| BackgroundUsing pedicle screw to do posterior fixation was most common procedure in spine surgery.The pedicle screw technic bring great improvement on treatment of trauma,degenerative disease,tumor and scolosis.The core of this technic is accurate placement of pedicle screw,the failure of placement may cause spine cord injury or nerve root defect.Currently,there are two ways to fix the pedicle screw,they are named as "∧" and Magerl according to different drilling points.However,most studies about these two methods were anatomic measurement and imaging observation,seldom studies evaluate the biomechanical effect.Objective(1).To find out the easy and safety method of the pedicle screws placement.(2).To biomechanically compare two different methods of lumbar pedicle screw placement,(3).Provide a biomechanical evidence on the ideal pedicle screw placement.Method12 adult human embalmed cadavers(L1～5) were used.Specimens were divided into two groups drilling points of "∧",drilling points of Magerl.The angle of pedicle screws of two groups were measured respectively.Using the 858 MTS material testing machine,the axial pullout test of pedicle screws was performed.The maximum pullout force were recorded and analyzed.After measurement of insert angle,the individual vertebrae were potted into a cylindrical fixture using PMMA.The construct was mounted onto a universal joint fixed to a platen,which was attached to the actuator of the test machine(MTS Corporation,Minneapolis,MN).The fixture allowed for rotation about the z-axis,and adjustment of the x-y plane to ensure vertical pullout alignment.The pedicle screw was attached to the load cell by a rod threaded to the head of the screw.The screw was loaded at a constant displacement rate of 5 mm/minute.Failure was defined as the point at which the load peaked and then decreased sharply with increasing displacement.Stiffness was defined as the linear portion of the load displacement curve and energy absorbed to failure as the area under the load-displacement curve.Friedman test and Wilcoxon Signed Ranks test were performed determine the differences between groups,with a significance level of 95%.ResultsThe imaging results show that all screws are within the pedicle and no pedicles broken.The kinds of screw placement were performed,and extraversion angle was measured.During the pullout test,the screw was pull out in the inserted direction.For the angle of screw placement,the group using "∧" as drilling points was significantly smaller than the group using Magerl(P＜0.05).the maximum pullout strength of "∧" group and Magerl group were 1008.2±122.4 N and 989.7±107.5 N respectively.There is no significant difference of the maximum pullout strength between two groups(P＞0.05) ConclusionIn summary,choosing the proper entry point to inserting pedicle screws is the first step to help avoid penetration of the pedicle wall.Understanding pedicle angles helps decrease the risk of pedicle violation during screw insertion.Magerl's entry point is further lateral,located at the nape of the neck of the superior articular process,so pedicle violations by pedicle screws have been reported to occur more often through the medial and lateral walls than the superior and inferior walls.The "∧" shape entrance point leads to a very high success rate of screw insertion through posterior approach in lumbar spine and has the advantage of small exposure and easily located.This study biomechnically compares the pull out strength of two most common screw placement methods.The results indicate that there are no significant difference between "∧" group and Magerl group.In conclusion,the "∧" method is recommended in clinic practice because it has advantage of easily operate and small exposure.
|
PDF Full Text Request