Biomechanical Study Of Shear Characteristics And Load Sharing Of Spinal Dynamic Fixation

Objective:1.To explore the establishment of multi segmental degenerative spondylolisthesis model and compare the shear characteristics of L4-L5 and L5-S1 segments.2.To evaluate the shear stability of three posterior dynamic fixation with PEEK rod,sliding rod,and rotating screw in the lumbosacral spine.3.To evaluate the stability and load sharing of three anterior dynamic fixation with PEEK rod,sliding rod,and rotating screw in the thoracolumbar spine.Methods:1.L4-L5 and L5-S1 segments in eight human cadaveric specimens(L4-S1)were subjected to 300 N axial compressive force and-50 N to 250 N anterior and posterior shear force.The anterior displacements under shear loading was tested and the shear rigidity of the segment was calculated in three states of intact,facet joint injury and disc injury.2.In eight models of L4-L5 segmental degenerative spondylolisthesis,the shear displacements of posterior fixation with rigid rod,PEEK rod,sliding rod,and rotating screw under shear loading were measured separately with or without intervertebral fusion device(Cage).The loading method of the shear force had been described in Chapter One.3.Eight human cadaveric specimens(T11-L3)were subjected to a pure moment of 5.0 Nm in flexion-extension,lateral bending and axial rotation,and flexion-extension with a compressive preload of 300 N.After the testing of intact state,an anterior reconstruction of anterior L1 corpectomy was performed using a intervertebral simulated titanium mesh embedded with a load-cell and T12-L2 segmental anterior fixation.With the full length of intervertebral titanium mesh or titanium mesh shortening 2mm(simulated bone graft subsidence),range of motion(ROM)and intervertebral loading of anterior fixation with rigid rod and PEEK rod,sliding rod,and rotating screw were measured respectively.Results:1.The anterior displacements of L4-L5 and L5-S1 segments under shear loading in the intervertebral disc injury state were 1.4mm and 1.2mm respectively,which were significantly higher than that in intact state(P<0.05).2.The anterior shear stiffness of L5-S1 segment in intact state was significantly higher than that of L4-L5 segment(P<0.05),but the anterior shear stiffness of L4-L5 segment was equivalent to that of the L5-S1 segment in the states of facet joint injury and disc injury.3.The anterior displacements of posterior fixation with PEEK rod and rotating screw under shear loading increased significantly compared with the rigid rod(P<0.05),and the displacement of the sliding rod construct was equivalent to that of the rigid fixation.4.The anterior shear displacements of all internal fixation modes after Cage implantation were less than that before Cage implantation and the dynamic fixation showed a more pronounced reduction.5.Regardless of the length of the intervertebral titanium mesh,the ROMs of the anterior PEEK rod and the rotating screw construct increased in all directions with various degrees compared with that of the rigid fixation,and the ROMs of the sliding rod construct decreased in flexion with the titanium mesh shorten and the preload,and had equal level or little increase in the remaining directions compared with that of the rigid fixation.6.Regardless of the length of the intervertebral titanium mesh,the intervertebral load values of three kinds of anterior dynamic fixation under preload of 300N were greater than those of the rigid fixation in neutral position or flexion-extension state.Conclusion:1.The grade ? degenerative spondylolisthesis model can be simultaneously established in the L4-L5 and L5-S1 segments.2.There was a better ability to resist shear in the L5-S1 segment than in the L4-L5 segment,but the two segments were comparable in resistance to shear in the injury states.3.The shear stability of the posterior fixation with PEEK rod and the rotating screw in the lumbosacrum was weaker than that of the rigid fixation,and the shear stability of the posterior sliding rod fixation was comparable to that of the rigid fixation.4.Intervertebral Cage implantation can enhance the shear stability of all forms of fixation,but Cage implantation had a greater impact on dynamic fixation compared with rigid fixation.5.The stability of the thoracolumbar anterior fixation with PEEK rod and rotating screw was weaker than that of the rigid fixation in the whole and the stability of the sliding rod was comparable to that of rigid fixation.6.The thoracolumbar anterior dynamic fixation with PEEK rod,sliding rod,and rotating screw had better intervertebral bone graft sharing than rigid fixation.