Platelet rich plasma ( platelet let rich plasma, PRP) is a platelet concentrate obtained by gradient centrifugation and separation of autologous whole blood, which is rich in platelets. When activated, platelets release a variety of growth factors that play an important role in promoting the proliferation, growth, differentiation and tissue formation of osteocytes and osteoblasts. Since Marx et al. first used PRP composite graft bone to repair mandibular defects in 1998, PRP has been gradually applied in tissue repair in the fields of dentistry, orthopedics, orthopedics, otolaryngology, and neurosurgery. In this paper, we review the isolation and preparation of PRP and its application in the field of orthopedics as well as its problems, and give an outlook on its application. 1. Separation and preparation of PRP PRP is a PLT concentrate that is separated from whole blood by density gradient centrifugation according to the different sedimentation coefficients of the components in blood, but there is no uniform preparation method. The number of PLT, concentration of various growth factors, and number of leukocytes in PRP prepared by different number of centrifuges, centrifugal force, centrifugal time, and different activation methods of PLT vary; and the biological effects of various surgical procedures and the time of application of PRP also produce different biological effects, thus creating a divergence in the biological effects of PRP. The preparation of PRP with different growth factor contents according to different physiopathological needs is the direction of future research. The preparation methods of PRP can be broadly divided into two types: manual preparation and fully automated preparation. The manual preparation process is tedious, but the required equipment is simple and easy to carry out. Fully automated preparation requires special equipment, and currently the common equipment includes SmartPReP system, Trissee system, Platele concentrate collection system, Curasau system, etc. Although the automatic platelet separator is easy to operate and highly automated, and the purity and concentration of the prepared PRP platelets are high, this method is generally used when the amount of blood used is large (generally above 150 ml) or when venous circulation channels need to be established, and is currently mainly used for platelet collection in blood banks to Currently, it is mainly used for platelet collection in blood banks for component transfusion, which limits its wide application in clinical practice due to its high cost. After one centrifugation, the blood can be divided into three layers, the bottom layer is the red blood cells with the largest settling coefficient, the top layer is the supernatant, and there is a thin layer at the junction, i.e. platelet-rich layer. After one centrifugation, the supernatant or red blood cell layer is discarded, and then the centrifugation force is changed again to separate more platelets. The two-centrifugation method is still the common method for the preparation of PRP. Liu Caixia et al. compared the effects of PRP prepared by different centrifugation force and centrifugation time on distraction osteogenesis in an animal model, and the results showed that the platelet count of PRP prepared by the Landesberg method with two centrifugations (200 × g and 10 min each) was significantly higher than that of whole blood, which was 6.17 times higher than that of whole blood; the platelet recovery rate was more than 86%, and the effect of promoting new bone production was more obvious. The platelet recovery rate was more than 86%, and the effect of promoting new bone production was more obvious. The platelet concentration of PRP obtained by centrifugation at a time < 5 min was not significantly different from that of whole blood when the centrifugation force was > 250 × g. Marx et al. found that the platelet concentration was highest in the erythrocyte layer 2 mm below the interface after one centrifugation, and the supernatant was discarded and centrifuged again at low speed for better platelet extraction. However, most scholars believe that the platelet recovery rate is higher by using the modified Appel method, i.e., first centrifugation at low speed and then aspiration of the entire supernatant and a small portion of red blood cells under the junction layer in another centrifuge tube, followed by high speed centrifugation. 2.The mechanism of action of platelet-rich plasma The action of PRP is accomplished through the interaction and mutual regulation of growth factors, which are secreted and immediately adhere to the surface of the target cell membrane and activate the cell membrane receptors. These membrane receptors in turn induce intrinsic signaling proteins that stimulate normal cellular gene sequence expression. Thus, the growth factors released by PRP do not enter the target cells and do not lead to changes in the genetic properties of the target cells, but only to an acceleration of the normal healing process. Although the mechanism of action of all cytokines involved in tissue repair and reconstruction is still unclear, some of the effects of cytokines on tissue repair and reconstruction have been clarified, such as: PDGF, one of the first growth factors to appear at the fracture site, can stimulate the mitosis of bone marrow stromal cells and increase the number of osteoblasts; stimulate the growth of endothelial