基于3D打印齿状突空心钉置入的数字化导航

doi:10.3969/j.issn.2095-4344.2015.35.023

中国组织工程研究 ›› 2015, Vol. 19 ›› Issue (35): 5697-5704.doi: 10.3969/j.issn.2095-4344.2015.35.023

• 骨与关节图像与影像 bone and joint imaging • 上一篇下一篇

基于3D打印齿状突空心钉置入的数字化导航

陈宣煌¹，张国栋¹，吴长福¹，林海滨¹，陈旭¹，余正希¹，孙宇庆²

¹福建医科大学教学医院，南方医科大学附属莆田医院，莆田学院附属医院骨科，福建省莆田市 351100；²北京大学第四临床医学院，北京积水潭医院脊柱外科，北京市 100035

收稿日期:2015-06-18 出版日期:2015-08-27 发布日期:2015-08-27
通讯作者: 孙宇庆，博士，北京大学第四临床医学院，北京积水潭医院脊柱外科，北京市 100035
作者简介:陈宣煌，男，1975年生，汉族，福建省人，2007年福建医科大学毕业，硕士，副主任医师，主要从事数字化骨科学研究。

Odontoid cannulated screw fixation using digital navigation based on three-dimensional printing technique

Chen Xuan-huang¹, Zhang Guo-dong¹, Wu Chang-fu¹, Lin Hai-bin¹, Chen Xu¹, Yu Zheng-xi¹, Sun Yu-qing²

¹ Department of Orthopedics, Teaching Hospital of Fujian Medical University; Affiliated Putian Hospital of Southern Medical University; Affiliated Hospital of Putian University, Putian 351100, Fujian Province, China
² Department of Spine Surgery, the Fourth Clinical Medical College of Peking University, Beijing Jishuitan Hospital, Beijing 100035, China

Received:2015-06-18 Online:2015-08-27 Published:2015-08-27
Contact: Sun Yu-qing, M.D., Department of Spine Surgery, the Fourth Clinical Medical College of Peking University, Beijing Jishuitan Hospital, Beijing 100035, China
About author:Chen Xuan-huang, Master, Associate chief physician, Department of Orthopedics, Teaching Hospital of Fujian Medical University; Affiliated Putian Hospital of Southern Medical University; Affiliated Hospital of Putian University, Putian 351100, Fujian Province, China

摘要/Abstract

摘要：

背景：齿状突骨折因部位特殊，毗邻重要解剖结构，置钉难度大，稍有偏移亦可能导致内固定强度下降甚至失效。提高置钉的安全性和准确性，确定方便推广实施的个体化手术显得尤为重要。
目的：探讨Mimics软件数字化设计结合3D打印模块进行齿状突空心钉置入导航的方法，探讨其可行性及准确性。
方法：将16具人尸体颈椎标本进行连续薄层CT扫描，采集Dicom格式图像。Mimics软件予以三维重建，设计C2椎体齿状突空心钉置钉钉道及支撑柱、分割可剥离骨面，设计带钉道的导航模块并3D打印。导航模块在尸体标本上进行置钉导航，观察卡位、置钉情况，以X射线、CT扫描评价置钉效果。
结果与结论：共制作16个导航模块，植入22枚螺钉，观察钉道及置钉后椎体周围骨质，未见爆裂。术后行X射线、CT扫描重建，发现所有螺钉的进钉点，进钉方向、长度均与Mimics软件中模拟的预定理想进钉点、方向和长度一致，导航模块和相对应的椎体前方骨性结构贴合紧密，嵌合度良好，在应用时卡位及稳定性良好。结果证实，在导航模块的辅助下，前路齿状突空心钉置钉精准。基于3D打印的数字化技术有望良好实现骨科内固定物置入导航并普及应用。

中国组织工程研究杂志出版内容重点：人工关节；骨植入物；脊柱；骨折；内固定；数字化骨科；组织工程

关键词: 植入物, 脊柱植入物, 数字化骨科, 齿状突骨折, 数字化设计, 3D打印, 手术导航, 空心钉

Abstract:

