# Biomechanical Study of Proximal Femur for Designing Stems for Total Hip Replacement
## 1 Introduction
The specific study of proximal femur biomechanics has led to interesting biomechanical models that have been used for years to design prosthesis for total hip replacement. In 1867, [PERSON] and [PERSON] presented their work to the Society for Natural Science about the internal architecture of the human femoral head and the related stress trajectories. [PERSON] demonstrated arched trabecular patterns in a sagittal femoral section and [PERSON] suggested that the patterns seemed to be associated with the principal stress produced by functional loading, inspired in a crane that he was designing with a similar shape [18]. In 1870, [PERSON] developed his \"trajectorial theory\", in which he proposed the path that physiological forces seemed to take, notably through the proximal femur, as shown by the trabecular patterns. [PERSON] emphasized the fact that these columns crossed at right angles [19]. In 1917, [PERSON] investigated femoral mechanics considering femur as an isolated beam structure whose main stresses occurred as a result of the body weight (BW) and did not consider the effect of surrounding muscles. He concluded that BW produces two different sets of stress lines: compressive stresses, along the medial side, and tensile stresses, on the lateral side [20]. [PERSON]'s model has been used for designing stems for total hip replacement. Consequently, the conventional stems use the medial side as a support because that set of lines transfers compressive loads. Some researchers have investigated the applicability of [PERSON]'s model in depth, with interesting conclusions. [PERSON] [21] proved, using [PERSON]'s model, that walking produces a force equal to 70% of its fatigue strength; therefore, the risk of fracture of the greater trochanter would be too high. Also, [PERSON] and [PERSON] [22] used parameters from [PERSON]'s model for predicting the femoral shape, but they found that the computer predicted an unnatural shape of the femur with an expanded diaphyseal diameter. In fact, [PERSON]'s model does not accurately describe the femoral biomechanics because it ignores the muscle action. If taken into account, the forces generated by the iliotibial band and by the vastus lateralis-gluteus medius complex create a tension band effect that converts the tensile stresses of the lateral femoral column into compressive stresses [22; 23]. This ascertains that cortical femur bone is undergoing compressive stresses in normal performance, in accordance with its histological characteristics. These differences between models are schematically presented in Figure 1.  
Consequently, the common processes and biomechanical models applied for designing artificial femoral stems should be changed, so as to transfer load, not only in the medial column, but also to the lateral regions. Accordingly, in this study we propose a biomechanical model that considers body weight and muscle action, a model which is helping to develop innovative hip prostheses designs, aimed at improved usability, in accordance with the increasing of physiological and mechanical demands from young and active people. In this study, we describe the proposed biomechanical model, which also takes into account cortical and trabecular regions and the orthotropic properties of bone. Besides, we apply the model to analyzing different loading cases, linked to daily activities of active patients (sitting down and standing up, walking, jogging, knee bending, walking up and down stairs, among others. The obtained simulation results, which show the stress state of intact bone according to a complete biomechanical model, can be used as a reference. We consider that these results can be employed for analyzing the changes produced by femoral implants on the stress state of healthy bone, and for predicting their potential impact on triggering stress shielding effects, as finally discussed. It is important to highlight that we concentrate on the proximal region, as it is the one suffering from long-term bone resorption in most state-of-the-art implants.