Study Of Nonlinear Viscoelastic Mechanical Properties Of Healthy And Degenerated Intervertebral Discs

Low back pain(LBP)is a very common clinical disease,which is usually caused by intervertebral disc degeneration.This disease has affected people’s work and life for a long time.Disc degeneration is a complex multifactorial process,and the tissue structure and mechanical properties of the disc are greatly altered after degeneration.Therefore,it is necessary to in-depth study of the difference between the non-linear viscoelastic properties of healthy and degenerated intervertebral discs in order to provide a theoretical basis for the clinical treatment of degenerative disc diseases.The nonlinear viscoelastic mechanical properties of healthy and degenerated intervertebral discs were investigated from both mechanical experiments and finite element simulations.The nonlinear viscoelastic properties of the intervertebral disc under tensile/compression loading were investigated by quasi-static loading,digital images,and fiber-optic grating techniques.The results showed that the elastic modulus of the intervertebral disc increased with increasing strain rate,with a significant strain rate dependence.At different loading rates,the tensile stiffness is one order of magnitude smaller than the compression stiffness,showing obvious tension-compression nonlinearity.Under tensile/compressive loading,the axial displacement distribution of the anterior and posterior fibrous rings of the disc remained basically the same,both being superficial AF>medial AF>deep AF.The radial displacement in the annulus fibrosus shows an opposite distribution under compressive loading,which is consistent with the stress distribution under compressive loading.Based on the experimental data,the viscoelastic constitutive relationship of the intervertebral disc at different strain rates is obtained.The fitting curves are well coupled with the experimental data.In the experiment of degenerative intervertebral disc,the oxtail vertebra was used as the experimental sample,and the degeneration model was made by injecting trypsin solution to explore the viscoelastic mechanical properties of the intervertebral disc before and after the degeneration.Firstly,the loading and unloading curves of intervertebral discs before and after degeneration at different strain rates were compared to analyze the changes of their apparent viscoelastic mechanical properties;The internal stress/strain distribution of the disc before and after degeneration was then tested by combining digital image technology and fiber grating technology.The results show that the modulus of elasticity and peak stress of the degenerated disc are significantly reduced,with the modulus of elasticity dropping to 50% of the normal value and the peak stress decreasing by about 55%;Degeneration will not change the distribution of the overall internal displacement of the intervertebral disc,but has a greater impact on the superficial and middle AF;The internal pressure in the center of the nucleus pulposus decreases,and the stress in the outer AF increases after degeneration.The finite element simulation was based on the CT data of the lumbar spine of the sheep,and the simulations were performed using ANSYS software to assign material properties to each part of the structure and apply load constraints to obtain the deformation,displacement,and stress-strain distribution of the intervertebral disc segments under vertical compressive loading.It was found that when the lumbar spine was subjected to vertical load,the entire disc segment was slightly deformed in retroversion,and the displacement field was distributed in an arc,with the maximum displacement occurring at the dorsal spinous process.The stress in the fiber ring is greater than that in the medullary nucleus,and the stress is concentrated in the dorsal outer layer as well as in the mid-layer region.