Citation, DOI, disclosures and article data
Citation:
Sheikh Z, Knipe H, Worsley C, et al. Extrusion index. Reference article, Radiopaedia.org (Accessed on 27 Apr 2024) https://doi.org/10.53347/rID-62947
Disclosures:
At the time the article was last revised Henry Knipe had the following disclosures:
- Integral Diagnostics, Shareholder (ongoing)
- Micro-X Ltd, Shareholder (ongoing)
These were assessed during peer review and were determined to
not be relevant to the changes that were made.
View Henry Knipe's current disclosures
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
History and etymology
This measurement is named after the Danish orthopedic surgeon Jørgen Reimers who described it in 1980 4.
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1. Tannast M, Hanke M, Zheng G, Steppacher S, Siebenrock K. What Are the Radiographic Reference Values for Acetabular Under- and Overcoverage? Clin Orthop Relat Res. 2015;473(4):1234-46. doi:10.1007/s11999-014-4038-3 - Pubmed
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2. Chiamil S & Abarca C. Imaging of the Hip: A Systematic Approach to the Young Adult Hip. Muscles Ligaments Tendons J. 2016;6(3):265-80. doi:10.11138/mltj/2016.6.3.265 - Pubmed
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3. Tannast M, Siebenrock K, Anderson S. Femoroacetabular Impingement: Radiographic Diagnosis--What the Radiologist Should Know. AJR Am J Roentgenol. 2007;188(6):1540-52. doi:10.2214/AJR.06.0921 - Pubmed
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4. Faraj S, Atherton W, Stott N. Inter- and Intra-Measurer Error in the Measurement of Reimers’ Hip Migration Percentage. The Journal of Bone and Joint Surgery British Volume. 2004;86-B(3):434-7. doi:10.1302/0301-620x.86b3.14094 - Pubmed
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