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

Abstract 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