A New Entrance Technique for C2 Pedicle Screw Placement and the Use in Patients With Atlantoaxial Instability.

Abstract

A prospective study and a technique note. To introduce a new entrance technique for C2 pedicle screw placement and to measure the related linear and angular parameters about the entrance point on computed tomography (CT) images. The safety of this technique for patients with atlantoaxial instability was also evaluated. Although earlier studies have introduced different methods for C2 pedicle screw placement, the entry points and the angular parameters may be variable. Few studies have established a fixed entry point on the basis of the anatomic structure of C2 for pedicle screw placement. A total of 60 dry C2 vertebrae were obtained for anatomic measurement in the study. The posterior bilateral nutrient foramens of C2 lamina were selected as the entry points for pedicle screw placement. The foramens were marked with needles and then the vertebrae underwent CT scan. The axial and sagittal planes of C2 pedicles were harvested and 4 linear and 2 angular parameters about the entry point were determined. After that, we used the entrance technique on 31 patients with atlantoaxial instability in a prospective study. CT of the cervical spine was performed to evaluate the safety of the entrance technique. The nutrient foramens exist in 97% of the left lamina and 93% of the right lamina of the C2 vertebra. The overall mean distance from the entry point (nutrient foramen) to the superior border of lamina (PSD), to the inferior border of lamina (PID), to the medial border of the pedicle (PMD), and the length of pedicle screw trajectory (PL, transit the pedicle center) were 3.32±0.63, 8.33±1.21, 6.85±1.00, and 24.47±1.51, respectively. The averaged transverse angle (α) on the axial plane and the superior angle (β) on the sagittal plane were 19.83±3.83 and 30.12±6.02 degrees, respectively. Then, 31 patients underwent bilateral C2 pedicle screw fixation without screw violation into the spinal canal or vertebral artery injury by the new entrance technique. The overall mean angles α and β and the length of the pedicle screw were 17.52±3.81 and 34.29±4.18 degrees and 25.85±2.06 mm, respectively. No statistical differences were found in these 3 parameters between the dry C2 vertebrae and the C2 vertebrae of patients who underwent the surgery (P>0.05). Using the posterior bilateral nutrient foramens of the C2 lamina as the entry point is a helpful intraoperative landmark for C2 pedicle screw placement.