First and second year — the floor first, then every step
A devastating ligamentous injury easily missed on CT: measure the basion–dens interval and the condyle–C1 interval, look for prevertebral swelling and the subarachnoid blood at the craniocervical junction — and get MRI of the ligaments.
Orient first
Basion–dens interval > 8.5–10 mm on CT (verify) and condyle–C1 interval > 1.5 mm (adults) or asymmetric suggest dissociation.
Children are at higher risk (relatively large head, horizontal facets).
Thin-section CT with sagittal and coronal reformats, bone and soft-tissue windows.
The manoeuvre
Sagittal midline reformat: basion–dens interval in mm.
Coronal and sagittal reformats: condyle–C1 interval each side in mm.
Soft-tissue window: prevertebral swelling and blood at the craniocervical junction.
Occipital condyle fractures.
What confirms it
Abnormal craniocervical intervals with ligamentous disruption on MRI.
What licenses you to exclude it
Normal intervals and ligaments on MRI exclude dissociation.
The classic misread
Reading a supine CT with traction reduced as normal.
Reporting the injury
Classification to use
Traynelis types by direction of displacement (anterior, longitudinal, posterior); the Harborview (Bellabarba) staging of craniocervical injury grades stability (verify).
Measurements — and how to take them
Basion–dens interval on the midline sagittal reformat; condyle–C1 interval on coronal and sagittal reformats (mean of four points where used).
What to report
Basion–dens interval, condyle–C1 intervals on both sides, prevertebral swelling, subarachnoid or epidural blood at the junction, occipital condyle fractures, ligament status on MRI.
How to report it
CT: "Basion–dens interval 12 mm and right condyle–C1 interval 3 mm with prevertebral haematoma — findings suspicious for craniocervical dissociation. MRI recommended urgently."
What not to report
Do not clear the junction on a reduced supine CT without measuring the intervals.
Associated injuries to look for
Vertebral artery injury, brainstem and cord injury, cranial nerve palsies, other cervical fractures.