Which radiographic sign supports the diagnosis of atlantoaxial instability in rheumatoid arthritis?

• A. Decreased atlantodental interval (ADI) on dynamic radiographs
• B. Basilar invagination
• C. Increased atlantodental interval (ADI) on dynamic radiographs
• D. Ossification of the posterior longitudinal ligament

Answer: (C)

In rheumatoid arthritis, instability at the atlantoaxial joint is most clearly shown by an increased atlantodental interval on dynamic radiographs. The condition often damages the transverse ligament, allowing the atlas to translate forward relative to the dens. Dynamic views, especially flexion and extension, reveal this instability: if the space between the anterior arch of C1 and the dens widens when the neck bends forward, that widening signifies anterior atlas translation and transverse ligament compromise. In adults, the normal ADI is roughly 2–3 mm; a measurement above the normal range on dynamic imaging indicates atlantoaxial instability.

Basilar invagination and ossification of the posterior longitudinal ligament are separate issues. Basilar invagination refers to upward migration of the odontoid into the skull base, which can co-occur but is not the primary sign of AAI. Ossification of the posterior longitudinal ligament causes different posterior compression along the spine and does not specifically indicate atlantoaxial instability.