## 3 Results
### Screw Failure Analyses  
Predicted screw axial forces differed substantially between loading conditions without and with muscle forces (Figure 5). Cortical screws 1-4 generally experienced higher, and Cortical screws 5-6 lower, axial forces for all load cases when muscle forces were included in the FE analyses. The one exception was Cortical screw 1 for the STUP 1 load case. Furthermore, while Cortical screws 1-4 were generally below the pullout failure limit without muscle forces, they all reached or exceeded the pullout failure limit for multiple load cases with muscle forces. The notable exception was again Cortical screw 1 for the STUP 1 load case, which dropped from above to below the pullout failure limit when muscle forces were included. In contrast, cancellous screws demonstrated no clear trends in axial forces related to the inclusion or exclusion of muscle forces in the FE analyses. Nonetheless, for all three cancellous screws, between one and three load cases produced an axial force that was at or above the pullout failure limit without muscle forces and that dropped to below the pullout failure limit with muscle forces. On average, cortical screws experienced a larger absolute percent change in axial force than did cancellous screws when muscle forces were added.  
Predicted screw peak von Mises stresses also differed substantially between loading conditions without and with muscle forces (Figure 6). Cortical screws 1-4 generally experienced slightly higher, and Cortical screws 5-6 slightly lower, peak von Mises stresses for all  
Figure 5: Predicted axial force in the free zone of each screw normalized by the respective pullout failure threshold under different loading conditions.  
load cases when muscle forces were included in the FE analyses. However, the corresponding absolute percent changes were much smaller than for axial forces. Similarly, cancellous screws generally demonstrated slightly lower peak von Mises stresses across load cases when muscle forces were included. The primary exception was Cancellous screw 3 for the GAIT 1 load case. However, the corresponding absolute percent changes were again small. Only for Cancellous screw 3 would inclusion of muscle forces alter the prediction of whether the peak von Mises stress would substantially exceed the screw's endurance limit. For this screw with and without muscle forces, the peak von Mises stress occurred at approximately the same location in the screw, where the screw became embedded in the bone (Figure 7). In addition to the changes in the maximum stress magnitude, the stress distribution patterns changed slightly between simulations without and with muscle forces, implying that the inclusion of muscle forces changed the load transfer direction within the post-operative pelvis. On average, cancellous screws showed a larger absolute percent increase in peak von Mises stress compared to cortical screws when muscle forces were added.  
When the FE results from different activities were weighted by annual load cycles, muscle forces were found to exert a considerable influence on predicted axial forces for most screws (Figure 8). The most vulnerable screw for pullout failure changed from Cancellous 3 without muscle forces to Cortical 3 with muscle forces. In contrast, muscle forces were found to exert a relatively small influence on predicted von Mises stresses. Both without and with muscle forces, the screw most vulnerable to fatigue failure was Cancellous 3.  
Figure 6: Predicted peak von Mises stress of each screw under different loading conditions.  
Figure 8: Predicted axial force normalized by pullout failure threshold (**top**) and peak von Mises stress (**bottom**) of each screw weighted by annual load cycles from all activities.  
Figure 7: Von Mises stress distribution in Cancellous screw 3 under different loading conditions. Due to the compressive forces applied within the pretension region, the stress distributions within the screw head and screw shank were discarded and considered non-physical.