[55, 107, 243]. Although they studied the isthmus rather than the pedicle, they correctly measured the angles of possible screw trajectory. Xu et al. [243] stated that the average angles for a pedicle screw are 33° medially and 20°, rostrocaudally. They also estimated the possible entry point location and created a placement algorithm. However, the same group, while strictly respecting their guidelines, found an unacceptably high rate of cortical breach in their later study [55]. Standardized algorith­mic approaches usually fail in C2 pedicle screw place­ment due to a great anatomical variability and also due to a number of possible screw trajectories. Therefore, Howington et al. [107] repeated the anatomical test on ten cadavers respecting the trajectory given by the sur­gical visualization of the pedicle. All their screws were correctly placed without cortical breach. The estab­lished medial inclination was on an average 35.2° and craniocaudal angle 38.8°. The notable, almost double, difference in craniocaudal angle measurements could be explained by the methodological variance in obtain­ing of their values. Xu et al. [243] used the line perpen­dicular to the axis of odontoid process as a reference whereas Howington et al. [107] used the plane of C2 body endplate. Although some discrepancies could be found in pure anatomical works, Sciubba et al. [201] confirmed clinically the prerequisite of Howington et al. in a large (single surgeon J.P. Wolinski) series of patients treated with C2 pedicle screws. They place C2 pedicle screws under a pure visual control with excellent clini­cal results. However, the same group recently published a retrospective analysis of 170 transpedicularly intro­duced screws (19.4% with fluoroscopical guidance) and found on the postoperative C2 pedicle focused coronal CT reconstructed images 25% of screw cortical breaches. Most of the cortical wall violations (67.4%) were lateral in FT with one recognized VA injury [5]. They also discovered that the correct screw placement is related to the surgeon’s experience. The average pedi­cle diameter measured on postoperative CT scans was found 6 mm in its thinnest portion, and the lateromedial angle of screw purchase was approximately 40°.

of C2 lamina to direct the screw into the C2 vertebral body while respecting the medial border of isthmus and pedicle to determine the medial angulation. They did not use fluoroscopy and the key point was clear ana­tomical visualization of medial border of isthmus/pedi­cle. Postoperative, thin sliced CT done in prospective manner discovered only 15% of cortical breaches. Only two screws (2%) were evaluated as more than half of diameter cortical breakthrough. Both perforations were without clinical consequences.

allows for a pedicle screw placement with expected/ intended cortical breach without the risk of arterial injury (Fig. 6.19). Pre-procedural knowledge of such variant can be important if other type of fixation is not possible or fails during the procedure. We will usually plan our entry points and trajectories virtually based on preoperative imaging. Virtual planning is only avoided in situations when unexpected extension of construct to C2 is required intraoperatively.

long isthmic screw trajectories rather than the actual pedicle screw. They also noted that the angle of pedicle screw is roughly parallel to the C2 spinous process. This is obviously incorrect from an anatomical point of view. There were no significant differences in 4 mm screw acceptability for both trajectories and the isth­mus height was, on an average, 6.6 mm (SD = 1.8). Despite the above, some interesting facts became apparent in this study. First, it was demonstrated that different entry points and directions can be used to pass the C2 isthmus safely. Second, the accuracy of preoperative planning of the possible screw trajectory can be improved with the use of three- rather than two­dimensional thin-cut CT images.