BACKGROUND: Odontoid fracture is very common in cervical spine injuries, the special position of odontoid process, which is adjacent to important anatomic structure, makes screw placement difficult, and a slight discrepancy in position and orientation of the inserted screw leads to a decrease in intensity of internal fixation, even invalid internal fixation. Therefore, it is very necessary to develop an individualized treatment protocol by which screws can be precisely and safely placed and which is worthy of clinical popularization.
OBJECTIVE: To study the navigation of Mimics software and three dimensional (3D)-printed module in anterior odontoid cannulated screw fixation and to investigate its feasibility and accuracy.
METHODS: Sixteen human cadaveric cervical spines were scanned by a continuous thin-slice CT scanner. Original DICOM CT images were three-dimensionally reconstructed using Mimics software. The screw channel and support column were designed for C2 vertebra odontoid cannulated screw fixation for odontoid fracture. Segmentation of bone surface was performed. Navigation modules with screw channel were built using 3D printing technique. Navigation modules were used to aid screw placement. Screw fitting and placement were evaluated using X-ray and CT scan.
RESULTS AND CONCLUSION: Totally 16 navigation modules were built and 22 screws were implanted. During and after screw placement, the cortical bone along screw channel and surrounding the vertebral body was not cracked. Postoperative X-ray and CT scans showed that some factors regarding screw placement such as entry point, orientation and depth of placement were consistent with those ideal factors simulated by Mimics software. The navigation modules were closely attached to the corresponding bony structure in front of the vertebral body, with a satisfactory gomphosis. Screw fitting and stability were good during application. These results verify that with the aid of navigation module, anterior odontoid cannulated screw fixation is reliable for treatment of odontoid fracture, which provides insights into the popularization of 3D printing-based digital navigation technique in orthopedic implantation.

中国组织工程研究杂志出版内容重点：人工关节；骨植入物；脊柱；骨折；内固定；数字化骨科；组织工程

Key words: Tissue Engineering, Internal Fixators, Cervical Vertebrae, Computer-Aided Design

中图分类号:

R318

陈宣煌，张国栋，吴长福，林海滨，陈旭，余正希，孙宇庆. 基于3D打印齿状突空心钉置入的数字化导航[J]. 中国组织工程研究, 2015, 19(35): 5697-5704.

Chen Xuan-huang, Zhang Guo-dong, Wu Chang-fu, Lin Hai-bin, Chen Xu, Yu Zheng-xi, Sun Yu-qing. Odontoid cannulated screw fixation using digital navigation based on three-dimensional printing technique[J]. Chinese Journal of Tissue Engineering Research, 2015, 19(35): 5697-5704.

