Basion-dens interval

Last revised by Francis Deng on 5 Apr 2022

The basion-dens interval (BDI) is the distance between the basion and the tip of the dens, used in the diagnosis of atlanto-occipital dissociation injuries.

It is the distance from the most inferior portion of the basion to the closest point of the superior aspect of the dens in the median (midsagittal) plane.

Normal values for adults are <12 mm on plain radiographs and <8.5 mm on CT 1. In children, >10 mm suggests atlanto-occipital dissociation 2.

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