## 4 Discussion
Clinical consensus suggests that the adjacent fracture rate did not increase with the augmentation procedure or the vertebral height reduction in treated vertebrae but did increase with the degree of osteoporosis [44]. Comparable strength and stiffness were also measured in both groups. It has been estimated that ultimate strength restoration compared with the intact group was 132.3% in the EVA(r) group and 136.5% in the OsseoFix(r) group. The results demonstrate that the EVA(r) vertebral body reduction implants are able to achieve effective endplate reduction and provide similar biomechanical stability to that of the OsseoFix(r) implant. Another issue of concern in vertebplasty is the refracture of cemented vertebrae, which occurs in up to 30% of osteoporotic patients [45; 46]; associated risk factors include intraosseous vacuum cleft, overcorrection of the anterior VBH, low BMD, and mostly inadequate cement filling pattern [45; 46]. The advantage of the EVA(r) device is not only the increased strength compared with the intact groups but also the enhanced symmetric distribution of cement into the surrounding trabeculae by the expanded blades. Thus, more interdigitated cement distribution resulting in less recollapse incidence could be expected [45; 46]. There was no significant difference in stiffness between the two augmentation groups; interestingly, the values in both groups were significantly lower than those in the intact group. Similar results were also observed for kyrophoplasty and verteboplasty in cadaveric biomechanical studies [47, 48]. The inversely proportional ultimate strength results indicate that vertebral body stiffness is not solely dependent on the mechanical properties of cement or implant. The cement filling volume percentage, cement distribution, heterogeneous osteopororotic region, and degree of decreased bone mineral density all contributed to the final stiffness of the augmented vertebral body. Because of the lower compression stiffness for vertebral augmentation with EVA(r) and OsseoFix(r), fewer adjacent vertebral fractures can be expected in our future study. The goal of this study was to determine whether the EVA(r) system is as effective as the OsseoFix(r) reduction system, which has been proven effective and safe in vertebral augmentation since 2009 in the U.S. and Europe [20, 27, 38]. No significant differences were seen between the two procedures for VBH restoration, ultimate strength, and stiffness. Nonetheless, the advantages in the novel design of EVA(r) are as follows: (1) Expanding mechanism: eight blades were gradually expanded following the forward advancing of the barrel relative to the central tube by the clockwise rotation of the expander handle. In contrast, a retrograde expansion of mesh is incorporated in the OsseoFix(r) design, which might cause fractured bony fragment shift posteriorly and create larger posterior defect, which could induce devastating posterior cement leakage. (2) The central tube design of the EVA(r) system provides higher strength than the hollow mesh in OsseoFix(r), but there is no significant difference in ultimate strength and stiffness in both the EVA(r) and OsseoFix(r) groups. Studies 23,25 showed that ultimate strength reached 120-127% of the intact level with only PMMA cement augmentation. In our study, the biomechanical performance (132.3% in the EVA(r) group and 136.5% in the OsseoFix(r) group) would mostly be affected by cement instead of implant, and we believe that the difference would appear if only comparing the implant; however, the comparison is non-practical clinically. Two shield designs (blades and central tube) preventing low-viscosity cement from leakage are rationally more effective than the mesh-only designs in OsseoFix(r)[49, 50]. There are some limitations in the current study. First, an axial compressive loading was used to create the fracture model in the current study, whereas the native trauma mechanism is much more complex.