图/表（结果） 1

参考文献

[1] Rajasekaran S, Kamath V, Avadhani A. Odontoid anterior screw fixation. Eur Spine J. 2010;19(2):339-340.
[2] Fountas KN, Kapsalaki EZ, Karampelas I, et al. Results of long-term follow-up in patients undergoing anterior screw fixation for type II and rostral type III odontoid fractures. Spine (Phila Pa 1976). 2005;30(6):661-669.
[3] Pryputniewicz DM, Hadley MN. Axis fractures. Neurosurgery. 2010;66(3 Suppl):68-82.
[4] Böhler J. Screw-osteosynthesis of fractures of the dens axis (author's transl). Unfallheilkunde. 1981;84(6):221-223.
[5] Koller H, Acosta F, Forstner R, et al. C2-fractures: part II. A morphometrical analysis of computerized atlantoaxial motion, anatomical alignment and related clinical outcomes. Eur Spine J. 2009;18(8):1135-1153.
[6] Osti M, Philipp H, Meusburger B, et al. Analysis of failure following anterior screw fixation of Type II odontoid fractures in geriatric patients. Eur Spine J. 2011;20(11):1915-1920.
[7] Brown GA, Firoozbakhsh K, DeCoster TA, et al. Rapid prototyping: the future of trauma surgery? J Bone Joint Surg Am. 2003;85-A Suppl 4:49-55.
[8] Paiva WS, Amorim R, Bezerra DA, et al. Aplication of the stereolithography technique in complex spine surgery. Arq Neuropsiquiatr. 2007;65(2B):443-445.
[9] Xie A, Fang C, Huang Y, et al. Application of three-dimensional reconstruction and visible simulation technique in reoperation of hepatolithiasis. J Gastroenterol Hepatol. 2013;28(2):248-254.
[10] White AP, Hashimoto R, Norvell DC, et al. M orbidity and mortality related to odontoid fracture surgery in the elderly population. Spine (Phila Pa 1976). 2010;35(9 Suppl): S146-S157.
[11] Dantas FL, Prandini MN, Caires AC, et al. Management of odontoid fractures using anterior screw fixation: analysis of 15 cases. Arq Neuropsiquiatr. 2002;60(3B):823-829.
[12] Ochoa G. Surgical management of odontoid fractures. Injury. 2005;36 Suppl 2:B54-864.
[13] Song KJ, Lee KB, Kim KN. Treatment of odontoid fractures with single anterior screw fixation. J Clin Neurosci. 2007; 14(9):824-830.
[14] Agrillo A, Russo N, Marotta N, et al. Treatment of remote type ii axis fractures in the elderly: feasibility of anterior odontoid screw fixation. Neurosurgery. 2008;63(6):1145-1150.
[15] Omeis I, Duggal N, Rubano J, et al. Surgical treatment of C2 fractures in the elderly: a multicenter retrospective analysis. J Spinal Disord Tech. 2009;22(2):91-95.
[16] Chiba K, Fujimura Y, Toyama Y, et al. Treatment protocol for fractures of the odontoid process. J Spinal Disord. 1996;9(4): 267-276.
[17] Wang MY. Cervical crossing laminar screws: early clinical results and complications. Neurosurgery. 2007;61(5 Suppl 2):311-325.
[18] Lantada AD, Morgado PL. Rapid prototyping for biomedical engineering: current capabilities and challenges. Annu Rev Biomed Eng. 2012;14:73-96.
[19] Magarelli N, Milano G, Baudi P, et al. Comparison between 2D and 3D computed tomography evaluation of glenoid bone defect in unilateral anterior gleno-humeral instability. Radiol Med. 2012;117(1):102-111.
[20] Wu ZX, Huang LY, Sang HX, et al. Accuracy and safety assessment of pedicle screw placement using the rapid prototyping technique in severe congenital scoliosis. J Spinal Disord Tech. 2011;24(7):444-450.
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引言

Odontoid fracture is very common in cervical spine injuries. Improper management of odontoid fracture can lead to unstable or dislocated atlantoaxial joint, and spinal cord injury, which result in paralysis, even threaten patient’s life. Anterior odontoid cannulated compression screw fixation is increasingly used in the clinic because of less bleeding, minimal invasion, stable fixation, and high rate of healing^[1]. In addition, short-lasting postoperative rigid neck immobilization can avoid the loss of normal physiological activities of atlantoaxial vertebra caused by posterior fusion and postoperative head and neck pains, and therefore this treatment method is widely accepted by patients^[2-3].

Anterior odontoid cannulated compression screw fixation for odontoid fracture was first used by Bohler in 1981^[4]. In this surgery, only the fractured stump is fixed, without destroying the normal structure of the atlantoaxial joint. Anterior odontoid cannulated compression screw fixation preserves intact axial rotation and motion function of the cervical spine without destroying the balance of normal biomechanics of the cervical spine, and has the advantages of stable
biomechanics, high bone fusion rate, little influences on postoperative activities of the neck, and low costs, which better solves the contradiction between stable reduction of fractures and range of motion^[5].

Biomechanically, this surgery is a “physiological reconstruction process”. However, the special position of odontoid process, which is adjacent to important anatomic structure, makes screw placement difficult, and a slight discrepancy in position and orientation of the inserted screw leads to a decrease in intensity of internal fixation even invalid internal fixation, which influences surgical curative effect. The important structures surrounding the odontoid process may be injured, leading to irreparable consequences.

The conventional barehanded screw placement technique should be performed based on two-dimensional image analysis, is not suitable for three-dimensional and individualized requirements, and often results in discrepancy in screw placement owing to operator’s experience, leading to catastrophic surgical complications. Anterior odontoid cannulated compression screw fixation is limited in hospitals where there are no expensive computer-based surgical guidance equipments because of repeated C-arm X-ray device use and required surgical experience and expertise. Therefore, it is very necessary to develop an individualized treatment protocol by which screws can be precisely and safely placed and which is worthy of clinical popularization.

