A patient with longstanding rheumatoid arthritis develops atlantoaxial instability with an atlantodental interval (ADI) of 8 mm on flexion radiograph. The threshold ADI at which posterior atlantoaxial fusion is indicated to prevent cord compression is:

• A >3 mm in adults
• B >10 mm in adults
• C >12 mm in adults
• D >5 mm in adults ✓

Explanation

In adults, the normal ADI is <3 mm; 3–5 mm indicates ligamentous laxity, and >5 mm indicates rupture of the transverse ligament of the atlas. In rheumatoid atlantoaxial instability, surgical stabilization (C1–C2 posterior fusion using Harms technique or Goel-Harms technique) is indicated when ADI >5 mm (or posterior atlantodental interval PADI <14 mm), or with neurological symptoms. An ADI of 8 mm in this patient clearly exceeds the surgical threshold.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.