| BackgroundSurgical interventions are common for degenerative cervical spine diseases. Due to the three-column fixation property, transpedicular screw fixation via the posterior approach has been shown to have superior stabilization capabilities in several biomechanical and clinical surveys. However, due to the posterior musculature stripping, the posterior approach can cause significant myofascial pain and lead to significant postoperative axial symptoms and neck pain. On the contrary.the anterior approach is less traumatic with no damage to the paravertebral muscles and allows for anterior instrumentation. However, since the screws in the anterior approach are anchored in the cancellous vertebral body, the biomechanical stability of anterior plate fixation is limited, leading to significant failure rates. Thus, for a successful>2-level corpectomy or operation for severe traumatic three-column instabilities, a combined anterior and posterior approach was found to be desirable, but it would require a secondary posterior approach procedure that might lead to a significant increase in morbidity.With the deepening of the research, anterior transpedicular screw (ATPS) in clinical application was reported by some studies. After corpectomy at one to three levels in9patients, ATPS were placed under visualization of the pedicles to affix fibular grafts to cervical pedicles. Some other studies demonstrated the anatomical feasibility of ATPS and found the pull-out strength of ATPS to be better than that of vertebral body screws. The ATPS technique merges the biomechanical merits of posterior transpedicular fixation with the surgical benefits of anterior-approach-only procedures, because it can increase initial construct stability in an anterior surgery which is believed to be best beneficial for some severe multilevel cervical instability.Accurate insertion is a key to successful application of ATPS in clinic. In fluoroscopy-guided manual insertion of ATPS, a percentage of78.3%was reported for correctly placed screws and non-critical pedicle breaches in the axial plane. Next one reported successful insertion of anterior pedicle screws in6patients with the aid of fluoroscopic images of the pedicle axis. However, the sample number was small and the surgery required much time and experienced physicians. Using the electronic conductivity device (ECD), found a high accuracy rate of ATPS insertion with no critical screw positioned in the axial or sagittal plane. However, the physical property of ECD prevents it from penetrating the dense cortical bone. Patient-specific drill templates were developed to assist screw insertion using3D reconstruction, computer aided design (CAD) and rapid prototyping (RP) techniques and good accuracy of screw insertion was obtained. However, these techniques have been used just in the posterior pedicle approach but not in the anterior pedicle approach. Also, the materials used in them, such as photosensitive resin, possess significant cytotoxicity. In the process of surgical drilling, debris of the drill template can get in contact with the wounded area, a potential danger if the debris cannot be totally rinsed.Therefore, accurate and bio-safe insertion of ATPS remains a challenge. To address this challenge, we first adopted a new strategy to construct a bio-safe drill template for ATPS insertion using3D reconstruction, RP production and reverse mold manufacture techniques. Secondly, we evaluated the accuracy of the drill template in assisting ATPS insertion.ObjectiveConsidering the good biomechanical property of cervical three-column fixation and relative instability of anterior cervical vertebral body fixation, anterior transpedicular screw (ATPS) is ideal for severe multilevel traumatic cervical instability. However,the accurate insertion of ATPS remains challenging. Here we constructed a patient-specific bio-safe drill template and evaluated its accuracy in assisting ATPS insertion.Methods1.Ethics statementEthical approval was obtained from the Human Research Ethics Committee, Southern Medical University, Guangzhou, China. The subjects gave informed consent. And all consent was written in nature regarding body donation for research.2.Specimens24formalin-preserved cervical vertebrae (C2-C7) from four human cadavers were CT scanned. The entire specimens were imaged using a Brilliance CT64-channel scanner (Philips, Eindhoven, The Netherlands). In-plane pixel size was0.5mm and slice thickness was0.705mm. Acquired data was saved in the common DICOM format.The following methods consist of two parts:1. The design and production of Patient-Specific Drill Template for Cervical Anterior approach;2.Cadaveric Kirschner wire insertion and assessment of accuracy of screw insertion.1.1Three-dimensional Reconstruction of Cervical Models with Virtual Pin TractsThe datasets of cervical specimens were processed and edited with Mimics software vl4.11(Materialise Corp., Leuven,Belgium). An interactive image processing strategy, such as "Threshold" and "Region growth", was used to segment the contours of each vertebra to obtain