Is the 4 mm height of the vertebral artery groove really a limitation of C1 pedicle screw insertion?

Purpose: To explore the feasibility and effectiveness of C1 pedicle screw fixation in patients whose atlas vertebral artery groove (defined as the C1 pedicle) height is less than 4 mm, but with a medullary canal.
Methods: From January 2010 to January 2013, 7 patients (6 males, 1 female) with atlantoaxial instability whose C1 pedicle height was less than 4.0 mm on one or both sides were treated by C1 pedicle screw fixation at our institution. Thirteen of the 14 C1 pedicles were less than 4.0 mm in height, but all had a medullary canal. Patients were followed up at regular intervals. Postoperative computed tomography (CT) scans were performed to assess if C1 pedicle screw placement was successful. Clinical outcomes were evaluated according to postoperative complications, the American Spinal Injury Association grading system, and bone graft status.
Results: Thirteen C1 pedicles with a height less than 4.0 mm were inserted by 13 3.5- or 4.0-mm-diameter pedicle screws, and one C1 pedicle whose height was 4.1 mm was inserted by a 4.0-mm-diameter pedicle screw. In addition, 14 pedicle screws were inserted in the axis. The mean follow-up period was 23 (range 8-38) months. No neurologic or vascular complications occurred in any of the seven patients. Postoperative CT three-dimensional reconstruction images showed that all 14 pedicle screws were inserted in the C1 pedicles without destruction of the atlas pedicle cortical bone. All patients demonstrated bony fusion 6 months postoperatively.
Conclusion: If there is a medullary canal in the C1 pedicle, a 3.5- or 4.0-mm-diameter pedicle screw can be safely inserted into the atlas and C1 pedicle screw fixation can be performed without any impact on fixation stability and clinical efficacy, even if the C1 pedicle height is less than 4.0 mm.