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.

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