C1C2 Harms-Goel fixation is useful for unstable upper cervical spine trauma, painful pseudarthrosis, painful osteoarthritis deformation. C2 pedicle screws remain challenging given the narrow surgical corridor, and unreliability of spinal navigation at this level.

Radiologic morphometric study analyzing the newly described « summit of isthmic crest » of C2 based on 56 consecutive CT scans of the neck with angiography, with standardized MPR analysis. Summit was defined as last sagittal view of isthmic crest. Two measures on axial view: summit-of-isthmic-crest to midline, and C2 pedicle converging angle. Three measures on sagittal view: summit-of-isthmic-crest to C2C3 joint, to upper isthmus, C2 pedicle ascending angle. Interobserver agreement was performed. Presence of high-riding-vertebral-artery was analyzed, and safety test was done using 3.5 × 20 mm screw.

Isthmic crest visible in 97.3% (109/112), hypoplastic in 2.7% (3/112). On axial view, mean summit-of-isthmic-crest to midline distance was 19.7 ± 2.2 mm, and was significantly higher in male, mean converging angle to align with C2 pedicle was 12.9 ± 5.4 degrees. On sagittal view, summit-of-isthmic-crest was 8.3 ± 1.6 mm distant to upper isthmus, which tended to be significantly higher in male, was 9.8 ± 2.3 mm distant to C2C3 joint, and mean ascending angle was 27.0 ± 7.0 °. Interobserver agreement was good (ICC 0.813) to excellent (0.921). High riding vertebral artery (HRVA) was associated with 3.5 × 20 mm screw cortical bone contact (OR 215, p < 0.001), and type 3 HRVA with trajectory breach (p = 0.0009).

Based on this study, t he summit of isthmic crest of the axis could be used as a reliable entry point for insertion of C2 pedicle screws.