Non-linear Finite Element Analysis For The Prediction Of Human Lumbar Strength And Its Application In Related Lumbar Diseases

In industrialized countries,70%of the population experienced low backpain once in their lives. The main cause of low back pain was lumbar degeneration.It was shown in epidemiologic and biomechanical studies that mechanical fatorsare quite important in lumar degernerative disease. Finite element method, asan effective alternative for in vitro biomechanical studies, has been usedextensively for the study of lumbar spine. However, there were still someproblems that should be solved. One of the problems was that the materialproperties and the geometry of the soft tissue were oversimplificated. Theconstitutive relation of the degenerative soft tissue was difficult todetermine. The aims of the present study were to develop a finite element modelof a lumbar motion segment which contained3D soft tissue models and complexmaterial properties. It was then modified to simulate three different scenariosof posterior lumbar interbody fusion (PLIF). Biomechanical evaluation of thethree surgical scenarios was performed. The models were also used to simulatedifferent grades of lumbar degeneration and investigate the effects ofdegenerative diseases on the mechanical behaviors of lumbar segments. Thisstudy composed of three sections:In the first section, a L4-L5lumbar segment model was established with3D solid tissues and validated in details. The L4-L5lumbar spinal segment modelwas established with MIMICS and ABAQUS softwares. The hyper-elastic material properties were assigned to all the surrounding tissues.10Nm pure momentswere applied to the upper vertebral body under the loading conditions ofextension, flexion, lateral bending and torsion, respectively. The intradiscalpressure and range of motion of the simulated results were compared with invitro experimental results to verify the finite element model. The simulatedresults were also compared with previous results of other finite element models.The results of comparisons showed that:1) the simulated ranges of motions werein a good agreement with the in vitro experimental data;2) Compared with thefinite element model with ligaments established by cable and spring elements,our model can more effectively reflect the actual mechanical behaviors of lumbarsegments.In the second section, a three-dimensional nonlinear L1-S1finite elementmodel (intact model) with the ligaments of solid elements was established. Thenthe model was modified to simulate three scenarios of PLIF.10Nm moments with400N preload were applied to the upper L1vertebral body under the loadingconditions of extension, flexion, lateral bending and torsion, respectively.The comparison among the intact model and three surgical models showed that:The lowest stresses on the bone grafts and the greatest stresses on endplatewere found in the model which used cages made of Ti. So this model was inferiorto the other two models. Both the models used cages made of PEEK (PCP) and themodel used autogenous iliac bone (PAIB) had their own relative merits.The PCPmodel obtained considerable stresses on the bone grafts and less stresses onligaments. But the changes of stresses on the adjacent discs and endplate wereminimal in the PAIB model. The findings provide theoretical basis for the choiceof a suitable surgical scenario for different patients.In the third section,eight L2-L3lumbar spinal segments with differentgrades of disc degeneration (healthy, mild, moderate, and severe) were builtto evaluate the effects of lumbar degenerative diseases on the mechanicalbehaviors of lumbar segments. The degenerative grading system covered threemain radiographic signs of disc degeneration: height loss, osteophyte formationand diffuse sclerosis. According to the previously described grading systemvertebral body and soft tissues were assigned different material properties. The range of motion, stresses on ligaments and facet compliant cartilage layers(FCCL), and the fiber strains were investigated under an axial compression loadof500N together with moments of7.5Nm,10Nm,15Nm. The comparison ofdifferent grades of degenerative models showed that: the average stresses onthe posterior ligaments were highest in the mildly degenerated model underflexion, but highest in the moderate degenerated model under lateral bendingand torsion. The average stresses on the FCCL increased with the increasingdegeneration and moments. With increasing degeneration, the region of highstresses was more concentrated. The maximum principle strains on the fibersdecreased with the increasing degeneration under extension and flexion. As themoments increased, the maximum principle strains on the fibers increased withthe increasing degeneration under lateral bending and torsion.The simulated results of our study provided theoretical basis for clinicalstaff to have a better understanding of the pathogenesis of lumbar degenerativediseases for more suitable treatment scheme. The model established in this studycan be used as the basis for further research. The modeling methods of softtissue can also be applied to other body parts such as knee joint, ankle jointand shoulder joint, in which the soft tissues are important for the functionsof joint. These would be a new perspective for clinics to understand the roleand pathogenesis of these soft tissues in the joints.