## 4 Discussion  
Vertebroplasty is often used to provide mechanical support to the anterior fractured column, thus reducing the rate of collapse. [PERSON] et al. [15] reported on a fractured vertebral collapse treated with short segment fixation without vertebroplasty. In this current study, of all the instrumented models simulated, only the TSF model was not supplemented with vertebroplasty, which lead to a noticeable reduction in the flexion stiffness of the fixed segments. The models that combined vertebroplasty with posterior fixation demonstrated a considerable increase in the flexion stiffness of the fracture site, which could be reasonably expected to reduce the rate of vertebral collapse. Collapse of the greater anterior column would increase the loading and stress on pedicle screws or rods [22; 23], and thus, the greatest von Mises stress was observed on the screws in the TSF model (i.e., without vertebplasty) in flexion (Figure 6). In contrast, the maximum von Mises stress on the pedicle screws decreased considerably when the loading was shared by the fractured anterior column supplemented with bone cement (AwC-TSF and AwC-TSF-S models). The range of motion of the instrumented segments in the AwC-TSF-S model was lower than in both the TSF and AwC-TSF models. The ROM is related to the displacement between the bone and screw in the bone tunnel, with the result that an increase in ROM means greater displacement or deformation at the bone-screw interface. The greater strain may lead to a \"windshield wiper effect\" with subsequent enlargement of the bone tunnel and eventual loosening of the graft [24; 25], [PERSON] et al. [24] reported that the use of cement-augmented screws significantly increased the number of load cycles achievable by the implanted segments in comparison to a non-augmented control group. In this current study, the AwC-TSF-S group had the lowest range of motion of the instrumented segments. The rigid anterior support and cement-augmented screws acted to reduce the migration of the bone-screw interface, which may slow the progression of screw loosening and screw cut out. The stress on the adjacent discs in the AwC model was similar to the intact INT model. This is likely because the mobility of the motion segments is not affected by the cement injected into the verteb during verteboplasty. Of the instrumented models, the AwC-TSF-S model had the greatest increase in stress on the adjacent disc, which was primarily due to the increased stiffness of the motion segment due to the presence of the spinal implant [26; 27; 28; 29]. Figure 6: Maximum von Mises stress on pedicle screws at L2 and L4. These findings demonstrate that the increased stiffness of the motion segments resulted in greater stress on the adjacent disc (L1/L2 and L4/L5). As mentioned previously, the use of cement augmentation does not noticeably limit spinal motion, and as such, the use of cement-augmented pedicle screws would not induce greater loading on the discs in comparison to non-augmented screws. Therefore, the mobility of the adjacent segments was similar between AwC-TSF and AwC-TSF-S. Vertebroplasty also did not have much influence on the loading of adjacent segment discs after fixation with rigid instrumentation. The maximum von Mises stress on the endplate of the AwC model was similar to the INT model, which is consistent with a study by [PERSON] et al. [30], and the von Mises stress on the adjacent vertebra was only slightly greater. In contrast, [PERSON] et al. [13] indicated that augmentation with bone cement increases stress on the adjacent endplate. The results of this current study differ from those reported by [PERSON], possibly because of differences in spinal levels, lordotic angle and loading conditions. One critical difference is that [PERSON] only created a 2-level spinal model, which is not large enough to incorporate a lordotic angle.