Transverse images of C1 and C2 with pedicle screw trajectories in place.

Grading of the screw position by Zdichavsky modified classification. Grade 1: fully contained within the pedicle and vertebral body. Grade 2a: medial pedicle wall penetration. Grade 2b: entirely medial to the pedicle wall. Grade 3a: partial lateral breaching. Grade 3b: full lateral breaching. Lateral breaching was defined as an intrusion into the C1 transverse foramen or C2 transverse foramen and breach of the C2 lateral cortex.</p

Screw insertion angles.

Screw insertion angles.</p

Screw length.

(XLSX)</p

Transverse CT image of C1, C2 cranial, and C2 caudal.

Transverse CT image of C1, C2 cranial, and C2 caudal.</p

Inserted screw angle.

(XLSX)</p

Evaluation of the bicortical screws.

Evaluation of the bicortical screws.</p

Signalments of the cadavers and screw sizes.

Signalments of the cadavers and screw sizes.</p

Screw length in millimeters determined using CT, bone models, and cadavers in the guide group.

Screw length in millimeters determined using CT, bone models, and cadavers in the guide group.</p

Zdichavsky modified classification.

(XLSX)</p

Application of the patient-specific guide to the cadaver.

(A) Patient-specific guide of the C1 and C2 and the temporary pins inserted. (B) The drill sleeve on the patient-specific guide and the creation of a drill tract using a drilling. (C) Removal of the guide and temporary pins. (D) The surgical site with the inserted screws 3–5 mm above the bone.</p