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°
- acetabular dysplasia (i.e. DDH)
- 1. Tannast M, Siebenrock KA, Anderson SE. Femoroacetabular impingement: radiographic diagnosis--what the radiologist should know. (2007) AJR. American journal of roentgenology. 188 (6): 1540-52. doi:10.2214/AJR.06.0921 - Pubmed
- 2. Moritz Tannast, Markus S. Hanke, Guoyan Zheng, Simon D. Steppacher, Klaus A. Siebenrock. What Are the Radiographic Reference Values for Acetabular Under- and Overcoverage?. (2015) Clinical Orthopaedics and Related Research®. 473 (4): 1234. doi:10.1007/s11999-014-4038-3 - Pubmed