The Influence Of Sacral Slope On The Screw Stress Of L₅ Isthmic Spondylolisthesis With Different Operations:A Finite Element Analysis

Objective: To develop different three-dimensional finite element models of postoperation L5 isthmic spondylolisthesis with different sacral slopes,and evaluate the stress of pedicle screws between each model.Methods:The L5- sacral segment finite element model was established by Mimics, Ansys,which was based on the lumbar CT of an adult male with L5 isthmic spondylolisthesis(Meyerding grade I °).We verified its effectiveness.The sacral slope(Ferguson’s method) of L5- sacral segments finite element model was changed into 35 degree, 40 degree, 45 degree, 50 degree, 55 degree.Resection the L5-S1 rear structure(including the lamina of L5, spinous process of L5, inferior articular process of L5) simulated surgical decompression since the bilateral spondylolysis of each model.we added pedicle screw to simulate lumbar Posterolateral Lumbar Fusion(PLF),and added pedicle screw and fusion cage to simulate Posterior Lumbar Interbody Fusion(PLIF) in each model.Applied 500 N preload on the L5 surface(average weight of the upper body, simulate neutral position), and then 15 Nm load torque was given to flexion, extension, lateral flexion and torsion in different directions(simulate flexion, extension, lateral flexion and torsion in daily activities).To compare the stress of pedicle screws with different sacral slope and different internal fixation in finite element solution.Results:Established L5- sacral segment finite element model, and verified to be valid.The maximal stress of pedicle screw fixation system of every model was all in sacral screw tail.The maximal stress of pedicle screw fixation system of nine model was in sacral screw tail, which was recorded in compression combined with flexion 1oading. The left one(sacral slope 35°, PLIF) was in sacral screw tail and was recorded in compression combined with torsion 1oading. The maximal von Mises stress of sacral screw tail in the Posterolateral Lumbar Fusion model was bigger than that of Posterior Lumbar Interbody Fusion model in same sacral slope.With the increase of the sacral slope, the maximal stress of pedicle screw fixation system rise.With the increase of the sacral slope, the maximal stress of pedicle screw fixation system after PLF and PLIF increase was recorded in compression 1oading, compression combined with flexion 1oading,compression combined with lateral flexion 1oading.With the increase of the sacral slope, the maximal stress of pedicle screw fixation system after PLIF decrease was recorded in compression combined with extension 1oading, and that after PLF rise after the first decreased, the turning point was sacral slope of 45 degree.With the increase of the sacral slope, the maximal stress of pedicle screw fixation system after PLF decrease was recorded in compression combined with torsion 1oading, and that after PLIF rise after the first drop, the turning point was sacral slope of 40 degree.Conclusion:Stress mainly concentrated in the sacral screw tail after PLF and PLIF,and corresponding risk of pedical screw breakage increase here. With the same sacral slope, the sacral screw tail breakage risk with PLF is higher than that of PLIF.With the increase of the sacral slope,the risk of pedical screw breakage with PLF and PLIF increase.