Atlanto occipital dislocation
Diagnosis and Classification
Diagnosis is made after appropriate imaging studies are obtained. Plain films and CT scans of the cervical spine are the most commonly obtained initial imaging studies.
There are several methods for determining if atlanto-occipital dislocation has occurred based on imaging studies. The first is the "BAI-BDI method". Two distances are measured. The BDI, or basion-dental interval, measures the distance between the most anterior and inferior portion of the skull (ie: "basion") and the tip of the dens, which is a superior extension of the second cervical vertebrae. The BDI should normally be less than 12mm. In the image below the BDI is 14mm and is indicative of atlanto-occipital dislocation.
The second distance that is measured is the BAI, or basion-axial interval. The BAI is the distance between the basion and a line drawn straight up from the posterior (ie: back) portion of the dens. The normal interval is less than 12mm. In the image below the BAI is 14.2mm indicating likely atlanto-occipital dislocation.
A second method of assessing atlanto-occipital dislocation is known as Power's ratio. Power's ratio is most useful if the dislocation is anterior (ie: the head moves forward relative to the spine). Two distances are measured. The first is the distance between the opisthion and the anterior ring of C1; the second is the distance between the basion and the posterior ring of C1. The second distance is then divided by the first to obtain the ratio. A "normal" ratio is less than 1.0; if the ratio is greater than 1.0 then suspicion for dislocation should be high.
X-rays and CT show bony anatomy very well, but do not demonstrate ligamentous injury. Many patients with suspected AOD will also have an MRI to assess damage to the "soft" tissues of the neck. MRI will usually show severe injury to the ligaments that connect the first and second cervical vertebrae to the skull base.
There are two commonly used classification systems for describing atlanto-occipital dislocations. The Traynelis system is purely descriptive and is based on which way the head moves relative to the spine.