The radiological evaluation of posterior occipital condyle-C1 lateral mass screw fixation for patients with basilar invagination and atlas assimilation.

Abstract

STUDY DESIGN: Retrospective radiological study. PURPOSE: To evaluate the anatomical feasibility of posterior occipital condyle-C1 lateral mass (OCLM) screw fixation in patients with basilar invagination (BI) and atlas assimilation (AA). METHODS: Thin-section cervical CTA of 73 patients (25 M / 48 F; 46.4 ± 12.4 year) was reconstructed. (1) Anatomical variables—hypoglossal canal height (HC), canal-to-insertion point height (HCH), upper (UL) and lower (LL) lines, internal (IH) and external (EH) heights—were measured. (2) Screw trajectories: cephalad angles (HCT/IT·CA), medial convergence angles (MMA, IMA, OMA) and lengths (MMAL, OMAL). 3-D CT classified V3 vertebral artery courses. Sex and side differences were tested (t-test/χ², P < 0.05). RESULTS: Mean HC 3.7 ± 1.1 mm, UL 12.2 ± 3.2 mm, LL 17.5 ± 3.2 mm, IH 11.5 ± 3.1 mm, EH 17.1 ± 3.6 mm; angles HCT·CA 61.4°, IT·CA 29.7°, MMA 27.7°, IMA − 9.6°, OMA 9.0°; MMAL 23.4 mm, OMAL 18.8 mm. Three VA risk types were identified: Type I 59.3% (low), Type II 31.9%, Type III 8.8% (high). In 10 of 140 sides, the occipital bone was positioned too low to permit screw insertion without resection.; posterior OCLM screws were feasible in 92.9%. CONCLUSIONS: Posterior OCLM screws can be safely inserted at the posterior midpoint; when occipital bone obscures this site, relocation to the posterior margin of the OCLM inferior endplate preserves feasibility > 90%.