# Role of Autologous Platelet Gel (APG) in Bone Healing: A Rabbit Study
## 4 Discussion  
In this study we observed that APG induced a higher quantity of lamellar bone and woven bone within the tibia defect as compared to the control. In this research, we aimed to further investigate bone regeneration enhanced by APG in a rabbit model using histological parameters. We calculated the percentages of marrow space and new bone formation in the tibia defects filled with APG. The results showed that the percentage of formation of new bone and marrow space was different between the two groups. Precisely, we found that, although some percentage of new bone in the APG group was higher than that of the control, when comparing the woven bone, we found that APG increased bone formation significantly more than in the control site. The present findings, when comparing the data obtained on bone formation, showed that the positive role of APG in bone regeneration is due to the formation of mature (lamellar) bone restricted to the cortical bone. These results confirm previous observations on the role of concentrate platelet on bone healing. APG possesses osteocoinductive properties and is capable of providing a three-dimensional structure for bone healing, thus, it may be speculated that concentrate platelet and fibrin may favor the natural process of bone formation and can provide a microenvironment to produce osteogenic activity. This hypothesis is also confirmed by data showing that APG showed greater osteoconductivity and had a greater percentage of woven bone when compared to the control site. Probably, fibrin matrix could hold many cytokines and growth factors and release them in the wound site for a prolonged time. In fact some authors find that fibrin matrix releases growth factors in the wound site over a prolonged time and accelerates bone repair [10; 17]. However, given the low number of rabbits used in the study, further studies are necessary to validate this hypothesis. Several authors have investigated the action of fibrin matrix in bone regeneration, either alone or in combination with other graft materials [18]. Platelet derivatives have been used in bone regeneration to improve bone quality and bone quantity [19; 20]. In fact, autologous platelet derivate, liberates different growth factors, such us vascular endothelial growth factor (VEGF), growth factor (PDGF) transforming growth factor beta (TGF-8), and others. The autologous platelet derivate gradually releases growth factors from the APG matrix and this liberation can last for up to seven days, which may improve tissue healing and better results in bone regeneration procedures [21; 22; 23; 24; 25]. In the present study, we chose New Zealand Rabbits due to the possibility of collecting an amount of blood as in humans, their bone turnover rate, and the ease of handling and availability, making them an appropriate model for the evaluation of bone regeneration. This model is frequently used for evaluating bone healing in implantology [26]. It was decided to evaluate the bone regeneration after four weeks because the rabbit has a high rate of bone turnover and reaches bone maturation after eight weeks [27]. In the present study, at the four-week healing interval, the APG group showed significantly more new bone formation in the cortical portion than  
Figure 6: Graph chart of the lamellar bone, woven bone, and medullary space of control group and APG group after four weeks (_t_ Student test). *_p_ < 0.05.  
control. In the present study we did not evaluate the differences from the other CP and our results are similar to [PERSON] et al. that reported the platelet concentrate increases new bone formation [10]. The use of fibrin matrix solely as a filling material for critical bone defects remains controversial. [PERSON] et al. reported positive effects of L-PRF when used in conjunction with titanium barriers [28]. The animal study conducted by [PERSON] et al.