In Figure A, the radiographic diagnosis of atlanto-occipital dissociation (AOD) was made and confirmed on computed tomography (CT, B). What objective criteria are used to establish this diagnosis?
AOD, also known as craniocervical dissociation, results from disruption of stabilizing ligaments at the skull base and typically occurs in the setting of high-speed motor vehicle accidents (MVAs).1 Ligamentous disruption results in anterior/posterior dislocation of the skull or vertical distraction. Although AOD is usually fatal or catastrophic, early recognition and prompt atlantoaxial fixation have led to improved outcomes. Although AOD may be readily apparent on lateral radiographs of the cervical spine, routine use of CT in trauma has allowed for more precise measurements.
The lateral radiograph in this patient following a high-speed MVA demonstrates anterior dislocation of the skull (yellow arrow, A) with respect to the cervical spine (red arrow, A). There is also significant increase in the C1-C2 interspinous distance (green arrow, A).
The sagittal CT image of the cervical spine (B) shows an increase in the condyle–C1 interval (CCI), which is measured in the sagittal plane at the center of the articulation between the condyle and C1 (yellow arrows). The CCI has been shown to be the most reliable parameter of AOD, with a distance > 4 mm considered abnormal. Some studies have reported a distance > 2.5 mm on either side, or a combined distance of > 5 mm by summing both sides (condylar sum), as abnormal. Additional measurements on CT are available and can be useful and complementary to the CCI measurement.1
Shelat N, Bennett DL. At the Viewbox: Atlanto-occipital Dissociation. J Am Osteopath Coll Radiol. 2016;5(4):27.