## 2 Materials and Methods
### Patient-Specific Finite Element Model  
An FE model of the post-operative ipsilesional hemipelvis assembly was constructed in Abaqus (Dassault Systemes, Waltham, MA, USA, [https://www.3 ds.com/products/simulia/abaqus](https://www.3 ds.com/products/simulia/abaqus)) to predict post-operative bone, implant, and screw stresses as previously described in [PERSON] et al. (2019) (Figure 3). The foundation of the FE model was the patient-specific geometric model of the post-operative pelvis construct, which consisted of 16 parts: the implant, nine screws, the cortical and trabecular portions of remaining iliac and ischial bones, a sphere representing the femoral head of the subject's hip joint replacement, and an intermediate liner between the acetabular cup and the femoral head (Figure 1b). After meshing of the geometry, the model consisted of 668,465 10-node quadratic tetrahedral elements (C3D10; average element size: 1.73 mm) with approximately 2.9 million degrees of freedom. The material properties of the patient's cortical bone, trabecular bone, custom implant, and fixation screws were defined in the post-operative FE model as described in [PERSON] et al. (2019). In brief, cortical bone, the metallic custom implant, and the metallic fixation screws were assigned homogeneous material properties based on values reported in the literature ([PERSON] et al., 2019; [PERSON] et al., 2020; [PERSON] et al., 2020). In contrast, trabecular bone was assigned heterogeneous material properties derived from the patient's pre-operative CT scan data. The image intensity of each pixel within the trabecular bone regions was extracted, calibrated ([PERSON] et al., 2019; [PERSON] et al., 2020), and converted to Young's modulus value using published empirical relationships ([PERSON] et al., 2020). The voxel-specific Young's modulus values were then mapped to each node associated with trabecular bone in the FE model. A comprehensive summary of the material properties for all FE model components may be found in [PERSON] et al. (2019). Figure 2: Hip joint contact forces for the seven load cases used in the FE analyses. **Upper left**: Gait; **Upper right**: Squatting down and up; **Lower left**: Stairs up; **Lower right**: Stairs down. The second half of the stairs down plot is grayed out due to the absence of experimental data for that portion of the motion. The x-, y-, and z-components of each load case were represented in the pelvic coordinate system, where x, y, and z indicate anterior-posterior, superior-inferior, and medial–lateral directions, respectively. _F_res indicates the magnitude of the resultant force. Physiological boundary conditions were applied to the post-operative FE model to account for the interactions between the hemipelvis and its surrounding anatomical structures. The model was constrained by three sets of springs representing the joints and ligaments that remained connected to the operated hemipelvis. Two sets of linear springs were defined at the sacroiliac and cubic joint surfaces to simulate the stiffness of the ligaments and cartilage at these joints. The total spring stiffness in the normal direction was 103.09 kN/mm at the sacroiliac joint and 4.24 kN/mm at the public joint [14; 23; 35; 36; 37], while the spring stiffness in the tangential direction was set to 10% of that in the normal direction. A third set of linear springs was defined at the medial margin of the ischial tuberosity to represent the sacrotuberous ligament. These springs were restricted to compressing only in the direction of the ligament with a total spring stiffness of 1.5 kN/mm [14; 23; 36]. Two types of modeling methods were used to describe the interactions between different FE model components. The first type of modeling method was a unilateral contact that was assumed to be \"hard\" in the normal direction and frictionless in the tangential direction. The contact model was implemented between the implant and resected bone, allowing them to come into contact with or separate from each other.