Three-dimensional spiral CT measurement of atlantal pedicle and its clinical application

The study aimed to establish the safe placement area and corresponding entry angle of atlantal pedicle screw using axial computed tomography (CT) measurement of atlas, in order to guide the clinical operation. Spiral thin-slice CT scan of atlas and three-dimensional reconstruction of 38 patients were randomly selected. Screw placement space was defined as the distance between the tangent lines of entry channel on the atlantal cross section and inner edge of transverse foramen and outer edge of spinal canal. Before operation, spiral CT measurement was used to determine the safe placement area, and the pipeline dredge method was used to conduct the internal fixation of atlantal pedicle screw for 7 patients. In CT measurements, the width of pedicle was 9.15±2.57 mm, which could safely accommodate screws with the diameter of 3.5 mm. The safe placement area was located in posterior arch of atlas (18.35±2.86 to 25.26±1.76 mm) away from the posterior tubercle, the entry angle ranged from 9.09±7.45° outward to 18.72±17.42° inward, and the length of screw channel ranged from 26.20±2.69 to 27.04±2.51 mm. The width of the safe placement area was up to 6.91±7.66 mm, and the angle of inclination on cross section was up to 27.81±10.32°. In conclusion, we identified a safe placement area for atlantal pedicle screw, where the screw was implanted inwards and outwards according to different entry points within the safe placement area. The detailed preoperative image measurement, determination of safe placement area and individual screw placement were found to be the key to a successful surgery.