The Basion-Dens Interval

Injuries at the craniocervical junction are important to recognize due to their critical clinical implications. The basion-dens interval (BDI) is a parameter used to define the normal articulation between the occiput and the cervical spine. Measured on a midline sagittal image, the distance between the inferior tip of the basion and the tip of the dens is less than 12 mm in 95% of adults. A value greater than 12 mm is concerning for atlanto-occipital dislocation.  

The above image is from a patient with bilateral atlanto-occipital dislocation with anterior translation of the occiput relative to the cervical spine.


REFERENCES
Deliganis AV, Baxter AB, Hanson JA, et al. Radiologic spectrum of craniocervical distraction injuries. Radiographics 2000;20:S237-50.