Acetabular index
Developmental
dysplasia of hip. Patient A: Bilateral femoral heads are located supero-lateral (red circles) to their normal positions (black circle). They are seen superior to Hilgenreiner"s lines and lateral to Perkin"s lines on both sides.
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).
Measurement
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.
Interpretation
- decreased: <3°
- in severe overcoverage, there may be a negative value
- pincer morphology in femoroacetabular impingement (FAI)
- normal range: 3-13°
- increased: >13°
- acetabular dysplasia (i.e. DDH)
