# Biomechanical Study of Proximal Femur for Designing Stems for Total Hip Replacement
## 2 Materials and Methods
### Post-Processing  
In order to perform results post-processing, the proximal femur was cut longitudinally using a parallel XZ plane, for measuring the maximum principal stress (MPS), a stress indicator that was used in this study according to examples from the literature [32; 33; 34]. The MPS was calculated on the medial and lateral sides and simulation results were analyzed accordingly.  
The medial side was considered to have three differentiated regions for evaluation purposes: the external cortical (EC), the internal cortical (IC) and the trabecular (T); while the lateral side was considered to have just two representative regions for the evaluation purposes: the internal cortical (IC) and the trabecular (T). In each region of interest, six control points were taken, point 1 being placed at the top of the femur and point 6 at the bottom, as depicted in Figure 5. Control points were used for plotting MPS, as further detailed in the following results and discussion section. Apart from employing the MPS values, the Tsai-Wu field was also obtained and represented for providing additional insight to the discussion of results.  
Apart from the presented proximal femur model, a final application case was also incorporated into the discussion and future proposals section to illustrate the potentials of the approach. In the application case, an unconventional prosthetic stem was implanted and simulated. Meshing procedures, bone properties, loads applied, and boundary conditions corresponded to aforementioned explanations.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline  & **Cycling** & \begin{tabular}{c} **Sitting** \\ **Down** \\ \end{tabular} & \begin{tabular}{c} **Standing** \\ **Up** \\ \end{tabular} & **Walking** & **Staying** & \begin{tabular}{c} **Stairs** \\ **Up** \\ \end{tabular} & **Knee** & \begin{tabular}{c} **Stairs** \\ **Bending** \\ \end{tabular} & \
\begin{tabular}{c} **Stairs** \\ **Down** \\ \end{tabular} &
\begin{tabular}{c} **Jogging** \\ **Fore** \\ \end{tabular} \\ \hline \(-\text{F}_{\text{X}}\) (N) & 299.5 & 714.4 & 1125.1 & 596.8 & 681.6 & 829.9 & 857 & 773.6 & 884.8 & - \\ \hline \(\text{F}_{\text{Y}}\) (N) & \(-\)41.4 & \(-\)62.7 & 49.9 & 17.1 & \(-\)35.7 & \(-\)48.5 & \(-\)37.1 & \(-\)55.6 & \(-\)15 & - \\ \hline \(-\text{F}_{\text{Z}}\) (N) & 805.7 & 1931.5 & 2481.2 & 1931.5 & 2280.2 & 2763.6 & 2054.9 & 2611 & 3222 & 2300 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Standardized loads for each of the daily activities considered for the study. * ISO 7206-4.  
Figure 4: **(A) Osteotomy level. (B) Acetabulum region where the load is applied. (C) Boundary conditions.**  
The implant stem was modeled with titanium alloy and meshes with a similar size to that employed for cortical and trabecular bones. In that final application case, gluing between bone regions and implant was also applied, as contact condition, which would represent the case of a cemented implant. Again, NX 10(r) (Siemens PLM Software Solutions, Plano, Texas, US) was used both for designing the implant and simulating its performance and effects on remaining bone.  
This final case was performed to show that the stress field of normal bone is importantly modified by the incorporation of prostheses, and that the results from the FEM analyses upon intact bone can be used for comparing stress fields before and after implantation, and, hence, selecting the most adequate implant geometries and materials.