5.1 The Choice Of Internal Fixation Of The Vertebrae
The fixation of the pivotal vertebrae is essential in cervical spine fixation surgery. There are mainly C1 / 2 joint screws and transpedicular fixation techniques. Wright and Lauryssen  found in clinical that the vertebral artery injury rate in the clinical application of transatlantoaxial joint fixation technology can reach 4.1%. According to anatomy of cadaveric specimens, it was found that 20% of the C2 vertebral artery foramen region had a tortuous deformity of the vertebral artery, which invaded the isthmus and lamina . Yan Ming  and other researchers found that 5% of patients had a cavity deformity in the vertebral pedicle, which increased the risk of axial pedicle screws. The cross-screw technique can effectively reduce the arterial injury rate, and its wide and thick laminae can ensure the effective biomechanical stability of laminar nails [16, 17, 18]. Cassinelli et al.  found that the thickness of the vertebral lamina of the axial vertebrae exceeds 5 mm to reach 70%, and the thickness of the vertebrae exceeds 4 mm to reach 92%. The actual feasible length of the lamina is 24 mm, and the specimens exceeding 20 mm reach 99%. This indicates that The fixation of axial lamina screws on the anatomical structure is practical and feasible for most people. Ma Xiangyang et al  found that the average thickness of the proximal end of the axial lamina exceeds 3 mm, the average thickness of the middle segment exceeds 5.9 mm, the average thickness of the distal lamina exceeds 5.59 mm, and the spinous process The average height of the base is more than 12.1 mm, the height of the lamina is more than 12 mm, and the average length of the lamina screw is more than 25 mm; this shows that most people's vertebral structures can accommodate adequate diameter and length of vertebral lamina nails. Considering the limitation of the spinal process of C2 on the angle of the laminar screw channel, Kabir and Casey  can remove the laminar cross screw directly in the direction of the laminar by biting off the spinous process of C2 and exposing the cancellous bone. The risk of penetrating the lamina cortex due to the angle deviation of the screw placement, but this technique has a certain effect on the stability of the inserted screw.
Spinal lamina nails are a reliable way to fix the posterior cervical route. Wright  suggested that based on the large vertebral lamina, it can provide stable biomechanics and avoid vertebral arteries. Confirmed clinically . The biomechanical effect can achieve the same effect as pedicle screw and transarticular screw fixation [22, 23, 24]. Jea  improved the single cortical screw fixation method in clinical application. Under the condition of choosing the same nail insertion method, the screw penetrated the lateral cortex of the axial lamina, and the needle exit point was located at the connection between the articular process and the lamina At the outer edge of the lamina, an ideal double cortical fixation is formed to achieve greater emphasis on biomechanical stability.
5.2 The Clinical Significance of Guide Assist in C2 Laminar Screw Internal Fixation
During C2 laminar screw fixation technique, the accuracy and safety of nail placement will be affected by the operator's operating experience, fluoroscopy effect, intraoperative position and other factors. The existing relevant clinical data shows that some cases have caused screws to invade the spinal cord due to the wrong orientation of the nail, causing high spinal cord injury [26, 27]. In addition, improper grasp of the screw length or direction during screw placement may also damage the vertebral artery . Notmell et al.  and rajasekaran et al.  assisted with the intraoperative fluoroscopic three-dimensional navigation system during clinical surgery, and completed a considerable number of lamina nail insertion surgery. Clinical follow-up results showed the accuracy of surgical nail placement Satisfaction greatly improves the success rate of the placement of the axial lamina nail. And some clinical data analysis results show that with the precise assistance of the intraoperative fluoroscopic 3D navigation system, the diameter of the inserted screw can be increased to 4 mm. However, this navigation system also has some shortcomings, such as the high cost of the entire 3D navigation system, which leads to increased surgical costs. At the same time, similar 3D navigation systems often require professional and technical personnel to operate, which increases the complexity of the operation, and most hospitals do not meet the configuration and application. condition.
Based on the application of three-dimensional data simulation and synthesis technology in orthopedics, some researchers have applied three-dimensional digital simulation modeling technology to the internal fixation of C2 lamina screws. By extracting the three-dimensional CT data of the cervical spine, a personalized guide device for assisting the placement of C2 laminar screws is established, which plays a certain role in improving the accuracy of the placement of axial laminar screws . Lu et al.  obtained the complete structure and nail path of the original data, and reconstructed the model and nail path from the thin-layer CT scan data. On the computer, the spiral track plate was locked and reconstructed by UG Imageware software. The structure of the posterior spine, reconstruction of the positioning track and the virtual screw mold, and finally the use of light curing rapid prototyping technology, using photosensitive resin as the material, to make customized guides. Through the analysis of ascending corpse accuracy and clinical application analysis, the customized guide based on this technology can effectively increase the efficiency of C2 intralaminar screw implantation . However, there are still some shortcomings with this technology : In the design process of the navigation template, there are many softwares that need to be used, and the data transmission conversion process is easy to be missing, which affects the accuracy of the final modeling. In addition, the requirements for software operation and professional spine surgery knowledge are extremely high, resulting in a long learning curve for design and operation. Because the principle of SLA technology is to use laser or ultraviolet radiation to irradiate the liquid photosensitive resin layer by layer, it has a higher working environment, and the liquid resin is easy to condense, which is not conducive to molding. The resin is easy to shrink and deform during the curing. In addition, higher design and production costs limit the development of this technology. At the same time, due to the complexity of the computer simulation design process and the time consuming process, it has hindered its application in emergency surgery. Furthermore, because the material is resin, its thermal decomposition products are biotoxic, and there are potential safety hazards in clinical application. And after molding, the position of the guide tube on the template cannot be changed, and the guide error caused by the material deformation is very easy.
