## 4 Discussion
Based on normalized weighted axial force, our FE models without and with muscle forces predicted that Cancellous screw 3 would be susceptible to pullout failure, which proved to be an accurate prediction. Following the completion of this study, the authors learned that the patient experienced a pullout failure of Cancellous screw 3 and eventually underwent revision surgery to fill in the resulting osseous defect 16 months after the initial surgery. While the FE model without muscle forces ranked Cancellous screw 3 as the mostsusceptible to pullout failure, the FE model with muscle forces ranked Cortical screw 3 as the most susceptible, with Cortical screws 1 and 2 also being susceptible. However, to date, none of those screws has pulled out. Our FE analyses predicted potential screw pullout (and fatigue) failures at a single snapshot in time and thus could not predict a possible sequence of failures. It was also possible that cortical screw failures would be more difficult to observe in X-ray images than are cancellous screw failures. Given that eight screws were used in the ilium region, failure of any of these screws might not be visible in imaging data as the remaining screws might maintain fixation visually despite the failure. Due to the lack of a better understanding of the post-operative status of retained muscles, we would suggest that FE analysis of screw fixation durability be performed using FE models with and without muscle forces. However, this limitation does not change the premise of our study, which was that predicted axial forces and von Mises stresses within each fixation screw may be sensitive to the inclusion of muscle forces in the FE model. Further investigation should focus on (1) estimating muscle forces using post-operative muscle-tendon model parameters in the NMS model and/or (2) exploring methods to apply muscle forces in a more realistic manner in the FE model. Because the hip joint contact forces used in our study were also estimated using the personalized NMS model, these forces were subject to the same potential issues as discussed above for muscle forces. In addition, at least for other joints, the prediction muscle forces from net joint moments using static optimization has been shown to underestimate joint contact forces [39]. Despite these factors, our hip joint contact forces estimated post-operatively remained comparable to reported hip joint contact forces measured in vivo [40]. To our knowledge, this is the first study to use personalized hip joint contact forces for pelvis FE analyses, as prior studies typically used hip joint contact forces derived from [PERSON] et al. [40]. In addition, no study has reported estimated post-operative muscle forces for a specific pelvic sarcoma patient. The primary reason is likely that previous studies have not combined a patient-specific NMS model with a patient-specific FE model for assessing the reconstructed pelvis of a pelvic sarcoma patient. In past similar studies, due to the lack of patient-specific experimental data, estimation of various FE model parameters had to rely on previously published data. However, we collected patient-specific pre- and post-operative experimental data to build the patient-specific FE model in a manner consistent with practices reported in similar studies. For example, the heterogeneous material properties of trabecular bone were estimated using the patient's pre-operative CT scan images along with established image processing techniques and strongly correlated empirical relations. Nonetheless, the actual material properties of the post-operative pelvic construct may differ significantly from our estimates. Due to the retrospective nature of this study and the use of a human subject, the material properties and other parameters in the patient-specific FE model could not be validated experimentally, nor could they be evaluated using a different implant design due to the patient specific nature of the implant. We made several necessary assumptions when applying the boundary conditions to our FE model.