Abstract
BACKGROUND: C2 transpedicle screw fixation for Hangman fractures has been paid more attention due to reliability and no loss of physiological function. However, there are lacks of biomechanical evidences for indication treatment because the fixation is single segmental.
OBJECTIVE: To investigate the biomechanical stability of C2 transpedicle screw fixation for Hangman fractures.
DESIGN, TIME and SETTING: This was a contrast study which was performed at the General Key Laboratory of Biomechanics, First Military Medical University of Chinese PLA from May to August 2004.
MATERIALS: AO-universal titanium alloy transpedicle screw of 18-25 mm in length and 3.5 mm in diameter was adopted in this study. Six fresh C1-C4 cervical vetebrae samples were ordinally made into type Ⅰ, ⅡA, and Ⅱ Hangman fracture models.
METHODS: After transpedicle screw fixation, Hangman fracture models were measured by non-destroyed style with spinal 3D motor equipment. Pure moment of couple (20 N · m) was exerted via loading disc to generate six physiological motions: anterior flexion/posterior extension, left/right lateral curvature, and left/right axial direction. Loading/unloading circulation was performed three times during each testing. Kinematics indicators were measured on the 3rd circulation.
MAIN OUTCOME MEASURES: Spinal motor images at zero load and maximal load were obtained with laser photoscanning (0.1% in precision), and the corresponding systematic software was adopted to calculate 3D range of movement.
RESULTS: The relative stability of type Ⅰ Hangman fracture models after C2 transpedicle screw fixation was 100.62% (inflexion), 96.91% (posterior extension), 99.19% (lateral curvature), and 97.12% (rotation) as compared to control group (P > 0.05). The relative stability of type Ⅱ Hangman fracture models after C2 transpedicle screw fixation was 47.84% (inflexion), 21.29% (posterior extension), 65.98% (lateral curvature), and 41.69% (rotation) as compared to control group (P < 0.05).
CONCLUSION: Biochemical evaluation suggests that type Ⅰ and ⅡA Hangman fractures do fit for C2 transpedicle screw fixation, and the fixation may generate well physiological fixation or stability. However, stability of type Ⅱ Hangman fracture is poor, so it is not suitably adopted single transpedicle screw fixation.

INTRODUCTION

Recently, Hangman fractures increasingly attack due to deceleration injury, such as road accident and crash[1-3]. C2 transpedicle screw fixation for Hang-man fractures has been paid more and more attention due to reliability and no loss of physiological func-tion. However, there are lack of biomechanical evi-dences for indication treatment because the fixation is single segmental[4-6]. This study was designed to investigate the biomechanical stability of C2 transpedicle screw fixation for Hangman fractures and provide biodynamic evidences for clinical treatment.

MATERIALS AND METHODS

Materials
This study was performed at the General Key Labo-ratory of Biomechanics, First Military Medical Uni-versity of Chinese PLA from May to August 2004. Fresh and freezing cervical vertebral samples de-rived from six young males who suffered from acci-dental death were provided by Department of Anatomy, First Military Medical University of Chi-nese PLA. Cervical vertebral trauma, degenerative disease, tumor, and hyperosteogeny were excluded. C1-4 cervical vertebrae were taken; paravertebral muscles, fat, and connective tissues were rejected; integrated ligament and articular capsule were re-served; complete samples were successfully estab-lished. C1 and C4 segments were embedded with poly-methyl methacrylate (PMMA) (self-coagulation, Shanghai Dentistry Factory) and maintained in hypothermia refrigerator at –20 ℃. Before the test, the segments were defrosted for 12-18 hours. Four specific tags were individually secured at top and ending part of testing segments (C2-3), and the fixing points were not affected by any loading force, such as transverse process, in order to sign active range of testing segments.

Preparation of fracture models
Pathological criteria of Levine-edwards fracture typing were standardized to prepare fracture models in various types[6]. Type Ⅰ fracture models: Isthmic portion between bilateral superior and inferior small articular processes was cut off with import high-speed cervical vertebrae saw. Type ⅡA fracture models: Bilateral interarticularis was damaged, and interarticular cartilage was cut off. Type Ⅱ fracture models: Based on type ⅡA, a 10-cm anterior longitudinal ligament of C2-3 was cut off, and annular fibrosus between bilateral anterior longitudinal ligament and interarticular cartilage was only reserved. Because testing segments of type II fracture models were connected by annular fibrosus between
bilateral anterior longitudinal ligament and interarticular car-tilage and by interarticular cartilage of atlanto-axial, it was not significant to establish type Ⅱ fracture models through damaging bilateral small articular capsule; furthermore, the stability was poor, and damage of posterior column structure was not suitable for transpedicle screw fixation. Therefore, type Ⅱ fracture was not simulated in this study.

Fixation and methods
Internal fixation: AO-universal titanium alloy transpedicle screw of 18-25 mm in length and 3.5 mm in diameter was adopted in this study. Needing point and angle: Needing point was located at 5 mm below superior border of odontoid verte-bral plate and at 7 mm cross with medial border of canalis spinalis. Needling angle was 30° in internal slope and 20° in superior slope[7-11]. The matching tools were used to internal fixation.

