The 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 morphology in 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.
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