The acetabular angle, also known as Sharp angle ⁶, is a radiographic measurement most commonly used to evaluate potential developmental dysplasia of the hip (DDH) in children and acetabular dysplasia in adults.

Usage

The acetabular angle is used in patients who have started to ossify the epiphysis since ossification diminishes the usefulness of ultrasound ^ref. After 10-13 years of age, it can replace the use of the acetabular index, which becomes difficult to measure after triradiate cartilage fusion ¹⁰.

It is one of the less commonly used measurements for assessing potential acetabular dysplasia in adults ¹¹.

Measurement

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 extending 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.

Interpretation

Pediatric

The acetabular angle using the Hilgenreiner line should be <30º at birth ⁹. The angle should become progressively shallower with age and should measure <22º at and beyond 1 year ⁹. 

Adult

• increased

  • reported between >42º ⁶ or ≥45º ^7,11

  • associated with acetabular dysplasia ¹¹

• borderline/indeterminage: between upper normal value and increased ^6,7

• normal: 33º to 38º ^6,8

• decreased: <32º ⁶

Related pathology

• increased acetabular angles

  • neuromuscular disorders

  • acetabular dysplasia

  decreased acetabular angles

  • Down syndrome

  • achondroplasia

See also

• beta angle

• alpha angle

• Hilgenreiner line