Radiology pearls 2 -craniocervical junction

Craniocervical junction: the skull base, atlas and axis form the craniocervical junction that in conjunction with associated ligaments acts as a single functional unit.
A. On sagittal CT images check for the following:

1. atlanto dental space should be less than 3 mm in adult and less than 5 mm in child
2. anterior cortex of the odontoid peg and posterior cortex of anterior arch of C1 are parallel
3. anterior aspects of the laminae of C1-C3 are in alignment, the spinolaminar line
4. bodies of C2 and C3 are in alignment with a normal disc space
5. no sublaxation or widening of the atlanto occipital joints
6. facet joints between C1/C2 and C2/C3 are aligned

B. On axial images check the following:

1. space between anterior arch of C1 and the odontoid peg should be less than 3 mm
2. C1 is symmetrically aligned around the odontoid peg
3. no significant rotation of C0/C1 or C1/C2
4. absence of soft tissue swelling

C. On coronal images, check the following:

1. space between odontoid peg and lateral mass of C1 is the same on both sides
2. there is no sublaxation or widening of the atlanto-occipital joints
3. the facet joints between C1/C2 and C2/C3 are aligned
4. the edge of the lateral mass of C1 does not over hang C2 at the facet joint

The major ligaments of the craniojunction are:

1. the paired alar ligaments that extend from the odontoid peg to the medial aspect of the occipital condyles
2. the posterior longitudinal ligament that runs posterior to the vertebral bodies and extends as the tectorial membrane to insert into basion
3. the anterior longitudinal ligament that runs anterior to the vertebral bodies and extends as anterior atlanto occipital membrane to insert into the basion
4. the transverse atlantal ligament that extends between the lateral masses of C1, passing posterior to the odontoid peg.

Occipitocervical dissociation

occipitocervical dissociation which is also known as atlanto-occipital sublaxation is easily missed on plain radiography, and is potentially fatal!

It maybe

• type 1 - anterior sublaxation
• type 2 -vertical distraction of atlanto-occipital joint more than 2 mm, or
• type 3 - posterior dislocation