A Novel C2 Screw Trajectory: Preliminary Anatomic Feasibility and Biomechanical Comparison

BACKGROUND: Pedicle screw and translaminar screw fixation in C2 may not be applicable in many patients with anatomic abnormalities or narrow laminar thickness and spinous process height. The aim of this study was to assess morphometric and mechanical feasibilities of a novel alternative screw trajectory that pierces the bifid base of C2. METHODS: Anatomic measurements that determined the feasibility of spinous process bifid base (SPB) screw fixation were assessed in 14 cadaveric C2 vertebrae. Pullout tests to assess ultimate fixation strength for 3 screw trajectories (transpedicular, translaminar, and SPB) were performed in cadaveric vertebrae for comparison. RESULTS: Anatomic measurements included mean spinous process height (10.4 +/- 4.2 mm) and mean bilateral bifid base length (10.1 + 2.2 mm) and thickness (left, 4.4 +/- 1.0 mm; right, 4.3 +/- 0.9 mm). In 64% (9/14) of specimens, bifid base length was >= 9 mm. Mean pullout strength for transpedicle, translaminar, and SPB screws in 9 viable specimens was 648 + 305 N, 628 divided by 417 N, and 755 + 279 N. CONCLUSIONS: SPB screw fixation may be viable anatomically and mechanically for C2 fixation. Feasibility of SPB screw fixation is determined by length, thickness, and mutual angle of the bilateral bifid bases. Patients with thin (<4 nun) and short (<9 nn) bifid bases are not likely to be suitable candidates. SPB screw fixation shows potential as an alternative approach or a salvage technique for patients with high-riding vertebral arteries or severely thin C2 lamina and warrants further investigation.