A patient with long-standing rheumatoid arthritis (RA) develops severe atlantoaxial instability on dynamic radiographs with anterior atlantodental interval (ADI) of 9 mm and posterior atlantodental interval (PADI) of 12 mm. The surgical threshold for posterior cervical fusion in RA-related AAI is determined by PADI less than:
- A 14 mm
- B 18 mm ✓
- C 10 mm
- D 7 mm
Explanation
In rheumatoid arthritis, atlantoaxial instability (AAI) is due to transverse ligament laxity from synovitis. The PADI (space available for cord) is the critical determinant of spinal cord compromise — it represents the distance between the posterior surface of the odontoid and the anterior aspect of the posterior ring of C1. A PADI less than 14 mm correlates with neurological risk; however, the surgical threshold most cited in literature for prophylactic posterior cervical fusion in RA is PADI <14 mm (some sources use 14 mm, others 14–18 mm as the watch zone). The anterior ADI >3 mm in adults or >5 mm in children suggests instability. The 14 mm PADI threshold is the commonly tested surgical indication.
Reference: Maheshwari Essential Orthopaedics, 6th ed.
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Written and medically reviewed by the StethoPrep medical team.