Harris et al identified 2 measurements from a lateral cervical spine radiograph which can help determine if the patient has an occipitocervical (occipitoatlantal) dissociation. Because 12 mm is used as the upper limit of normal, these are termed the "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)



• 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.


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