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Acetabular index

Acetabular index, also called acetabular roof angle or Tönnis angle, is a radiographic measurement of femoral head bony coverage by the acetabulum. It is useful in assessing for developmental dysplasia of the hip (DDH) as well as pincer-type femoroacetabular impingement (FAI).


It is measured on AP pelvic radiograph. First, a line is drawn running through the medial edge of the sclerotic acetabular zone and through the lateral sourcil. Then a horizontal line is drawn and an angle is measured between these two lines.


  • decreased: <3°
    • in severe overcoverage, there may be a negative value
    • pincer-type FAI
  • normal range: 3-13° 2
  • increased: >13°
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Article information

rID: 74583
Synonyms or Alternate Spellings:
  • Tönnis angle
  • Tonnis angle
  • Acetabular roof angle

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