Introduction

Pedicle screws are commonly used for rigid fixation in spine surgeries[1-3].They provide robust and quick stability.Thoracic fractures represent 20%to 30%of vertebral fractures and this spinal segment has very little mobility.The deformities in the sagittal plane are often less dramatic when compared to the lumbar spine.However,neurological complications are more likely due to the presence of the spinal cord throughout the thoracic segment and small diameter of the spinal canal[4].Several techniques have been developed to increase the accuracy of pedicle screw placement,including intraoperative CT,image-assisted navigation and intraoperative fluoroscopy in the thoracic spine[5-7].Moreover,some techniques of freehand pedicle screw placement in the thoracic spine primarily are based on medial angulation and various entry points[8-11].Although the accuracy of screw placement was increased　slightly,　these　techniques　increased radiation exposure and operative time to patients and surgeons[8-9].Roy-Camille et al.[2]proposed a screw pedicle placement method that the screws were parallel to the vertebral endplate,and the entry point was located in the intersections of the thoracic transverse process midline and the facet joint midline.

Kim et al.[12]indicated the entry points of the thoracic pedicle screw placement in different levels:1.Proximal thoracic(spine T1,T2):the intersections of the bisected transverse process and lamina at the lateral pars.2.Midthoracic(spine T7-T9):the intersections of the proximal edge of the transverse process and lamina,just lateral to the mid-portion of the base of the superior articular process.3.Lower thoracic(spine T11-T12):the intersections of the bisected transverse process and lamina at the lateral aspect of the pars.

In the present study,we described our experience with pedicle screw placement in the thoracic spine using a free hand technique by a novel entry point.In this technique,pedicle screws were placed via the approach between the multifidus and the longissimus,focusing on the entry point which was located on the intersection of the line of the lateral edge of the inferior articular process and the bisected transverse processes.The study was aimed to evaluate the accuracy,safety and biomechanical stability of this technique.

宋代徐铉校定本《说文解字·卷十·夭部》对“乔”字的说解为：“高而曲也。从夭，从高省。《诗》曰：‘南有乔木。’巨娇切。”清代段玉裁的《说文解字注》这样写到：“按乔不专谓木。浅人以说木则作‘桥’，如《郑风》‘山有桥松’是也。以说山则作‘峤’，释‘山锐而高峤’是也。”可见，“乔”字刚开始是表示树木或其枝干高大弯曲的。元以后，“乔”字主要作形容词和动词，有贬低的感情色彩，它的意义也发生了一定的引申变化。

Materials and methods

Patients

Normal thoracic CT images at the transverse plane were obtained from the Imaging Department of the authors' affiliated hospital(76 patients,including 40 males and 36 females,median age:46 years;range:18-70 years)were recruited.The images of spine fractures,spondylolisthesis,tumor,and severe degeneration were excluded.This clinical study was approved by the Ethical Committee of the First Affiliated Hospital of Nanjing Medical University,and written informed consent was obtained from all patients.

这是商景兰对于自我价值的理解和思考，是女性觉醒的可贵声音。以“才”为归结点，商景兰把现世幸福的失落，在“立言”以求不朽中得到了超越，从而将文学活动作为生命苦难的感性慰藉，升华到扬名后世的理性追求。商景兰后半生的生命探索与价值追寻，至此划上了圆满的句号。

Radiological measurements

从以上分析不难看出，经济是旅游的表象，文化是旅游的本质。从旅游发展史看出，各个时期的旅游活动都有其独特的表现形式及其不同的主题倾向，但在本质上具有一个共同之处，即旅游者在旅游活动中追求的是文化享受③。

The distance was measured from the traditional entry point to the vertebral anterior edge(L).Then,we measured the angle relationship between the approach via this entry point and the sagittal plane.This angle was called transverse pedicle angle(TPA)(E).

Fig.1　Two different entry points in the thoracic spine.The novel entry point(P)is located on the junction point of the line of the lateral edge of the inferior articular process and the bisected transverse processes.The traditional entry point(P’)in this study is the point created by the intersection of the bisected transverse process and the midline of the superior facets.

Type BLR-1 biomechanical testing machine(maximum power 20kN,precision grade 0.5,date of 2005～02)was used in the study.

