| Objectives: To investigate the safe directions of screw trajectory in atlas via posterior arch and lateral mass and its clinical significance.Methods: Lateral radiographs and CT axial scans of atlases were performed in 30 normal cases. The thinnest height of posterior arch (PH), the maximum inclination of screw projection relative to sagittal plane (βmax, γmax), and the maximum medial angle of screw projection relative to axial plane (αmax) were evaluated radiologically. 21 patients operated with screw fixation in atlas via posterior arch and lateral mass were evaluated as the operating influences of screws on surrounding structures and postoperative neural symptoms. Preoperative and postoperative radiographs and CT scans of 13 patients were performed and some related parameters were measured to evaluate the safety of the screw placements.Results: ①The maximum angle of screw projection to sagittal plane is about 10° cephalad or 6° caudal, which is related to the thinnest height of posterior arch.?When the entry point moves along the posterior arch from the medial to the lateral, the medial angle of screw projection relative to axial plane varies from 0° to 30°, correspondingly. ?The postoperative directions of screw trajectory had some differences from the preoperative one, but all were in the estimation range. ?A11 screws were placed successfully in all patients of our group, and the postoperative radiographs and CT scans verified that all of the atlantal screws were in areas in common use and no neural or vascular injury was observed.Conclusion: The direction of screw trajectory in atlas via posterior arch and lateral mass is adjustable in a safe range in sagittal and axial plane, which we should make full use of to enhance the safety and manipuility of screw placement.
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