Establishments Of The Finite Element Model And Biomechanical Analysis Of Oblique Lumbar Inter-Body Fusion

[Background]:Oblique lumbar inter body fusion(OLIF)was first reported by Silvestre in 2012.It’s minimally invasive incision surgical approach from the oblique posture of lumbar,which can significantly reduce surgical trauma,operation time and intraoperative bleeding.However,there are some complications,especially the end plate fracture and cage displacement,which limit its wide clinical application.The end plate fracture and cage displacement often lead to postoperative patients with severe low back pain,lower limb radiation pain,decreased muscle strength and other symptoms.As a result,a revision surgery often needed,but the etiology is not clear.Finite element analysis can be used to disassemble the complex biomechanical situation of the lumbar spine into finite elements,and use finite number of small elements to calculate infinite approximation of the real physical system,to compare and analyze the changes of the biomechanics of the lumbar spine after operation.[Objective]:By establishing the digital finite element model of normal lumbar spine before operation and after OLIF stand-alone,the biomechanical changes of lumbar after operation were analyzed,and the possible causes of superior and inferior end plate fractures of fusion segments were discussed.[Method]:The finite element model of the normal lumbar spine before operation was established and verified.Based on the verified model,a single segment OLIF stand-alone model was established to simulate the changes of biomechanical properties before and after surgery under the conditions of flexion,extension,lateral bending and axial rotation.[Results]:1.The finite element model established in this study is consistent with the experimental results of domestic and foreign experts in vivo and in vitro,which proves that the finite element model established in this study is effective.2.Compared with the preoperative lumbar range of motion(ROM),the flexion,extension,lateral bending and axial rotation ROM after OLIF were reduced by 85.2%,54.2%,62.5%and 71.4%respectively.3.The displacement of L4 vertebral body measured by the model after OLIF stand-alone was 1.10 mm in flexion,1.48 mm in extension,0.57 mm in left bending,0.66 mm in right bending,0.33mm in left axis rotation and 0.32 mm in right axis rotation.Part of the displacement activity were still remained after operation,and the maximum was later extension,but all of them were less than 3 mm.4.The maximum stress of L4 lower end plate increased to 42.7,49.7,38.5,43.0,23.5 and 20.4MPa in the process of forward bending,backward extension,left and right side bending and left and right rotation,respectively.The maximum stress of L5 upper end plate increased to 47.5,46.5,47.3,45.2,29.1 and 24.2MPa respectively.Meanwhile,the stress of OLIF cage was 53.5,70.0,49.3,49.3,35.5 and 31.9MPa.The yield stress of cortical bone in normal people is 60MPa,and that of osteoporosis is lower,so the maximum stress during flexion and extension is close to that of osteoporosis.[Conclusion]:1.The established lumbar model is validated by previous studies and can be used for further finite element study.2.The activities of flexion,extension,lateral bending and rotation of lumbar spine after single-stage OLIF stand-alone operation were significantly reduced compared with those before operation,and the activities were limited,but the reduction of extension was relatively small,and the ROM was still large.There is a certain degree of displacement and activity of lumbar vertebrae after single segment OLIF stand-alone operation,especially during posterior extension.It is suggested that external fixation should be worn to limit lumbar vertebrae activity in the early down movement.3.After operation,the maximum stress of the upper and lower end plates of the fusion segment increased significantly,especially during flexion and extension.When the maximum stress of the end plate approaches the yield stress of the cortical bone,it may lead to the fracture of the end plate and the displacement of the fusion device,especially in the patients with basic diseases of osteoporosis.It is suggested that the patients with osteoporosis should carry out the posterior pedicle screw rod system assisted internal fixation.