====== McRae’s line ======

The [[opisthion]] is the median (midline) point of the posterior margin of the [[foramen magnum]].

{{::opisthion.png?300|}}

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)
((McRae DL. The Significance of Abnormalities of the Cervical Spine. AJR. 1960; 70:23–46)).
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
((Cronin CG, Lohan DG, Mhuircheartigh JN, Meehan CP, Murphy JM, Roche C. MRI evaluation and meas- urement of the normal odontoid peg position. Clin Radiol. 2007; 62:897–903)).

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

[[McRae’s line]]

Chamberlain line