Atlas-dens interval instability: what measurement indicates instability?

• A. greater than 1 mm
• B. greater than 2 mm
• C. greater than 4 mm
• D. greater than 3 mm

Answer: (D)

The key idea is that instability at the atlas-dens interval is defined by the anterior atlantodental interval exceeding a normal threshold. The distance between the anterior arch of the atlas (C1) and the dens of the axis (C2) should be small. In adults, a normal interval is up to about 3 mm. When this distance is greater than 3 mm, it suggests laxity or rupture of the transverse ligament, allowing abnormal forward movement of C1 on C2 and signaling atlantoaxial instability, which can threaten the spinal cord.

Dynamic imaging, like flexion-extension views, is often used because instability may be more evident with movement than in a neutral position. Contextually, in children the threshold can be higher (the normal range is a bit larger), but for adults the >3 mm criterion is the standard indicator of instability.

So, the measurement that indicates atlas-dens interval instability is an anterior atlantodental interval greater than 3 mm.