Quantitated by measuring the basal angle, which on plain x-rays, measured the angle between lines drawn from the nasion to center of sella and then to the anterior foramen magnum, 1) but on MRI was felt to be better represented by the angle between a line drawn along the floor of the anterior fossa to the dorsum sellae and a second line drawn along the posterior clivus. 2) Normal mean basal angle: 130°. Platybasia: >145° (abnormally obtuse basal angle).
1. McRae’s line:
No part of odontoid should be above this line (the most accurate measure for BI)
Less than 3 mm or half of dens should be above this line, with 6 mm being definitely pathologic. Seldom used because the opisthion is often hard to see on plain film and may also be invaginated. On CT 6) and MRI 7) the normal odontoid tip is 1.4 mm (± 2.4) below the line
3. McGregor’s line:
It refers to a line connecting posterior edge of the hard palate to the most caudal point of the occipital curve. If the tip of the dens lies more than 4.5 mm above this line it is indicative of basilar invagination.
joins the digastric notches.The normal distance from this line to the middle of the atlanto-occipital joint is 10 mm (decreased in BI) 8).
joins tips of mastoid processes. The odontoid tip averages 2 mm above this line (range: 3 mm below to 10 mm above) and this line should cross the atlanto-occipital joint.
Erosion of the tip of the odontoid, commonly seen in rheumatoid arthritis (RA), obviates use of any measurement that is based on the location of the tip of the odontoid 9). For this reason, other measures have been developed, including the Clark station, 10).
Redlund-Johnell criteria, 11) and Ranawat criteria 12). Since even these methods will miss up to 6% of cases of BI in RA, 13), it is recommended that suspicious cases be investigated further (e.g.with CT and/or MRI).
MRI: optimal for demonstrating brainstem impingement, poor for showing bone.
Cervicomedullary angle: the angle between a line drawn through the long axis of the medulla on a sagittal MRI and a line drawn through the cervical spinal cord. The normal CMA is 135 – 170 ° . A CMA < 135 ° correlates with signs of cervicomedullary compression, myelopathy or C2 radiculopathy 14).
CT: primarily done to assess bony anatomy (erosion, fractures…).
CTA should be performed when surgery is contemplated, to show detail of VA anatomy.
Myelography (water soluble) with CT: also good for delineating bony pathology.