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Open Access Highly Accessed Research article

Hip abductor moment arm - a mathematical analysis for proximal femoral replacement

Eric R Henderson1*, German A Marulanda1, David Cheong2, H Thomas Temple3 and G Douglas Letson2

Author Affiliations

1 Department of Orthopaedics and Sports Medicine, 13220 Laurel Drive, University of South Florida, Tampa, Florida, 33612, USA

2 Sarcoma Division, 12902 Magnolia Drive, H. Lee Moffitt Cancer & Research Institute, Tampa, Florida, 33612, USA

3 Orthopaedic Oncology Division, Department of Orthopaedic Surgery, University of Miami, Miami, Florida, USA

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Journal of Orthopaedic Surgery and Research 2011, 6:6  doi:10.1186/1749-799X-6-6

Published: 25 January 2011

Abstract

Background

Patients undergoing proximal femoral replacement for tumor resection often have compromised hip abductor muscles resulting in a Trendelenberg limp and hip instability. Commercially available proximal femoral prostheses offer several designs with varying sites of attachment for the abductor muscles, however, no analyses of these configurations have been performed to determine which design provides the longest moment arm for the hip abductor muscles during normal function.

Methods

This study analyzed hip abductor moment arm through hip adduction and abduction with a trigonometric mathematical model to evaluate the effects of alterations in anatomy and proximal femoral prosthesis design. Prosthesis dimensions were taken from technical schematics that were obtained from the prosthesis manufacturers. Manufacturers who contributed schematics for this investigation were Stryker Orthopaedics and Biomet.

Results

Superior and lateral displacement of the greater trochanter increased the hip abductor mechanical advantage for single-leg stance and adduction and preserved moment arm in the setting of Trendelenberg gait. Hip joint medialization resulted in less variance of the abductor moment arm through coronal motion. The Stryker GMRS endoprosthesis provided the longest moment arm in single-leg stance.

Conclusions

Hip abductor moment arm varies substantially throughout the hip's range of motion in the coronal plane. Selection of a proximal femur endoprosthesis with an abductor muscle insertion that is located superiorly and laterally will optimize hip abductor moment arm in single-leg stance compared to one located inferiorly or medially.