Biomechanical analysis

Fig.2　The transverse pedicle angle(TPA)via two different entry points.A:the TPA via the traditional entry point for screw pedicle placement(E).B:the TPAvia the novel entry point for screw pedicle placement(E').

Fig.3　The measurement method of the distance from the entry point to the vertebral anterior edge.A:The left line is the screw length through the traditional entry point(L).B:the right line is the screw length through the novel entry point(L').

The thoracic pedicle anatomy of 6 cadaveric specimens was studied at the authors’affiliated institution.Three males and 3 females were included.The age of the specimens ranged from 38 to 80 years with an average age of 52 years.None of the cadavers had a history of spinal trauma,infection,neoplasm or developmental malformation.The study protocol was approved by the local institutional review board at the authors' affiliated institution and the study was carried out in accordance with the institutional and state guidelines regarding use of human cadavers.The spinal surgical instruments were provided by Shanghai Sanyou.The types of screw are shown as follows(diameter,length):(4.0 mm,30 mm),(4.0 mm,35 mm),(4.5 mm,35 mm),(4.5 mm,40 mm),(5.0 mm,40 mm),and(5.0 mm,45 mm).

The measurement method for the novel entry point was similar to the one mentioned above.We measured the distance from the novel entry point to the vertebral anterior edge(L').Also,the angle relationship between the approach via the novel entry point and the sagittal plane was measured,and was called TPA(E').The measurement method of TPA via the two different entry point is shown in Fig.2.The measurement method of the distance from the entry point to the vertebral anterior edge is shown in Fig.3.

由图8可知，无论是数据通道还是导频通道，码相位比较法比门限法的发现概率要高。在使用本文设置参数的情况下，在使用码相位比较法进行捕获判决时，数据通道的发现概率比导频通道的发现概率高。

Group A used the traditional entry point on the left sides.Group B used the novel entry point on the right sides.The choice of the pedicle screws in each segment is shown in the Table 1.All operations were completed by one surgeon,and after location,we observed the pedicle screws in three-dimensional C arm to compare the success rate between the two methods(Fig.4).The definition of success:the screws were located in the pedicle cortex totally.The definition of failure:the screws punctured through the pedicle cortical bone.Subsequently,the specimens were fixed in a special mold,using the electronic universal testing machine BLR-1,with 5 mm/minute speed to test maximum axial pullout strength of the two methods(Fig.5).

Table 1　The choice of the pedicle screws in each segment

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Fig.4　The pedicle screws in the three-dimensional C arm.A:Lateral X-ray film after screw placement;B:frontal X-ray film after screw placement;C:transect X-ray film after screw placement.

Statistical analysis

The longest average screw length was[L=(33.73±4.55)mm,L'=(40.10±5.63)mm]at T11 level in males and[L=(28.77±1.51)mm,L'=(31.28±1.51)mm]at T11 level in females.The shortest average length[L=(24.56±2.13)mm,L'=(30.98±2.77)mm]in males and[L=(24.57±0.83)mm,L'=(26.60±1.22)mm]in females was at the segment of T1.There was a significant increase in screw length from the T1 to T11 thoracic vertebrae(P＜0.05).The screw length of male vertebrae were significantly longer than that of female vertebrae in the majority thoracic levels(P＜0.05)except the levels of T1 and T2.The results of the TPA and the screw length are shown in Table 2.

第一个前提，对无人驾驶时代的准确把握。根据美国机动车工程师协会(SAE)关于无人驾驶分级的理论，第一等级、第二等级无人驾驶汽车目前实现的可能性很大，根据现在各个汽车制造商以及互联网巨头的研发势头以及各国法律对无人驾驶合法性的认可，可以预见未来几年内，道路上行驶的机动车主体有：传统汽车、第一等级的无人驾驶汽车、第二等级的无人驾驶汽车、第三等级的无人驾驶汽车、第四等级的无人驾驶汽车甚至包括第五等级的汽车，如果自动驾驶功能的汽车在道路上行驶占主导地位，数量与规模远超传统汽车，则无人驾驶时代来临基本上可以断定。这是分析不同类型汽车交通肇事罪构成的前提。

Results

TPA

The widest TPA was(E=24.56°±2.13°,E'=30.98°±2.77°)at the T1 level in males and(E=31.88°±1.49°;E'=34.30°±1.38°)in females.The narrowest TPA was(E=-4.54°±1.35°;E'=-2.54°±1.85°)at the T11 level in males and(E=-2.54°±1.85°,E'=-0.07°±1.23°)in females(Table 2).Statistical analysis showed that the TPA gradually decreased from T1 to T11 and the TPA of the novel entry point was significantly greater than that of the traditional group(P＜0.05).Statistically significant difference in TPA was found between females and males at the level of T1,T4,T5 and T10(P＜0.05).

