Several diagnostic criteria for atlanto-occipital dislocation have been proposed, including Wholey’s dens-basion interval, the Harris method, and the Powers ratio; however, these criteria are subject to observer error and lack the sensitivity and specificity to be useful in clinical practice. Traynelis et al.(50) classified these injuries according to the relative displacement of the occiput with respect to the atlas; however, this also provides limited clinical discrimination. A simpler and more robust test is the occipital condyle-C1 interval (CCI) defined by Pang et al (38,39) This measure is easily calculated from sagittal and coronal reformatted CT and has been demonstrated to have greater sensitivity than previous tests.
Wackenheim clivus line
- Dens below clival line:The odontoid should be tangential to or below the line that extends along the dorsal clivus (28).
- Posterior C2 body:A line along the posterior aspect of the C2 body should pass <12 mm from the basion, and the basion to the tip of the dens should be <12 mm.
- 31% sensitivity: This method has a sensitivity of 31% (28).
Wackenheim’s and Harris’s lines: Wackenheim’s line extends from the line overlying the dorsal surface of the clivus. Harris’s line extends rostrally from the dorsal surface of the dens (28).
- Basion to C1 divided by opisthion to arch: The ratio of the distance from the basion to the anterior midportion of the lamina of C1 (A – B) divided by the distance from the opisthion to the posterior midportion of the arch (C – D)should be <1 (28).
- 37.5% sensitivity
Powers’ ratio: Line from A to B divided by line from C to D (AB/CD) should be <1 (28).
Occipital condyle to C1 interval
- Width of joint space: The width is the distance across the atlanto-occipital joint space, measured at 4 points on the right and left side in both the coronal and the sagittal planes. An average distance of separation of occipital condyle and C1 facet of ≥4 mm yields 100% sensitivity and specificity for atlanto-occipital dislocation. This measure was introduced by Pang et al (38,39) in 2007 and was created to evaluate CT scans for evidence of atlanto-occipital instability.
Condyle-C1 interval in the sagittal and coronal planes: Four points (arrowheads) equidistant from each other are picked for measurement of the joint interval to compensate for the slight unevenness of the joint space (38).
- Ruptured tectorial membrane:This finding has a 71% sensitivity.
- Blood surrounding spinal cord:This finding has a 63% sensitivity.
- Extra-axial C1-C2 hemorrhage:This finding has a 75% sensitivity.
- None of these specific:None of above are specific for atlanto-occipital dislocations.
Atlanto-occipital dislocation: There is evidence of ligamentous disruption at the craniocervical junction. Signal change can be seen at the cervico-medullary junction.