Powers ratio

Last revised by Dr Fabio Macori on 12 Jun 2022

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 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.

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Cases and figures

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