The emergence and continuous development of digital three-dimensional reconstruction technique provide novel solutions for modern orthopedic surgery. Mimics is an encouraging software that can help reconstruct three-dimensional models of the surgical region based on original thin-layer CT scans, providing intuitive and valuable information. This can raise the precision and safety of screw placement[6]. Brown et al^[7] applied three-dimensional (3D) printing technique in 117 cases who received complex orthopedic surgery and they concluded that the object models printed by 3D technique benefited for preoperative evaluation of the severity of disease, helped accurately measure the size and length of required internal implants, predict the position of implantation and develop operative scheme, and aided intraoperative adjustment. Increasing attention has been paid to 3D printing technique used for individualized module production for orthopedic diseases^[8]. Computer-aided surgical navigation technology is important in reducing operation time and the risks produced during operation^[9].

Based on the highly developed medical imaging technique, computer technique and 3D printing technique, this study built navigation modules using Mimics software to aid anterior odontoid cannulated compression screw fixation for odontoid fracture in 16 human cadaveric cervical spines. Satisfactory effects were acquired.

中国组织工程研究杂志出版内容重点：人工关节；骨植入物；脊柱；骨折；内固定；数字化骨科；组织工程

材料方法

Design
This was a method validation study.
Time and setting
The study was performed in the Department of Orthopedics, Affiliated Putian Hospital of Southern Medical University from January to June 2014.

Materials
Sixteen formalin-soaked adult cadaveric cervical spines without odontoid vertebra fracture, bony structure defects or space-occupying lesions were included in this study. These specimens (10 males and 6 females, with mean age at death 48.6 years, range, 36-71 years) were provided by Center for Clinical Anatomical Research of PLA Nanjing Military Region, China. Single screw implantation was performed in 10 human cadaveric cervical spines and double screw implantation in 6 human cadaveric cervical spines.

Windows 7 Ultimate 64-bit computer running Mimics 14.0 software was used (Materialise, Leuven, Belgium; provided by Department of Human Anatomy, Southern Medical University of China). Hardware included a 3D printer (MakerBot Replicator2, Brooklyn, NY, USA), Dell Precision M6800 workstation, and Seagate SATA 1T hard disk. There were 64-row 128-slice volumetric CT scanner, 3.5 mm titanium hollow compression screw, 2.0-3.5 mm diameter hollow tap, electric drill and 2.0-3.5 mm drill tip, 1.2 mm Kirschner wire, which was provided by Beijing Libeier Bio-engineering Institute Co., Ltd., Beijing, China.

Methods
Specimen management
CT scanning: Human cadaveric cervical spines were scanned by 128-slice CT scanner using the scan parameters including tube voltage 130 kV, tube current 37.8 mAs, and slice thickness 0.75 mm. DICOM images (512 pxl × 512 pxl) were saved.

Cervical spine specimen preparation: Through an anterior approach, the neck of the human cadaveric cervical spine was in a hyperextension position and the lower mandible was raised. A transverse skin incision was made on the right side of the neck at C3-4 intervertebral space level. After the platysma was incised and bluntly dissociated till the inferior border of the C2 vertebral body. The bone surface of the C2 vertebral body in the longus colli was exposed. The midline of cervical vertebrae was located and the midpoint of the inferior-anterior margin of the C2 vertebral body was determined.

Three-dimensional reconstruction
DICOM images of CT scans of the cervical spine were directly read, imported and organized by Mimics 14.0 software. Thresholding can be used to create selection masks. That is to say, without compromising the appearance of three-dimensional modules, thresholds can be decreased, extremely low threshold value possibly leads to an increase in mask pixel, and extremely high threshold value may result in a decrease in mask pixel. In this study, the threshold values were designated according to the Bone (CT) Scale in Mimics, i.e., 226-1 448 Hu. Segmentation of adjacent vertebrae was performed. The redundant data of each fault image were discarded by selective processing and noise reduction. Three-dimensional models of the cervical vertebrae were primarily acquired. Three-dimensional reconstruction of Mask was performed for better understanding surgical process and surgical location. Specific regions of interest can be edited with 3D tools using Mimics medical image processing software to separate individually and specify any color. The masks and 3D models created during all processes should be named in time to avoid errors. The temporally generated masks and 3D models can be deleted.

