From the Research
Atlanto-Axial Joint Widening Threshold
The millimeter threshold for considering atlanto-axial (A-A) joint widening on an X-ray as abnormal is not consistently defined across studies. However, some studies provide insight into the normal ranges and thresholds for atlantoaxial interval and atlantodental interval measurements.
- The upper limit of the normal range of values for the atlantoaxial interval in adults was found to be 3.34 mm on the right and 3.39 mm on the left 1.
- More than 95% of the pediatric population was found to have an atlantoaxial interval of less than 3.9 mm on either side 1.
- A study on pediatric patients with Trisomy 21 considered atlantoaxial instability (AAI) to be present when the atlantodens interval (ADI) was ≥6 mm or the space available for the cord at C1 (SAC) was ≤14 mm 2.
- Another study proposed that the atlantodental interval (ADI) should be considered abnormal when it exceeds 6 mm 3.
- A three-dimensional study of the atlantodental interval in a normal Chinese population found that the anterior atlantodental interval (AADI) was 1.83 ± 0.46 mm (0.9-3.4 mm) in males and 1.63 ± 0.43 mm (0.5-3.2 mm) in females, with a 95% confidence interval for AADI of 1.75-1.90 mm in males and 1.54-1.72 mm in females 4.
Key Findings
- The normal range of atlantoaxial interval and atlantodental interval measurements varies across studies and populations.
- Thresholds for considering atlanto-axial joint widening as abnormal range from 3.34 mm to 6 mm, depending on the study and population.
- The choice of threshold may depend on the specific population being studied and the clinical context in which the measurement is being used 1, 2, 3, 4.