Requirement: anteroposterior (AP) radiograph of the pelvis with the patient in the supine position and the legs in the neutral position
Landmarks:
(1) point A = lateralmost point of the superior acetabular rim (roof)
(2) point B = lowest point of the acetabular margin to which the acetabular notch extends (seen on the X-ray as a bright spot or small, lucent incisure)
(3) point M = midpoint on the line AB
(4) point C = point on acetabulum identified by a line drawn from M (line CM) perpendicular to line AB
The ACM angle is the angle between lines CM and a line drawn from C to A (CA).
The CAM angle = 90° - (ACM angle), since the AMC angle is 90°.
Interpretation:
• The normal ACM angle is 40 to 50° according to Idelberger and Frank.
• The angle in a dysplastic hip is > 50° according to Idelberger and Frank.
• Busse et al and Bruckl et al derived the following criteria for a dysplastic hip:
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Age
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Dysplastic Range
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<= 16 years
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>= 53°
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17 - 20 years
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>= 52°
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>= 21 years
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>= 49°
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