The Biomechanical Study Of The Sagittal Axial Dynamic Pedicle Screw And Rod System

Part I Evaluation of the biomechanical stability of unstable lumbar Dynamically fixed by the sagittal axial dynamic pedicle screw and rod systemObjectiveTo explore effect on the stability of a single segmental unstable lumbar fi xed through the sagittal axial dynamic pedicle screw and rod system.MethodThe sagittal axial dynamic pedicle screw is made of titanium alloy,the scr ews remain flexibility of sagittal axis after locking the connecting rods,the sagi ttal plane activity was less than 20°.6 specimens of calf thoracic and lumbar s pine(T11～L5)were selected,each specimen is correspondingly made into a fin e spine,semi-laminectomy and unilateral dynamic fixation(L3 and L4 left semi-laminectomy was proceeded,and two sagittal axial dynamic pedicle screws wer e placed on left side of L3 and L4,and the connecting rods were locked),ins tability after semi-laminectomy(the connecting rod from the previous model wa s removed),total laminectomy and bilateral dynamic fixation(L3 and L4 total laminectomy was proceed and two sagittal axial pedicle screws were placed on right side,bilaterally joined and locked),total laminectomy and bilateral rig id fixation(four sagittal axial dynamic pedicle screws were replaced by tradition al pedicle screws from the previous model,bilaterally joined and locked),insta bility after total laminectomy(the connecting rods from the previous model wa s removed).meanwhile,pure 8Nm were acted on flexion and extension,lateral bending,rotation,in order to measure the range of motion.ResultsThe range of movement of specimens in the three plane was significantly increased in the semi-laminectomy and total laminectomy than the fine spine group(P<0.05).Compared to the fine spinal group,the range of movement of flexion and extension,lateral bending and rotation were significantly decrease d in the semi-laminectomy and unilateral dynamic fixation group(P<0.05).Com pared to the fine spinal group,the range of movement of flexion and extensio n,lateral bending and rotation direction were significantly decreased in total la minectomy and bilateral dynamic fixation group and total laminectomy and bila teral rigid fixation group(P<0.05).Comparing the total laminectomy and bilater al dynamic fixation group and the total laminectomy and bilateral rigid fixation group in the range of movement of flexion and extension,lateral bending,the former has a significant superiority(P<0.05),but there was no significant diffe rence in the range of movement of rotation.(P>0.05).ConclusionCompared to a fine spine,the sagittal axial dynamic pedicle screw and ro d system would significantly decrease the range of movement of flexion,latera 1 bending and rotation in fixing unilateral semi-laminectomy and bilateral lami nectomy unstable lumbar,providing immediate stability,meanwhile remain the appropriate range of movement.Part II Finite element analysis of the sagittal axial dynamic pedicle screw and rod system for unstable lumbarObjectiveTo analyse the biomechanical characteristics of the lumbar instability mode 1 with the sagittal axial dynamic pedicle screw and rod system by the finite el ement method,providing the theoretical and biomechanical basis for the clinica 1 application of the dynamic internal fixation system.MethodTo establish normal lumbar model and L4 and L5 instability model by usi ng the lumbar CT data of a healthy Chinese male adult volunteer,which is pre vious acquisited.The finite element models were calculated and established on the basis of the material parameters of the sagittal axial dynamic pedicle sere w and rod system and the strong pedicle screw and rod system,and introduce them into the verified L4 and L5 instability model to establish the single-seg ment dynamic fixation model and the single-segment strong fixation model.Under the constraints of L5 vertebral endplate,under axial 500N load and 10N m torque,five working conditions of normal physiological axial load,fle xion,extension,lateral bending and rotation of the human lumbar were simulat ed respectively.L4 and L5 axial displacement were recorded respectively in th e four models of axial load working conditions and the range of movement of the L4 and L5 segment(fixed segment)and the L3 and L4(adjacent segment),the intervertebral disc stress,the stress of the facet joints in adjacent segment s and the stress of the apparatus were recorded respectively in the four models of the latter four working conditions to analyze and compareResultsUsing the sagittal axial dynamic pedicle screw and rod system to fix unst able lumbar allows the fixed segment to has more proper activity than the rigi d fixation group,yet less than the fine spine group,meanwhile has less effec ts in the adjacent segment lumbar activity than the rigid fixation group,also i n which fixed segment intervertebral disc stress was more than rigid fixation g roup,while device stress was less than the rigid pedicle screw fixation system,adjacent segmental lumbar intervertebral disc and facet joint stress was less th an the rigid fixation group.ConclusionIn Solving the trouble of lumbar instability,the sagittal axial dynamic ped icle screw and rod system could not only remains the fixed segment proper ac tivity,but also provide immediate stability;further more,it could participate in bearing spinal load with the spine,effectively avoid internal fixation failure a nd other complications caused by the postoperative stress shielding and the dev ice stress concentration.meanwhile effectively lower stress of the adjacent inter vertebral disc and intervertebral joint,lower influence of adjacent segment lum bar activities than strong fixation group,theoretically helps reducing and prev enting degeneration of adjacent segments.