Effects of spinal pedicle screw internal fixation at different extraversion angles Biomechanical evaluation / 中国组织工程研究

ABSTRACT

BACKGROUND: It has been demonstrated that pedicle screw internal fixation influencing factors involve screw structural morphology, thread parameter, bone density, moment size for tightening screw during operation, and depth of screw placement. There is little known about the correlation of pullout strength of spinal pedicle screw with device for transverse traction to extraversion angle. OBJECTIVE: To assess effect of the spinal pedicle screw with device for transverse traction on pullout strength at different extraversion angles. DESIGN: Repeated measurement. SETTING: Center for Bone Joint, the First Affiliated Hospital of Nanjing Medical University. PARTICIPANTS: This study was performed at Laboratory for Material Mechanics, Hehai University between June and November 2003. A total of 18 adult dried lumbar vertebrae (L1-5) were provided by Department of Anatomy, Nanjing Medical University, and recruited for this study. The protocol was approved by the hospital's Ethics Committee. The pedicle screw was made of stainless steel. Each pedicle screw had a diameter of 5.5 nun, total length of 150 nun (thread part 50 into included), and the same thread parameter. Electrical universal material machine (EW type) was provided by Laboratory for Material Mechanics of Hehai University. METHODS: Bone density was measured with a single photon bone density determinator. According to the bone density, the lumbar vertebrae were numbered and randomly divided into 3 groups with 6 lumbar vertebrae in each extraversion angle 5 ° group, extraversion angle 15° group, and extraversion angle 30° group. ① Installation of pedicle screw and clamping apparatus According to Wein-Stein method, one entry-point was selected at each side of lumbar vertebra, and at the sametime, extraversion angle 5°, 15° ,and 30° were respectively defined for extraversion angle 5° , 15° ,and 30° groups. A 50 mm-depth pinhole was drilled with a drill bit with a diameter of 3.0 nun. Pedicle screw was screwed into 50 nun, and its end part was connected to the device for transverse traction. Spinal vertebrae and the device for transverse traction were fixed with a specially made clamping apparatus. ② Determination of pullout strength and observation of pedicle and vertebral injury Spinal vertebrae, on which pedicle screw and device for transverse traction were installed, was placed on a EW electrical universal material machine together with clamping apparatus for determining the pullout strength of pedicle screw. Sensor was connected to a computer to draw strength-displacement curve. The wave crest of the curve was considered the maximum pullout strength. At the same time, injuries to pedicle and vertebra caused by pullout of pedicle screw were observed. MAIN OUTCOME MEASURES: Pullout strength and injuries to pedicle and vertebra. RESULTS:①The mean maximum pullout strength of pedicle screw was respectively 0.878 167, 1.420 333, and 2.154 167 KN for extraversion angle 5° , 15 ° , and 30° groups. There was significant difference among the 3 groups (F = 12.554 22, P < 0.01). ② In the extraversion angle 5° group, 4 patients presented with cortical bone fracture which occurred at the entrance for pedicle screw, and 2 patients presented with fragmentation of junctional zone between pedicle and vertebral posterior edge; In the extraversion angle 15° group, 1 patient presented with cortical bone fracture which occurred at the entrance for pedicle screw, 4 patients presented with fragmentation of junctional zone between pedicle and vertebral posterior edge, and 1 patient presented with vertebral posterior coronal fragmentation; In the extraversion angle 30° group, 1 patient presented with cortical bone fracture which occurred at the entrance for pedicle screw, 2 patients presented with fragmentation of junctional zone between pedicle and vertebral posterior edge, and 3 patients presented with vertebral posterior coronal fragmentation. There was statistical significance in the intergroup rank-sum test (P < 0.01).CONCLUSION: During application of pedicle screw with device for transverse traction, proper accrescence of extraversion angle can increase pullout strength of the screw and enhance fixative strength, and excessive extraversion angle easily injures vertebra.