In rheumatoid arthritis, atlantoaxial instability occurs due to erosion of the transverse ligament. The standard radiological assessment for instability is the atlanto-dens interval (ADI). In adults, the ADI value indicating instability is:
- A >2.5 mm on flexion lateral view
- B >3.5 mm on flexion lateral view ✓
- C >5 mm on lateral neutral view
- D >10 mm on extension lateral view
Explanation
The atlanto-dens interval (ADI) is measured on a lateral flexion cervical X-ray as the distance between the posterior surface of the anterior arch of C1 and the anterior surface of the odontoid process. In adults (with non-elastic ligaments), an ADI >3.5 mm on flexion views is considered abnormal and indicates transverse ligament incompetence. In children, >5 mm is the threshold due to ligamentous laxity. The space available for the cord (SAC = 34 mm minus ADI) <14 mm indicates high risk of spinal cord compression. RA-related atlantoaxial instability with ADI >8–10 mm and neurological symptoms warrants posterior C1-C2 fusion.
Reference: Maheshwari Essential Orthopaedics, 6th ed.
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Written and medically reviewed by the StethoPrep medical team.