The Evolution And The Related Biomechanical Mechanism Of Degenerative Lumbar Scoliosis Via Finite Element Analysis

Objective: To investigate the evolution and the related biomechanical mechanism ofdegenerative lumbar scoliosis via finite element analysis.Methods: To acquire image data of the lumbar by CT scan, then to utilize Simpleware toconstruct the healthy lumbar model, the degenerative lumbar model, the lumbar scoliosissimulation model and the degenerative lumbar scoliosis model.Given800N axial load or800N axial load combined with10N.m moment on the superior end plate of L1vertebrae,to observe the stress distribution and load on the part of the lumbar,then to seewheather there is a progress that load or stress is transferred from the anterior column tothe posterior structures.By contrast,we identify the difference of the stress distributionand load transmission of the lumbar to understand the biomechanical mechanism of thedegeneration lumbar scoliosis.Through the method, we can provide a scientific basis forthe prevention and surgical treatment of clinical degenerative lumbar scoliosis.Results:(1) The nucleus pulposus play an important role of bearing weight in healthylumbar due to high intervertebral disc pressure which can transfer axial load to the centerof the adjacent endplate and vertebrae, the contact froce of the facet joints is low. Thepressure stress in the nucleus pulposus is high,but the von mises stress is very low inannulus fibrosus or facet joints.(2) With the disc degeneration, the character of lumbarloading changed, axial load is transmitted mainly by the annulus fibrosus and is inclined tobe transfered to the rear of the vertebrae, outer edge of the vertebrae and the facet joints.Stress increases in the posterior structures obviously,and pressure stress decreases in thenucleus pulposus followed by the decline in weight-bearing capability and the endpatebearing less weight. The stress is much higher in annulus fibrosus, outer edge of thevertebrae,facet joints, but lower in nucleus pulposus. So the endplate and the intervertebraldisc bear less weight and the contact force of facet joints increases.(3) After the formationof degenerative lumbar scoliosis, loading decreases in the anterior column,and the stressincreases in the posterior structures such as facet joints.The deformation of the facet jointsdue to compression keep itself out of capability of limitting rotation and blocking listhesis.Conclusion: The evolution of the degenerative lumbar scoliosis began with lumbar discasymmetric degeneration, stress is the initiatial and determined factors. In the health, thenucleus pulposus play an important role in weight bearing, transfering the most axial loadto the central endplate, while the remaining load is transfered by the annulus fibrous.Theasymmetric degeneration in the disc caused stress concentration,fiber cross-link broken, annulus ground substance changed,and produced a lot of cells apoptosis.So the asymmetricdisc degeneration makes it similar to the solid biomechanical behavior. In degeneration,the nucleus pulposus is dehydrated, its loading capability declines and the main load istransferred to the peripheral annulus fibrous.So the spinal loading site is transfered to theouter edge of the vertebrae and posterior structures. With the eccentric compressive stress,the vertebral wedging deformation appears on the concave side and facet compressivedeformation appears on the convex side, and it decreases in the effect of the facet joints ofthe convex side to limit the vertebral rotation and listhesis due to the shear forces act whenloaded, the vertebrae had rotatory subluxation and translation to the convex side.So thespine was imbalanced by asymmetric load, and the weight bearing line of force would beoffsetted by lumbar itself. The facet joints was in subluxation and translation with seriousimbalance due to the shear forces, the destructed disc has exacerbated severely asymmetricdeformity of the lumbar structures in the continuous progress of the degenerative lumbarscoliosis. Lumbar imbalance would be offsetted to reconstruct the lumbar balance byvertebral hyperplasia deformation,vertebral rotatory dislocation and decompensation of theinferior segmental reverse scoliosis.Therefore, the vicious cycle of " stressconcentration,asymmetric degeneration,imbalanced loading,stress injury, structuraldeformation of lumbar spine, lumbar balance reconstruct " was in the evolution ofdegenerative lumbar scoliosis. Severe deformities and abnormalities of the lumbar activityleads to stenosis of spinal neural pathways, mechanical imbalance of themuscle,osteoarthritis of facet joints. The patients had low back pain and neurogenicclaudication. Generally, postoperative efficacy is satisfying and much better via surgicaltreatment. The key point is thorough nerve decompression, to reconstruct the lumbarlordosis via the rotatory technology,to correct the vertebral rotation and listhesis withscoliosis deformity, to stabilize lumbar vertebrae, to restore lumbar spinal capability ofweight-bearing and to make it vertical the loading line.