# Biomechanical Study of Proximal Femur for Designing Stems for Total Hip Replacement
## 4 Discussion and Future Proposals  
The graphs from groups 1, 2 and 3 show higher values for the cortical stresses than for the trabecular ones, which can be explained because cortical bone has a higher Young's modulus and, according to the mechanical concepts, the stiffer material withstands more load. Likewise, global post processing results, which show the stress fields provide similar information. In addition, these graphs show a higher stress level on the medial than on the lateral side, but when both sides are closer to each other the lateral stress tends to increase. Group 4 behaves in the opposite way to the other groups, maintaining higher stress for the cortical bone but with the lateral more loaded than the medial side. The femoral neck and subtrochanter region are zones with the most risk of fracture according to the Tsai-Wu field (Figures 14-17); the representative loads that increase the risk of fracture are jogging and ISO force. [PERSON] et al. (2019) used three proximal femurs for testing the femoral strength in one leg position and applying the load on femora heads and concluded that rupture of proximal femur begins in the neck, therefore transercvical neck fracture is one of the causes of hip replacement. The neck plays an important role in the biomechanics of femur because it enables the correct transfer of load from the head to the rest of femur; when femoral neck is removed (Figures 4A and 18) the load transfer changes and this leads to the beginning of stress shielding problems. Another aspect promoting stress shielding is connected to differences between the mechanical properties of bone and those from the biomaterials used as substitutes. For instance, the difference between the Young's modulus of Ti-6 Al-4V, frequently used to manufacture stems, and bone is responsible for the stress shielding effect common in femoral prostheses. This effect is studied, to advance potential applications of the proposed modeling approach, by introducing a short titanium stem (Figure 18B) into the femoral cavity (Figures 4A and 18A) and simulating intact (Figure 4C) and implanted femur (Figure 18C). Stand up load is chosen as a representative case. The trabecular region is fixed to the cortical one and the implant is fixed to the bone in the implant position. Being common alloys nearly ten times stiffer than bone, their employment importantly modifies the stress field of implanted femurs, with respect to intact ones, as shown in the example of Figure 19, which shows higher stresses in the prosthesis region. The osteotomy level also plays a significant part in the biomechanics of proximal femur, as in some surgeries part of the greater trochanter is cut to correctly fit the stem in spite of importantly modifying the stress fields with respect to those present in intact bones, as consequence of the loose of the iliotibial band. Therefore, we envision that the proposed approach may be also useful for surgical planning tasks towards personalized healthcare and user specific designs of implantable devices. Figure 18: (**A**) Exploded view of the implanted femur. (**B**) Short stem. (**C**) Boundary conditions. Relative micro-displacements between bone and implant should also be evaluated because they affect the osseointegration process, and, consequently, have an impact in the long-term fixation or secondary stability that reduces aseptic loosening, one of the main causes of stem failure [35]. In the authors' opinion, the biomechanical behavior of stems should be evaluated using different daily activities so as to quantify the potential stress shielding, the difference between intact and implanted femur stresses divided by stress in the intact femur [36], because the aseptic loosening of the implant is influenced by bone resorption and it appears due to stress shielding [37]. Then, comparative studies between designed stems should be performed.