## 4 Discussion
In this study, the stress distributions of the adjacent lower T11 and upper L1 endplate also showed no significant difference among the surgical intervention groups, implying no significant effect on the risks of adjacent fracture. This study also showed that compression fractures treated with VS have the highest stiffness of 4690.34 N/mm. According to a study on the stress burden of the spine after VP and BKP by [PERSON] et al., the stiffness and strength of the vertebrae increase as the bone cement volume increases [8]. Their probabilistic and sensitivity study suggested that in cement augmentation procedures, the maximum stresses moderately depend on the injected cement volume. The VS group has the most amount of cement augmentation and VP has the least amount. According to [PERSON] et al., during intraoperative period, VS (SpineJack) could preserve the maximum height gain significantly better than BKP, which creates cavity absence of load-bearing after balloon deflation and before cement injection [28]. Furthermore, the additional support of titanium alloy implant also contributes to a higher stiffness for the VS group. Interestingly, the results of this biomechanical study also suggest a superior body height maintenance ratio for the VS surgical model due to a higher stiffness. However, several previous articles have reported the possible risk factors for recollapse of cemented vertebrae after percutaneous cement augmentation. In 2011, [PERSON] et al. retrospectively reviewed 1800 patients after a 2-year follow-up. The incidence of refracture of the same vertebra after VP with an incidence rate of 0.56% [29]. Osteonecrosis, greater anterior vertebral height restoration, lesser kybosis angle correction and cystic filling pattern were found to be significant risk factors for recollapse. As for BKP, [PERSON] et al. reported a 10% incidence recurrent fracture after BKP of a previously operated vertebra primarily within the first 90 days after surgery [30]. The study by [PERSON] et al. demonstrated the presence of intervertebral cleft and non-PMMA-endplate-contact may contribute to future recompression of BKP treated vertebrae [31]. [PERSON] et al. conducted a risk factor analysis for re-collapse of cemented vertebrae after percutaneous VP or BKP [32]. Low bone mineral density, percutaneous BKP, and low volume cement injection were found to be associated with high risk of recollapse. Therefore, cautious interpretation of patients with the above risk factors is imperative to prevent deterioration of fracture conditions. There are several limitations to this study that need to be considered. The creation of a finite element model requires several simplifications and assumptions, including a homogeneous cement filling model and orthotropic material simulation. Physiological boundary conditions, cortical and trabecular bone region distinction, subsequent bone remodeling and pre-existing damage may be neglected. Furthermore, as of many other biomechanical studies, our finite element analysis was limited to a single thoracic vertebral segment and may not be generalized to other vertebrae. Despite the shortcomings mentioned above, this is the first study comparing the stress distribution and maximum von Mises stresses on the treated and adjacent vertebrae among VP, BKP, and VS using finite element analysis. This biomechanical study can improve the understanding of cement augmentation on vertebral compression fracture. To make the results from this finite element analysis clinically applicable, we encourage future multicenter studies with long term follow up to confirm these findings. The interpretation of this finite element analysis can only explain immediate post-operative results but cannot predict long-term clinical effects. According to current literature review, all three surgical procedures lead to good clinical results. This is in line with the low stress distribution values found in our results.