## 4 Discussion  
The FE simulation results indicated that a high-stress concentration existed in the iliac fixation area. In contrast, a low-stress region was found at the pubis and ischium fixation area (Figure 4). This high-stress concentration at the iliac fixation area has been previously reported [27; 28]. Based on this stress distribution, we could conclude that the iliac fixation area bore most of the compressive load, and thus that the region of the ilium required more stable fixation with the implant than the pubis and ischium regions after type II tumor resection. The important finding of this study was that the bridge component in the horseshoe plate model reduced the stress concentration by as much as 21% at the iliac fixation area (Figure 4). This indicates that the bridge component dissipated a large portion of the compressive load to the surrounding bones, and thus reduced the risk of fracture at the iliac fixation area. While this is the FE simulation result from only one patient's pelvic implant, this study suggests that the bridge component of the periacetabular implant fixation would enhance the mechanical stability after type II resection. In future work, the mechanical stability of the horseshoe-shaped plate model should be evaluated in order to determine the suitability of this model for periacetabular implants in more patients. This study demonstrated that the horseshoe-shaped plate model exhibited a more stable fixation than the conventional straight-type plate model in the pelvic implant fixation. For the fixation of a pelvic bone fracture, previous investigators have developed various fixation systems by incorporating screws or another plate into the fixation system with a column plate [13; 14; 29; 30]. In addition to the pelvic implant fixation, our horseshoe-shaped plate model could be beneficial to enhance the stabilities of the fixation system in the pelvic bone fracture. In future studies, we will investigate stable fixation systems for the pelvic bone fracture by comparing the biomechanical stabilities between the horseshoe-shaped plate model and other conventional fixation systems. In this study, the porous structure was applied on the surface of the titanium implants in order to reduce the stress shielding effect. The titanium implant is much stiffer than the surrounding bones, and the stress transfer between an implant device and a bone is not homogeneous [31]. The surrounding bone is then stress shielded and experiences abnormally low levels of stress, which can lead to the resorption of the bone, and again, loosening of the implant [32]. The porous titanium material has the advantages of enhancing the bone-implant interface strength by promoting bone and soft tissue  
Figure 6: Post-operative medical images of the pelvis area obtained from a patient: (**a**) plain radiograph and (**b**) 3D CT images. ingrowth, and of reducing the bone-implant strength mismatch [33]. In future studies, the porous design, such as pores, holes, or lattice structure, should be investigated in order to enhance the mechanical properties as well as osteointegration. Our FE simulation approach has several limitations. First, the FE simulation model was created with a uniform thickness of the cortical bone. In an actual pelvic bone, the thickness of the cortical bone varies across the pelvic bone in order to adjust to load transfers [28]. This variable thickness of the cortical bone would affect the stress distribution in the pelvic bone and the implant. Therefore, the mechanical stability of the horseshoe-shaped plate model should be evaluated further with various cortical bone thicknesses via FE simulations. Second, in the FE model, the contact type between the pelvis and implant was set to \"bonded\", and the effects of the screws on the stress distribution in the pelvis bone were not considered. In future research, the stress concentration at the screw holes should be investigated with the horseshoe-shaped plate model.