# Bone Tissue Condition during Osteosynthesis of a
## 5 Conclusions  
The search for biodegradable materials for use in osteosynthesis is one of the most promising areas in traumatology. A conventionally \"ideal\" composite should be completely metabolized by the body, without exerting local and systemic pathological effects, have the properties of \"biocompatibility\" with bone tissue, have sufficient strength, and at the same time, participate in accelerating the processes of bone tissue regeneration [27]. Partially magnesium biodegradable structures meet the listed requirements, but the issue of corrosion forces researchers to continue the search for coatings that can not only stop the corrosion of implants but can also contribute to the acceleration of osteogenesis [28,29]. One of the promising directions in the creation of such coatings is the use of hydroxyapatite coatings. According to [PERSON] et al. [15], a significantly lower rate of biocorrosion was observed in comparison with uncoated magnesium implants when rats were implanted in the skull bone of magnesium implants coated with hydroxyapatite. The same authors, in an experiment on rabbits, established a more active formation of bone tissue around magnesium implants with a hydroxyapatite coating during osteosynthesis of a tibial defect in comparison with uncoated magnesium implants at 6 and 12 weeks of the study. In our study in rats with experimental osteoporosis, the use of magnesium implants with hydroxyapatite coatings for osteosynthesis of a femoral fracture also led to accelerated bone regeneration compared to rats without the use of implants. An interesting fact is that in the case of using magnesium implants with an anti-corrosive coating on the 14 th and 30 th days of the experiment, the osteosynthesis processes were more pronounced, in comparison not only with animals without implants, but also in comparison with constructions with hydroxyapatite coating. Similar data were obtained in the work of [PERSON], [PERSON] et al. [30] in an experiment on rabbits, in which an artificially created defect of the tibia was coated with polytetrafluoroethylene-coated hydroxyapatite granules in one group and uncoated hydroxyapatite granules in the other. A month after the operation, the rabbits of the first group showed a more pronounced increase in the volumetric density of the cancellous bone in comparison with the group without polytetrafluoroethylene coating. Figure 8: Eroded surface dynamics (ES). Figure 9: Dynamics of the number of osteoclasts (N.Oc). The mechanisms of the higher regenerative potential of the bone in the presence of polytetrafluoroethylene coating require deeper research. One of the possible options for the implementation of the coating effect can be the more pronounced angiogenesis obtained in our research at the fracture site in this group of animals, which is evidenced by the indicators of the average volumetric density of the vessels (Table 1). According to our data, the average volumetric vascular density was significantly higher in the group of rats with a magnesium implant with an anti-corrosive coating on the 14 th, 30 th, and 60 th days of the experiment, than in the rats with the implant with hydroxyapatite application. It is known that the formation of endochondral bone depends on the intensity of angiogenesis and stops if it is blocked, which is shown in models of experimental pathological conditions. In the experiment, inhibition of chondrocyte VEGF signals by blocking antibodies to a related receptor on developing vessels in the metaphysis leads to a stop not only of bone growth but also of chondrogenesis zones. A similar picture is observed in rickets, which can be mimicked by microsurgical ablation of the vascular system of the metaphysis [31]. Thus, according to our data, the most harmonious processes of the regenerative potential of bone tissue and its surroundings during osteosynthesis of fractures in difficult conditions of osteoporosis were observed when using biodegradable magnesium alloy implants with a hydroxyapatite coating, sealed with polytetrafluoroethylene.