## 4 Discussion
### Load Distribution and PVMS on Posterior Fixation Constructs and Allograft Spacers  
In the current study, the PFCs functioned better in extension than tension during the flexion mode regardless of the LMS and PS. The Type 2 model (PS-only) had lower overall loading on the allograft spacer when compared to other models, except during lateral bending. During flexion, the Type 1 model demonstrated the highest allograft spacer loading, which indicated that the LMS alone could be insufficient to support the vertebral body-allograft spacer constructs under specific circumstances, such as loading in osteoporosis patients. Interestingly, in lateral bending mode, the Type 3 and 4 models, which had support from the PS on the compressed side or the opposite side, demonstrated the lowest loading on the allograft spacers. These data suggest that, on both the compressive and distractive sides, the PS hold the C5-6-allograft spacer construct tightly and resist the lateral bending forces.  
In axial rotation, the type 3 model had the highest PVMS on the PFCs, which occurred around the LMS neck due to the direction of the counterclockwise rotation force and relatively weaker support from the LMS on the opposite side. Therefore, the stress on the PS side increased to compensate the weaker support from the LMS. The stress on the PS side also increased but was widely distributed over the entire body of the PS. However, on the LMS side, in the near cortex, which was in contact with the LMS, there was increased bone-interface stress. In this area, screw toggling and loosening are  
Figure 6: Posterior and lateral views of the Peak von Mises stress distributions on the screw-bone interface. *Red arrows indicate the highest stress distributions. Type 1: all LMS, Type 2: all PS, Type 3: left (Lt) PS and right (Rt) LMS, Type 4: Lt LMS and Rt PS, Type 5: upper PS and lower LMS, and Type 6: upper LMS and lower PS.  
expected (Figures 2, 3, 5 and 6). In contrast, for the Type 4 model, the PVMS on the allograft spacer was the highest. The PVMS on the PFC also increased near the LMS neck area due to the screw direction, which allowed for the counterclockwise rotation of the vertebra that could displace the screws.  
It is anticipated that, in flexion mode, the allograft spacer PVMSs are higher than in the PFCs. However, in extension mode, after ipsilateral hybrid insertion of the LMS and the PS (Type 3 and 4 models), the allograft spacer PVMSs increased due to asymmetric physiologic loading when compared to the LMS and PS-only models. In the proximal and distal combinations (Type 5 and 6 models), however, both the PVMS on the allograft spacers and on the PFCs increased. For example, in the Type 6 model, the C6 PS had a stronger column fixation when compared to the C5 LMS. The C6 PS was a source of stiffness against LMS fixation, and the PVMS was highest on the rod near the PS head. In the Type 5 model, the PVMS on the PFCs was the highest on the rods near the distal screws (Figure 6). During lateral bending, the Type 2 and 3 models were supported by the PS on the compressed side and demonstrated the lowest PVMSs on both the allograft spacers and PFCs. When combined with the LMS or when the LMS was used alone on the compressed side, the PVMSs on the allograft spacers and PFCs increased (Figures 2 and 3).  
The distribution patterns of the PVMSs on the PFCs are clinically meaningful (Figure 6). Most of LMS exhibited higher PVMSs around the LMS neck area (Type 1, 3, and 4 models), which is the most common area for bone-screw interface loosening. In the PS, the highest PVMSs regions were at the screw neck insertion point and at the pedicle screw-shaft interface. In contrast to the LMS, in the PS, the range of bone-screw contacts are wider and have a decreased risk of bone-screw failure caused by toggling motions.