## 7 Discussion  
It is evident that different researchers use different techniques and properties to characterize the proximal humerus, but every one of them follows the same basis. The techniques used range from the simplest to the most complex, and are mainly affected by the researcher's resources, leading to attempts to further simplify the bone characterization process. However, some characteristics are unavoidable in any study of the proximal humerus, such as the bone anatomy, which has to be the same in any model, unlike the bone geometry, which will be specific to the patient of the study. The muscles cannot be ignored but can be simplified, which generally occurs due to the complexity of simulating their behavior. Researchers tend to simplify the forces and torques generated from the muscle's attachments using force vectors and movement constraints. The most accurate results would be obtained with the conditions closest to reality. Thus, the best way to simulate natural shoulder movements (abduction, flexion, internal rotation, etc.) is to consider the muscles involved (supraspinatus, infraspinatus, subscapularis, etc. ), as well the joint reaction forces generated, as in the research by [PERSON] et al. [42]. However, this is not the only way. [PERSON] et al. [67] validated the use of a vector force in a coordinate system for flexion and abduction in elderly subjects; later, this technique was replicated in other studies, such as those by [PERSON] et al. [27], [PERSON] et al. [66], and [PERSON] et al [3]. \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Reference** & **Mesh Type** & **Element Type** & \
\begin{tabular}{c} **Number of Nodes** \\ **per Element** \\ \end{tabular} &
\begin{tabular}{c} **Element Edge** \\ **Length (mm)** \\ \end{tabular} \\ \hline [PERSON] et al. [31] & Linear & \(\sim\) & (mean) 0.85 \\ [PERSON] et al. [65] & Quadratic & \(\sim\) & 1–5 (mean 1.5) \\ [PERSON] et al. [3] & Quadratic & \(\sim\) & \(\sim\) \\ [PERSON] et al. [28] & Quadratic & 10 & 1–1.5 \\ [PERSON] et al. [66] & Tetrahedral & Quadratic & \(\sim\) & 2 \\ [PERSON] et al. [42] & Quadratic & \(\sim\) & 0.2–2 \\ [PERSON] et al. [20] & Quadratic & 8 & 3 \\ [PERSON] et al. [33] & Quadratic & \(\sim\) & (mean) 0.97 \\ [PERSON] et al. [27] & Quadratic & \(\sim\) & 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Parameters used for meshing proximal humerus three-dimensional (3D) models in biomechanical studies. The bone composition of the proximal humerus is one of the most modified and simplified characteristics in studies, due to the anisotropic nature of bones. For the sake of the studies, the use of isotropic nature has been validated and accepted for biomechanics testing of the proximal humerus and other bones. There are a few ways to simplify the humerus model bone composition, such as making a solid humerus model and using the law of mixtures [68] for composite materials to combine the properties of cortical and cancellous bone. This will not give the best results, as there is no validation process and the model differs substantially from a real bone. However, if precision of the results is sought, the separation of both tissues seems a better option and closer to reality, according to the correlation values obtained from validation tests. The assignment of mechanical properties, especially the elastic modulus, will vary considerably between studies due to the particularities of the subject-specific study model. There are different formulas to convert vBMD values into the elastic modulus. Some other proposed formulas were evaluated in a previous [PERSON] et al. study [22], which concluded that the best correlation to experimental observations was the Keyak and Falkinstein equation [59] that he used for his proposal. The [PERSON] et al. formula [56] also refers to the [PERSON] and [PERSON] research, but is based primarily on the research by [PERSON] et al. [61].