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.
- decreased: <3°
- 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