Aim of the present study was to evaluate the influence on the global model's accuracy of the strategy adopted to define the average element Young's modulus in subject-specific finite element models of bones from computed tomography data. The classic strategy of calculating the Young's modulus from an average element density and the one that averages the Young's moduli directly derived from each voxel Hounsfield Unit were considered. These strategies were applied to the finite element model of a real human femur. The accuracy of the superficial stress and strain predictions was evaluated against experimentally measured values in 13 strain-gauge locations for five different loading conditions. The results obtained for the two material distributions were statistically different. Both models predicted very accurately the superficial stresses, with regression coefficients higher than 0.9 and slopes not significantly different from unity. The second strategy definitely improved the strains prediction accuracy: the regression coefficient raised from 0.69 to 0.79; the average and peak errors decreased from 45.1% to 31.3% and from 228% to 134% of the maximum measured strain, respectively. The stress fields predicted inside the bone were also significantly different. A new software implementing the second strategy was made available in the public domain.

The assignment of bone tissue material properties is a fundamental step in the generation of subject-specific finite element models from computed tomography data. Aim of the present work is to investigate the influence of the material mapping algorithm on the results predicted by the finite element analysis. Two models, a coarse and a refined one, of a human ileum, femur and tibia, were generated from CT data and used for the tests. In addition a convergence analysis was carried out for the femur model, using six refinement levels, to verify whether the inclusion of the material properties would significantly alter the convergence behaviour of the mesh. The results showed that the choice of the mapping algorithm influences the material distribution. However, this did not always propagate into the finite element results. The difference between the maximum Von Mises stress remained always lower than 10%, apart one case when it reached the 13%. However, the global behaviour of the meshes showed more marked differences between the two algorithms: in the finer meshes of the two long bones 20-30% of the bone volume showed differences in the predicted Von Mises stresses greater than 10%. The convergence behaviour of the model was not worsened by the introduction of inhomogeneous material properties. The software was made available in the public domain.