**5. Conclusions**

This research work has proposed new tools and concepts that facilitate the custom design of short femoral stems. The application of elliptical adjustment has proven to be very useful for studying femoral morphology, assessing implantability and designing and selecting the customized implant. This instrument can be used in the design of conventional femoral stems or other prostheses in arthroplasty because human error is eliminated when trying to empirically fit an ellipse to the bone cavity.

The implantability has been defined, which is based on the integration of the anatomical parameters with the factors related to surgery, to verify that the stems designed adapt to the femoral cavity and are implantable. Consequently, three geometries for each case study have been designed and evaluated using the finite element method. To analyze the results of the simulations, a methodology based on regression graphs, scatter plots and strain maps that integrate the study of shielding, bone remodeling and femoral biomechanics has been proposed and proven to be more effective and provide more information to the designer compared to the conventional methodology. Based on this, the V3 stem has been selected for having low shielding, keeping the femur within the dead zone and being light in relation to the other geometries, and the Ti21S material, because it restores femoral biomechanics, reduces shielding and does not present the adverse effects of Ti6Al4V related to Alzheimer's disease and cytotoxicity caused by vanadium.

It has been proven, through analysis, that customized implants restore the patient's functional mobility, improving their quality of life, because they reproduce the physiological distribution of the femur, in a scaled form, but subsequent optimization is necessary to resemble more closely the mechanical response before surgery. Likewise, it has been shown that the ISO force, being a high energy mechanism, better distributing the load along the femur and facilitating prototype testing and analysis by the finite element method, should be the load used to evaluate the mechanical response of the femur to the insertion of short prostheses.

**Author Contributions:** Conceptualization, methodology and formal analysis, W.S.-R., C.O., A.D.L.; design and simulation, W.S.-R.; writing—original draft, W.S.-R.; writing—review and editing, C.O., A.D.L.; supervision, C.O., A.D.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Fondo Nacional de Desarrollo Científico, Tecnológico y de Innovación Tecnológica (FONDECYT) of Peru by contract N◦316-2019, which afforded W.S.R. the opportunity to remain in the Product Development Laboratory at Universidad Politécnica de Madrid for three months.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** The Cancer Imaging Archive (TCIA) is a service that de-identifies and hosts a large archive of medical images of cancer, accessible for public download after informed consent from patients. The data are organized as "collections"—typically, patients' imaging related by a common disease (e.g., lung cancer), image modality or type (MRI, CT, digital histopathology, etc.) or research focus. DICOM is the primary file format used by TCIA for radiology imaging. Supporting data related to the images, such as patient outcomes, treatment details, genomics and expert analyses, are also provided when available. In this study, two samples of hip images from TCIA were used as input for the design and simulation studies.

**Data Availability Statement:** To promote collaborative work and encourage the development and improvement of the proposed design methodology, the STL files of both geometric cases, including their cortical and osteotomized bone, and the short stems are openly available in https://www. thingiverse.com/thing:5187570 (accessed on 3 January 2022).

**Acknowledgments:** The authors express their gratitude to the members of the Grupo de Biomecánica at Universidad de Piura and División de Ingeniería de Máquinas at Universidad Politécnica de Madrid, especially Maritza Requejo, Mar Cogollo, Pedro Ortego, Isabel Moscol, Gustavo Grosso and Javier Tuesta, for inspiring and supporting us to conduct research that benefits society. The authors acknowledge the advice of the reviewers and their relevant questions, which helped us to present a more detailed and consistent paper, and to better describe the limitations and current challenges.

**Conflicts of Interest:** The authors declare no conflict of interest.
