McRae’s line
The opisthion is the median (midline) point of the posterior margin of the foramen magnum.
It is one of the skull landmarks, craniometric points for radiological or anthropological skull measurement.
Drawn across foramen magnum (tip of clivus (basion) to opisthion) 1). The mean position of the odontoid tip below the line is 5 mm (± 1.8 mm SD) on CT and 4.6 mm (± 2.6 mm SD) on MRI 2).
No part of odontoid should be above this line (the most accurate measure for BI).
Various lines and measurements using the opisthion are made in the diagnosis of atlanto-occipital dissociation (AOD) injuries, platybasia and basilar invagination:
Powers ratio
Boogards angle
Chamberlain line