The vertebral artery and the cervical pedicle: morphometric analysis of a critical neighborhood.

OBJECT The purpose of this retrospective study was to quantify the anatomical relationship between the vertebral artery (VA), the cervical pedicle, and its surrounding structures, including the incidence of irregularities. Additionally, data delineating a "safe zone," and these data's application during instrumentation with transpedicular cervical screw fixation were considered. The anatomical proximity of the VA to the cervical pedicle prevents spine surgeons from preferring cervical pedicle screws (CPSs) over lateral mass screws at levels C3-6. Accurate placement of CPSs is often difficult to determine, because this definition can vary between 1 and 4 mm of lateral "noncritical" and "critical" pedicle breaches. No previous study in a western population has investigated the VA's proximity to the cervical pedicle, its percentage of occupancy in the transverse foramen (TF), and the incidence of irregular VA pathways. METHODS One hundred twenty-seven consecutive patients who underwent CT angiography of the neck were enrolled in this study. The measurements included the following: medial pedicle border to VA; lateral pedicle border to VA; pedicle diameter (PD); sagittal diameter of the VA; coronal diameter of the VA; sagittal diameter of the TF; and coronal diameter of the TF. The cross-sections of the VA and the TF were measured to determine the occupation ratio of the VA. In addition, a safe zone was defined based on all lateral pedicle border to VA measurements in which the VA was within the TF. The level of entry of the VA into the TF as well as irregularities of the VA and the cervical pedicles were recorded. RESULTS Vertebral artery dominance on the left side was seen in 69.3% of cases. The mean PD increased from 4.9 to 6.5 mm (from C-3 to C-7, respectively). Statistically significantly bigger PDs were seen in males. The mean PD at C-2 was 5.6 mm. Entry of the VA at C-6 was seen in approximately 80% of cases. The TF occupation ratio of the VA was found to be the greatest in C-4 and C-7 (37.1 and 74.2%, respectively). The safe zone increased from C-2 to C-6 (1.1 to 1.7 mm, respectively), but was only 0.65 mm at C-7. In 23.6% of cases, an irregular pathway of the VA or irregular anatomy of a cervical pedicle was seen, with the highest incidence of irregularities found at C-2. CONCLUSIONS Computed tomography angiography is a valuable tool that can help determine the relationships between cervical pedicles and the VA as well as irregular VA pathways. Pedicle diameter, safe zone, and occupational ratio of the VA in the foramen determine the risk associated with instrumentation and should be assessed individually. Based on the authors' measurements, C-4 and C-7 can be considered critical levels for CPS placement. Because of this and the high incidence of irregular VA pathways and different entry points, it may be helpful to review neck CT angiography studies before considering posterior instrumentation procedures in the cervical spine.