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 1) and MRI 2) the normal odontoid tip is 1.4 mm (± 2.4) below the line
It helps to recognise basilar invagination which is said to be present if the tip of the dens is >3 mm above this line.
McGregor developed a modification of the Chamberlain line for when the opisthion could not be identified on plain radiographs.
see McGregor's line.