| BackgroundCervical pedicle screw technique is one of many methods of treatment of cervical diseases as an effective means. However, compared with the thoracolumbar, cervical pedicle tiny, anatomical variations and large complex surrounding anatomical structures, pedicle screw placement is difficult and with high risk. Currently, cervical pedicle screw placement methods are anatomic studies, radiography techniques and computer-assisted surgery system, etc. Although anatomic method to determine entrance point through vertebral laminae anatomical structure, but mainly rely on the experience of the surgeon, and have a high incidence of pedicle screw misplacement. radiography techniques has shortcomings like surgery for a long time and both the patients and the surgeons have to suffer from a large amount of X-ray radiation. Computer-assisted navigation provides accurate real-time and multi-angle information, but the equipment is expensive, complicated to operate and the accuracy is not high enough, thus this method does not been widely promoted.In recent years, with the rapid prototyping (RP) technology is widely used in the medical field, some academics have suggested the principle of customized pedicle screw insertion, which is to design customized drill template based on the anatomy of different pedicle in position, direction and depth. First,3D reconstruction model of each cervical vertebra was obtained. Then through the use of reverse engineering software to extract the surface contour of the patient’s 3D model of the vertebra to calculate the optimal screw channel location. After that, design a customized drill templates based on the vertebra laminae. Finally, rapid prototyping machine converted into physical drill template. The drill templates with preplanned trajectories may allow precise drilling and screw placement in the cervical spine while reducing the risk of spinal cord injury. Moreover, the drill template is simplicity of application, as long as the template to fit closely with vertebral laminae. Through this method, fluoroscopy is not necessary during screw insertion. Thus, the radiation exposure could be considerably reduced for patients and doctors. RP technology is a well-accepted technology to produce the drill templates. However, The material of drill templates, such as polymer resin, possess significant cytotoxicity. In the process of surgical, debris, caused by the action of drilling, could get in contact with the wounded area. There are a potential danger if the debris remain in the body. Furthermore, this material had a low hardness. The drill channel would shift during drilling thus affect the accuracy of the drill templates.How to make a both non-toxic to the human body, but also high-precision of the drill template become the focus of this study. In this study, we explored a novel method for making customized drill template, aimed at finding a new feasible method of produced the drill template. As the reverse engineering of molding technology, in addition to the existing rapid prototyping (RP) technology, the Computer numerical control (CNC) technology is more mature. CNC technology, invented in 1952, through the development of several generations of CNC machine tools, especially the appearance five-axis machine, which can process complex parts. Today, CNC machining is the most accurate process capable of producing objects out of any material.Therefore, this study would use a three-dimensional reconstruction technique, reverse engineering, CNC machining technology and RP technology to design and produce to customized drill template and evaluated its accuracy in assisting cervical pedicle insertion, which compared with that by RP technique.ObjectiveConsidering the good biomechanical property of cervical three-column fixation and relative instability of posterior cervical vertebral body fixation, pedicle screw fixation of posterior cervical is widely used in degenerative, traumatic, orthopedic, tumor, etc. Therefore, accurate and security insertion of the pedicle screw remains challenging. To address this challenge, we developed a new drill template with th computer numerical control (CNC) technique, and evaluated its accuracy in assisting cervical pedicle insertion, which compared with that by RP technique.MethodEthics StatementEthical approval was obtained from the Human Research Ethics Committee, Southern Medical University, Guangzhou, China. The subjects gave informed consent. And all consent wrote in nature regarding body donation for research. Specimen and ImagingThree formalin-preserved lower cervical cadavers (range C3-C7) were obtained. The soft tissue in posterior surface of the specimens was cleared. The specimens were imaged using a Brilliance CT 64-channel scanner (Philips, Eindhoven, The Netherlands). In-plane pixel size was 0.5 mm and slice thickness was 0.625 mm. All the cervical vertebrae used after the CT scan images showed no significant bone defects.The following description is divided into two parts:1.The design and production of customized drill template; 2. Cadaveric K-wire insertion and assessment of accuracy of screw insertion.1. The design and production of customized drill template;1.1 Design three-dimensional models with a virtual cervical screw pinThe datasets of cervical specimens were processed and edited with Mimics software v 14.11 (Materialise Corp., Leuven,Belgium).3D model of cervical vertebra was obtained from the 2D CT images Then exported the 3D model in STL format to Geomagic studio 12(Raindrop GeomagicCorp,US)software to determine the optimal screw size and orientation. The 2 cylinders,2 mm in diameter and pre-designed in Unigraphics NX 6.0 (Siemens PLM Software, Plano, TX), were imported into Geomagic studio 12 where it could be freely translated and rotated. It was made sure that the cylinders were located at the central cervical pedicle by visual observation.1.2 Design customized drill templateThe initial template was design by the CAD software UG6.0 and save it as STL format. Then import it into Geomagic Studio 12 software. In this software, move the template, so that the template surface can be in contact with the lamina, and the drill pin from the interior through the drill template. By Boolean subtraction operation, get consistent