Ratcheting Behavior Of Intervertebral Discs Under Cyclic Compression

Lumbar disc degeneration was the pathological basis of lumbar disc herniation and low back pain.The causes of human disc degeneration were complex,such as genetic,nutritional,aging,trauma and biomechanical history.Much research has been addressed to investigate the biomechanical properties of intervertebral disc,such as compression,stretching and bending,but the research on ratcheting behavior was relatively lacking.Ratcheting behavior was a kind of phenomenon that the material deforms and accumulates continuously due to the cyclic load,which exists in many materials.The human body was in the mechanical environment of cyclic load in daily activities.It is of great significance to study the ratcheting behavior of intervertebral disc to reveal the process of disc degeneration.In this paper,the ratcheting behavior of lumbar disc under cyclic compression was studied by experiment and finite element simulation.1.The ratcheting behavior of the lumbar intervertebral disc was studied by the cyclic compression experiment of triangular waveform on L₁-L₇ of the sheep lumbar spine sample.The effects of the peak stress,the stress rate and the segments on the ratcheting behavior were discussed.The results showed that ratcheting strain evolution of lumbar disc includes two stages:Transient and quasi steady.Ratcheting strain and ratcheting strain rate increased with the increase of stress peak（R=0.962,P=0.004）,and decreased with the increase of stress rate（R=-0.876,P=0.019）.The compression stiffness increases with the increase of stress rate（R=0.964,P=0.004）or stress peak（R=0.838,P=0.037）.Compared with L_5-6,the ratcheting strain of L_6-7 was smaller（P=0.04）,indicating that the lumbar disc in this segment was more resistant to ratcheting.2.Based on the ratcheting prediction constitutive model URM,by adding the material coefficient k=0.01,the constitutive equation which can predict the ratchet strain evolution of lumbar intervertebral disc was established,and the predicted results were in good agreement with the experimental data.3.Based on imaging and anatomy,the finite element model L_4-5 of lumbar intervertebral disc was established.The ratcheting behavior of L_4-5 was simulated under the condition of high frequency cyclic compression.The simulation results were in accordance with the experimental results,which verified the validity of the model.The results showed that the internal pressure of high frequency cyclic compression increased with the increase of the number of cycles.4.The ratcheting behavior of healthy and degenerative lumbar disc under the condition of low frequency cyclic compression was simulated,and the parameters such as displacement,internal pressure and total stress were analyzed.The results showed that the displacement and effective stress of lumbar disc increase with the number of turns,while the pore pressure,internal pressure and total stress decrease with the number of turns.By comparing healthy and degenerative lumbar discs,it is found that the stress of degenerative lumbar disc is mainly distributed in the position near the abdomen of the inner fiber ring,while the stress of healthy lumbar disc is mainly distributed in the central position of the nucleus pulposus.