• Xiangyang Ma, Shizhen Zhong, Jingfa Liu, Qingshui Yin, Dachuan Xu, Zihai Ding, and Zhonghua Li.
• Department of Orthopaedic Surgery, Guangzhou General Hospital of Guangzhou Military Command, Guangzhou Guangdong 510010, PR China.
• Zhongguo Xiu Fu Chong Jian Wai Ke Za Zhi. 2004 Sep 1; 18 (5): 392-5.

ObjectiveTo assess the possibility of placing the posterior pedicle screw on atlas.MethodsTwenty human cadaver specimens were used to insert pedicle screws in atlas, through the posterior arch or the pedicle of C1 into the lateral mass. The screw entry point was on the posterior surface of C1 posterior arch and at the intersection of the vertical line through the center of C2 inferior articular process and the horizontal line at least 3 mm below the superior rim of the C1 lamina. The screw of 3.5 mm in diameter was placed in a direction of 10 degrees medial angle and 5 dgrees upward angle. After placement of C1 pedicle screw, the distance from C1 screw entry point to the medial-lateral midpoint of C1 pedicle, the maximum length of screw trajectory and the actual screw trajectory angles were measured. The direction of screw penetrating through the cortical of C1 pedicle or lateral mass and the injuries to the vertebral artery and spinal cord were observed.ResultsForty pedicle screws were placed on atlas, the mean distance from C1 screw entry point to the medial-lateral midpoint of C1 pedicle was (2.20 +/- 0.42)mm, the maximum length of screw trajectory averaged (30.51 +/- 1.59)mm, and the actual screw trajectory angle measured (9.7 +/- 0.67)degrees in a medial direction and (4.6 +/- 0.59)degrees in a upward direction. Only 1 screw penetrated the upper cortical bone of the atlas pedicle because the upward angle was too large, and 8 screws were inserted so deep that the inferior cortical bone of the C1 lateral mass was penetrated. But no injuries to the vertebral artery and spinal cord were observed.ConclusionC1 posterior pedicle screw fixation is quite accessible and safe, but the surgeons should pay attention to the angle and the depth of the screw placement.

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