The Secure Region Of Screw Fixation In Atlas Via Posterior Arch And Lateral Mass And Its Clinical Application

Objective To investigate the secure region for screw fixation in atlas via posterior arch and lateral mass by radiologic measurements, and conduct the operative procedure.Method The dimensions of the pedicles were determined in 30 patients without atlas deformity from CT scan and three dimensional reconstruction images of the upper cervical spine. The space available for the screw (SAS) was defined as the perpendicular distance between two lines tangential to the spinal canal and the transverse foramen, respectively. SAS was evaluated at 0°(SAS1) and 10°(SAS2) insertion angles. The location of screws in 13 cases of operation were observed by CT scan images measurements of the cervical spine.Results SAS1 at 0° insertion angle was (7.81 ± 1.28) mm,SAS2 at 10° insertion angle was (9.69±1.23) mm, with significant difference between SAS1 and SAS2. The distance from the midline of pedicle to the midline of posterior arch and the distance from the center line of C2 inferior articular process to the midline of posterior arch were obtained, with insignificant difference. The entry point which the midline of SAS2 is located in the posterior arch at the point of 2.0 mm lateral to the midline of pedicle. 26 pedicle screws via posterior arch were inserted correctly, without cortex breakage and nerve or vessel injury in 13 clinical practices. Conclusion Screw fixation in atlas via posterior arch and lateral mass does notfocus on one point. There is a safe region. Wider space available for the screw can be acquired when transverse angle is 10° .It is necessary to confirm the safe area by using axial CT of Atlas before screw fixation .