Experimental Study And Finite Element Simulation On Human Lower Lumbar Spine

With the development of modern society,people's living habits and ways of working change dramatically.Sedentariness makes the lumbar spine in a fixed long-term stress state,and some special industries could cause lumbar spinal fracture.These problems have led to an increasing number of patients and types of lumbar spine disease.The age of patients tends to be lower,and the categories of diseases are more complex,which need more research urgently.Human lumbar spine is the main weight-bearing structure in complicated stress conditions.Degeneration diseases are more likely to happen in this place,so the biomechanical analysis of lumbar spine should be paid more attention.The purpose of this research was to understand the biomechanical properties of lumbar spine,and to help orthopedic surgeons ward off disease by making more accurate decisions and reasonable options for operation procedures.This dissertation analyzed the biomechanical response of lumbar spine through theoretical,finite element(FE)and experimental analysis,and gave assistance to spinal clinical diagnosis.An anisotropic visco-hyperelastic constitutive model was developed for human ligaments based on continuum mechanics theory.The strain energy function was divided into a hyperelastic part and a viscoelastic part,which can be denoted by strain or strain rate invariants.Considering the anatomical structure and deformation feature of ligaments,the hyperelastic strain energy part could be characterized by the energy from matrix deformation and fiber elongation;the viscoelastic energy part was mainly provided by matrix and fiber.The proposed strain energy function contained six material parameters,which can be determined by quasi-static and medium strain rate experimental data.High strain rate results were predicted,and then the constitutive model was applied to five types of ligaments.The results indicated that the constitutive model could accurately reflect the mechanical properties of ligament under different strain rates.For FE analysis aspect,MIMICS was used to generate geometric surfaces of L3-L5 lumbar segments based on CT images.Then HYPERMESH was employed to establish according FE model.Finally,the biomechanical analysis of the lumbar segment was implemented in ABAQUS.User defined material subroutine UANISOHYPER_INV was introduced to define material properties of intervertebral disc and ligaments on ABAQUS platform.Threshold segmentation approach was imported to determine the material parameters of cortical and cancellous bone.After applying loads and boundary conditions,flexion,extension,lateral bending and torsion were simulated.The predicted value of range of motion(ROM)and intradiscal pressure were compared with in vitro experimental results to verify the present model.The human pelvis plays an important role in mechanism of lumbar compensation.In order to maintain sagittal balance,sacral slope angle changes.So based on previous research,three FE models with different sacral slope angles were developed.Then biomechanical responses of the three models were analyzed.The results showed that the ROM and stress distribution of the lower lumbar spine could be influenced by different sacral slope angles.Besides,by inserting a PEEK cage with screw-rod system fixation,an operation method was simulated on L3-L5 segment to treat low lumbar spine instability.The stress distribution and maximum value of vertebra,nucleus pulposus,endplate,annulus fibrosis,cage and pedicel screw-rod system with different cage locations were analyzed.For experimental study,human lumbar specimens were chosen to test.The change of ROMs within three models were measured,which were intact model,instability model with the articular capsule and more than 50% of the facet joint on one side of the L3-L4 dissected and unilateral pedicle screw fixation(UPSF)with TLIF model.At the same time,biomechanical responses of different cage location were also tested.Considering both experimental data and FE results,the 45° cage location was better than other two methods,which had lower stress value and might be beneficial to maintain sagittal and coronal plane balances.The FE model of the human lumbar spine in the present study can be used to investigate other lumbar diseases.It can provide a reference for clinical surgeon to understand the pathogenesis of lumbar degenerative diseases,and make suitable operative schemes and rehabilitation programs.The modeling method of ligament can be applied to other material with a fiber-reinforced character.This would develop a new method for investigating the mechanical properties of relative materials.