Powers ratio

Radiopaedia • CC-by-nc-sa 3.0 • de

The Powers ratio is a measurement of the relationship of the foramen magnum to the atlas, used in the diagnosis of atlanto-occipital dissociation injuries.

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 and <0.9 on CT . 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 .