Spinal Cord Medial Safe Zone for C2 Pedicle Instrumentation: An MRI Measurement Analysis.

Study design Retrospective observational study. Objectives To investigate the spinal cord safety margins for C2 instrumentation. Summary of background data Intraoperative spinal cord injury during C2 spine surgery is a rare, but potentially life-threatening complication. Pre-operative planning for C2 instrumentation mainly focuses on C2 pedicle bony dimensions on CT and the vertebral artery location and few studies have evaluated C2 spinal cord safety margins. Methods We measured two distances in C2 bilaterally: 1) C2 pedicle to dura distance (P-D), defined as a transverse line that measured the shortest distance between the medial wall of the C2 pedicle and the dural sac, 2) C2 pedicle to spinal cord (P-SC), defined as a transverse line that measured the shortest distance between the medial wall of the C2 pedicle and spinal cord. We defined the distances above 4 mm as safe for instrumentation. Result A total of 146 patients (mean age 71.2, 50.7% female) were included. The average distances were 5.5 mm for C2 left P-D, 5.9 mm for C2 right P-D, 10.1 mm for C2 left P-SC and 10.6 mm for C2 right P-SC. Twenty eight (21.4%) patients had C2 P-D distances under 4 mm and out of those 2 (7%) patients had distances under 2 mm. There were more female patients with C2 P-D distances under 4 mm compared to males. No patient had C2 P-SC distances under 4 mm. Conclusion We demonstrated that around 20% of patients had C2 P-D distance below 4 mm, but no patient had C2 P-SC distance less than 4 mm. Since a lateral misplacement can lead to a potentially fatal vertebral artery injury, medial screw trajectory is recommended for C2 pedicle instrumentation with consideration of these safety margins.Level of Evidence: 3.