The basion-axial interval (BAI) 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 <12 mm on plain radiographs. This value was found to be unreliable on CT where the basion-axial interval ranged from -8.7 mm to 26 mm in normal subjects 1.
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