Design of virtual screw channel and support column and segmentation of bone surface
Using the “MedCAD module” of Mimics software, a “Cylinder” was newly created and its attribute was defined with “Radius” being 1.5 mm. The “Cylinder” was the screw channel pole, which can simulate the channel for screw passing through (Figure 1A). In the transparent view, CAD Objects were selected and the radius of the “Cylinder” was adjusted to be 2 mm. The diameter of the “Cylinder” can be rotated at any angle or increased. Caution was taken to avoid destroying the bone surrounding the screw channel, and the scheduled ideal position of the entry point was located by repeated adjustments. The entry point was about at 1 mm posterior to the midpoint of inferior-anterior margin of the C2 vertebral body. The distance between entry point to the tip of the odontoid process and the axis for screw insertion were determined using the Measurement Distance function. The support column (radius 7 mm) was designed based on the screw channel pole (Figure 1B).

Within the range inside the attached part of bilateral longus colli before the C2 vertebral body and below the upper transverse ridge line (i.e., body surface can be dissected in the practical work), bone surface was segmented through the use of “Cut Orthogonal to Screen”. A solid block was established on the bone surface. The ventral surface of the solid block had a reversed structure relative to the anatomical structure of the body surface, which benefits for better fixing the base plate of the navigation module, like an anastomosis between Chinese words “凹” and “凸”. The solid block can be panned and copied till ideal thickness (Figure 1C).

Design of navigation module with screw channel
The navigation module was composed of a screw channel pole, a support column and a solid object. After Boolean operation, a prototype of navigation module was obtained. After trimming unrelated parts, an ideal navigation module with screw channel was achieved (Figure 2).

Production of navigation modules by 3D printer and fitting observation
The STL format file of the navigation modules was processed by 3D printing software MakerWare version 2.4.1.24. Under high print precision mode (print precision: 0.1 mm), polylactic acid navigation module products were 3D printed, screw channel was dredged and the fitting of navigation module product was observed (Figure 3).

Screw channel determination and screw placement in human cadaveric cervical spines using navigation modules
The navigation module was closely attached to the surface of the bone of the C2 vertebral body in the field of operation and fixed by compression using fingers. The Kirschner wire was advanced through the navigation module till the posterior-anterior margin of the C2 vertebral body.

An expected length was drilled using an electric drill. After removal of navigation base plate and Kirschner wire, the pore was expanded using a tap. Then, the screw with known length and diameter was screwed along the screw channel. During the process of screw placement, no X-ray or other auxiliary means were used to confirm or amend screw channel (Figure 4).

Evaluation of screw placement
After screw insertion, the entry point and orientation as well as whether screws penetrate the cortical bone were observed. X-ray and CT scans were used to determine whether the entry point, orientation, depth of placement and diameter of screws were consistent with those ideal parameters simulated by Mimics software.

Main outcome measures
X-ray and CT scans were used to determine the accuracy rate of screw placement.

中国组织工程研究杂志出版内容重点：人工关节；骨植入物；脊柱；骨折；内固定；数字化骨科；组织工程

讨论

Current anterior approaches for odontoid fracture and their shortages

Odontoid fracture accounts for 9-15% of all cervical spine structures^[10], with a mortality rate of 5-10%^[11]. There is a consensus that the surgical indications of odontoid fracture^{[1, 3, 12]} include unstable Anderson II odontoid fracture, in particular those displaced fracture (displaced over 4-6 mm in distance or > 10° in angle); unstable comminuted Anderson III odontoid fracture; odontoid fracture accompanied by rupture of transverse ligament of the atlas; odontoid fracture accompanied by atlas fracture, leading to occipitocervical instability; displaced fracture for which reduction is impossible, nerve compression-caused neurological disorder; obsolete odontoid fracture, bone nonunion, or atlantoaxial instability. Surgical contraindications include (1) comminuted fracture of the base of odontoid process, severe osteoporosis; (2) rupture of ligamentum transversum; (3) oblique fracture. In addition, it is disputed whether long-term (> 6 months) odontoid fracture is a contraindication^{[3, 13-14]}.

