## 4 Discussion
#### Hybrid Proposal  
The hybrid profile in activities 2-4 and 6 showed stress reductions of 1.96-9.6% (see Table 6) for the different activities when compared with the reference stem (P2- trapezoidal); this reduction was possible by combining the characteristics of the stems with better results obtained from the multi-objective analysis. The maximum stress in the hybrid profile occurred in the medial neck area for the first five activities. The lowest strain in all the activities was obtained from the hybrid profile. The mechanical behavior of the hybrid stem suggests a decrease in the deformations at the prosthesis base for all the evaluated loads. A reduction in strains is a potential advantage for patients with osteoporosis because fractures cannot occur with small strains [58; 59]. An increment of 5.22 mm in the proximal lateral dimension of the cross-section area (dimension between point M5 and L5 of the same plane) of the hybrid profile was obtained due to the new geometry (the final dimensions were 27.57 mm). Despite modifications in the dimensions around the middle and top area of the stem, the distal section embedded in the bone did not suffer any change. The increment in the dimensions around the proximal section could be a potential limitation for the end-user because it will depend on the particular morphology of the potential user; however, it does not seem to affect the implantation of the stem in the bone. According to the studies presented by [PERSON] et al. [60], the femur has dimensions of 33.8 \(\pm\) 1.3 mm in the proximal part (medullary transverse diameter in the lesser trochanter). The work presented by [PERSON] et al. [61] said that the femur had dimensions of 25.47 \(\pm\) 3.60 mm, which guarantees that the hybrid implant must fit in the femur. However, it could be limited to the specific anatomical restrictions of each patient. This increment (5.22 mm) will be useful in overweight patients, according to [PERSON] et al. [62], which suggests that a thin stem (neck and body) is not the best option for these patients. Improvement in the stability and the fixation of the stem in the bone are the main concerns for the surgeons during THR [42]. The hybrid design proposes an increment in the proximal area and the advantage of being an almost straight implant with a trapezoidal cross-sectional area that will aid in the stem stability and fixation. A stem elasticity modulus that approximates as much as possible the modulus of elasticity of the bone is desirable. Less deformation is obtained with a rigid material; therefore, the load distribution is smaller towards the spongy bone. [PERSON] et al. [2] described that in the neck area of the implant, the stress behavior is practically the same; either stainless steel (SS316L) or titanium (Ti6 Al4V) can be used. This behavior is caused because it is the area where the implant does not contact the bone. Therefore, it does not distribute the load to the bone. This behavior could be observed in the different prosthesis configurations assessed in this research. Currently different metals are used for the femoral componente of the THR and their muduli of elasticity is higher than cortical bone, therforepotential problems could be generated, such as as aseptic loosening [29]. SS316L is the material with the best strain results, and titanium is a better option in terms of stress but is more expensive. Currently, ASTM F2996-13 and ISO 7206-4:2010(E) are the norms used for the hip implant assessment. However, differences in the loads and boundary conditions between the norms and the methodology used in this work seem to make the comparison difficult. To perform the comparison, the percentage difference in stress between the hybrid and the reference stem was used (Table 6). As a result, the hybrid implant showed a stress reduction of 9.6% compared to the reference implant (P2-T) in activity 4, considering the titanium alloy (Ti6 Al4V) as the material and the methodology used for the authors of this work.