Watanabe instability index is measured from lateral flexion and extension radiographs.
Minimum and maximum distances are measured from the posterior border of the C2 body to the posterior arc of the atlas.
The instability index is calculated by the following equation:
Instability index = maximum distance ? minimum distance + maximum distance × 100(%)
If the instability index is more than 40% the patient is very likely to have neurological symptom« Back to Glossary Index