Basion-Dental Interval (BDI) and Basion-Posterior Axial Line Interval (BAI) of Harris for Evaluating a Patient with a High Cervical Spine Injury (Rule of Twelves)

 

Examination: lateral supine radiograph with the subject supine

 

Landmarks and reference lines:

(1) basion (along the anterior border of the foramen magnum)

(2) tip of the dens

(3) posterior axial line running along the vertebral body of C2

 

basion-dental interval (BDI) =

= (distance in mm from the basion to the tip of the dens)

 

basion-posterior axial line interval (BAI) =

= (horizontal distance in mm from the basion to the posterior axial line, with the lines at 90 degrees to each other)

 

Interpretation:

• The BAI is <= 12 mm in 98% of normal adults.

• The BDI is <= 12 mm in 95% of normal adults.

• Either measurement > 12 mm suggests occipitocervical (occipitoatlantal) dissociation.

• A significant cervical injury is not excluded if the measurements are normal.