Three Dimensional CT Study Of Posterior Atlantal Screw Channel And Its Clinical Application

BackgroundIn recent years, the applied posterior atlantal screw fixation technology mainly includes atlantal mass screw fixation and atlantal pedicle screw fixation. Due to its superb biomechanical property and good clinical effects, the technology has been widely applied in the treatment of the atlantoaxial instability caused by trauma, inflammation, and deformity. Due to special anatomic form, complicated structure and great variability, as well as the fact that it is adjacent to important nerves, blood vessels and other tissues such as cervical cord, vertebral artery, and etc., it is very difficult to implant screws on atlas with great risk. Therefore, quantitative measurement of anatomical structure parameters related to atlantal implant has great significance for the improvement of safety and accuracy of atlantal screw fixation technologyAlong with the continuous development of modern digital medicine, the computer assisted surgery technology has been playing a more and more important role in the research of spine surgery. On the basis of assimilation of research fruits and methods of various disciplines, the paper combines modern medical imaging, computer3D reconstruction measurement, and reverse engineering technologies to conduct digital3D positioning and quantitative analysis on posterior atlantal screw fixation trajectory, to study the anatomical structure factors affecting the placement of screws, and to lay a theoretical basis for intra-operative safe and accurate screw fixation.Objectives1. CT image data of people’s atlas are collected to reconstruct an atlantal3D Model by using reverse engineering software Mimics, and the form of atlas ponticulus and its occurrence rate are observed.2. The relevant osseous parameters of atlantal pedicle screw placement are measured on the established atlantal3D model, to study the law of changes of screw trajectory parameters along with the change of transverse section angle, and to explore the ideal atlantal pedicle screw implant method.3. To measure the relevant parameters of atlantal lateral mass screw fixation using CT three-dimensional reconstruction measurement technology, and to study and analyze the anatomical structure factors that can influence the screw placement.4. To design and manufacture the personal drill guiding template (PDGT) of the atlantal pedicle screw by using three-dimensional reconstruction measurement and CAD-RE-RP technologies, and to discuss the application value and clinical significance of the template in atlantal pedicle screw fixation.Methods1. Input the CT scanning image data of atlas into the software Mimics10.01for three-dimensional model reconstruction. Then observe the form of atlas ponticulus and its occurrence rate.2. Based on the analysis of the atlantal three-dimensional model and a series of space construction measures with auxiliary dot, line and plane, to conduct3D measurement on the8anatomical structure data of the atlantal pedicle screw fixation and to analyze the digital anatomical characteristics of the change of screw trajectory parameters along with the change of transverse section angle. Then conduct3D measurement and analysis on the7anatomical structure data of the atlantal lateral mass screw fixation.3. To design the bilateral positioning guiding pipe and the intermediate connection parts of Personal drill guiding template(PDGT) of the atlantal pedicle screw with CAD-RE-RP technology, to form PDGT3D models after data integration and then manufacture the PDGT that is able to match with the posterior structure of atlas bone specimen using the3D printer.Results1. The atlantal3D anatomical model based on CT two-dimensional topographic image is established, and the number of atlas ponticulus is54with the occurrence rate of17.8%, among which the posterior ponticulus is41with the occurrence rate of13.5%,, and the occurrence rate for the posterior ponticulus Ⅰ～Ⅳ is3.9%,3.0%,1.6%and4.9%respectively. There are13lateral ponticulus, and its occurrence rate is4.3%.2. The height of the posterior arch at the bottom of the atlantal vertebral artery groove is (4.52±0.99)mm, among which42are less than3.5mm, occupying a percentage of13.8among the total number, and94are less than4.0mm, accounting for30.9%. There are no statistical differences between the left and the right side of the posterior arch in terms of both height and width, however, distinctive differences exist between males and females, and the data of female is lower than that of male. Furthermore, distinctive differences also appear among the three groups with different height, and it shows an increasing trend along with the increase of height.3. The leaning angle of the atlantal pedicle screw trajectory ranges from0°to15°with a uniform interval of5°and the changing rules of parameters from the4directions of screw trajectory are as follows:the screw trajectory available space width (SAS)from the left to the right side shows an increasing trend within the leaning angle from0°to15°, and after the analysis of the comparison between any two different angles, it shows that SAS has no distinctive differences