## 3 Results
### Analysis of the Mode of Action of the SJ and KP  
Suppose it is only desired to strengthen the weakened (demineralized) structure of the verteba without moving the bone fragments. In the case of using the SJ system, it can be observed that the pushing soles of the implant remain approximately parallel. If, however, an attempt is made to restore the height of the verteb, it is observed that, after the introduction of the expansion element (SpineJack or balloon) and its operation, to create the cavity to restore this height, a cavity is produced in the body of the verteb (as seen from the sagittal view) with a quite pronounced conical shape. These cavities were created both due to the damage to the internal bone structure (under the mechanical action of the balloon or the implant) and due to the displacement of the upper and lower plates of the verteb. In analyzing the shapes of the cavities obtained, it can be seen that they have larger openings toward the anterior part of the verteb. In the case of S, it can be seen from the moment that the device is opened that its soles tend to act uniformly on the fragment from the top and the bottom due to the constructive parallelism. However, in the end, these soles lose their parallelism and end up in inclined positions with a pronounced conical shape, as shown in Figure 1. This aspect is easily observable in works [13; 14; 15; 16; 17; 18]. The same is observed in the case of KP, as shown in Figure 2. Since there is pneumatic action, the pressure in the balloon is constant, generating equal pushing forces through the balloon's walls in all directions. When only the reinforcement of the verteb is desired (for cases of a nonmobile fracture), the shape of the balloon is approximately symmetrical along its length. Thus, the resulting cavity has a relatively constant height when viewed from the side, as shown in Figure 2.  
Sometimes, bone fragment displacement is desired in mobile fractures. In the end, the creation of a cavity with a variable opening can also be observed. In other words, under the action of a relatively evenly distributed force toward the surfaces of the two plates (upper and lower) of the fractured verteb, they do not move linearly but (as expected) have a rotational movement (the opposite of the one produced at the time of fracture). This can be observed in works [19; 20; 21; 22; 23; 24; 25]. Kinematically, it seems that during the restoration of its shape (height), the verteb behaves like a joint with free ends toward the anterior part and the point of rotation  
Figure 1: Schematic representations of the SpineJack action: (**a**) theoretical operation of the device; (**b**) the deformed aspect of the device in the action of restoring the height of the verteb. Figure 2: Kyphoplasty action: (**a**) only the reinforcement of the verteb—the shape of the balloon is approximately symmetrical along its length; (**b**) the shape of the cavity obtained in an attempt to restore the height of the verteb. somewhere in the posterior area of the vertebral body. This explains the more significant displacement in the anterior part of the vertebral plates. In both cases (S) or KP), part of the energy (the force developed by the system) is dissipated in the entrance area of the S) or the balloon. In this portion, where the cavity created has a shape with small dimensions, close to the diameter of the hole made to access the body of the vertebra, practically, the pushing forces, no matter how great, do not produce significant displacements from a geometric point of view or with an anatomical effect. Thus, we conclude that the strength requirement is in the anterior part of the vertebra, namely, in the collapsed area, not in the posterior, intact area. However, both systems act both on the areas affected by the fracture that must be moved and on the unbroken (non-fractured) area that does not require movement.