Modified and traditional iliolumbar pedicle screw-rod system for the fixation of vertically unstable posterior pelvic ring dislocation:a biomechanical comparative study
Objective To compare the biomechanical properties of the modified and traditional iliolumbar pedicle screw-rod system for the fixation of vertically unstable posterior pelvic ring dislocation.Methods The CT images of the lumbar spine,pelvis,and femur from a 31-year-old healthy male volunteer were used to establish models of posterior pelvic ring dislocation and unilateral and bilateral fixation models of modified and traditional iliolumbar pedicle screw-rod systems with the three-dimensional virtual software using the finite element method.By restraining the distal ends of the femurs of both sides and applying a vertical load of 500 N to the L,vertebral body,the maximum stress of the intact pelvis model and the stiffness of the posterior pelvic ring dislocation model were evaluated to validate the reliability of the models.After the validation of the models,a follower load of 400 N was applied in the direction of the L1 vertebral curve to simulate the upper body weight;the maximum vertical displacement,the maximum posterior displacement,the maximum right displacement,and the overall maximum displacement of the sacrum of the modified and traditional iliolumbar pedicle screw-rod system models were compared.Additionally,stress distributions of the implant and the pelvis were also compared.Results(1)The maximum stress of the intact pelvis model was 22.0 MPa,with the stiffness of the posterior pelvic ring dislocation model as 180.03 N/mm.(2)In terms of the maximum vertical displacement of the sacrum,the bilateral modified iliolumbar pedicle screw-rod system was similar to the bilateral traditional iliolumbar pedicle screw-rod system.However,the maximum posterior displacement,the maximum right displacement,and the overall maximum displacement of the sacrum in the bilateral modified iliolumbar pedicle screw-rod system were 0.57 mm,0.19 mm and 0.68 inm respectively,which were markedly smaller than those of the bilateral traditional iliolumbar pedicle screw-rod system(1.38 mm,0.26 mm and 1.41 mm respectively).(3)The maximum vertical displacement,the maximum posterior displacement,the maximum right displacement,and the overall maximum displacement of the sacrum in the unilateral modified iliolumbar pedicle screw-rod system were 0.70 mm,0.73 mm,0.17 mm and 0.88 mm respectively,which were markedly smaller than those of the unilateral traditional iliolumbar pedicle screw-rod system(1.17 nim,2.21 mm,0.31 mm and 2.29 mm respectively).(4)The stress of the implant was concentrated on the connecting rod between the iliac pedicle screw-rod and the lumbar pedicle screw-rod in the modified iliolumbar pedicle screw-rod system,but it was mainly concentrated on the connecting rod at the upper edge of the iliac screw in the traditional iliolumbar pedicle screw-rod system.The maximum stress of the implant was 340.84 MPa in the bilateral modified iliolumbar pedicle screw-rod system,which was 30.4％less than that in the bilateral traditional iliolumbar pedicle screw-rod system(489.77 MPa),and was 351.23 MPa in the unilateral modified iliolumbar pedicle screw-rod system,which was 15.5％less than that in the unilateral traditional iliolumbar pedicle screw-rod system(415.82 MPa).(5)In both modified and traditional iliolumbar pedicle screw-rod systems,the stress of the pelvis was concentrated on the contact area between the iliac screw and the cortical bone.The maximum stress of the pelvis was 19.21 MPa in the bilateral modified iliolumbar pedicle screw-rod system,which was 78.1％less than that in the bilateral traditional iliolumbar pedicle screw-rod system(87.59 MPa),and was 39.91 MPa in the unilateral modified iliolumbar pedicle screw-rod system,which was 70.4％less than that in the unilateral traditional iliolumbar pedicle screw-rod system(134.98 MPa).Conclusion Compared with the traditional iliolumbar pedicle screw-rod system,the modified iliolumbar pedicle screw-rod system can effectively reduce the displacements of all directions in vertically unstable posterior pelvic ring dislocation and significantly reduce the stress of the implant and the pelvis,thus having good biomechanical stability.
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