Cranial Settling in RA¶

Definition¶

Cranial settling (also called basilar invagination or vertical subluxation) occurs when erosion of the lateral masses of C1 and/or the occipital condyles allows the skull and C1 to settle downward relative to C2, causing the odontoid process to migrate superiorly into the foramen magnum. This is the most dangerous cervical complication of RA because it can compress the brainstem and upper cervical cord.

Pathophysiology¶

Progressive erosion of the lateral masses of C1 by rheumatoid pannus causes loss of the bony support between the occiput and C2. The skull "settles" downward, and the odontoid effectively protrudes upward into the foramen magnum. This results in compression of the cervicomedullary junction — the junction between the brainstem and upper cervical cord.

Imaging Measurements¶

Several measurements assess cranial settling:

McGregor line — A line from the posterior hard palate to the most caudal point of the occiput on lateral radiograph. The odontoid tip should not project more than 4.5 mm above this line.
Ranawat criterion — The distance from the center of the C2 pedicle to the transverse axis of C1 on lateral radiograph. Values <13 mm in men or <13 mm in women indicate cranial settling.
Clark station — Divides the odontoid into three equal segments. The anterior arch of C1 normally aligns with Station 1 (upper third). Alignment with Station 2 or 3 indicates progressive cranial settling.

Imaging Findings¶

MRI¶

Odontoid tip projecting above the foramen magnum
Cervicomedullary junction compression
Pannus around the odontoid and craniocervical junction
Cord signal change (myelopathy)
Associated atlantoaxial subluxation is common

CT¶

Erosion of the C1 lateral masses and occipital condyles
Superior migration of the odontoid relative to skull base landmarks
Sagittal and coronal reformats best demonstrate the vertical migration

Clinical Pearl

Cranial settling can be insidious and difficult to detect on plain radiographs because the cranial settling masks the atlantoaxial subluxation — as the skull settles downward, the ADI may paradoxically normalize even though instability worsens. Serial imaging with CT or MRI is essential for RA patients with progressive myelopathy or upper motor neuron signs, even if the ADI appears normal.

Management¶

Occipitocervical fusion is typically required because the craniocervical junction has been destabilized
Preoperative traction may be attempted to reduce the cranial settling
This is a high-risk surgery with significant morbidity

Key Points¶

Most dangerous cervical complication of RA — compresses the brainstem and upper cord
Caused by erosion of C1 lateral masses allowing the skull to settle downward
Odontoid migrates superiorly into the foramen magnum
McGregor line, Ranawat criterion, and Clark station are measurement tools
ADI may paradoxically appear normal — do not rely on ADI alone
Occipitocervical fusion is the definitive treatment

References¶

Donnally CJ III, Munakomi S, Varacallo MA. Basilar Invagination. In: StatPearls. Treasure Island (FL): StatPearls Publishing; updated 2023 Aug 13. https://www.ncbi.nlm.nih.gov/books/NBK448153/
Cranial settling - rheumatoid arthritis. Radiopaedia.org. https://radiopaedia.org/cases/cranial-settling-rheumatoid-arthritis
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;83(2):194-200. https://pubmed.ncbi.nlm.nih.gov/11216680/
Kawaida H, Sakou T, Morizono Y. Vertical settling in rheumatoid arthritis: diagnostic value of the Ranawat and Redlund-Johnell methods. Clin Orthop Relat Res. 1989;(239):128-135. https://pubmed.ncbi.nlm.nih.gov/2912612/
Mańczak M, Gasik R. Cervical spine instability in the course of rheumatoid arthritis — imaging methods. Reumatologia. 2017;55(4):201-207. https://pmc.ncbi.nlm.nih.gov/articles/PMC5647537/
Ellatif M, Sharif B, Baxter D, Saifuddin A. Update on imaging of the cervical spine in rheumatoid arthritis. Skeletal Radiol. 2022;51(8):1535-1551. https://pubmed.ncbi.nlm.nih.gov/35146552/
Joaquim AF, Ghizoni E, Tedeschi H, Appenzeller S, Riew KD. Radiological evaluation of cervical spine involvement in rheumatoid arthritis. Neurosurg Focus. 2015;38(4):E4. https://pubmed.ncbi.nlm.nih.gov/25828498/