Computed tomography in the determination of transarticular C1-C2 screw length.

This study evaluated the significance of computed tomographic (CT) measurements of the upper cervical vertebrae and their clinical implications in transarticular C1-C2 screw placement. In the first part of the study, analysis of axial CT scans of the atlas of 46 patients who had a normal C1-C2 region was performed. Measurements included the vertical distance between the middle of the ventral cortex of the lateral mass and the anterior-most point of the anterior tubercle, and the angle of the anterior ring of C1 relative to the frontal plane. In the second part, axial CT scans of the upper cervical spine were performed in seven cadaveric cervical spines and analyzed using the same criteria. Using the Magerl technique of transarticular C1-C2 screw placement, one screw was placed in each cervical spine. Following each placement, a strict lateral radiograph was taken and the distance between the tip of the screw and the anterior-most point of the anterior tubercle of C1 was measured. Analysis of the cervical cadaveric specimens showed the vertical distance between the middle of the ventral cortex and the anterior-most part of the anterior tubercle when measured on CT scan corresponded to the distance measured on lateral radiographs after placement of the C1-C2 transarticular screw. The study of the 46 patients with normal C1-C2 region had shown the mean values of linear and angular measurements to be greater in males than in females, although no significant difference was found between the two groups (P>.05). The mean distance between the anterior-most point of the anterior tubercle and the middle of the ventral cortex of the lateral mass was 6.5+/-1 mm, and the mean transverse angle of the anterior ring relative to the frontal plane was 22 degrees+/-3.1 degrees. Axial CT evaluation of the individual anatomic relationships of the atlas is simple and may be a useful guide in the determination of the length of the transarticular screw when performed during surgery under lateral fluoroscopic control.