IS Case 15: Distracted Occipitoatlantal Dissociation

Lisa Siripun, MD

Imaging Sciences URMC

Imaging Sciences URMC 2008
Publication Date: 2009-05-18


Patient is a 6-year-old girl status post motor vehicle accident with severe head trauma.


Both images show separation of the occipital bone and C1 with increased basion dens interval.


Distracted Occipitoatlantal Dissociation


Occipitoatlantal dissociation refers to abnormal separation of the occipitoatlantal junction and can include partial (subluxation) or complete (dislocation) of the occipitoatlantal articulations. This condition is more common in children, since the pediatric occipital condyles are small and almost horizontal and lack inherent stability;

Normal relationship between the occipital condyles and the superior articular facets of the atlas is maintained by the ligamentous structure that extend between the atlas and the occiput (anterior atlantooccipital membrane, cruciform ligament) as well as the axis and the occiput (tectorial membrane, apical dental ligament). Consequently, vector forces that exceed the physiologic tolerance of these ligaments result primarily in ligamentous disruption.

The methods of determining occipitoatlantal dislocation are:

  1. The distance between the basion-dental interval (BDI) which should not normally exceed 14 mm in children.
  2. The distance between the basion and the rostral extension of the posterior cortical margin of the of the axis body defines the basion-axial interval (BAI) which should not normally exceed 12 mm.
  3. Power ratio: ratio of the distance between the basion (B) to the midpoint of spinolaminar line of C1 (C) to the distance between the midpoint of the posterior margin of the anterior tubercle of C1 (A) and the opisthion (o) or BC/AO. The ratio is more than 1 in occipitoatlanal dislocation. Power ratio is usually normal in distracted occipitoatlantal dissociation (as opposed to dislocation) as seen in this patient.


  1. Harris J, Mirvis E. The Radiology of the Acute Cervical Spine Trauma, 3rd ed. Lippincott Williams & Wilkins, 1996.

2 images