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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 2. However, Powers ratio is not useful in diagnosing posterior dissociation and vertical distraction injuries 2.
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 C, Bertozzi J, Martinez C, Whitlow J. Reassessment of the Craniocervical Junction: Normal Values on CT. AJNR Am J Neuroradiol. 2007;28(9):1819-23. doi:10.3174/ajnr.a0660 - Pubmed