## 4 Discussion  
This study investigated the influence of muscle forces on FE predictions of fixation screw durability in a custom pelvic implant. Predicted screw axial forces and von Mises stresses were compared to failure limits with and without estimated muscle forces applied. The post-operative muscle forces estimates were obtained from a personalized NMS model of the same patient for whom the FE model of the post-operative pelvis was developed. Muscle forces were found to have a considerable influence on the predicted durability analysis of the fixation screws. They generally had a greater influence on predicted screw axial force than on predicted screw von Mises stress. This finding suggests that assessment of the likelihood of pullout failure would be more sensitive to the inclusion of muscle forces than would assessment of the likelihood of fatigue failure. However, whether exclusion or inclusion of muscle forces would yield more conservative screw failure predictions was inconclusive, as some screws were predicted to be more susceptible to failure with muscle forces while others were predicted to be less susceptible with muscle forces. Several studies have developed FE models to assess the structural integrity of the pelvis reconstructed with a custom implant [6; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21]; however, muscle forces were often omitted, likely due to challenges in predicting post-operative muscle forces following significant anatomical changes. Because the accuracy of an FE analysis depends heavily on the imposed boundary and loading conditions, it is crucial to evaluate how exclusion of muscle forces impacts the accuracy of the FE results. The findings of the study suggest that inclusion of muscle forces redirects the load transfer between the hip joint and the reconstructed pelvis. The predicted axial forces and von Mises stresses of the screws inserted through the posterior flange (i.e., Cortical screws 5-6) were generally decreased by the inclusion of the muscle forces. In contrast, the predicted axial forces and von Mises stresses of the screws inserted through the anterior flange (i.e., Cortical screws 1-4) were generally increased. This second observation agrees with the findings of a previous study by [PERSON] and [PERSON] [22], where FE models of a healthy pelvis with muscle forces included were found to exhibit elevated stresses near the posterior part of the ilium. The influence of muscle forces on the predicted axial force and von Mises stress in the cancellous screws was less consistent. This difference may be associated with the longer length and closer proximity to the hip joint of the cancellous screws compared to the cortical screws. While our study demonstrated that predicted screw axial forces and von Mises stresses were sensitive to the inclusion of muscle forces, the findings were limited by how post-operative muscles were modeled in the NMS and the FE models. These muscles were assumed to remain attached to bone at their original origins in both the NMS and FE models, and the estimated muscle forces were applied as surface tractions directly on the surface of the remaining bone in their respective muscle attachment areas. However, during the surgery, each of the 12 retained muscles was detached temporarily at its origin to gain access to the tumor and later reattached to a polypropylene mesh sutured to the remaining pelvic bone. The detachment and reattachment of muscles may have moved the muscle origins and also caused the muscles to pull on a flexible surface rather than on rigid bone. Furthermore, the effect of muscle detachment and subsequent reattachment on muscle strength was not considered. The retained muscles in the post-operative NMS model were defined to have similar strength, optimal muscle fiber length, and tendon slack length properties to those in the pre-operative model. However, the assumption that the retained muscles were not impacted substantially by the surgery requires more thorough investigation.