The acetabular angle is a plain film measurement used when evaluating developmental dysplasia of the hip (DDH). It is most useful in patients who have started to ossify the epiphysis since ossification negates the usefulness of ultrasound.
The angle is formed by a horizontal line connecting both triradiate cartilages (Hilgenreiner line) and a second line which extends along the acetabular roof. In the adult where the triradiate cartilages are fused and therefore inapparent the inferior margin of the pelvic tear drop is used instead. This of course shifts the horizontal line inferiorly and changes the value of the angle.
Normal values in children
The acetabular angle using Hilgenreiner's line should be less than 28 degrees at birth. The angle should become progressively shallower with age.
Normal values in adults
The normal range is 33 to 38 degress. Angles above 47 degrees are seen in patients with acetabular dysplasia. A measurement between 39 and 46 degrees is indeterminate.
Additionally, having the center beam above or below the pubis can alter the measurment due to geometric distortion. However a 5 cm error only changes them measurement by 4º. This does highlight that these measurements should not be performed on an abdominal radiograph.
Conditions associated with increased acetabular angles
- neuromuscular disorders
- developmental dysplasia of the hip (DDH)