The basion-axial interval (BAI), as the name suggests, is the horizontal distance between the basion and the posterior cortex of the axis, used in the diagnosis of atlanto-occipital dissociation injuries.
It is the distance (in mm) between the basion and the superior extension of the posterior cortical margin of the body of the axis in the median (midsagittal) plane.
Normal values are < 12mm on plain radiographs and were not found to be reliable of on CT 1 where they range from -8.7mm to 26mm in normal subjects.
- 1. Rojas CA, Bertozzi JC, Martinez CR et-al. Reassessment of the craniocervical junction: normal values on CT. AJNR Am J Neuroradiol. 2007;28 (9): 1819-23. AJNR Am J Neuroradiol (full text) - doi:10.3174/ajnr.A0660 - Pubmed citation