Foramen Magnum Angle: A New Parameter for Basilar Invagination of Type B.

Objective: To describe the foramen magnum angle (FMA) as a new parameter for basilar invagination (BI) type B.

Methods: The FMA was performed on sagittal slice of magnetic resonance imaging (MRI) as a line from the hard palate to the opisthion (angle vertex), and another line from the opisthion to the basion. The MRIs from 31 participants with BI type B and 96 controls were used. Intraclass correlation coefficient, descriptive data, and receiver operating characteristic (ROC) curve were used for statistical analysis at the 95% confidence interval.

Results: The interobserver agreement of the FMA was 0.952. Patients with BI type B had a FMA significantly greater (25.9° ± 9.3°) than control participants (11.6° ± 4.9°) (P < 0.001). The area under the ROC curve showed a diagnostic value of 0.947. The FMA showed sensitivity 0.900 and specificity 0.854 for the cutoff criterion 17° (P < 0.001).

Conclusions: The FMA had an optimal diagnostic value that provided complementary evidence to investigate BI type B.