| Lumbar disc herniation(LDH)is a very common clinical spinal pathology,generally due to degeneration of the disc that changes the thickness of the endplate or annulus fibrosus,resulting in the gradual extrusion of the nucleus pulposus into the fissure between the disc fibrous rings under prolonged mechanical loading,resulting in herniation.The rupture of a herniated lumbar disc is usually closely related to the stress concentration caused by internal stress-strain changes after lumbar disc degeneration,so an in-depth study of the stress and strain behavior of the lumbar disc is needed to provide a theoretical basis for subsequent treatment and prevention.In this study,the rupture mechanism of the lumbar disc is discussed from multiple perspectives through in vitro experiments,and the stress concentration of the lumbar disc during compression before and after the onset of mild injury is investigated by finite element simulation to infer the rupture mechanism of the lumbar disc.Fresh lumbar vertebrae of sheep slaughtered locally for 4-6 hours were selected,and the treated intact segments L4-5 were selected by macroscopic observation as samples for the experimental group,and the remaining segments were used as the validation group,and the prepared samples were divided into two groups: healthy and mildly ruptured discs.(1)The experiments were taken to obtain the properties by quasi-static loading before and after compressive fatigue loading.The experimental results showed that the experimentally obtained stress-strain curves all showed nonlinear properties.The Young’s modulus,yield stress,yield strain,and proportional limit of the healthy samples were increased after fatigue loading;the mildly ruptured discs were compressively fatigued and showed an increase in Young’s modulus,but the proportional limit,yield stress,and yield strain were decreased after fatigue.The stress-strain curves of healthy and mildly ruptured discs were fitted using the elastic principal model,and the results showed that the principal equation could fit the stress-strain curves of the discs well before and after fatigue.(2)The annulus fibrosus of L4-5 segment,which was dissected along the sagittal plane in parallel,was loaded quasi-statically before and after the compressive fatigue loading experiment,and digital image correlation techniques were used to observe and measure the displacement changes inside the disc during the compression process.Optical fibers were inserted into different positions of the L4-5 segment disc to measure the stress distribution inside the disc under quasi-static compression.The data results showed that the strains in the medial annulus fibrosus were significantly greater than the strains in the lateral annulus fibrosus during quasi-static loading,and that fatigue would have an effect on the distribution of internal strains in the disc under quasi-static vertical compression,but the overall trend and characteristics did not change before and after fatigue.The internal stress distribution in the lumbar disc under vertical loading was also not uniformly distributed.During loading,the stress was greatest at the posterior part,and the stress concentration caused the rupture and protrusion of the disc.(3)Reverse engineering was used to validate the results,and Ansys was used to build a finite element model of the lumbar intervertebral disc of L4-L5.A vertical load of 600 N was applied to the model to study the mechanical properties of the intervertebral disc under a vertical load of 600 N.The simulation result curves were compared with the experimental curves to verify the accuracy of the experimental results.Meanwhile,considering the effect of early rupture on disc rupture,a quasi-static simulation was performed after establishing a mildly ruptured lumbar disc model and applying the same load to verify the effect of mild rupture on the mechanical properties of the lumbar disc.The distribution of stress and strain of the disc under different conditions was analyzed. |
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