A 58-year-old woman with 20-year history of rheumatoid arthritis reports occipital headache worsened by coughing and episodes of upper limb paresthesia. Lateral cervical radiograph shows anterior atlantodental interval within normal limits. Sagittal reconstruction shows the odontoid tip projecting well above a line drawn from the basion to the opisthion of the foramen magnum. Which complication has occurred?

• A Anterior atlantoaxial subluxation
• B Vertical translocation (basilar invagination) of the odontoid ✓
• C Subaxial subluxation at C5-C6
• D Posterior atlantoaxial subluxation

Explanation

Rheumatoid pannus erodes the odontoid process and the lateral masses of B1, allowing the skull to settle onto the spine so the odontoid migrates cranially through the foramen magnum. Projection of the odontoid tip more than about 3 mm above Chamberlain's line (basion to opisthion) confirms basilar invagination, the most dangerous form of rheumatoid cervical instability because it compresses the brainstem and vertebral arteries. The normal anterior atlantodental interval excludes the far more commonly tested anterior subluxation.

Reference: Campbell's Operative Orthopaedics, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.