Basilar Invagination: A Tilt of the Foramen Magnum.

Objective: Congenital basilar invagination (BI) is a craniocervical deformity marked by odontoid prolapse into the skull base. The foramen magnum angle (FMA), which is formed by the Chamberlain's line and McRae's line, has not been fully studied. The study aimed to investigate the FMA and its relationship with other craniocervical parameters.

Methods: Participants were divided into control, type A BI, and type B BI groups. Parameters included Chamberlain line violation, atlantodental interval, clivus height, clivus anteroposterior dimension, FMA, basal angle, clivo-axial angle, head and neck flexion angle, Boogard's angle, and subaxial cervical spine lordosis angle. A comparison of these parameters among the 3 groups and correlation analysis between FMA and other parameters were performed. The significance level was set at P < 0.05.

Results: A total of 111 controls, 111 type A BI patients, and 62 type B BI patients were enrolled. The FMAs in the control, type A BI, and type B BI groups were 6.21° (3.67°, 8.71°), 22.16° ± 6.61°, and 22.39° (17.27°, 31.08°), respectively. Correlation analysis revealed correlations between the FMA and other variables. In the 2 BI subgroups, FMA was significantly correlated with Chamberlain line violation, clivus height, clivus anteroposterior dimension, basal angle, clivo-axial angle, and Boogard's angle.

Conclusions: The FMA in patients with BI was approximately 22° and approximately 6° in controls, indicating that the foramen magnum in BI had a greater tilt. As a pathological condition, FMA can reflect the degree of BI. Clivus hypogenesis is a reason for the excessive tilt of the FM.