Predicting the yield of the proximal femur using high-order finite-element analysis with inhomogeneous orthotropic material properties.

High-order finite-element (FE) analyses with inhomogeneous isotropic material properties have been shown to predict the strains and displacements on the surface of the proximal femur with high accuracy when compared with in vitro experiments. The same FE models with inhomogeneous orthotropic material properties produce results similar to those obtained with isotropic material properties. Herein, we investigate the yield prediction capabilities of these models using four different yield criteria, and the spread in the predicted load between the isotropic and orthotropic material models. Subject-specific high-order FE models of two human femurs were generated from CT scans with inhomogeneous orthotropic or isotropic material properties, and loaded by a simple compression force at the head. Computed strains and stresses by both the orthotropic and isotropic FE models were used to determine the load that predicts 'yielding' by four different 'yield criteria': von Mises, Drucker-Prager, maximum principal stress and maximum principal strain. One of the femurs was loaded by a simple load until fracture, and the force resulting in yielding was compared with the FE predicted force. The surface average of the 'maximum principal strain' criterion in conjunction with the orthotropic FE model best predicts both the yield force and fracture location compared with other criteria. There is a non-negligible influence on the predictions if orthotropic or isotropic material properties are applied to the FE model. All stress-based investigated 'yield criteria' have a small spread in the predicted failure. Because only one experiment was performed with a rather simplified loading configuration, the conclusions of this work cannot be claimed to be either reliable or sufficient, and future experiments should be performed to further substantiate the conclusions.