Which term describes the distance between the atlas and axis used to assess instability at the craniovertebral junction?

• A. Nuchal distance
• B. Foraminal height
• C. Atlas-Dens Interval
• D. Cervical lordosis angle

Answer: (C)

The Atlas-Dens Interval captures the distance between the atlas (C1) and the axis (C2) to assess instability at the craniovertebral junction. It’s measured as the gap between the posterior surface of the anterior arch of C1 and the anterior surface of the dens (odontoid) of C2 on lateral neck radiographs. This interval changes with instability, especially when there’s ligamentous injury like a transverse ligament rupture, making a larger ADI a red flag for potential instability. Normal values are about 3 mm or less in adults and up to about 5 mm in children; values exceeding these thresholds, particularly with increased widening on flexion radiographs, indicate atlantoaxial instability. Other measurements refer to different structures or regions and do not specifically quantify the C1–C2 relationship.