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 mm, total length of 150 mm (thread part 50 mm 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 same time, 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 mm. Pedicle screw was screwed into 50 mm, 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: ① The 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.

INTRODUCTION

Trans-pedicle internal fixation is an important milestone in the developmental history of spine surgery. Since it is purposed, trans-pedicle internal fixation has been widely accepted. Trans-pedicle internal fixation system has good biological features and clinical application effects. How to enhance the effect of trans-pedicle internal fixation is an important problem concerned by each clinical physician.
This study aimed to observe the effects of different extraversion angles on the pullout strength of pedicle screw with device for transverse traction, so as to provide theoretical evidence for clinical practice.

SUBJECTS AND METHODS

Subjects
This study was performed at Laboratory for Material Mechanics (state-level key laboratory), 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 mm, total length of 150 mm (thread part 50 mm included), and the same thread parameter. Electrical universal material machine (EW type) was provided by Laboratory for Material Mechanics of Hehai University.

Methods
Grouping
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. All the vertebrae subjected to internal fixation of the same batch of pedicle screw made of stainless steel.

Installation of pedicle screw and clamping apparatus
According to Wein-Stein method[1], one entry-point was selected at each side of lumbar vertebra, and at the same time, 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 mm. Pedicle screw was screwed into 50 mm, 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.

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 (loading speed: 3 mm/min) 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 maximum pullout strength. At the same time, injuries to pedicle and vertebra caused by pullout of pedicle screw were observed.

Statistical analysis
Statistical analysis was performed by the first author with Microsoft Office excel 2003.

RESULTS

Determination of maximum pullout strength
In the extraversion angle 5°group, maximum pullout strength of pedicle screw was respectively 0.653, 0.683, 0.695, 0.969, 1.106 , and 1.163 KN, averaging 0.878 167 KN.
In the extraversion angle 15°group, maximum pullout strength of pedicle screw was respectively 1.044, 1.064, 1.372, 1.522, 1.563, and 1.975 KN, averaging 1.420 333 KN.
In the extraversion angle 30°group, maximum pullout strength of pedicle screw was respectively 1.306, 1.669, 1.987, 2.169, 2.887, and 2.907 KN, averaging 2.154 167 KN. There was significant difference among the 3 groups （F =12.554 22, P < 0.01）.
Injuries to pedicle and vertebra caused by pullout of pedicle screw (Table 1)

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 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 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.

DISCUSSION

Pedicle is the hardest part of vertebrae. All strength, which transmits from posterior part of spine to vertebrae, goes through this point. Trans-pedicle spinal internal fixation system is designed based on the principle that pedicle screw is installed into vertebra through a pedicle, and provides three-column fixation of spine for many years.
Lim et al [2] confirmed that axial stability increased after application of transversal connector bar of the pedicle screw system. Lynn et al [3] reported that two transversal connector bars could obviously increase rotational and bending strength. It has been accepted that the diameter of screw is most important for pullout strength of the pedicle screw [4]. It has been also proved that pedicle screw internal fixation influencing factors involve screw structural morphology, thread parameter, bone density, moment size for tightening screw during the operation, and depth of screw placement [5]. Some scholars also confirmed that the included angle between two screws had no influence on pullout strength, while forward anteromedial included angle of screw could increase the pullout strength [4].
The present study demonstrated that when pedicle screw diameter, thread parameter, bone density, and depth of screw placement were controlled, and simultaneously, longitudinal pullout strength （Fcosθ）of pedicle screw was defined, pullout strength of pedicle screw, on which, a device for transverse traction was installed, was related to the extraversion angle. It also demonstrated that with extraversion angle increasing (range 5°-30°), pullout strength (F) of pedicle screw increased and, partial strength to vertebra (Fsinθ)also increased during pullout of pedicle screw that easily injures the vertebra. Therefore, in the trans-pedicle screw internal fixation, extraversion angle about 15°can provide a very big pullout strength and concomitantly does not produce excessive stress to vertebra.