Experimental grouping and procedures
Samples were divided into four groups based on reduction of dynamic functional effect caused by the next testing procedure: control group (complete samples), type Ⅰ fracture C2 transpedicle screw fixation group, type ⅡA fracture C2 transpedicle screw fixation group, and type Ⅱ fracture C2 transpedicle screw fixation group.

Biodynamic test
After transpedicle screw fixation, Hangman fracture models were measured by non-destroyed style with spinal 3D motor equipment. Pure moment of couple (20 N · m) was exerted via loading disc to generate six physiological motions: anterior flexion/posterior extension, left/right lateral curvature, and left/right axial direction. Laser scanning apparatus (General Biodynamics Laboratory of the First Military Medical Uni-versity, 0.1% in precision) was used to intake spinal motor image from load 0 to the maximal load value. Moreover, the corresponding software system was used to analyze images to calculate 3D movement range. Loading/unloading circulation was performed three times during each testing. Kinematics indicators were measured on the 3rd circulation to reduce the effect of viscoelasticity. ROM value was measured in different procedures, and increase of ROM value expressed as stability descent[12-13].

Statistical analysis
SPSS 11.0 software was used by authors in this study. Ex-perimental data were analyzed with one-way ANOVA and compared with LSD test between two groups. α=0.05 was regarded as the significant difference.

RESULTS

3D movement range of C2-3 samples (Table 1)

Relative stability of the three types of fractures after transpedicle screw fixation compared to control group[14]
Q=Rn/R0 ×100% (R0: ROM in the control group; Rn: ROM in the three types of fractures)

The results showed that there were significant differences in the relative stability of type Ⅰ-Ⅱ fracture models before and after transpedicle screw fixation. However, there were no significant differences in ROM value between typeⅠfracture fixation group and control group. The relative stability of Hangman fracture models after C2 transpedicle screw fixation was 100.62% (inflexion), 96.91% (posterior extension), 99.19% (lateral curvature), and 97.12% (rotation), which was not significantly different from control group. The relative stability of type Ⅱ Hangman fracture models after C2 transpedicle screw fixation was 47.84% (inflexion), 21.29% (posterior extension), 65.98% (lateral curvature), and 41.69% (rotation) as compared to control group. The relative stability in the type Ⅱ fracture fixation group was significantly poorer than that in the types I fracture fixation group and type ⅡA fracture fixation group. However, there were no signifi-cant differences except rotation between type Ⅰ fracture fixation group and type ⅡA fracture fixation group. Particu-larly, the function of rotation in the type ⅡA fracture fixation group was poorer.

DISCUSSION

Superiority and feasibility of C2 transpedicle screw fixation for the treatment of Hangman fractures
Spinal fixation trends to remain spinal motor function to a maximal degree, and more and more scholars think that fixed confluence segments may be reduced as much as pos-sible under a permissible condition[15-17]. A research has shown that transpedicle screw fixation in dentata is reliable and easy[18]. Single-segmental C2 transpedicle screw fixa-tion for the treatment of Hangman fractures achieves imme-diate reposition, correcting abnormity, and pressurizing fixation. In the phase of fracture reposition, vertebral ante-location of dentata resisted by inferior articular process of axis is recovered, and C1,2 rotation and C2,3 function are remainted without any damages of physiological function[19]. Transpedicle screw passing through pedicle of vertebral arch and vertebra of dentata has a long tract and a tri-collumn effect. So the fixation intensity is reliable and surgery is in advantage of less wound and early rehabilita-tion. In recent years, C2 transpedicle screw fixation has been paid more and more attention. Transpedicle screw fixation perforation may cause a series of severe complications, such as nervous vascular injury. Generally speaking, C2 transpedicle screw fixation is safer than C3–7 transpedicle screw fixation. Albumi et al[20] found that probability of screw perforating pedicle of vertebral arch is low, and there are no severe complications. Recently, scholar at home and abroad report that there are no severe complications during the application of C2 transpedicle screw fixation. Surgical achievement depends on enough preoperative preparation, sufficient consideration of individual differences, and image system.

Stability of different types of fracture after transpedicle screw fixation
Results in this study showed that there were no significant differences in 3D movement range between type Ⅰ fracture group and control group after C2,3 transpedicle screw fixation because of complete rear structure of vertebra. Therefore, the fixation was informed and satisfactory to physiological fixa-tion. Anti-displace of type ⅡA fracture was stable, but anti-angulation was not stable[21]. Because rear structure of anterior longitudinal ligament, small articular capsule, and ligament supraspinale was complete. There were no signifi-cant differences in lateral curvature, flexion and extension between type ⅡA fracture group and control group; however, rotation (61.43%) was lower than that in the control group. Stability among vertebra was well and generally satisfactory to single-segmental fixation. Type Ⅱ fracture was a unstable fracture[21]. Because isthmic protion, anterior and posterior interarticular cartilage, and disci intervertebrales were dam-aged, the stability was poor. After transpedicle screw fixation, type Ⅱ fracture was only as 47.84%, 21.29%, 65.98%, and 41.69% as normal status. The stability among vertebra was poor and not satisfactory to transpedicle screw fixation alone. Type Ⅵ fracture was quite unstable[21]. It was not simulated in this study due to damage of small articular process fracture or dislocation and vertebral plate and soft tissue injury in an extensive region. Transpedicle screw fixation was proper for single-segmental fixation due to loss of support.

