Impact of anatomical variations in basilar invagination on anterior atlantoaxial fixation: a computed tomography-based analysis of screw trajectories

Background Basilar Invagination (BI) is a rare yet clinically significant craniovertebral junction anomaly, associated with a wide range of complex anatomical variations. The atlantoaxial fixation technique is an effective surgical procedure for the treatment of BI. Compared with normal subjects, variations of anatomical features in BI patients fundamentally alter critical screw trajectory parameters, leading to conventional screw trajectories being unsuitable. However, few studies have addressed this issue. In this study, CT-based simulated anterior screw placement was employed to compare screw trajectory parameters between BI patients and controls. The study aimed to clarify the impact of anatomical variations on surgical screw placement and provide clinical guidance. Methods From June 2013 to December 2024, 150 BI patients and 150 asymptomatic subjects underwent CT-based simulation of four anterior fixation configurations: C1 anterior lateral mass screws (C1ALMS), C2 anterior pedicular screws (C2APS), C2 anterior transarticular screws (C2ATS), and C2 vertebral screws (C2VS). Trajectory length (TL), coronal inclination angle (CIA), sagittal inclination angle (SIA), and articular surface angle (ATSA) were measured and compared. Results Significant differences were observed in screw trajectory parameters between BI patients and controls. The BI group demonstrated: (1) increased CIA for C1ALMS (p < 0.05); (2) for C2APS/C2ATS, 5–8° elevation in SIA and ATSA (p < 0.05), with C2APS showing 3° CIA reduction but C2ATS exhibiting 4° CIA increase (p < 0.05); (3) all screw trajectories exhibited shortening exceeding 10% (2–3 mm) (p < 0.05). Subgroup analyses of BI revealed: in atlantooccipital assimilation cases, C1ALMS showed reduced TL and CIA (p < 0.05); with C2-3 fusion, C2APS displayed decreased CIA but increased SIA (p < 0.05); and the ATSA of C2APS was greater in Type A BI than in Type B BI (p < 0.05). Conclusions Due to anatomical variations, screw trajectories in BI patients differ significantly from those in normal individuals. When BI is accompanied by atlantooccipital assimilation, C2-3 fusion and atlantoaxial dislocation, screw trajectory parameters including screw length and inclination angles demonstrate significantly increased variability. Surgeons must adapt screw length and angles to address anatomical variations for safe placement.