Study On Mechanical Behavior Of Degenerative Lumbar Intervertebral Disc After Nucleotomy

The lumbar intervertebral disc connects the upper and lower vertebrae of the lumbar spine,which has the function of transferring load and has a certain degree of mobility in different directions.Long-term mechanical load resulted in stress concentration,tissue water loss,proteoglycan content reduction and nucleus pulposus hardening.Under the action of external force,the annulus fibrosus(AF)is squeezed to bulge or even rupture,and the nucleus pulposus(NP)prolapses or flows out,which leads to lumbar intervertebral disc herniation.Nucleotomy is often used to treat patients who are ineffective in conservative treatment of lumbar disc herniation.Because of the degeneration of intervertebral disc and nucleotomy,its water content decreases,and the material properties,mechanical environment and load-bearing mode change.Therefore,it is necessary to study the biomechanical characteristics of lumbar intervertebral disc after degeneration and removal of nucleus pulposus,and provide theoretical basis for clinical treatment of lumbar disc herniation.The effects of degeneration and removal of nucleus pulposus on the mechanical properties of lumbar intervertebral disc was studied by mechanical experiments and finite element in this paper.Porcine lumbar intervertebral discs were used as study subjects.The discs after nucleotomy were treated with trypsin as the experimental group and normal discs as the control group.In order to ensure the normal restraint in the sagittal plane of the test samples in mechanical experiments and to restore the mechanical environment better,an experimental auxiliary device was designed and manufactured.Experimental group and the control group were applied compressive load at different magnitudes and different rates.The stress-strain properties,instantaneous elastic modulus and creep properties of intervertebral disc were obtained.The creep constitutive model was established.It is found that the relationship between stress and strain of the two groups showed a nonlinear with the increase of the loading rate.The strain of the experimental group was larger than that of the control group and increased obviously with the increase of the loading rate and the compressive load.The effects of load and loading rate on the instantaneous modulus of elasticity in the experimental group were greater than that in the control group.The viscoelastic properties of the experimental group were reduced by the decrease of the liquid content in the disc,the elasticity and the creep variable were increased.Three parameter solid model can be still used to describe the creep constitutive model and predict the creep behavior of lumbar intervertebral disc after nucleotomy.The radial strain distribution of lumbar intervertebral disc under axial compression load was studied by digital image correlation(DIC)technique.Experiments showed that the intervertebral discs of both groups expanded outward,and the extent of expansion in the posterior region was larger than that of the anterior region.In the experimental group,theouter region of the AF expanded outward,and the extent of expansion was greater than that of the control group;the inner AF expanded to the nucleus pulposus area,and the extent of expansion was less than that of the control group.The strain increases nonlinearly with the stress,and the strain growth rate in the experimental group is more obvious than that of the control group.A porous elastic finite element model of L3-L4 segment of lumbar spine was established by the ABAQUS software,and C3D10 MP pore element was used in the model.The creep response of lumbar intervertebral disc under compressive loads of different magnitude and combined action of different loads were studied.The simulation results show that Mises stress of the AF was greater than that of the nucleus pulposus under compressive load,and stress concentrates on the outer region of posterior AF.After removing the nucleus pulposus,stress concentrates on the inner region of posterior AF,and the value also increased.The axial creep curve of lumbar intervertebral disc increase exponentially.Creep strain and its growth rate increases after removal of nucleus pulposus,which is basically consistent with the experimental results.Under the combined action of compression and flexion,the posterior region of lumbar intervertebral disc was pulled while the anterior was compressed.Under the combined action of compression and extension,the posterior region of lumbar intervertebral disc was compressed while the anterior was pulled.Under the combined action of compression and left torsion,the left posterior region of lumbar intervertebral disc was compressed while the right anterior region was pulled.After removal of nucleus pulposus,the axial stress of compressive side increased significantly.At the beginning of creep,strain increases rapidly with time.After 1000 s,the creep curve tends to be flat.After removal of nucleus pulposus,the creep strain values in compression region increased significantly,while the creep strain values in tension region remained basically constant.