Fig.5　The electronic universal testing machine BLR-1

Screws lengths

SPSS 13.0.was employed to conduct statistical analysis.All continuous data were presented as mean-±standard deviation(SD)and all categorical data as percentages or numbers.Statistical analyses for comparisons between groups were performed using the independent Students' t-test,the frequencies and the failure rates between two groups were compared using the chi-square test,and P＜0.05 was considered statistically significant.

The conventional entry point in this study was the point created by the intersection of the bisected transverse process and the midline of the superior facets.The novel entry point in this study was located on the intersection of the line of the lateral edge of the inferior articular process and the bisected transverse processes.The novel entry point(P)and the traditional entry point(P')are shown in Fig.1.

Success rates

From T1 to T11 levels,excluding failure screws,there were no significant differences in axial pullout strength between the novel entry point group and the conventional entry point(P＞0.05).In the novel entry point group,T11 had the largest average maximum axial pullout strength(604.71±223.06),and T1 had the smallest average maximum axial pullout strength(323.53±208.78).In the traditional entry point group,the largest average maximum axial pullout strength was in the level of T11(539.22±184.15),and the smallest average maximum axial pullout strength was in the level of T2(219.18±89.32).The average maximum axial pullout strength of the two groups is shown in Table 4.

Table 2　Measurement of the transverse pedicle angle and the screw length for male and female

*P＜0.05;**P＜0.01

Axial pullout strengths

A total of 132 screws were implanted in 6 cadavers(66 from novel entry points and 66 from traditional entry points).The mean age,weight and height of thecadavers in the two groups were similar.The failure rates of the two different techniques are shown in Table 3.In detail,the failure rate was 12.1%in the novel entry point group,and 15.1%in the traditional entry point group.The results revealed no significant differences between the two groups.The pedicle screw failure rate was the highest in T4 and T5 for both techniques(33.3%in the novel entry point group,and 41.6%in the traditional entry point group).

还有一次，我一个人在家。妈妈临出门时告诉我：“10点到11点是你的自由阅读时间，11点以后要去室外活动，顺便买一瓶酱油回来。”可是，当我看《皮皮鲁外传》时，又被那精彩的故事吸引……妈妈回来时，见我还在看书，严肃地说：“以后，我要把你的书全没收了，你看看你，这么小的年龄就戴上了眼镜！酱油呢？”我连忙站起来，扔下一句“马上就去”就一溜烟跑出去了。

Discussion

Transpedicular screw fixation is becoming more common in spine surgery[13-15].However,this procedure is technically challenging and brings along complications,particularly in the thoracic spine.It is proved that minimally invasive surgical techniques bring many bene fits to patients.However,diminished visualization of anatomy may increase intraoperative complications for surgeons and patients[16-17].Accordingly,it is imperative to evaluate varying surgical methods and navigation aids so as to optimize clinical outcomes.Image navigation systems have been considered to improve the accuracy of transpedicular screw placement.Substantial research suggests that intraoperative CT navigation improves the accuracy of pedicle screw placement[18-21].However,this surgical method will cause substantial radiation exposure to surgeons and patients.The ideal pedicle screw placement technology has the characteristics of constant needle entry points,less radiation exposure,little trauma,and high accuracy rates[22].Based on the previous studies,we improved a minimally invasive thoracic pedicle screw technique,with minimally invasive incision,exposing the inferior articular process and transverse process.We chose the outer edge of the extension line of the inferior articular process and the middle line of the transverse process as the entry points.After screw placement,the position was con firmed by X-ray examination,and radiation exposure was significantly reduced.

