In evaluating for atlantoaxial instability, which imaging approach is most informative?

• A. CT scan without dynamic views
• B. Static MRI
• C. Dynamic radiographs measuring ADI
• D. Skull radiographs

Answer: (C)

Atlantoaxial instability is about abnormal motion between the atlas and axis, so the imaging method must reveal actual movement at the C1–C2 joint. Dynamic radiographs with flexion and extension views let you measure the atlas–dens interval (ADI). An abnormal increase in ADI during flexion confirms instability, making this approach the most informative. Static imaging like a single CT or static MRI shows anatomy at one position but misses how the joint moves; skull radiographs don’t adequately assess the C1–C2 relationship. Therefore, dynamic radiographs measuring ADI best reveal instability. In adults, an ADI above about 3 mm is typically abnormal; in children, up to about 5 mm can be normal, so age matters in interpretation.