ABSTRACT
Objective To investigate the safe region for screw fixation in atlas lateral mass by ra- diologic measurements to conduct the operative procedure.Methods The dimensions of the pedicles were determined in 30 patients with normal atlas by 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 opera- tion were observed by CT scan images measurements of the cervical spine.Results SASI at 0?inser- tion angle was(7.81?1.28)mm and SAS2 at 10?insertion angle was(9.69?1.23)mm,with statisti- cal difference(P＜0.01).For SAS1,the distance from the midline of pedicle to the midline of posterior arch and the distance from the center of C2 inferior articular process to the midline of posterior arch showed insignificant statistical difference.However,the entry point at the posterior arch of the midline of SAS2 was located at 2.0 mm lateral to the midline of ASAI.Twenty-six pedicle screws in 13 cases were inserted correctly,without cortex breakage,or nerve and vessel injury.Conclusions There is a safe region for screw fixation in atlas lateral mass.Wider space available for the screw can be acquired when transverse angle is 10?.It is necessary to confirm the safe region by using axial CT of atlas before screw fixation.