Biomechanics Of Reinserted Pedicle Screws Using The Previous Entry Point And Trajectory

BackgroundAs the development of spinal internal fixation system,pedicle screw instrumentation is considered standard for treatment of spinal degenerative diseases,fractures,tumors,and deformities.Although pedicle screw instrumentation is strong,malposition of a screw,resulting in a lateral wall breach in the pedicle and penetrating in intervertebral space,occurs frequently,especially with freehand technique.When pedicle screw malposition occurs during surgery,the screw is removed and reinserted along a correct entry point and trajectory.Many studies reported that the biomechanical strength of a redirected pedicle screw is less than that of a correctly placed screw.In order to inspect the channel or change a longer pedicle screw,the pedicle screw is often removed and reinserted using the previous entry point and trajectory.However,no study has reported the biomechanical strength of pedicle screws inserted to various depths,followed by removal and reinsertion using the previous entry point and trajectory.PurposeThis study compared the pullout strength of reinserted pedicle screws using the previous entry point and trajectory with those of correctly inserted pedicle screws.And it explored the feasibility of reinserted pedicle screws using the previous entry point and trajectory to provide the biomechanical basis in clinical practice.MethodThe study used 18 lumbar vertebrae(L1-6)from 3 fresh calf sp:ines.Before testing,the vertebrae were thawed at room temperature and soft tissue was removed.The specimens were divided randomly into 3 groups.Each pedicle screw(6.5×40-mm)was inserted using freehand technique under direct visualization.A control screw was inserted correctly along the axis of one pedicle,while an experimental screw was reinserted using the previous entry point and trajectory in the other pedicle.The experimental screw was removed after being completely inserted in group A and after 80%of the total trajectory was reached in group B,and was removed after 60%of the total trajectory was reached in group C.The pullout strength of the pedicle screws was measured using MTS machine.All analyses were performed using Microsoft Excel 2007 and SPSS 19.0 sofitware.Differences in biomechanical properties were analyzed using the paired Milcoxon rank sum test.Significance was set for a P value<0.05.ResultEach pedicle screw was in a good position.There was no pedicle screw penetrating in intervertebral space or lateral wall breach.There were no significant differences in pedicle screw axial pullout strength between reinserted screws and correct screws in the 3 groups(PA=0.4635 PB=0.753,Pc=0.753).Stiffiness measurement increased for the reinserted screw compared with that of the control screw.Fracturing was observed between the vertebral body and pedicle when the terminal end of the pedicle screw passed the edge of the vertebral body.ConclusionThe biomechanical study showed that the pullout strength was not found to be significantly greater between correctly inserted screws and reinserted screws using the previous entry point and trajectory.Theoretically,the pedicle screw can be removed when necessary and reinserted using the previous entry point and trajectory.It had the biomechanical value of clinical safety and replacing a longer pedicle screw.