Powers ratio

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:

  1. basion (A) and the posterior spinolaminar line of the atlas (B) and,
  2. 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.

Article information

rID: 38038
Section: Signs
Tag: eponym
Synonyms or Alternate Spellings:
  • Power ratio
  • Power's ratio

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Cases and figures

  • Figure 1: base of skull lines (annotated images)
    Drag here to reorder.
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