Cervical Sagittal Alignment in Chiari Malformation type I with Basilar Invagination Type B: An Observational Study Using Upright X-Radiographs

Abstract

Objective: Basilar invagination (BI) frequently coexists with Chiari malformation type I (CMI), yet its impact on global cervical sagittal alignment remains unclear. This study aimed to quantitatively evaluate the cervical sagittal alignment in adult CMI patients with type B BI, with a focus on subaxial curvature. Methods: One hundred and eight adult CMI patients (35 CMI patients with type B BI for CMI-BI group and 35 patients without for CMI-only group) and 53 controls were retrospectively included. Upright neutral lateral cervical radiographs were obtained. Craniocervical parameters, including the foramen magnum angle (FMA), basal angle (BA), clivus-axial angle (CXA), and horizontal visual axis angle (HVA), as well as cervical sagittal alignment parameters (C0–2, C2–7, and C0–7 angles, T1 slope, and C2–7 sagittal vertical axis (SVA)), were measured. Intergroup comparisons were performed using one-way analysis of variance or the Kruskal–Wallis test. Results: Significant intergroup differences were observed in multiple parameters. FMA and BA demonstrated a stepwise increase from controls to CMI-only patients, reaching the highest values in the CMI-BI group, whereas CXA and HVA decreased, indicating progressive craniocervical kyphosis. Upper cervical lordosis increased in both CMI groups, while lower cervical lordosis was markedly increased only in CMI-BI patients. The whole cervical lordosis increased synchronously, whereas T1 slope and C2–7 SVA remained unchanged. Conclusion: Under physiological load, CMI patients with type B BI demonstrate a compensatory alignment characterized by increased lordosis in both upper and subaxial cervical segments, despite craniocervical junction kyphosis, while maintaining the entire sagittal balance. The discovery of this compensatory mechanism underscores the importance of comprehensive preoperative evaluation and highlights the potential need to consider existing spinal alignment when performing fusion surgery.