The extrusion index is a radiographic measurement of femoral head bony coverage by the acetabulum. It is useful in assessing for developmental dysplasia as well as femoroacetabular impingement.
It is calculated by dividing the horizontal distance of the lateral femoral head that is uncovered by acetabulum by the total horizontal distance or width of the femoral head. This is expressed as a percentage.
The index can be calculated in young adults or older children in whom the pelvic growth plates have not completely fused but who have fully developed femoral capital epiphyses.
Values between 17-27% are considered within the normal range 1.
Increased acetabular index
- acetabular dysplasia (i.e. developmental dysplasia of the hip)
Reduced acetabular index
- pincer type impingement
- falsely high index in patients with concurrent CAM lesions
- falsely low in patient's with developing or abnormal femoral capital epiphyses
- 1. 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
- 2. Chiamil SM, Abarca CA. Imaging of the hip: a systematic approach to the young adult hip. (2016) Muscles, ligaments and tendons journal. 6 (3): 265-280. doi:10.11138/mltj/2016.6.3.265 - Pubmed
- 3. 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