Research On Strain Rate-Dependent Mechanical Behavior Of The Intervertebral Discunder Flexion

In recent years,with the change of human work and lifestyle,the incidence of low back pain is gradually increasing,while the age of onset is decreasing.This disease seriously affects the normal work and life of patients,and causes great economic and mental burdens to individuals and families.The most closely related cause of low back pain is age-related intervertebral disc degeneration,which accounts for about 40% of its etiology.The main function of the intervertebral disc is to transmit and absorb mechanical loads,while maintaining the lumbar motion segments to achieve compression,extension,flexion,extension,lateral bending and rotation.The IVD would be vulnerable to failure and degeneration when the above function is completed.Both mechanics and biology are considered to be the main causes of intervertebral disc degeneration,while an increasing number of scholars believe that cellular physiology is strongly influenced by mechanical loading.Therefore,it is necessary to analyze the underlying mechanisms of mechanical influences in intervertebral disc degeneration.In this study,the strain rate-dependent mechanical behavior of the intervertebral disc under flexion was investigated,while the effect of degeneration on its viscoelastic mechanical behavior was also explored.Mechanical tests on healthy intervertebral discs: The test samples were divided into 3 groups.In group a,the test samples were used to test the mechanical behavior of the intervertebral disc rupture at different strain rates.In group b,the test samples were used to test the mechanical behavior of fatigue injury on the intervertebral disc under different strain rates.In group c,the test samples were used to test the internal displacement distribution of the intervertebral disc at different strain rates.The results as follows:(1)The cracking phenomenon occurred at fast and medium loading rates,but not at slow loading rates.In addition,the stress-strain curve of the intervertebral disc shows different multi-segment features at different strain rates.(2)The yield stress,compressive strength and Young’s modulus all increase with the increase of the strain rate,and the yield strain decreases with the increase of the strain rate.The logarithm of the Young’s modulus in the intervertebral disc is approximately linear with the logarithm of the strain rate under different strain rates.(3)The intervertebral disc showed a significant rate correlation during before and after fatigue loading.In addition,the stiffness of the loading and unloading curves is inconsistent,forming a hysteresis loop,which is caused by the viscoelastic effect.(4)The strain rate has no significant effect on the axial displacement distribution inside the intervertebral disc.(5)Based on the experimental data,the constitutive relationship of the intervertebral disc at different strain rates is obtained.The fitting curve is well coupled with the experimental curve,and the fitting parameters are approximately linear with the logarithm of the strain rate.Mechanical tests on degenerated discs: Degenerative bovine lumbar IVD samples were made by different concentrations of trypsin solution,and quasi-static loading unloading tests were carried out on the test samples.In addition,the digital image correlation technique was used to test the internal displacement distribution,while He staining was used to observe the pathological changes of the IVD.The area of the hysteresis loop formed by the loading-unloading curve decreased with increasing strain rate.There was an approximately linear relationship between the log of elastic modulus and the log of strain rates,while trypsin injection did not change the slope of the linear relationship curve between the two parameters.In addition,degeneration significantly affects the radial displacement distribution in the posterior annulus fibrosus.