The significant differences between types 1 and 2 basilar invagination - a case- control study based on the sphenoid bone.

Basilar invagination (BI) is a congenital malformation of the skull base, in which underdevelopment of the cranial base displaces the odontoid process cranially and posteriorly. Few studies have examined the role of sphenoid bone in BI, despite its central position occupying large portion of the skull base. The objective of this study is to compare the craniometric measurements centered on the sphenoid bone between the two types of BI, and normal controls. This is a cross-sectional study based on MRI and CT databases from authors practice, from 1985 to June 2024. The craniometrics of the sphenoid bone were measured in BI subtypes and controls. A total of 106 images were selected for this study, comprising 34 BI1 patients, 38 BI2 patients, and 34 control individuals. In the horizontal plane, the sphenoid crest-clivus distance in BI2 was significantly greater than BI1 and control group, with no significant difference between BI1 and control group. For all vertical measurements, such as basisphenoid length, BI2 values were significantly shorter compared to BI1 and control group, again with no significant difference between BI1 and control group. There were no significant differences among the groups in the sphenoid angle. Craniometric analysis focused on the sphenoid bone supports a clear distinction in BI2 patients, suggesting more extensive and pronounced cranial changes. In contrast, the changes in BI1 patients were not significantly different from those in normal subjects, indicating that these malformations are primarily associated with the underlying cranio-cervical segmentation defect.