IS Case 177: Basilar invagination of dens

Christopher Mosley, MD and Bret Birrer, MS4

Imaging Sciences URMC 2008
Publication Date: 2009-05-21


Patient is a 79-year-old female presenting with gait disturbance, urinary incontinence, and depressed affect. Patient also complained of neck pain. Clinical suspicion for normal pressure hydrocephalus.


C1-2 dislocation with basilar invagination of the dens.


Basilar invagination of dens


Basilar invagination was first described by Anders Adolph Retzius and Frederik Theodor Berg in 1855. Basilar invagination is the most common observed malformation of the craniocervical junction. The condition has been associated with a variety of symptoms, including: neck pain, cough, headache, lower cranial nerve palsies, and corticospinal signs. In addition, basilar invagination may be a cause of hydrocephalus, cerebellar dysfunction, syringomyelia and syringobulbia. The presentations of neurological signs are variable. The malformation has been associated with other anomalies of the notochord and craniovertebral junction, include occipitalization of the atlas, Klippel-Feil anomaly, Chiari type I malformation and syringomyelia. In addition, basilar invagination can be acquired in the setting of Paget’s disease, osteomalacia, osteogenesis imperfecta and rheumatoid arthritis.

Assessing the position of the dens relative to the foramen magnum has classically been determined using Chamberlain's line, McGregor's line, and McRae's line. Chamberlain's line is a line between the posterior aspect of the hard palate and the posterior margin of the foramen magnum. The tip of the dens should be below this line and not more than 5mm above it. McGregor's line is a line from the posterior aspect of the hard palate to the lowest point of the occiput of the skull. The tip of the dens should be no more than 7mm above this line. McRae's line is a line between the anterior and posterior aspects of the foramen magnum margin. The tip of the dens should always be below this line.

Multiple plain film radiographic criteria exist in the literature. A 2001 study of basilar invagination of the dens [2] identified that the use of the Clark station, the Redlund-Johnell criterion, and the Ranawat criterion have a sensitivity of 95%, with a negative predictive value of 91% for diagnosing basilar invagination of the dens, when at least one of these tests was positive.

Clark station describes the location of the atlas in relationship to the odontoid process in the sagittal plane. A diagnosis of basilar invagination is made when the anterior ring of the atlas is level with the middle third (station II) or the caudal third (station III) of the odontoid process.

Redlund-Johnell criterion refers to the distance between the McGregor line and the midpoint of the caudal margin of the second cervical vertebral body. A measurement less than 34 mm in males and less than 29 mm in females indicates basilar invagination.

Lastly, Ranawat criterion refers to the distance between the center of the second cervical pedicle and the transverse axis of the atlas measured along the axis of the odontoid process. A measurement less than 15 mm in males and less than 13 mm in females indicates basilar invagination. Currently, the diagnosis ultimately is made by MRI or CT.


  1. Pearce JM. Platybasia and basilar invagination. Eur Neurol. 2007;58(1):62-4. [PMID: 17483591]
  2. Riew KD, Hilibrand AS, Palumbo MA, Sethi N, Bohlman HH. Diagnosing basilar invagination in the rheumatoid patient. The reliability of radiographic criteria. J Bone Joint Surg Am. 2001 Feb;83-A(2):194-200. [PMID: 11216680]
  3. Katz DS, Math KR, Groskin SA (eds). Radiology Secrets. Philadelphia, Hanley & Belfus, 1998: 335-342.

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