Mechanical Analysis Of Spinal Internal Fixation Based On Finite Element Simulation

Objective: The common complications after lumbar fusion and fixation are adjacent segment degeneration(ASD),and the most common fixation failures are screw loosening and screw rod fracture.ASD is usually associated with abnormal stress in various tissues at the fusion adjacent segment and compensatory increase in range of motion(ROM),and screw loosening and screw rod fracture mostly result from internal fixation system factors,such as the stress shielding effect caused by excessive structural rigidity,which eventually makes the fixation system fail.The first part of this paper aimed to investigate the mechanical effects of pedicle screw placement technique on adjacent segments.The second part of this paper starts with changing the pedicle screw structure to reduce the stiffness of the fixation system.Methods: In both parts of this paper,finite element method was used to conduct the research.In the first part,the mechanical effects of different pedicle screw placement techniques on fusion adjacent segments were comparatively analyzed using finite element method.A normal male volunteer without lumbar series disease was selected to collect the CT data of his lumbar spine and construct the complete L1-S1 3D finite element model using software such as mimics,Geomagic wrap,and Solid Works,then the established finite element model was compared with other scholars finite element simulation data and in vitro biomechanical test results to verify the validity of the constructed model.A postoperative simulation of posterior lumbar interbody fusion(PLIF)at L4-L5 segment was performed on the basis of a validated model,and models a,B,and C were constructed with the traditional stapling technique at L4-L5 fusion segment.A finite element analysis was conducted based on these pedicle screw placement models,a load of 500 N was applied on the upper surface of the L1 vertebral body to simulate the weight of the human body,a reference point was established on the L1 upper surface,and six operating conditions of the lumbar spine under the natural state were simulated by applying a torque of 10 N.m on this point.Disc,endplate,and facet maximum stress as well as vertebral ROM were compared between fusion fixed and adjacent segments.In the second part,five screws were tested by mechanical simulation using finite element method to find out the best hollow diameter screw.Screws with hollow diameters of 0 mm(solid),0.5 mm,1 mm,2 mm,and 3 mm and a cuboid test block were constructed in Solid Works,and mechanical finite element simulations of three medical metal materials(Ti-6Al-4V,Co-28Cr-6Mo,and Ti2448)were performed for each of the five hollow screws in finite element software.On the basis of the ability of hollow structures to reduce screw stiffness as determined by stiffness calculations,five screws were tested for torsional and bending properties.Results: The first part study results for the established lumbar model were validated for use in subsequent studies of pin placement models.Compared with model A,the maximum stress of the facet joint decreased by 43.9% under the forward bending working condition and 17.3% under the left twisting working condition in model C.The maximum stress on the intervertebral disc of the three screw placement models increased compared with that of the normal model.In segments L3-L4,the disc stress of model C at the left lateral curvature and twisting and active is larger than that of model A,while the maximum stress of the disc between model B and model C at each operating condition is not significantly different.In the L5-S1 section,the three models show no significant difference at all operating conditions;The stresses of the upper endplate at the L4 vertebral body under the forward flexion and right bending working conditions of model C are reduced by 18.6% and 16.6% compared with model A in turn,the trends of model B and model C are the same,and the maximum stress of model C is smaller than model A for the lower endplate at the L5 vertebral body,except under the back extension working condition,the maximum stresses of the other five cases are smaller than model B,while those of model B under most working conditions are slightly larger than model A.The ROM of the normal model was smaller at the fusion segment than the three pedicle screw placement models and the opposite was true at the fixed segment,where both model B and model C had model A smaller ROM than model A and model C performed more significantly.The screw to cortical bone stress difference is also smaller for materials with a small elastic modulus when the hollow diameter is constant.The Ti2448 material screw showed the smallest difference between screw and cortical bone stress at all hollow diameters compared to the Co-28Cr-6Mo material and Ti-6Al-4V material screws.Conclusions: All three pedicle screw placement models were able to affect disc,endplate,facet joint,and ROM at adjacent segments to a greater extent than the upper adjacent segment,and models B and C had a compensatory increase in activity at adjacent segments relative to model A.When the hollow diameter was lower than the screw diameter by 16.7%.The highest flexural strength was found for the hollow diameter of 1 mm for Ti-6Al-4V screws and 2 mm for Co-28Cr-6Mo screws,while the best flexural strength for Ti2448 material screws was between 1 mm and 2 mm hollow diameter.