## 1 Introduction  
The surgical treatment of pelvic sarcoma, a malignant tumor in the pelvic region, typically involves partial pelvic resection, which has a high complication rate. When the lower limb can be preserved following tumor resection, the procedure is known as an internal hemipseudectomy. Following partial resection, the pelvis is left with a significant bony defect. Custom pelvic implants are an increasingly popular option for reconstruction of this defect. These implants are typically designed to recapitulate the patient's original pelvic anatomy (as determined from medical imaging data) and are manufactured using 3D-printing technology. Compared to patients who do not receive custom implant reconstruction, those who receive reconstruction often achieve a relatively normal post-operative gait pattern without a limb length discrepancy [1; 2]. However, custom implant reconstruction carries an increased risk of complications, such as infections [1; 2; 3] and mechanical failure of the fixation screws [2; 4; 5; 6; 7; 8; 9; 10]. More specifically, screw pullout and fatigue failures, evidenced by screw loosening or breakage, may necessitate a revision surgery [9]. Currently, screw placement largely relies on the operating surgeon's discretion, with the structural integrity and biomechanical behavior of the post-operative pelvic construct rarely being studied. Objective engineering assessments of screw placement could potentially reduce screw failures and improve the durability of custom pelvic implants. Patient-specific finite element (FE) models have been employed to analyze the durability of custom pelvic implants within the post-operative pelvic construct [6; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21]. In these studies, each patient-specific FE model is created by performing a virtual surgery on a patient-specific geometric model of the pre-operative pelvis. This virtual surgery involves \"resecting\" the pre-operative pelvis bone geometry according to the surgical plan and then \"installing\" the custom implant geometry, using simplified geometric models of the fixation screws. The resulting post-operative pelvic construct consists of geometric models of the post-operative pelvic bone, the metallic custom implant, and the metallic fixation screws. The construct is then converted into an FE model by meshing the geometry, adding material properties, and applying boundary and loading conditions. While previous studies have made significant progress in constructing realistic post-operative pelvis + implant FE models capable of identifying potential screw failures, they have omitted realistic muscle forces from the loading conditions [6; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21]. Most studies have assumed that the impact of post-operative muscle forces is negligible, arguing that numerous muscles are often disrupted or removed to achieve clear margins around the tumor during internal hemiplectomy [13; 21]. In a healthy hemipelvis, 21 different muscles are involved in mobilizing the hip joint, and several studies have demonstrated the importance of including muscle forces in FE analyses of the healthy pelvis [22; 23]. However, to the authors' knowledge, no study has investigated the effect of muscle forces on the predicted durability of the fixation screws used to secure a custom pelvic implant to the remaining pelvic bone, even though the assumption of negligible muscle influence has not been evaluated. The challenges involved in estimating post-operative muscle forces likely explain their exclusion from FE analyses of the post-operative pelvis. Although muscle forces have been estimated in the pelvic region for healthy subjects [22], such estimates have yet to be reported for pelvic sarcoma patients following reconstruction with a custom implant. Ideally, post-operative muscle forces could be reliably estimated based on pre-operative gait data, and two recent studies using data from the same pelvic sarcoma patient modeled in this study have made strides in this direction.