Abstract
BACKGROUND: Locking pedicle screw system is commonly used in clinic, but it often suppresses spinal longitudinal growth of adolescent at growth phase. Thus, a pedicle screw system that can reduce even avoid the inhibition to spinal growth is needed.
OBJECTIVE: To compare the biomechanical performance of sliding instrumentation of pedicle screw system and traditional locking pedicle screw system.
DESIGN: Comparative observation.
SETTING: Department of Orthopedics, Xinqiao Hospital of Third Military Medical University of Chinese PLA, and Department of Orthopedics, the 211 Hospital of Chinese PLA.
MATERIALS: The experiment was performed at Department of Material Science, Harbin Institute of Technology on June 29th, 2007. Self-designed sliding pedicle screw system was made of Ti alloy by Wujin No. 3 Medical Instrument Factory Co., Ltd., Jiangsu Province. It consisted of sliding pedicle screw, orthopaedic rod and transversal coupling device. Twelve samples of fresh porcine spine were selected, and muscles attached on vertebral bodies of T1-L5 were removed carefully but integrity of main ligament and precessus articularis posterior was retained.
METHODS: The samples were randomly divided into sliding system group and locking system group with 6 samples in each group. Partial vertebral plate and surrounding ligaments of T12 as well as bilateral facet joints between T11-12 and T12-Ll were removed to induce spinal destabilization, then sliding pedicle screw system and locking pedicle screw system were respectively fixed onto T10, T12, and L2 vertebral bodies of two groups. The samples then were fixed into fixture, and put onto INSTAON-4505 axial compressor. The strain gauge was connected with YJ-31 static electricity resistance strain gauge instrument human to simulate human spinal load, and the center of gravity was loaded to induce forward flexion, backward extension, lateral flexion and axial construction. Load of 100, 200, 300, 400 and 500 N was given gradually, and displacement of T12 was measured under different loads.
MAIN OUTCOME MEASURES: ①Changes in principal stress and displacement under forward flexion, backward extension, lateral flexion and axial construction; ②Spinal fixation intensity and rigidity.
RESULTS: No statistical difference was detected in main straining, displacement of apical vertebrae and intensity of fixation between sliding system group and locking system group under forward flexion, backward extension, lateral flexion and axial construction (P > 0.05).
CONCLUSION: Sliding pedicle screw system has identical biomechanical stability as locking system. Furthermore, in sliding pedicle screw system, the screw and rod are coupled by sliding pattern, which extend along with spinal growth. It can be used to treat scoliosis at growth phase.

INTRODUCTION

Pedicle screw instrumentation is frequently used to correct scoliosis in clinic. However, the locking pedicle screw suppresses spinal longitudinal growth of adolescent in growth and development stage, even results in iatrogenic trunk shortening or crankshaft[1-5]. In this study, we designed a novel sliding instrumentation of pedicle screw system based on the structural characteristics of domestic and foreign pedicle screw system to hope it can reduce or prevent the above-mentioned problems. In addition, we compared the mechanical performance index, intensity and rigidity of fixation with locking pedicle screw system.

MATERIALS AND METHODS

Materials
The experiment was performed at Department of Material Science, Harbin Institute of Technology on June 29th, 2007. Self-designed sliding pedicle screw system was made of Ti alloy by Wujin No. 3 Medical Instrument Factory Co., Ltd., Jiangsu Province, consisted of sliding pedicle screw system (Figure 1), orthopaedic rod (Figure 2) and transversal connection device (Figure 3). The assembly is presented in Figure 4. In the sliding lumbar pedicle screw system, every screw fixed on the pedicle was integrated solidly by orthopaedic rod, which maintained the correction force of coronal and sagittal planes; the longitudinal axis range of every screw was unlimited, which kept longitudinal growth; in addition, bushing of transversal stem could freely slide, and the sliding range and speed were regulated by pedicular growth speed and width. Twelve samples of fresh porcine spine were selected, and muscles attached on T1-L5 were removed carefully to retain integrity of main ligament and precessus articularis posterior. Instrument of INATAON-4505 axial compressor and YJ-31 static electricity resistance strain gauge instrument were employed.