REFERENCES

1 Weinstein JN, Spratt KF, Spengler D, et al. Spinal pedicle fixation: Reliability and validity of roentgenogram-based assessment and surgical factors on successful screw placement. Spine 1988;13(9): 1012-1018
2 Lim TH, Eck JC, An HS et al. Biomechanics of transfixation in pedicle screw instrumentation. Spine 1996;21(19):2224-2229
3 Lynn G, Mukherjee DP, Kruse RN, et al. Mechanical stability of thoracolumbar pedicle screw fixation: the effect of crosslinks. Spine 1997;22(14):1568-1573
4 Krag MH, Beynnon BD, Pope MH, et al. An internal fixator for posterior application to short segments of the thoracia, lumbar, or lumbosacral spine. Clin Orthop 1986;(203):75-98
5 Wang C, Liu WJ. Biomechanical evaluation of pedicle screw internal fixation effects. Yiyong Shengwu Lixue 1998;13(3):179-184

不同外倾角脊柱椎弓根螺钉置入机体后的内固定效果：生物力学评价★

陈 群1，金正帅2，曹晓健2，江 泉3
1南京医科大学第一附属医院骨关节中心，江苏省南京市 210029；2南京医科大学第一附属医院骨科， 江苏省南京市 210029；3河海大学材料力学实验室，江苏省南京市 210029
陈 群★，男，1969年生，江苏省南京市人，汉族，1992年南京医科大学毕业，硕士，副主任医师，主要从事骨与关节疾病的治疗和研究。
摘要
背景：有研究表明，影响椎弓根螺钉置入机体后固定效果的因素有螺钉的结构形态、螺纹参数、骨密度，手术时螺钉拧紧力矩大小，以及进钉的深度等。脊柱椎弓根螺钉并横向牵引装置的把持力与外倾角是否也有一定关系？

目的：观察不同外倾角对椎弓根螺钉并横向牵引装置的把持力的影响。
设计：重复测量实验。
单位：南京医科大学第一附属医院骨关节中心。
对象：实验于2003-06/11在河海大学材料力学实验室完成，选择18只成人干燥腰椎（L1~5节段）椎体标本，标本均为南京医科大学解剖学教研室提供，实验经过医院伦理委员会批准。实验用椎弓根螺钉为不锈钢材料，直径5.5 mm，螺纹参数相同，钉长150 mm，其中螺纹部分长50 mm。EW型电子万能材料实验机为河海大学材料力学实验室提供。
方法：用单光子骨密度测定仪测定骨密度后按骨密度编号，完全随机法将椎体分为外倾角5°组、外倾角15°组及外倾角30°组，每组6只。① 椎弓根螺钉及夹具的安装：在同一腰椎两侧进针点按Wein-stein法，取外倾角5°组外倾角5°，外倾角15°组外倾角15°，外倾角30°组外倾角30°。针孔准备用3.0mm钻头打孔深50mm，拧入椎弓根螺钉50mm，钉尾部用横向牵引装置连接。用特制夹具固定脊椎椎体和DTTs。② 把持力测定及椎弓根和椎体损伤观察：分别将已安装好椎弓根螺钉及横向牵引装置的脊椎椎骨连同夹具一起安置到EW型电子万能材料实验机上，测定椎弓根螺钉的把持力。传感器与电脑相连，并绘出力－位移曲线。以曲线波峰为最大把持力。同时观察椎弓根螺钉拔出时椎弓根和椎体损伤情况。
主要观察指标：把持力检测结果及椎弓根和椎体损伤情况。
结果：①最大把持力：外倾角5°组、外倾角15°组及外倾角30°组椎弓根螺钉平均最大把持力分别为0.878 167，1.420 333，2.154 167 KN。组间比较差异有显著性意义（F = 12.554 22， P < 0.01）。②椎弓根和椎体损伤情况：外倾角5°组4例椎弓根入口处骨皮质骨折，2例椎弓根与椎体后缘交界区断裂；外倾角15°组1例椎弓根入口处骨皮质骨折，4例椎弓根与椎体后缘交界区断裂，1例椎体后缘冠状面断裂；外倾角30°组1例椎弓根入口处骨皮质骨折，2例椎弓根与椎体后缘交界区断裂，3例椎体后缘冠状面断裂。组间秩和检验，差异有显著性意义（P < 0.01）。
结论：在应用经椎弓根螺钉置入机体内固定并加用横向牵引装置时，适当增加外倾角，可增大螺钉的把持力，提高固定强度，外倾角过大则易对椎体造成破坏。
关键词：外倾角；椎弓根螺钉；横向牵引装置；内固定；生物力学； 把持力
中图分类号: R318 文献标识码: A 文章编号: 1673-8225(2008)13-02573-03
陈群，金正帅，曹晓健，江泉.不同外倾角脊柱椎弓根螺钉置入机体后的内固定效果：生物力学评价[J].中国组织工程研究与临床康复，2008，12(13):2573-2575
[www.zglckf.com/zglckf/ejournal/upfiles/08-13/13k-2573(ps).pdf]
（Edited by Dong J/Song LP/Wang L）