The atlantoaxial joint plays an important role in the rotation of the cervical spine. After odontoid fracture, the coupling movement of axial rotation, flexion-extension and lateral flexion of the atlantoaxial joint is most obvious^[11]. Therefore, recovery of the pivot role of the odontoid process in the occipital-cervical region is a key to treat odontoid fracture^[15]. In terms of surgical approach and methods, there is no obvious difference in bone union between anterior cannulated screw fixation and posterior C1-2 fusion^[15]. Anterior odontoid cannulated compression screw fixation is a highly efficient safe treatment method of type II odontoid fracture^[10], while posterior atlantoaxial fusion technique is difficult to be applied in the majority of basic-level hospitals owing to its strict requirements. Chiba et al ^[16] reported that anterior odontoid cannulated compression screw fixation almost completely reserves the full range of motion of the cervical spine, in particular the rotation function, and therefore, the incidence of frequent neck pains is markedly decreased compared to posterior fusion. The anatomical structure of the cervical spine is special and complex, so some important organs, such as vertebral artery, superior laryngeal nerve, and recurrent laryngeal nerve, are easily injured owing to a small space for operation, resulting in many surgical difficulties and high risk. During the whole process of screw placement, fluoroscopic monitoring is required to avoid the displacement of guide spin, but it is not always easy to reach the theoretical optimal position. Therefore, in the clinical work, the operators are required to use caution and to have abundant clinical experience and skills. Computer-aided surgical navigation technology meets the principle of three-dimensional screw placement and greatly increases the precision of screw placement, but its operation is complex and costs much and therefore is limited in clinical popularization in basic-level hospitals^[17-18]. A practical and simple method is necessary in the clinic to aid screw placement. This method should have the following advantages: independent of operator’s experience, high accuracy of screw placement, short operation time, reduced intraoperative X-ray device use, low costs, and easy to be popularized.

Application of digital design in the treatment of odontoid fracture
The MedCAD function of Mimics can visually simulate the odontoid screw fixation. First, the entry point and orientation of screw can be simulated. According to measurement parameters, the diameter and length of placement of cannulated screws are designed. The simulated odontoid vertebra can be rotated at any angle, thus, the structure of screw channel, orientation of screw and the anatomical relationship with adjacent structures can be observed from all angles. Once the screw penetrates the cortical bone of the odontoid process at one point, the orientation of screw insertion can be adjusted till the satisfactory position, even the theoretical optimal position. Therefore, the Mimics software provides insights into selection and preparation of therapeutic protocol for clinical surgeons^[19].

Application of 3D printing technique for odontoid fracture
3D printing technique sets up a bridge between virtual design and clinical practice and has been widely recognized as a method that is accordant with the concept of individualized medicine. Paiva et al^[8] applied 3D printing technique in one patient with cervical Ewing’s sarcoma and they considered that this technique benefited for preoperative scheme preparation and intraoperative risk evaluation.

The individualized digital navigation screw fixation is a simple and cheap method that is performed based on location of anatomic landmark. This method can reduce intraoperative X-ray exposure, can be repeatedly used, has a relatively short learning curve, and is safe and highly accurate^[20-21]. There is a “concave-convex”-shaped bone surface in the area surrounded by bilateral longus colli, upper transverse ridge line and inferior-anterior margin of C2 vertebral body. The bone surface appears to be a “triangle” small convex with outward concave surface. This is different from other planar bone surface in the other cervical bodies. This distinct bony structure is used as the substrate region of navigation module and is also the region that can be stripped during the operation. In this surgical region, the soft tissue of the cortical bone is easily to be stripped, which makes the navigation module accurately attaching to the C2 vertebral body surface possible and increases the brake’s stability of navigation module.

Advantages of digitally navigated odontoid cannulated compression screw fixation based on 3D printing technique

The Mimics software helps design module: The Mimics software has been widely used in daily teaching and research, is easily learned and convenient for popularization because of its short learning curve. It can to the largest degree reduce data loss risks produced during transformation by multiple software formats^[22].
Achieve one-time successful implantation and reduce radiation damage: avoid the formation of false or excessively wide screw channel caused by determining entry point or adjusting guide pin orientation too many times; ensure suitable and firm fixation of screws; reduce the incidence of screw loosening, shedding and fragmentation; avoid the occurrence of a series of adverse events caused by repeated operations, including iatrogenic injury of C2-3 intervertebral discs, destroyed blood supply around the fracture line, worsened displacement and rotation of distal end of fractured odontoid process and spinal cord injury; reduce X-ray device use and avoid operator’s and patient’s long-term exposure to X-rays.