Methods
The upper and lower parts of samples were embedded with dental base acrylic resin powder and formed a platform for stable precise loading. The samples were randomly divided into sliding system group and locking system group with 6 samples in each group. Partial vertebral plate and surrounding ligaments of T12 as well as bilateral facet joints between T11-12 and T12-Ll were removed to induce spinal destabilization, then sliding pedicle screw system and locking pedicle screw system were respectively fixed onto T10, T12, and L2 vertebral bodies of two groups. The samples then were fixed into fixture, and put onto INSTAON-4505 axial compressor. The strain gauge was connected with YJ-31 static electricity resistance strain gauge instrument human to simulate human spinal load, and the center of gravity was loaded to induce forward flexion, backward extension, lateral flexion and axial construction. Load of 100, 200, 300, 400 and 500 N was given gradually, and displacement of T12 was measured under different loads.
Statistical analysis
Data were analyzed using SOX-2 software.

RESULTS

Comparison of mechanical performance indexes between two groups
The load-displacement in sliding system group and locking system group were measured. Principal stress of T12 under physiologic load was shown in Table 1, and displacement changes in Table 2. The results of two tables suggest ①Load, principal stress and displacement display linear alterations under physiologic load. Principal stress and displacement increased with load, and restored after unloading; ②There was no significant difference in principal stress and displacement changes between sliding system group and locking system group under forward flexion, backward extension, lateral flexion and axial construction (P > 0.05); ③Stress peak and displacement peak occur under lateral flexion condition.

Comparison of fixation intensity and rigidity for spine in sliding system group and locking system group
Under 500 N load, fixation intensity (σ) was calculated according to Generalized Hook's law (σ=E·ε) and rigidity was calculated by the ratio of load and displacement (Table 3). The results show that there was no significant difference in fixation intensity and rigidity between sliding system group and locking system group under forward flexion, backward extension, lateral flexion and axial construction (P > 0.05).

DISCUSSION

In the recent 30 years, the studies and application of spinal pedicle screw fixation is rapid developing. However, locking pedicle screw system is still predominantly used in clinic. Because the screw and rod are fixed, there are many problems for the correction in adolescent idiopathic scoliosis [1-2]. More than ten vertebral bodies need to be fixed for patients with scoliosis, so when these vertebral bodies are fixed by locking and bone graft fusion, spine growth will be suppressed. This is not accorded with human body physiological development rules [3]. It is demonstrated that for scoliosis patients at growth phase, correction effect is lost, and Cobb angel increased, even trunk shortening and crankshaft are developed [4-5]. Additional foreign researches suggest crankshaft incidence in younger patients is relatively higher [6-7]. Thus an internal fixation system that can correct scoliosis but not influence spinal growth is necessary. In this study, we modified pedicle screw-rod structure and transversal connection stem to make locking screw-rod connection become sliding one, which makes each pedicle screw longitudinally slide along orthopaedic rod and avoids the limitation to spinal growth. Transversal connection stem is axis-cuff structured, and it can slide along vertebral transversal growth but not limit the growth of two vertebral bodies. The modified sliding pedicle screw system can achieve better stability and effective correction besides the advantages of original system such as correction of spinal deformity and spinal rotation, and restoration of spinal physiologic curve [8-9].
Biomechanical studies need to be conducted to examine whether correction effect and stability characteristics of sliding lumbar pedicle screw system that has been used for scoliosis at growth phase are similar to commonly used locking pedicle screw system [10-11]. The results of this study suggest that there is no significant difference in principal stress, relative displacement, stable intensity and rigidity between these two systems. In sliding lumbar pedicle screw system, every screw fixed on pedicle is integrated solidly by orthopaedic rod, which maintains the correction force of coronal and sagittal planes; the longitudinal axis range of every screw is unlimited, which keep longitudinal growth; in addition, bushing of transversal stem can freely slide, and the sliding range and speed are regulated by pedicular growth speed and width. Therefore, the transversal stem can control longitudinally interlocking displacement of two orthopaedic rods, and rotation of coronal and sagittal planes. Furthermore, the connection does not inhibit pedicular transversal growth. This indicates the feasibility of sliding lumbar pedicle screw system as surgery for scoliosis at growth phase [12-13].
In this study, the influences of sliding and locking lumbar pedicle screw systems on lumbar fixation intensity, rigidity and stability are compared to improve the original locking system to well fix and correct adolescent scoliosis and allow normal growth and development. In traditional posterior approach correction for scoliosis, bone graft fusion is conducted in certain segmental range [14]. While in this study, the self-designed sliding pedicle screw system can conduct bone graft fusion only in 3-5 segments even omit this process [15-16]. Sliding pedicle screw system provides reliable mechanical intensity during correction, so it can efficiently correct scoliosis, especially adolescent idiopathic scoliosis. This system can automatically shift as spinal growth, and can be regarded as an alternative for treating scoliosis.