The TPAwith this novel entry point of thoracic screw placement is an important factor for the accuracy of screw placement.Cinotti et al.[23]pointed out that different thoracic pedicle TPAs in different segments were an important factor in the failure of the screw placement.According to our studies,the widest average angle was 33.72±4.68 degrees(T1 level)in males and 34.30±1.38 degrees(T1 level)in females.The narrowest average angle was 2.54±1.85 degrees(T11 level)in males and 0.07±1.23 degrees(T11 level)in female.TPA decreased from T1 to T11 levels.TPA had no significantly differences between males and females with the exception of the average TPA on T4 level which was significantly wider in females than in males.The finding is similar to previous studies that the narrowest angle was always found at T11 level.Compared with the traditional entry points,the novelentry point had the characteristics of more partial entry points,larger TPA,and a lower probability for the screw to penetrate through the vertebral body.Thus,this new entry point for pedicle screw placement was better in safety and stability.

Table 3　The number of failed pedicle screws of the two methods

Table 4　The axial pullout strength of the two kinds of screw pedicle placement

According to a previous study[24],the length of pedicle screw had a significant influence on the biomechanical stability.Weinstein et al.[25]found that the fixation strength increased by 15%-20%when the screws were in the vertebral cancellous bone,increased by another 16%while the screws were in the cortical bone,and increased by another 20%-25%when the screws penetrated through the cortical bone.Failure in axial pullout was tested using the electronic universal testing machine BLR-1.Longer screws showed higher pullout strength than all the other screws.Mckinley's study[26]con firmed that the 35 mm long screw bears the bending moment increased by 16%when compared to the 40 mm screw in the vertebral body.Barber et al.[27]showed that paired pedicle screws at 30 degrees of convergence offered more resistance to axial pullout(28.6%on average)and sustained higher loads at the clinical threshold of loosening(101%on average)than paired pedicle screws placed in parallel.Therefore,longer screw length and larger TPA make the stability of pedicle screw fixation better.Our study suggested that the screws of the new entry point group are longer than those of the traditional group.Therefore,compared with the greater screw length and the larger TPA,the novel entry point for pedicle screw placement has better stability than the traditional group.

Almost all published studies showed that the breach rate of the thoracic level is the highest in the spinal segments[28].In our study,we developed a new entry point with the aim of improving the accuracy and safety of pedicle screw technique.The results demonstrated that pedicle screws can be accurately and safely inserted with the new entry point.According to the statistical data,we can conclude that this new entry point significantly improved the accuracy rate of pedicle screw placement compared to the traditional entry point.The failure rate of screw placement was relatively higher at the level of T4-T5 in both groups.The reason was that the level of T4-T5 showed smaller diameters,the relatively thinner pedicles and lower angles of convergence than other pedicles[29],it is difficult to make correct placement of pedicle screws.Therefore,it is not recommended to refer to this new entry point at the T4-T5 level.To ensure the safety and the accuracy of pedicle screw placement,radiographic data should be studied carefully before the operations to evaluate the anatomic relationship of pedicle diameter,pedicle length,and the TPA.

In conclusion,our study con firmed that this new entry point can significantly improve the accuracy rate and make the screws more stable in the process of pedicle screw placement.Moreover,this new entry point can avoid extensive exposure of the paraspinal muscles,ligaments and the facet joint,without affecting the stability of the spine.Therefore,the novel entry point of pedicle screw placement is safter,simpler,more accurate,and more stable than the traditional entry point in the thoracic spine.

Acknowledgements

This work was supported by the National Natural Science Foundation of China(Grant#81371968,#81371969,#81401791);the Program for Development of Innovative Research Team in the First Affiliated Hospital of NJMU(No.IRT-015),and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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[26]McKinley TO,McLain RF,Yerby SA,et al.Characteristics of pedicle screw loading.Effect of surgical technique on intravertebral and intrapedicular bending moments[J].Spine(Phila Pa 1976),1999,24(1):18-24.,discussion 25.

[27]Barber JW,Boden SD,Ganey T,et al.Biomechanical study of lumbar pedicle screws:does convergence affect axial pullout strength[J]?J Spinal Disord,1998,11(3):215-220.

[28]Karapinar L,Erel N,Ozturk H,et al.Pedicle screw placement with a free hand technique in thoracolumbar spine:is it safe[J]?J Spinal Disord Tech,2008,21(1):63-67.

[29]Panjabi MM,Goel V,Oxland T,et al.Human lumbar vertebrae.Quantitative three-dimensional anatomy[J].Spine(Phila Pa 1976),1992,17(3):299-306.