Anatomic considerations for the pedicle screw placement in the first cervical vertebra

Study Design. Anatomic bony measurements were manually performed on 50 dry atlantoaxial vertebral complexes with an electronic digital caliper, and a reliable landmark for insertion of a pedicle screw in C1 vertebra was described and evaluated. Objectives. To investigate the feasibility of placing a screw in the C1 pedicle and evaluate the reliability of the C2 lateral mass as a landmark for determining the optimal site of screw entry. Summary of Background Data. The use of C1-C2 pedicle screws with the additional rods or plates has been an alternative method of stabilization for patients who are unsuitable for C1-C2 transarticular screw stabilization. However, the optimal portal of entry for the C1 pedicle screw is difficult to determine during surgery in the absence of reliable reference landmarks and sufficient relevant anatomic studies. Methods. We manually measured 11 linear anatomic parameters related to the C1 lateral mass, its pedicle and posterior arch under the groove on 50 pairs of dry C1-C2 complexes harvested from cadavers. We also treated five patients with atlantoaxial instability with C1 and C2 pedicle screw fixation; the entry point of the C1 pedicle screw was ascertained by using C2 lateral mass as a landmark, and the position of the C1 pedicle screws in each case was evaluated with postoperative radiographs and computed tomography scans. Results. The mean mediolateral width and rostrocaudal height of C1 pedicle were 8.57 and 5.83 mm, respectively. The mean rostrocaudal height of C1 posterior lamina at the reference entry point for the screw was 4.59 mm. The mean rostrocaudal height of the C1 posterior arch under the vertebral artery groove was 3.88 mm at its medial one-third and 4.25 mm at its lateral one-third. The thinnest external diameter of the screw tract that was recorded was < 4 mm in six (12%) specimens of C1 vertebrae. The center of C2 lateral mass was 1.51 mm lateral to the sagittal plane of the mediolateral midpoint of the C1 pedicle. The distance from the suggested screw entry point to C1 posterior tubercle was 22.15 mm, and the mean length of the screw tract was 28.55 mm. Ten C1 pedicle screws were placed exactly, without neural or vascular injury in all five patients. Conclusion. The heights of the C1 pedicle, the posterior arch under the groove and the posterior lamina at the screw entry point are the major determinants for the possibility of placing pedicle screws in C1 of a given patient. This study indicates that it is feasible to place a 3.5-mm pedicle screw safely in C1 in most patients, and the lateral mass of C2 is a reliable anatomic landmark that can be easily identified to help the surgeon determine the optimal screw entry portal conveniently during surgery.