Acetabular angle

Last revised by Dr Henry Knipe on 20 Jul 2022

The acetabular angle, also known as the Sharp angle 6, is a radiographic measurement most commonly used when evaluating for 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.

In children, the angle is formed by a horizontal plane of the pelvis defined by a line connecting both triradiate cartilages (Hilgenreiner line) and a second line that extends through to the lateral aspect of the acetabular roof 7,8.

In adults, where the triradiate cartilages are fused and therefore inapparent, the horizontal plane of the pelvis defined as the inferior margin of the pelvic teardrop is used instead 7. This, of course, shifts the horizontal line inferiorly and changes the value of the angle ref.

The acetabular angle using the Hilgenreiner line should be <28º at birth ref. The angle should become progressively shallower with age and should measure <22º at and beyond 1 year of age ref

The normal range is 33º to 38º 6,8. An increased acetabular angle defining acetabular dysplasia varies between authors and is reported at >42º 6 or ≥45º 7. When the angle is between the upper end of the normal range and an increased acetabular angle the terms "borderline" or "indeterminate" can be used 6,7. A decreased acetabular angle is <32º 6.

Having the center beam above or below the pubis can alter the measurement due to geometric distortion. Although a 5 cm error in beam alignment only changes the measurement by 4º, this does mean that these measurements should not be performed on an abdominal radiograph ref.

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Cases and figures

  • Figure 1: acetabular angle
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  • Figure 2: measurement techniques
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