## 4 Discussion
First, three sets of linear springs were used to represent the sacroiliac joint, public joint, and the sacrotuberous ligaments, respectively, where the stiffness of these various sets of springs was obtained from values reported in the literature. Second, a \"hard\" contact model was implemented between the implant and resected bone, and a tied constraint was applied between the femoral head and liner of the total hip replacement and between the liner and the custom implant acetabular cup. Third, a screw model using Abaqus' built-in functions was used to model the interaction between the fixation screws and implant. Indeed, these assumptions may have affected the FE results presented in this study. However, since it is currently impossible to conduct in vivo experiments on a human subject to measure bone screw stresses and strains, it is not currently possible to validate our simulation results experimentally. Nevertheless, our study demonstrated that the influence of post-operative muscle forces should not be assumed to be negligible. In conclusion, this study evaluated the influence of muscle forces on the predicted likelihood of screw failures using a patient-specific post-operative FE model of a pelvis reconstructed with a custom implant. We found that estimated fixation screw axial forces but not von Mises stresses were sensitive to the inclusion of estimated muscle forces in the FE analyses. Though it remains unclear whether including or excluding muscle forces would yield more conservative predictions of screw failures, our findings suggest that FE analyses of screw durability in custom pelvic implants should be performed both without and with muscle forces included. By including patient-specific muscle and hip joint contact forces estimated by a patient-specific NMS model, this study marks an important step toward building a comprehensive patient-specific FE framework for evaluating the structural integrity of the reconstructed pelvis following pelvic sarcoma surgery. Conceptualization, B.J.F. and V.O.L. ; methodology, Y.Z., A.B.-N., M.R.W.B. and J.E.A. ; software, Y.Z. and A.B.-N.; validation, Y.Z. ; formal analysis, Y.Z. ; investigation, Y.Z. and A.B.-N.; resources, M.R.W.B., G.L. and B.J.F. ; data curation, Y.Z., G.L. and B.J.F. ; writing--original draft preparation, Y.Z. ; writing--review and editing, A.B.-N., M.R.W.B., J.E.A., G.L., V.O.L. and B.J.F. ; visualization, Y.Z. ; supervision, M.R.W.B., J.E.A. and B.J.F. ; project administration, B.J.F. ; funding acquisition, V.O.L. and B.J.F. All authors have read and agreed to the published version of the manuscript. This research was funded by the Cancer Prevention and Research Institute of Texas (CPRIT) under grant number RR170026 and by Rice University. The study was conducted in accordance with the Declaration of Helsinki and approved by the Institutional Review Boards (or Ethics Committees) of the University of Texas Health Science Center (ethic committee name: Committee for the Protection of Human Subjects (CPHS); approval code: HSC-MS-19-0805; approval date: 27 April 2022), MD Anderson Cancer Center (ethic committee name: Human Research Protection Program (HRPP); approval code: 2018-0216; approval date: 4 August 2023), and Rice University (ethic committee name: Institutional Review Board; approval code: IRB-FY2021-272, approval date: 26 March 2021). Written informed consent was obtained from all subjects involved in the study. The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author. The authors would like to express their gratitude to [PERSON] for providing the editorial support needed to complete this manuscript. The authors declare no conflicts of interest. Figure 1: Muscle contributions to hip joint contact forces throughout a gait cycle. The red dashed lines correspond to GAIT 1 and GAIT 2. Figure A2: Muscle contributions to hip joint contact forces throughout a squant cycle. The red dashed lines correspond to SQDN and SQUP.