Acetabular index

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

Measurement

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.

Interpretation

Different normal ranges have been published by different authors:

• decreased acetabular inclination: <0° ⁵ or <3° 

  • values can be negative

  • at risk for pincer morphology of femoroacetabular impingement (FAI)

• normal range: 0-10° ⁵ or 3-13° ² 

• increased acetabular inclination: >10° ⁵ or >13° ⁴

  • acetabular dysplasia

History and etymology

The acetabular index has been first described by the American Orthopaedic Surgeons Samuel Kleinberg and Herman Lieberman in 1936 ^6,7. An exact definition of the measurement has been made by the German orthopaedic surgeon Dietrich Tönnis in 1984 ^8,9.

Practical points

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