Atlantodental Interval Measurement in Rheumatoid Arthritis
The atlantodental interval (ADI) in patients with rheumatoid arthritis and atlantoaxial instability is measured as the distance from the posterior surface of the anterior arch of the atlas (C1) to the anterior margin of the dens (odontoid process of C2), making option (e) the correct answer.
Anatomical Landmarks and Measurement Technique
The ADI measurement specifically captures the space between:
- Posterior surface of the anterior arch of atlas (C1) - the back wall of the front ring of C1
- Anterior margin of the dens - the front surface of the odontoid peg
This measurement is critical because anterior atlantoaxial subluxation is the most common form of cervical instability in RA patients 1, 2, 3. The inflammatory synovitis in RA destroys the transverse ligament that normally holds the dens against the anterior arch of C1, allowing C1 to slide forward relative to C2.
Normal Values and Pathological Thresholds
Based on multidetector CT measurements in adults 4:
- Normal ADI ranges: 0.85-3.12 mm in adults aged 18-39 years
- ADI decreases with age: 1.77 mm in young adults (18-24 years) to 1.06 mm in those >70 years
- Pathological widening in RA: ADI of 8-9 mm indicates severe instability 3
Clinical significance thresholds:
- ADI >3 mm suggests transverse ligament insufficiency
- ADI >5 mm indicates severe instability requiring surgical consideration
- Dynamic change (ADIΔ) between flexion and extension >3 mm confirms instability 5
Diagnostic Imaging Approach
Essential imaging protocol for RA patients with neck pain:
Lateral cervical radiographs in flexion and extension - the standard screening tool 3
- Critical pitfall: Ensure adequate flexion is achieved during imaging; inadequate motion limits diagnostic utility 3
- Measure ADI on the flexion view where anterior subluxation is maximal
CT or MRI when instability suspected 6
- CT provides precise bony measurements 4
- MRI evaluates spinal cord compression and pannus formation
Dynamic measurements 5:
- ADI in flexion (ADIf)
- ADI in extension (ADIe)
- Change between positions (ADIΔ)
Clinical Examination Pearls
Sharp-Purser test: Place one hand on the patient's forehead and thumb of the other hand on the spinous process of C2. As the patient flexes the neck, apply posterior pressure on the forehead. A positive test (excessive anterior-posterior motion or "clunk") suggests atlantoaxial instability 3.
Red flags requiring urgent evaluation:
- Occipital headache radiating anteriorly
- Neurological symptoms (myelopathy signs, paresthesias)
- Vertebrobasilar insufficiency symptoms (dizziness, drop attacks)
- Progressive neck pain despite medical management
Why Other Options Are Incorrect
Options (a), (b), (c), and (d) describe anatomically impossible or clinically irrelevant measurements. The posterior arch of the atlas is located posteriorly and cannot be used to measure the anterior atlantodental space. The measurement must capture the anterior relationship between C1 and the dens, which is what becomes pathologically widened in RA-associated anterior atlantoaxial subluxation 1, 2.