Extrusion index

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The extrusion index is a radiographic measurement of femoral head bony coverage by the acetabulum. It is useful in assessing for developmental dysplasia of the hip as well as femoroacetabular impingement.

Measurement

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.

Interpretation

<17%: pincer morphology (femoroacetabular impingement)
17-27%: normal
>27%: acetabular dysplasia

Potential pitfalls include:

falsely high index in patients with concurrent cam morphology
falsely low in patient's with developing or abnormal femoral capital epiphyses