the3D-reconstructed models.3D reconstructions were obtained from the2D CT images. The2cylinders,2mm in diameter and pre-designed in Unigraphics NX6.0(Siemens PLM Software, Piano, TX), were imported into Mimics software 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. With the tool "subtraction" under "Boolean operation" in the Geomagic studio software,3D models of cervical vertebrae with bilateral pin tracts were obtained and saved in group1in stereolithography file format (.stl) supported by many software packages and widely used for rapid prototyping and computer-aided manufacturing.1.2Production of Bio-safe Drill TemplateThe ".stl" files of cervical vertebrae were processed using the software Zprint7.10(Z Corporation, Burlington, MA) and printed on the Z Corporation3D printer Spectrum ZTM510(Z Corporation, Burlington, MA). The3D models were virtually cut into thin layers of0.0875mm intervals with the Zprint7.10software and transferred into the Spectrum Z510for rapid prototyping (Figure2). A Kirschner wire was then inserted into the pin tract of the RP model. Polymethylmethacrylate (PMMA), which is usually used as bone cement and has good biocompatibility, was used to construct the drill templates (Figure2). Also, to allow for convenience and easy handling, a grip was created at the top of the drill template. In addition, since the surgery field was narrow, the base of the drill template was not allowed to exceed the juncture of the vertebral body and the transverse process. After the PMMA was solidified, the Kirschner wire was pulled out to finalize the drill template.2.1Cadaveric Kirschner Wire InsertionAnterior soft tissue was removed from the vertebrae. The drill template was put in place by hand and compressed slightly to the anterior surface of cervical vertebrae. A2-mm-diameter Kirschner wire was then drilled into the cervical pedicle with the assistance of the drill template, a total of48Kirschner wire were inserted. Then the Kirschner wire were pulled out respectively before the image acquisition (Since the Kirschner wires produced image artifacts)2.2Secondary3D Reconstruction of Cervical Models with Pin TractsAfter all pin tracts were drilled, the cadaveric cervical specimens were scanned with the same CT scanner using the same parameters. Since the Kirschner wires produced image artifacts, they were pulled out before the image acquisition.3Dmodels of each vertebra with pin tracts were obtained with same segmentation and reconstruction strategy, and saved in group2in ".stl" file format.2.3Assessment of Accuracy of Screw InsertionAccuracy of ATPS insertion with the assistance of the drill templates was evaluated by a reverse engineering process using the software Geomagic studio, version11(Geomagic, Inc., Morrisville,NC). The3D models of groups1and2were imported into the Geomagic software, and 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. Aligned with the pin tract of the3D model of group2, a predesigned3D screw model (3.5mm in diameter) was imported into Mimics to simulate the screw insertion. A grading was used to distinguish non-critical and critical screw positions. Briefly, the grading consists of the following:Grade1:Screw positioned at the center of the pedicle.Grade2:Less than one-third of the screw cross-section(≤1.2mm with a3.5-mm diameter screw) penetrating the cortex.Grade3:Between one-third and one-half of the screw cross-section penetrating the cortex (or deviation<2mm).Grade4:More than one-half of the screw cross-section penetrating the cortex (or deviation≥2mm).Grade5:Deviation equal or greater than the screw diameter Non-critical pedicle breaches corresponded to grades1and2.Critical pedicle breaches, with the potential of posing a risk to the vertebral artery (VA), nerve root or dural sac, corresponded to grades3-5.2.4Statistical 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<0.05was considered as statistically significant. And, 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.Results1.The3D-reconstructed models of cervical vertebrae were obtained with Mimics software;2. Three-dimensional Reconstruction of Cervical Models with Virtual Pin Tracts were obtained with Geomagic and UG software;3. The drill templates were constructed with the RP models;4.48ATPS screws (replace with Kirschner wires) were inserted successfully with the assistance of patient-specific drill templates. Calculation showed no significant difference between the deviations towards lateral and medial in the axial plane (P=0.797). There was no significant difference either between the deviations towards superior and inferior in the sagittal plane (P=0.741). The absolute deviation value in axial plane and sagittal plane were0.82±0.75mm、1.10±0.96mm respectively.The screws in a noncritical position were44/48(91.7%) in the Virtual experiment of screw insertion.ConclusionIn this in vitro study, the patient-specific bio-safe drill template we constructed is compatible, easy-to-apply and accurate. Further research should be done to test its clinical applications. |
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