Extrusion index

Last revised by Henry Knipe on 4 Oct 2023

The extrusion index, also known as the (Reimers) hip migration index, is a radiographic measurement of femoral head bony coverage by the acetabulum. It is useful in assessing for developmental dysplasia of the hip, femoroacetabular impingement, and assessment of hip subluxation in cerebral palsy.

It is calculated by dividing the horizontal distance of the lateral femoral head that is uncovered by the 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. 

Potential pitfalls include ref:

  • falsely high index in patients with concurrent cam morphology

  • falsely low in patients with developing or abnormal femoral capital epiphyses

  • true change between studies is estimated to be >10% (range 8.3-13%) with change <10% possibly due to measurement or other errors 4

This measurement is named after the Danish orthopedic surgeon Jørgen Reimers who described it in 1980 4.

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