Nail placement by the auxiliary guide can effectively reduce this complication and improve the efficiency of nail placement. Most of the existing vertebral guides are individual models [11, 12, 13], which need to be customized in advance, are expensive, and require a long time to prepare before surgery. Improper cleaning of soft tissue or damage to the lamina structure will affect the accuracy of the guide. In addition, the insufficient hardness of polyethylene and other materials will lead to structural instability. Errors in 3D simulation and insufficient fixed points will affect the accuracy of the guide. Therefore, this study aims to develop a guide device that is simple in operation, stable in structure, accurate in guidance, safe in use, and reusable for clinical application.
In order to insert the laminar screw more accurately and safely, this experiment takes the needle point of the axial laminar nail as the marked point, and uses the arc slide centripetal principle combined with the anatomical structure of the axial lamina and spinous process, through 3D printing The technology designed a new type of lamina screw guide for the insertion of lamina screws. The accuracy test of nail placement was performed in vitro, and the clinical application analysis was performed to verify its safety and accuracy.
5.3 Structure principle of bidirectional slide rail guide system for vertebral lamina nail
There are three main indicators to determine the accuracy and safety of the placement of the axial lamina nail, namely, the needle entry point, the needle exit point and the inclination angle, as long as the appropriate needle entry point and needle exit point are locked during the nail placement process Obtain a safe and effective nail path, and it is easy to determine the proper needle entry point during the nail placement process, and the needle exit point is often difficult to control. During the guiding process, the guide takes the needle point of the axial lamina as the marking point, and locks the needle point first, but because of the difference in the anatomy of the vertebrae of different patients, the needle insertion point of the axial lamina needs to be adjusted accordingly. In order to achieve a proper needle entry point, in order to solve this problem, the guide sleeve of this guide must always take the needle exit point as the center of the guide sleeve during the adjustment process and point to the needle exit point. The main body of the guide is provided with an arc-shaped slide rail, and the guide sleeve can be finely adjusted to the center of the slide rail to ensure that the guide sleeve always takes the needle point as the center of the circle and points to the needle point. The guide consists of a fixed frame body, a two-way guide sleeve, an arc-shaped centripetal slide rail, a slide rail connection ring and other parts; the fixed frame body includes an arc-shaped two-way fixed frame: the upper end is a spinous process locking sleeve, which can penetrate into the Kline The needle fixes the guide on the spinous process, and the middle is an arc-shaped centripetal slide rail. The guide can be fixed to the inside of the slide rail through the slide rail connection ring to ensure the centripetality of the sleeve; at the same time, in order to obtain better stability The main body of this guide adopts a metal structure to reduce deformation during operation.
5.4 The accuracy of C2 laminar staple bidirectional slide-type guide system
In vitro experiments and clinical application studies have shown that the new axis laminar nail guide assisted nail placement has higher safety than free hand nail placement, and locks the needle entry point and the needle exit point to ensure a higher double cortical rate, Effectively increase the mechanical strength of the inserted screw, and at the same time, in terms of accuracy, the new pivotal laminar screw guide assists the placement of the needle and the inclination angle is closer to the ideal nail path, because the guide can lock the needle The point and the needle out point can lock the safety nail path more effectively, with higher accuracy and safety.
The guide is a metal frame structure with high hardness and low deformation rate, which greatly improves the structural stability. The bidirectional slide rail guide does not require repeated fluoroscopy during navigation operation, which can greatly reduce the operation time and reduce the amount of radiation. Because it can be reused, it is superior to custom guides in cost. By guiding the positioning of the nail entry point and the needle exit position at the same time, this guider improves the double cortex rate, and then assists the two cross-fixation brackets to fix the guider on the spinous process, which can firmly stabilize the guider and avoid guiding errors. The guide tube needs to be in contact with the laminar line point, and it can be effectively and accurately guided when the soft tissue is left or the laminar structure is damaged. The entire operation is completed under visual inspection. The experimental results also show that the guide is accurate and reliable. The great clinical value provides an economical and reliable auxiliary method for the placement of axial lamina screws.