Atlantoaxial Subluxation in RA¶

Definition¶

Atlantoaxial subluxation (AAS) is the most common cervical spine manifestation of rheumatoid arthritis, occurring in approximately 25–50% of RA patients. It results from pannus formation and erosion of the transverse ligament and odontoid process, allowing excessive anterior translation of C1 on C2.

Types¶

Anterior AAS (Most Common — 70%)¶

Forward displacement of C1 on C2 due to transverse ligament incompetence. Measured by the anterior atlanto-dental interval (ADI):

• Normal ADI — ≤3 mm in adults, ≤5 mm in children
• ADI 3–6 mm — Transverse ligament laxity
• ADI 6–9 mm — Transverse ligament rupture with alar ligament integrity
• ADI >9 mm — All ligamentous restraints compromised; high risk of cord compression

Posterior AAS (Rare)¶

Posterior subluxation of C1 on C2, usually from odontoid erosion or fracture through a weakened dens.

Lateral AAS¶

Lateral offset of C1 on C2, often asymmetric due to unilateral lateral mass erosion.

Rotatory AAS¶

Rotational subluxation of C1 on C2, causing fixed torticollis.

Imaging Findings¶

Radiography¶

• Lateral view — Increased ADI (>3 mm). Best demonstrated on flexion views which increase the subluxation.
• Posterior atlanto-dental interval (PADI) — The distance from the posterior odontoid to the anterior margin of the posterior arch of C1. PADI <14 mm correlates with cord compression and is a more clinically relevant measurement than ADI.

MRI¶

• Pannus surrounding the odontoid — intermediate signal soft tissue, enhancing on post-contrast
• Cord compression between the pannus/odontoid anteriorly and the posterior arch of C1 posteriorly
• Cord signal change (T2 hyperintensity indicating myelopathy)

CT¶

• Erosion of the odontoid process
• Lateral mass erosion
• Degree of subluxation in neutral position

Management¶

• Conservative — Rigid cervical collar for mild, stable subluxation without myelopathy
• Surgical — C1–C2 posterior fusion (or occipitocervical fusion) for progressive subluxation, PADI <14 mm, or myelopathy. Disease-modifying anti-rheumatic drugs (DMARDs) have reduced the incidence and progression of AAS.

Key Points¶

• Most common cervical manifestation of RA (25–50%)
• ADI >3 mm on lateral flexion radiograph indicates subluxation
• PADI <14 mm correlates with cord compression — more clinically useful than ADI
• MRI shows pannus around the odontoid and evaluates cord compression
• Surgical fusion is indicated for progressive instability or myelopathy

References¶

1. Lacy J, Bajaj J, Gillis CC. Atlantoaxial Instability. In: StatPearls. Treasure Island (FL): StatPearls Publishing; updated 2023 Jun 12. https://www.ncbi.nlm.nih.gov/books/NBK519563/
2. Morimoto LR, Kase DT, Esmanhotto PG, et al. Imaging Assessment of Nontraumatic Pathologic Conditions at the Craniovertebral Junction: A Comprehensive Review. RadioGraphics. 2024;44(5):e230137. https://pubs.rsna.org/doi/full/10.1148/rg.230137
3. Siempis T, Tsakiris C, Anastasia Z, Alexiou GA, Voulgaris S, Argyropoulou MI. Radiological assessment and surgical management of cervical spine involvement in patients with rheumatoid arthritis. Rheumatol Int. 2022;43(2):195–208. https://pmc.ncbi.nlm.nih.gov/articles/PMC9898347/
4. 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/
5. Jurik AG. Imaging the spine in arthritis—a pictorial review. Insights Imaging. 2011;2(2):177–191. https://pmc.ncbi.nlm.nih.gov/articles/PMC3259357/
6. Gillick JL, Wainwright J, Das K. Rheumatoid Arthritis and the Cervical Spine: A Review on the Role of Surgery. Int J Rheumatol. 2015;2015:252456. https://pmc.ncbi.nlm.nih.gov/articles/PMC4553335/
7. Atlantoaxial subluxation - rheumatoid arthritis. Radiopaedia.org. https://radiopaedia.org/cases/atlantoaxial-subluxation-rheumatoid-arthritis-1

Related Articles¶

• Rheumatoid Arthritis — Cervical Spine
• Cranial Settling in RA