Finite Element Analysis Of The Effects Of Spondylolysis Of L5 On The Biomechanical Behavior Of The Lumbar Spine

Background:Lumbar spondylolysis is more common in spinal disease and have a higher prevalence in adolescents and adults, especially in athletes. According to statistics, about 50% of patients with low back pain in athletes are correlated to spondylolysis.L5 isthmus is the site most likely to fracture because of the special anatomy of the lower lumbar.There are still debate on the etiology of spondylolysis which many scholars have been trying to explore.Currently the finite element analysis of lumbar spondylysis is more focused on the etiology and mechanism of spondylolisthesis.Although there are reports about the stability of spondylolysis on the biomechanical behavior of the lumbar spine,but most remain in the study of specimens.Objects:To investigate the effects of pars defect at L5 on the biomechanical behavior of the lumbar spinal motion segment,an anatomic detailed finite element model of the lower lumbosacral spine and three finite element models of lower lumbosacral spine with a pars defect at L5 were developed.Materials and Methods:The finite element model was constructed by three dimensional digital medical modeling method using MIMICS and ABAQUS software based on lumbar spine CT images of a young healthy man.This model was validated by comparing the average stiffness tested in flexion,extension,lateral bending and torsion loading under physiological loading with the corresponding in vitro results published by other scholars.Three finite element models of lower lumbosacral spine with a pars defect at L5 was established by cutting of the isthmus of L5 and replacing the isthmics by fibrous tissue material.Uniaxial forces with a 500N axial compression was applied to the surperior surface of L4;torques for flexion,extension,left lateral bending and left axial rotation with a 10N?m was applied to the finite element model.The biomechanical parameters of two models such as the angle-displacement of lumbar spine,Von Mises stresses in the annulus fibrosus,intradiscal-pressure in the nucleus pulposus,the forces in the facet joints and isthmics were measured.Results:A finite element model of youg normal lower lumbosacral spine has been developed and validated.All of average stiffness were within or even very close to the results of other scholars. This proves that the method of constructing the finite element model and material properties used in the model were effective which can be used for biomechanical Analysis. A finite element model of lower lumbosacral spine with a pars defect at L5 were developed. According to established finite element models, the angle-displacement of lumbar spine increased in the model of spondylolysis comparing with the model without pars defect; both stress in the annulus fibrosus and intradiscal-pressure in the nucleus pulposus increased in the L4/L5 and L5/S1 disk of L5 vertebra with a pars defect,especially L5/S1 disk,comparing with the model without a pars defect; The stress on the facet of the both superior articular processes of the L5 vertebrae increased comparing with the model without pars defect and the stress on the facet of the both inferior articular processes of the L5 vertebrae decreased comparing with the model without pars defect; The pars interarticularis had higher stress under flexion,extension,lateral bending and axial rotation postures than standing position,which had the maximum stress in the rear protraction.Conclusions:The accurately anatomical finite element models of the lower lumbosacral spine including normal and spondylolysis have been developed. The normal model was validated by comparing with the corresponding in vitro results published by many other scholars.It was convincing. From the analysis of the finite element model of youg normal lower lumbosacral,the lang-term hazard stress of pars interarticularis can cause fatigue fracture. From the analysis of the two finite element models,the angle-displacement of lumbar spine increased after the isthmus crack, this will result in lumbar spine instability; both stress in the annulus fibrosus and intradiscal-pressure in the nucleus pulposus increased in the upper and lower disk of the vertebra after the isthmus crack; The stress on the facet of the both superior articular processes of the vertebrae increased after the isthmus cracked,but the stress on the facet of the both inferior articular processes of the vertebrae decreased because of pars broken.The lower disk exist high stress zone.,which can accelerate the degeneration of intervertebral disc.