What is the normal upper limit for the atlas-dens interval (ADI) in adults?

• A. 2 mm
• B. 4 mm
• C. 3 mm
• D. 5 mm

Answer: (C)

The atlas-dens interval shows how stable the C1–C2 junction is by measuring the space between the anterior arch of the atlas and the dens. In adults, this distance should be very small, with a normal upper limit of about 3 millimeters on a neutral lateral radiograph. Keeping this interval within 3 mm reflects an intact transverse ligament and stable atlantoaxial alignment. When the ADI exceeds roughly 3 mm, it suggests instability at the C1–C2 level, which can arise from transverse ligament disruption or inflammatory conditions and may require further evaluation or intervention. In children, a larger limit (around 5 mm) is often accepted due to greater ligamentous laxity, but in adults, 3 mm is the standard cutoff. Values of 4 or 5 mm would be considered abnormal, indicating instability, while 2 mm is tighter but not the commonly quoted upper limit for adults.