the MRI graded severity of disc degeneration and disc height narrowing.Overall,the presence of intradiscal vacuum phenomenon is strongly associated with disc degeneration and the progression of lumbar spinal stenosis. |
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