with cervical lamina rear anatomy template. Though Boolean subtraction operation, a drill template with patient-specific posterior vertebral surface and optimal trajectory were designed1.3 Product customized drill templates with CNC technologyThe model of drill template was saved as IGS format, then import it to UG6.0 software for process design. First, cutting out the outline of the drill template on the blank. Secondly, fine processing irregular surfaces combined with lamina. The third step, drilling holes with a 2mm twist bits. The whole process takes only one clamping. Save the process design as TXT format, import it into VMC650 axis CNC machine tools (Baoji Machine Tool, China). Clamping a 70mm×50mm×12mm steel blank on the control desk, cut it as designed. After processing is complete, clean the drill template and set aside.1.4 Product customized drill templates with RP technologyImport the drill template data in STL format into Materialise Magics V13 software (Materialise company, Belgium) to run the virtual cutting process with cutting thickness 0.05mm, then transferred to RS6000 Stereo lithography Appearance rapid prototyping machine(RPM) to print.2. Cadaveric K-wire insertion and assessment of accuracy of screw insertion.2.1 Cadaveric K-wire InsertionPosterior soft tissue was removed from the vertebrae. The drill template was put in place by hand and compressed slightly to the posterior surface of cervical vertebrae.A 2-mm-diameter K-wire was then drilled into the cervical pedicle with the assistance of the drill template. One side of the cervical pedicle using CNC drill template (CNC group) and the other side using RP drill template (RP group) were randomly assigned. 30 K-Wires were inserted into levels C3-C7 with screw.2.2 Secondary 3D reconstruction of cervical models with pedicle screw channelsAfter all pin tracts were drilled, the cadaveric cervical specimens were scanned with the same CT scanner using the same parameters. Because the K-wires could produce image artifacts, they were pulled out before the image acquisition.3D models of each vertebra with pin tracts were obtained with same segmentation and reconstruction strategy.2.3 Accuracy analysis of screw placementAccuracy of the K-wires insertion with the assistance of the drill templates was evaluated by the reverse engineering process using the software Geomagic studio 12.The 3D models of the pre-operation and post-operation were imported into the Geomagic software. The deviations at the middle point of the pedicle in the axial and sagittal planes were calculated. The axial plane’s deviations towards the lateral side were recorded as positive values and the deviations towards the medial side as negative values. The sagittal plane’s deviations towards the superior and inferior sides were recorded as positive and negative values, respectively.In the post-operation date, a pre-designed 3D screw model(3.5mm in diameter) was imported into Mimics to simulate the screw insertion(figure 4).A classification was used to distinguish non-critical and critical screw positions. In this classification, there are five main categories for screw misplacement:Grade 1:Screw positioned at the center of the pedicle.Grade 2:Less than one-third of the screw cross-section(≤1.2 mm with a 3.5-mm diameter screw) penetrating the cortex.Grade 3:Between one-third and one-half of the screw cross-section penetrating the cortex (or deviation,<2 mm).Grade 4:More than one-half of the screw cross-section penetrating the cortex (or deviation≥2 mm).Grade 5:Deviation equal or greater than the screw diameter.Non-critical pedicle breaches corresponded to grades 1 and 2. Critical pedicle breaches, with the potential of posing a risk to the vertebral artery (VA),nerve root or dural sac, corresponded to grades 3-52.4 Statistical analysisIndependent-sample T test was used to analyze the screw direction differences between the deviations towards lateral and medial in the axial plane and towards superior and inferior in the sagittal plane. A P value< 0.05 was considered as statistically significant. To show the real deviations, the absolute deviation values were calculated to get their means and standard deviations in the axial and sagittal planes, respectively. We also used two Independent Mann-Whitney U test(for numerical data) to evaluate the grade of screw insertion differences between two groups. A P value <0.05 was considered to be statistically significant.Result:According to CNC processing characteristics, while successfully designed in line with CNC processing characteristics and meet the pedicle screw drill template requirements.15 drill templates was successfully produced by CNC machine. All the templates were found to be fitting to its corresponding vertebral laminate appropriately without any free movement.The K-wires were inserted into the cervical pedicle easily with the assistance of the drill templates.In both groups 30 K-wires were inserted. The absolute deviation in axial plane was 0.45±0.24mm in CNC Group and 0.63±0.26mm in RP Group. Calculation showed no significant difference between the deviations towards lateral and medial in the axial plane (t=-1.975, P=0.058).The absolute deviation value in the sagittal plane was 0.41±0.18mm in CNC Group, and 0.41±0.24mm in RP Group. There was no significant difference either between the deviations towards superior and inferior in the sagittal plane (t=-0.28, P=0.978).In the simulated insertion of 3.5 mm-diameter screw,14 screw position were in grade 1 (93.3%),1 was in grade 1 (6.6%)in CNC Group.12 pedicle screw positions were in grade 1(80.0%),3 were in grade 2 (20.0%) in RP Group. All the screws were in a noncritical position. Calculation showed the simulated insertion of pedicle screw had significant difference between two groups (p<0.05).ConclusionThis is the first use of CNC technology to produce metal drill template, which drill template produced with high precision and short processing aging. During the specimens in vitro experiments, drill template produced by CNC machine assisted pedicle screw and lamina linked closely together, simple experimental operation, with high accuracy and safety. Clinical application stage of the technology needs further study. |
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