Clinical significance
The results suggested that type Ⅰ fracture was satisfactory to physiological fixation after transpedicle screw fixation; type ⅡA fracture had a well stability after transpedicle screw fixa-tion and generally satisfied by single-segmental fixation; type Ⅱ fracture and type Ⅵ fracture were not proper for sin-gle-segmental fixation. Clinical therapy should be chosen according to biodynamical studies. However, types of Hang-man fractures were complex, in particular, type ⅡA fracture and type Ⅱ fracture were easily interfered by numerous fac-tors and clinical experiences during diagnosis. Although type ⅡA fracture was a subtype of type Ⅱ fracture, they were apparently different due to complete anterior longitudinal fas-cia, vertebral stability, and therapy choice. Types of fracture were determined with X-ray radiation and CT lamellar scan-ning reconstruction, or even dynamic X-ray examination at C2,3 vertebra preoperatively; furthermore, injured degree of rear structure and vertebral stability needed to be analyzed by euthyphoria to determine which it was used during the surgery, single-segmental or double-segmental fixation.

REFERENCES

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C2椎弓根螺钉置入内固定治疗Hangman骨折的生物力学评价**★

李凭跃1，尹庆水1，夏 虹1，吴增辉1，昌耘冰1，艾福志1，张 余1，赵卫东2
1解放军广州军区广州总医院全军创伤骨科中心，广东省广州市 　510010；2解放军第一军医大学全军生物力学实验室，广东省广州市 510515
李凭跃★，男，1976年生，江西省九江市人，汉族，解放军第一军医大学毕业，硕士，主治医师，主要从事上颈椎研究。
广东省自然科学基金团队项目（20023001）*；广东省医学科研基金（B2007145）*

摘要
背景：Ｃ2椎弓根钉置入内固定治疗Hangman骨折因固定可靠，无生理功能的丢失而受到众多学者的青睐，根据文献报道，单节段固定治疗Hangman骨折的适应证差异较大，并且缺乏生物力学依据。
目的：评价C2椎弓根螺钉置入内固定治疗Hangman骨折的生物力学性能。
设计、时间和地点：2004-05/08在解放军第一军医大学全军生物力学重点实验室完成的对比观察实验。

材料：材料采用AO通用的钛合金颈椎椎弓根螺钉，长度18~25 mm，直径3.5 mm。6具新鲜C1~C4颈椎标本依次制成Ⅰ型、ⅡA型和Ⅱ型Hangman骨折模型。
方法：椎弓根螺钉固定后在非破坏方式下用脊柱三维运动实验机进行测量,通过加载盘对标本施加2.0 N·m的纯力偶矩,使标本产生前屈/后伸、左/右侧弯和左/右轴向旋转6种生理运动;每次测试重复3次加载/卸载循环，在第3次循环时进行运动学测量。
主要观察指标：由激光扫描仪(精度0.1%)摄取在零载荷和最大载荷时的脊柱运动图像,并用相应软件系统进行图像分析,计算出标本的三维运动范围。
结果：Ⅰ型骨折C2椎弓根螺钉固定后相对稳定性在屈曲、后伸、侧弯及旋转达到了对照组的100.62%，96.91%，99.19%，97.12%（P > 0.05）。ⅡA型骨折C2椎弓根螺钉固定后旋转稳定性达到了对照组的 61.86 %（P < 0.05）。Ⅱ型骨折 C2椎弓根螺钉固定后相对稳定性在屈曲、后伸、侧弯及旋转为对照组的 47.84 %，21.29 %，65.98 %，41.69 % （P < 0.05）。
结论：生物力学评估提示，HangmanⅠ型、ⅡA型骨折基本适合C2椎弓根螺钉置入治疗，置入固定后骨折可达生理性固定或稳定性较好; Ⅱ型骨折固定后稳定性较差, 不适合单纯椎弓根钉内固定置入。
关键词：Hangman骨折; 枢椎; 椎弓根; 螺钉; 内固定
中图分类号: R687 文献标识码: A 文章编号: 1673-8225(2008)17-03381-04
李凭跃，尹庆水，夏虹，吴增辉，昌耘冰，艾福志，张余，赵卫东.C2椎弓根螺钉置入内固定治疗Hangman骨折的生物力学评价[J].中国组织工程研究与临床康复，2008,12(17):3381-3384
[www.zglckf.com/zglckf/ejournal/upfiles/08-17/17k-3381(ps).pdf]
（Edited by Ding C/Cui X/Song LP/Wang L）