Strain distribution within the human femur due to physiological and simplified loading: finite element analysis using the muscle standardized femur model

The aim of the current work was to study the effect of simplified loading on strain distribution within the intact femur using the Muscle Standardized Femur finite element model and to investigate whether the interaction between the intact human femur and the muscles which are attached to the bone surface could accurately be represented by concentrated forces, applied through the centroids of their attachment areas. An instant at 10 per cent of the gait cycle during level walking was selected as the reference physiological load case; nine load cases were analysed. Comparison of the calculated results for the physiological load case with muscle forces uniformly distributed over their attachment areas showed good agreement with in vivo measurements of strain values and femoral head displacement in humans. Simplified load cases generated unrealistic displacement results and high strain magnitudes, exceeding the physiological range. It was found that when muscles with large attachment areas are included in the model and the muscle forces are simplified, stress and strain distributions will be affected not only on the external bone surface in the vicinity of the load application node, but also on the internal surface of the cortical bone. However, applying muscle forces as concentrated loads at the centroids of the attachment areas can serve as first indicators of the physiological stress and strain levels, if results from nodes and elements in the vicinity of the load application nodes are discarded. Omitting muscle forces or fixing the femur in mid-shaft leads to large unphysiological strain values.