Sensitivity of femoral strain calculations to anatomical scaling errors in musculoskeletal models of movement

The determination of femoral strain in post-menopausal women is important for studying bone fragility. Femoral strain can be calculated using a reference musculoskeletal model scaled to participant anatomies (referred to as scaled-generic) combined with finite-element models. However, anthropometric errors committed while scaling affect the calculation of femoral strains. We assessed the sensitivity of femoral strain calculations to scaled-generic anthropometric errors. We obtained CT images of the pelves and femora of 10 healthy post-menopausal women and collected gait data from each participant during six weight-bearing tasks. Scaled-generic musculoslceletal models were generated using skin-mounted marker distances. Image-based models were created by modifying the scaled-generic models using muscle and joint parameters obtained from the CT data. Scaled-generic and image-based muscle and hip joint forces were determined by optimisation. A finite-element model of each femur was generated from the CT images, and both image-based and scaled-generic principal strains were computed in 32 regions throughout the femur. The intra-participant regional RMS error increased from 380 mu epsilon (R-2=0.92, p <0.001) to 4064 mu epsilon (R-2=0.48, p <0.001), representing 5.2% and 55.6% of the tensile yield strain in bone, respectively. The peak strain difference increased from 2821 mu epsilon in the proximal region to 34,166 mu epsilon at the distal end of the femur. The inter-participant RMS error throughout the 32 femoral regions was 430 mu epsilon (R-2=0.95, p < 0.001), representing 5.9% of bone tensile yield strain. We conclude that scaledgeneric models can be used for determining cohort-based averages of femoral strain whereas imagebased models are better suited for calculating participant-specific strains throughout the femur. Crown Copyright (C) 2015 Published by Elsevier Ltd. All rights reserved.