Inserted screw angle.

(XLSX)</p

Evaluation of the bicortical screws.

Evaluation of the bicortical screws.</p

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

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

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

Signalments of the cadavers and screw sizes.

Signalments of the cadavers and screw sizes.</p

Screw insertion angles.

Screw insertion angles.</p

Screw length.

(XLSX)</p

Degrees of the vertebral canal penetration (modified Zdichavsky classification).

Degrees of the vertebral canal penetration (modified Zdichavsky classification).</p

The C1 and C2 guides on the 3D-printed bone model and the drill sleeves.

Dorsal view (A) and lateral view (B) of the patient-specific guides on the bone model. The drill sleeve at the C1 guide hole (C). (D) The drill sleeve at the cranial hole of the C2 guide. (E) The drill sleeve at the caudal hole of the C2 guide.</p

The manufacturing process of the patient-specific guide based on the bone model.

Drill guide template for C1 (A) and C2 (F) before making holes. Each hole with a diameter similar to the drill sleeve (B, G and H). A temporary pinhole in the center of the C1 guide, and (C) two temporary pin holes on both sides of the C2 body (I). Completed design of the C1 (D and E) and C2 (J and K) patient-specific drill guide.</p