Radiographic lines - Basilar invagination

Five lines have been used to evaluate basilar invagination on frontal and lateral images, based on the relative projection of the odontoid process to these lines.

Lateral image

	Reference points	Basilar invagination threshold	Misc.
McRae line	basion to the opisthion	tip of the dens crosses this line	Chiari I: cerebellar tonsils position below >5 mm with associated findings/symptoms.
Chamberlain's line	back of hard palate to the opisthion	tip of the dens above >3 mm	#ThAC
McGregor line	back of hard palate to the most caudal point of the occipital curve	tip of the dens above >4.5-5 mm	modification of Chamberlain line, used when opisthion cannot be identify

Frontal image

	Reference points	Basilar invagination threshold	Misc.
Digastric line	right and left digastric grooves	tip of the dens below < 11 mm	also called biventer line
Bimastoid line	inferior tips of the mastoid processes	tip of the dens above >10 mm