To increase surgical precision and achieve individualized treatment: The optimal diameter, length of placement and orientation of screws can be preoperatively determined, and the screws are implanted by module navigation. Thus, effort should be made to minimize the discrepancy from theoretical situations. The location by navigation modules cannot be influenced by body position change and the relative movement of adjacent vertebral bodies during the operation. Preoperative accurate design and simulation of single or double screw placement can reduce surgery difficulties and greatly increase success rate. Operation time, blood loss, anesthesia time and surgery cost are reduced, which embodies the principle of individualization.

Reduce operation difficulties and facilitate popularization: in the digitally navigated odontoid cannulated compression screw fixation, application of navigation module is simple and convenient. Accurate location and orientation of screw can be achieved by module closely attaching to the exposed bone surface. Screw channel preparation does not depend on surgeon’s experience, and screw placement can be simply operated. Digitally navigated odontoid cannulated compression screw fixation can be popularized only based on being familiar with the knowledge regarding anterior approach of the neck and the basic equipments for orthopedic operations. 3D printing is fast and simple to use, costs less, and can provide individualized modules.

Several suggestions for digitally navigated odontoid cannulated compression screw fixation based on 3D printing technique
CT scan layer thickness and quality of specimens: the thinner layer, the higher level of integrity of three-dimensional reconstruction models, and the more satisfactory attachment of the navigation module designed according to the bone surface of reconstructed module to cadaveric vertebral spine, on the contrary, thick layers lead to rough surface of three-dimensional models, resulting in unstable attachment of guiding module and finally influencing the optimization of screw placement. If the sclerotin of cadavers is poor, scan data can be supplemented by the Mimics software, thus a bias between virtual design and practical specimen possibly occurs.

Management of bone surface soft tissue: when the base plate of the module is designed, soft tissue attachment should be excluded, and the body surface which can be stripped with small ridge line gradient amplitude and large area for braking is selected. During the operation, soft tissue on the bone surface should be stripped as possible, without destroying bony structure.

Management prior to digitally navigated screw placement: the length of the support column of the navigation channel should be appropriate, deviated screw channel may be occurs if the support column is too short, but too long support column is likely to be broken by surrounding tissue. Preoperative preparation is necessary to make the screw channel of the navigation module product unobstructed. Caution should be taken when drilling a screw channel using an electric drill along the targeting pore to achieve the orientation designed by the module. 3D printed navigation module product has a pedestal to support, which can be directly stripped. The possible granular or stripped printing materials should be removed to make the base plate of the bone smooth. The specification of placed screws is determined by the measured data according to navigation modules, ensuring reliable screw fixation.

During the operation, any tiny displacement of bone surface or navigation module will influence the accuracy of screw channel. After screw placement, the surrounding contact should be observed. When single screw is placed, constant tight compression is required; when double screws are placed, after the Kirschner wire is detained on one side, the C2 vertebral body and navigation module are ensured in the same screw channel, thus the slight movement produced during the process of Kirschner wire insertion on the other side can be reduced, making sure the stability of navigation modules, achieving optimal screw trajectory, and increasing the accuracy of double screw placement.

This study investigated anterior odontoid cannulated compression screw fixation using individualized navigation module. Individualized navigation modules were designed according to the practical situations of different bone surfaces based on the principle of reverse engineering. Under the assistance of 3D printed modules, high accuracy of anterior odontoid screw fixation was achieved in human cadavers. In this study, odontoid fracture models were not developed, and anterior odontoid cannulated compression screw fixation was only preliminarily investigated from the perspective of screw placement methodology. However, in the clinical work, many operation skills regarding odontoid fracture are needed to be summarized and applied. With constant development and in depth research of digital medicine, the present findings provide insights into screw placement in the cervical vertebrae, thoracic vertebrae, lumbar vertebrae
and other pedicles of vertebral arch or into internal implantation in other regions and demonstrate wide clinical application prospects.

中国组织工程研究杂志出版内容重点：人工关节；骨植入物；脊柱；骨折；内固定；数字化骨科；组织工程

基于3D打印齿状突空心钉置入的数字化导航

Odontoid cannulated screw fixation using digital navigation based on three-dimensional printing technique

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