Lines of the Skull Base
Contents
Basilar Invagination, Basilar Impression and Platybasia

Basilar invagination and basilar impression (we will use the term BI for both) are an uncommon syndrome that occurs when the superior part of the odontoid (part of the C2 vertebra) migrates upward. For the most part, the terms basilar invagination and basilar impression are often used interchangably because in both cases there is upwards migration of the upper cervical spine.



Basilar invagination is defined as superior migration of vertebral elements, most commonly the odontoid process, due to softening of bones at the base of the skull. Examples of basilar invagination include rheumatoid arthritis, hyperparathyroidism, Paget's disease, osteogenesis imperfecta, rickets, Hurler's syndrome, and Hadju-Cheney syndrome. These are diseases where the underlying bone is abnormal which leads to the superior migration of the vertebral elements.



Basilar impression is the upward displacement of vertebral elements, usually the odontoid process, into the normal foramen magnum. The skull base is normal in basilar impression. Examples of basilar impression are Down's syndrome, Klippel Feil syndrome, and Chiari malformation where the bone mineralization is normal but there is superior migration of vertebral elements.



Hence, the technical difference between basilar invagination and basilar impression is due to whether bone is normal or not.



Both basilar invagination and basilar impression can result in stenosis of the foramen magnum, and compression of the lower brainstem which can be manifested clinically as sudden death due to fatal brainstem compression.



Platybasia is defined as an increase in the basal angle of the skull which is due to flattening of the base of the skull. Remember that "platy" is Latin for "flat" like the bill of a platypus. Platybasia is diagnosed when, on a lateral skull film or sagital CT, the angle of the line drawn from the nasion to the dorsum sella to a line drawn along the plane of the clivus to the anterior margin of the foramen magnum is greater than 143 degrees. Platybasia itself is not a disease.



Cranial settling is caused by destruction of both lateral masses of C1 and resultant collapse of the occipitoatlantal and atlantoaxial articulations. This leads to inferior migration of the atlas relative to the odontoid process. A clue to the diagnosis of cranial settling is recognizing that the anterior arch of C1 is articulating with the base of the odontoid process or the body of C2 rather than its normal articulation with the mid to superior portion of the odontoid process. A method to differentiate cranial settling from BI is that in cranial settling the tip of the odontoid is below McRae's line. Cranial settling typically occurs in advanced rheumatoid arthritis where pannus erodes the lateral masses of C1. Pannus also increases the predental space (normal measuring <2.5mm) which is the definition of anterior atlantoaxial subluxation.



Normal Relationship


Plain film and CT: the skullbase in relation to the cervical spine.


Chamberlain's Line


Chamberlain's line is a line drawn on lateral skull radiograph between the posterior end of the hard palate to the posterior lip of the foramen magnum. In about 50% of normal subjects the tip of the odontoid is at, or below, Chamberlain's line.



If the tip of the dens is greater than 3mm above Chamberlain's line then the presence of basilar invagination is present. Another rule of thumb is if more than half of the odontoid process is above this line then basilar invagination is likely present.



Plain film and CT: demonstration of Chamberlain's Line.


McGregor's Line

McGregor's line extends from the upper surface of the posterior edge of the hard palate to the most caudal point of the occipital curve. If the tip of the odontoid process is more than 4.5mm above McGregor's line then basilar invagination is likely.



McGregor's Line is preferred over Chamberlain's line as it is easier to use.



Plain film and CT: demonstration of McGregor's Line.


McRae's line

McRae's line is defined by the opening of the foramen magnum. If the tip of the dens migrates above this line then impaction is present.



Plain film and CT: demonstration of McRae's line.


Occipitoatlantal Dissociations

There are a number of methods to assess for occipitoatlantal dissociations, all of which are measured from the sagittal plane on the radiograph or sagittal CT.



Scout and CT example of craniocervical dislocation.


Harris Lines

The Harris Lines have also been referred to as the BDI/BAI or the Rule of Twelve. The basion-posterior axial line interval (BAI) is drawn along the posterior aspect of the dens (the posterior axial line) and a measurement between this line and the tip of the basion is performed.



The basion-dental interval (BDI) is the distance measured between the tip of the basion and the tip of the dens.



When the the BDI and BAI to be greater than 12 mm then occipitoatlantal dissociations has occurred.



The Harris Line is believed to be the most useful, sensitive, and reproducible radiographic parameters for detecting and characterizing occipitocervical dissociation and dislocations.



Sagittal CT images: Left measures the basion-posterior axial line interval which is denoted by the small horizontal red line. The right image demonstrates measurement of the basion-dental interval which is denoted by the vertical red line. If either of these distances are greater than 12 mm then the diagnosis of occipitocervical dissocation is fairly certain.



Power's Rule

The Powers ratio can be measured to determine if there is anterior occipitoatlantal dissociation. The Powers ratio is the distance between the basion and the posterior spinolaminar line of C1 (BC) divided by the distance between the anterior arch of C1 and the opisthion (AO). If the Power's Rule (BC)/(AO) is greater than 1 then anterior occipitoatlantal dissocation has likely occurred.



The accuracy of the Powers ratio is controversial since it can be difficult to locate the position of the basion and the opisthion on the lateral skull radiograph. It is easier to obtain on sagittal CT. Both methods are demonstrated below.



Plain film and CT demonstration of measuring the Powers ratio. If the Power's Rule (BC)/(AO) is greater than 1 then anterior occipitoatlantal dissocation has likely occurred





Normal lateral skull radiograph