The acetabular angle is a radiographic measurement used when evaluating potential developmental dysplasia of the hip (DDH). It is most useful in patients who have started to ossify the epiphysis since ossification diminishes 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 roofs. In the adult, where the triradiate cartilages are fused and therefore inapparent, the inferior margin of the pelvic teardrop 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 28º at birth. The angle should become progressively shallower with age and should measure less than 22º at and beyond 1 year of age.
Normal values in adults
The normal range is 33º to 38º. Angles above 47º are seen in patients with acetabular dysplasia. A measurement between 39º and 46º is indeterminate.
Additionally, having the center beam above or below the pubis can alter the measurement due to geometric distortion. However, a 5 cm error in beam alignment only changes the 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)