Requirements: A lateral X-ray of the cervical spine and base of the skull, taken with the cervical spine in extension. Additional films may be taken with the cervical spine in neutral position and in flexion.
Landmarks:
(1) basion = midpoint of the anterior border of the foramen magnum
(2) opisthion = midpoint of the posterior border of the foramen magnum
(3) midpoint of the anterior surface of the posterior arch of C1
(4) midpoint of the posterior surface of the anterior arch of C1
Measurements:
(1) line BC = distance from the basion to the anterior surface of the posterior arch of C1
(2) line OA = distance from the opisthion to the posterior surface of the anterior arch of C1
Powers ratio in extension =
= (measurement for line BC in mm) / (measurement for line OA in mm)
Interpretation:
• A ratio >= 0.55 is considered normal (up to 2 SD below the mean for the normal study population evaluated)
• A ratio in the Powers ratio < 0.55 indicates that the patient is at risk for POAH.
Limitations:
• The landmarks taken for the measurements have to be constant for reproducible results.