REFERENCES

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滑动椎弓根钉棒系统的研制和生物力学测试☆

陈庆贺1,2， 周 跃1，卢 淼2，高吉昌2，王 仑2，张文进2
1解放军第三军医大学新桥医院骨科，重庆市 400037；2解放军第二一一医院骨科，黑龙江省哈尔滨市 150080
陈庆贺☆，男，1961年生，黑龙江省哈尔滨市人，汉族，解放军第三军医大学在读博士，主任医师，教授，硕士生导师，主要从事脊柱外科研究。
通讯作者：周 跃，博士生导师，解放军第三军医大学新桥医院骨科，重庆市 400037
摘要
背景：现今临床上常用的椎弓根钉内固定系统为钉棒锁定结构系统，妨碍了处于生长发育期青少年脊柱的纵向生长，所以迫切需要一种能够滑动的椎弓根钉内固定系统，以减少或避免对脊柱生长的影响。
目的：对比观察自行研制的滑动椎弓根钉棒系统与传统锁定椎弓根钉棒系统固定脊椎的力学性能。
设计：对比观察。
单位：解放军第三军医大学新桥医院骨科，解放军第二一一医院骨科。
材料：实验于2007-06-29在哈尔滨工业大学材料科学院完成。自行设计的滑动椎弓根钉棒系统，材质是钛合金，由江苏省常州市武进第三医疗器械厂生产，包括滑动椎弓根钉、 矫形棒和横向连接装置三部分。新鲜猪脊椎标本12具，仔细剔除T1~L5椎体所附肌肉，保留主要韧带及后关节突结构的完整性。
方法：将标本随机分成滑动组和锁定组，每组6具标本。切除Tl2椎骨的部分椎板、周围韧带及T11~12、T12~Ll椎间双侧小关节造成脊柱失稳，再将滑动椎弓根钉系统和锁定椎弓根钉系统固定于各组的T10，T12，L2椎体上。将标本稳妥安装在夹具内，置于INSTAON-4505轴向压缩机上。将应变片连接于YJ-31静电电阻应变仪上，模拟人体脊柱载荷并加载于重心点，以便造成前屈、后伸、侧屈、轴向压缩等运动状态。载荷在100，200，300，400，500 N实施分级加载，在不同情况下测量T12椎体的位移。
主要观察指标：①在前屈、后伸、侧屈、轴向压缩情况下，主应变及位移变化。②脊柱固定强度和刚度。
结果：滑动组和锁定组在屈伸、侧屈、轴向压缩情况下，其主应变、位移变化及固定强度差异均无显著性性意义 (P > 0.05)。
结论：滑动椎弓根钉棒系统可以获得与锁定椎弓根钉棒系统相同的生物力学稳定性。滑动椎弓根钉系统将椎弓根钉与矫形棒之间的连接设计为滑动式，使之可随脊柱生长而延长，用于生长发育期脊柱侧凸的的治疗具有可行性。
关键词：滑动椎弓根钉；脊柱侧凸；生物力学；骨科植入体
中图分类号: R318 文献标识码: A 文章编号: 1673-8225(2008)13-02569-04
陈庆贺，周跃，卢淼，高吉昌，王仑，张文进.滑动椎弓根钉棒系统的研制和生物力学测试[J].中国组织工程研究与临床康复，2008，12(13):2569-2572
[www.zglckf.com/zglckf/ejournal/upfiles/08-13/13k-2569(ps).pdf]
（Edited by Liu Z/Su LL/Wang L）