Medially-shifted rather than high-riding vertebral arteries preclude safe pedicle screw insertion

• 200 C2 pedicles were morphologically analyzed using multiplanar reconstructed CT images.
• A medially-shifted vertebral artery (VA) precludes safe C2 pedicle screw insertion.
• A high-riding VA does not necessarily preclude safe C2 pedicle screw insertion.
• To avoid VA injury, an axial CT scan should be evaluated before screw insertion.

We enrolled 100 patients who underwent preoperative CT angiography before cervical spine instrumentation and investigated the morphology of the C2 pedicle from the perspective of pedicle screw (PS) trajectory using volume rendering and multiplanar reconstruction. The narrowest portion of the pedicle was identified as the pedicle isthmus. Safe C2 PS insertion was regarded to be not feasible when the height of the medullary cavity of the pedicle isthmus and/or width of the medullary cavity of the pedicle isthmus was ⩽4 mm. Forty-five (22.5%) pedicles were ⩽4 mm in width, and safe insertion of a PS was determined to be not feasible. Among these, seven pedicles were ⩽4 mm in both height and width. The remaining 38 pedicles were ⩽4 mm in width with heights >4 mm. There was no pedicle with a width >4 mm and height <4 mm. In other words, short pedicles were always concomitantly narrow. Therefore, the seven pedicles ⩽4 mm in both height and width were considered to be morphologically narrow. The heights of the pedicle isthmus were not limited by the vertebral artery groove (VAG) and safe C2 PS insertion can be considered feasible where the VAG is marginally cranial, whereas the widths of the pedicle isthmus are limited by the VAG. Therefore, safe C2 PS insertion is precluded only when the VAG courses cranially and medially. It is a medially-shifted, rather than a high-riding, vertebral artery that precludes safe C2 PS insertion. Therefore to avoid vertebral artery injury an axial CT scan, parallel to the pedicle axis, should be evaluated before C2 PS insertion.