Acetabular index

Last revised by Henry Knipe on 20 Jan 2024

The acetabular index, also known as the acetabular roof angle or Tönnis angle, is a radiographic measurement of acetabular inclination. It is useful in assessing for developmental dysplasia of the hip (DDH) as well as pincer morphology in femoroacetabular impingement (FAI).

The acetabular index is measured on AP pelvic radiographs. First, a line is drawn from the medial edge of the acetabular sourcil and through to the most lateral aspect of the sourcil followed by a second line drawn in the horizontal plane of the pelvis and the angle is measured between these two lines 3,4.

Different normal ranges have been published by different authors:

  • when the medial edge of the sourcil is blurred it makes this measurement difficult or impossible 4

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