## 4 Discussion  
In this study, a series of finite element simulations was performed in order to evaluate the effects on the insert and bones of different configurations of RH TKA prosthesis, in terms of valgus and varus deformities, lengths of femoral stems, and fixation approaches. The results found returned that, overall, the tibio-femoral contact forces and areas are not considerably influenced by variations of stem length and fixation approach while varus/valgus variations lead to relative changes (varying according to the parameter analyzed and to the gravity of the deformity). Moreover, the femoral bone stress distribution was determined to be influenced by the variations in the prosthesis design and fixation, with further differences found in relation to the region of interest considered. Addressing more in depth, the outcomes of the tibio-femoral interface, the reason behind the lower sensibility of the contact area to varus/valgus angles and the overall insensibility to the variations in stem length and fixation approach is indeed to be found in the design of the RH TKA itself. In detail the central constraint, which allows internal-external rotations and superior-inferior translations, does not enable any other degree of freedom and thus contributes to maintain the tibio-femoral interactions to be more constant. For this reason, indeed, changes in the contact area are remarkably low in varus deformities ranging from 0\({}^{\circ}\) to 30\({}^{\circ}\) (less than 20% variation in terms of contact area, while slightly higher values are found in terms of force), even if the difference in the input forces is greater than 35%. These outcomes are then in agreement with several clinical studies focused on endo-model hinged TKAs, which reported its stability and absence of cases of wear due to overloading of the polyethylene [13; 15; 49]. Moreover, a recently published study [50] on the use of the rotating hinge and followed over 10-years concluded that using a specific RH TKA design with less rotational constraint has better clinical and survival outcomes than implants with greater rotational constraint (such as one specific CCK addressed in the study) in case of varus and valgus deformities. Addressing then the outcomes in terms of femoral bone stress, the results highlighted how these values are influenced more by the stem length and fixation rather than the values of varus and valgus deformities; these results are indeed in agreement with that found in the literature concerning the effects of stem design [20; 21] and moreover demonstrate the ability of this prosthesis to maintain similar outcomes in terms of bone-prosthesis interactions, despite the different levels of deformity of the patient joint. It is however to be mentioned that this study presents a series of limitations. Firstly, only one implant model was considered, with a single size and typology being analyzed; a single angle of fiction was then simulated for all of the configurations. These two restrictions may indeed represent a limitation for the generalization of the results, but it is to be highlighted that these choices allowed to perform a comparative study focused on the specific parameters taken into consideration, thus obtaining meaningful information that can therefore represent an interesting insight for the surgeons during the decision-making process. Addressing the finite element models, it is to be mentioned that the material models used to simulate the bones in this study implied a series of assumptions: indeed, no bone activity was considered in the simulations and therefore the variations in femoral mechanic characteristic, usually occurring in response to the different loading conditionsthe joint undergoes [51; 52; 53; 54], are not taken into account. Despite these latter limitations, however, these models were developed based on previously published and validated ones [21] and therefore the results they provided can be considered reliable, as they are furthermore in agreement with the literature [2; 20; 21].