The ratio, AB/CD, is measured as the ratio of the distance in the median (midsagittal) plane between the:
- basion (A) and the posterior spinolaminar line of the atlas (B) and,
- opisthion (C) and the anterior arch of the atlas (D)
Normal values are <1 on plain radiographs 1 and <0.9 on CT 2. If this ratio is >1, then the anterior atlanto-occipital dissociation should be suspected.
History and etymology
Barry Powers, American radiologist, described his ratio in a paper published in 1979 1.
- 1. Powers B, Miller MD, Kramer RS et-al. Traumatic anterior atlanto-occipital dislocation. Neurosurgery. 1979;4 (1): 12-7. Pubmed citation
- 2. Rojas CA, Bertozzi JC, Martinez CR et-al. Reassessment of the craniocervical junction: normal values on CT. AJNR Am J Neuroradiol. 2007;28 (9): 1819-23. AJNR Am J Neuroradiol (full text) - doi:10.3174/ajnr.A0660 - Pubmed citation