between10°and15°leaning angle, while distinctive differences appear on the rest comparing pairs; The bilateral screw trajectory length reaches the largest value at0°leaning angle and differences appear compared with the rest3angles, but there are no distinctive differences between the rest3angles, and the average values of the screw trajectory length among the4different leaning angles are nearly the same, with a difference less than0.5mm; The distance from screw entrance point to the posterior midline shows an increasing trend within0°～15°leaning angles; the distance from screw entrance point to the superior margin of the posterior arch shows a decreasing trend within0°～15°leaning angles, where distinctive differences occur through comparison between any two different angles. The distance from screw entrance point to the inferior margin of the posterior arch shows no distinctive differences within0°～15°leaning angles. No distinctive differences occur between the left side and the right side. Comparing the distance from axis lateral mass midline to the middle point of atlas posterior tubercle and the distance from screw entrance point to the posterior midline, it can be found that when the leaning angle is10°, the two values show no statistical significance, while when the screw implants from the rest three angles, the difference between the two values is distinctive.4. Measurement of the relevant parameters of the atlas lateral mass screw:the height of the lateral mass at the inferior margin of the posterior arch is (4.53±0.89) mm, among which35are less than3.5mm, accounts for11.5%of the total. The width of the lateral mass at the inferior margin of the posterior arch is (9.38±1.21) mm, the largest leaning angle of the screw trajectory is (26.69±3.32)°, the largest length of the screw trajectory is (22.56±2.08) mm, and the length of the screw trajectory with0°leaning angle is (15.75±1.76) mm. All the indexes have distinctive differences between the two genders except for the largest leaning angle of the screw trajectory, and the difference is that the index value of females is lower than that of males; all the indexes have distinctive differences among the three height groups except for the largest leaning angle of the screw trajectory, and the difference is that the index value increases along with the height; there are no statistical differences between the parameters at the left and the right sides. The shortest distance between the internal carotid artery and the atlantal mass is (3.53±1.38) mm, among which37are less than2.0mm, accounting for12.2%of the total. The leaning angle of the screw trajectory is (-9.65±7.22)°hen the axis of the screw trajectory is tangent to the inside margin of the internal carotid artery,21(6.9%)are larger than0°, and the largest angle is8.3°.5. A set of personal drill guiding template (PDGT) for atlas pedicle screw is designed and manufactured, and after matching it with the physical specimen in vitro, it is found that PDGT fits closely to the posterior surface of the posterior arch and can guide the implant of the atlas pedicle screw.Conclusions1. There exits certain atlas ponticulus in people’s atlas, among which the atlas ponticulus type Ⅱ to type Ⅳ may influence the pedicle screw implant.2. The height of the posterior arch at the bottom of the atlantal vertebral artery groove is the major restrictive anatomic factor for the pedicle screw fixation, the measurement values of males are larger than that of females and show an increasing trend along with the increase of the height.3. Under the condition of no atlantal dislocation,2mm above the intersection point between the longitudinal vertical via the inside and outside margin midpoint of atlantal lateral mass and inferior margin of atlantal posterior arch can be chosen as the atlas pedicle screw entrance point. The entrance point is more accurate with10°leaning angle, where more screw placement space and larger safety area can be achieved, and angle of sagittal view could be nailed in referring to axial plane of atlantal posterior arch under oblique perspective of the intraoperative X-line.4. The height of lateral mass at the inferior margin of atlantal posterior arch is the major restrictive factor in anatomy. Within certain limits, the adoption of appropriate leaning angle can increase the screw trajectory length of atlantal lateral mass screw, and a certain leaning angle is conducive to avoid the cervical arterial injury from atlantal double cortical lateral mass screws.5. To pay attention to individual differences of atlantal anatomical structure before operation, and each patient’s personal parameters of screw fixation can be obtained through atlantal CT three-dimensional reconstruction, such as screw entrance point and screw trajectory angle etc., so as to improve the accuracy and security of atlantal screw fixation. 6. The personal drill guiding template (PDGT) for atlas pedicle screw is a simple, convenient, scientific and reliable auxiliary means of atlas pedicle personal screw fixation with strong practicability. Though currently with some shortcomings, it is still of high clinical application value and broad prospects for development.