cells and promote capillary production in the recipient area; also stimulate It can also stimulate the chemotaxis of monocyte macrophages. As a mitogenic and chemotactic factor, TGF-β is highly expressed in traumatic bone tissue, promoting osteoblast chemotaxis and proliferation, and increasing collagen synthesis. IGF promotes the proliferation and migration of osteoblasts and enhances the viability of osteoblasts; VEGF induces the proliferation and migration of endothelial cells, thus promoting the formation of new blood vessels. In addition, activated PLT also releases a large number of proteins, which are important for tissue regeneration. Thrombin can recruit vascular endothelial cells from surrounding tissues and enhance their viability. Under human umbilical vein 3D culture conditions, thrombin can stimulate fibroblast proliferation and neocapillary formation, while inducing negative feedback, thus limiting neocapillary synthesis. Fibronectin stimulates migration of keratinized cells and achieves cell-cell interaction, which is important for cell morphology recovery. 3.Application of PRP in the field of orthopedics 3.1 Bone defect repair The repair of bone defects has always been one of the difficult problems faced by orthopedic clinics. Although autologous bone grafting can achieve satisfactory results, the source of bone is limited, and bone extraction not only requires additional surgical operations, but also increases patients’ pain and causes many postoperative complications and additional injuries. The creation and development of tissue engineering has provided new ideas and methods for the repair of bone defects. Biomaterials with composite osteoblasts and/or growth factors have good osteoinductive properties and have promising applications in the repair of bone defects. However, most of the growth factors are prepared in vitro, and most of them are single factors, which are complicated and expensive to prepare.Assoian was the first to discover that PRP extracted from plasma contains multiple growth factors, which provides a wide range of application prospects for bone defect repair.Marx was the first to apply PRP for bone defect repair in clinical studies, and his results showed that PRP composite graft bone repair was 1.162 times faster than graft alone. In the study of the repair of mandibular defects in dogs, Kovacs et al. found that the biomaterial group with composite PRP was superior to the group with biomaterial alone by both bone density evaluation and histological evaluation, and concluded that PRP had a restorative effect on bone defects. 3.2 Applications in spinal fusion The study of PRP has opened a new way for spinal fusion, which solves the defects of limited autologous bone source, immune rejection of allogeneic bone, no osteoinductive activity of biomaterials, complicated and expensive production of single growth factor, significantly promotes osteogenesis, accelerates bone healing ability, improves spinal fusion rate, and promotes the recovery of patients’ postoperative conditions and quality of life. Hee et al. reported that the early fusion rate of autologous iliac bone composite growth factor was higher than that of autologous iliac bone graft alone. Castro et al. found that the fusion rate in the composite PRP group was 19% lower than that in the control group in a study of lumbar interbody fusion with a lumbar foraminal approach, which may be related to the biomechanical environment of the lumbar spine, the technique of preparing the PRP, and the number of PLTs. The reasons for this may be related to the biomechanical environment of the lumbar spine, the technique of PRP preparation, the number and function of PLT, and the concentration of growth factors. 3.3 Meniscal articular cartilage injury and repair Usually, the damaged articular cartilage itself has only a very weak regenerative repair ability, and how to repair the damaged articular cartilage, restore the integrity of the joint surface, rebuild the joint function and prevent joint degeneration is a hot spot of research in regenerative medicine. In a rabbit model of total cartilage injury, Cugat et al. first applied local injection of autologous PRP and found that the biomechanical behavior of cartilage was significantly improved, chondrocytes proliferated, and cartilage damage was significantly repaired. The patients were treated with intra-articular PRP injections (5 ml of PRP per injection, 3 injections in 21 days), and the injection port was aseptically bandaged and the patients were instructed to flex and extend the knee joint several times. Everts et al. applied PRP after unilateral total knee replacement surgery and followed up for 5 months, and found that the degree of joint fibrosis was significantly reduced and the range of motion of the joint was significantly better after PRP application compared with the control group. The chondrocytes extracted were treated with different concentrations of PRP and their proliferation was observed. 10 days later, the results showed that 30% PRP significantly promoted the proliferation of human chondrocytes, and the cell proliferation was not only affected by PRP but also positively correlated with the increase of PRP concentration. As mentioned above, PRP can be a therapeutic option when repairing cartilage 3.4 Repairing ligament/tendon injuries Tendon tissue is composed of tendon cells, fibrillar collagen and water, and lacks its own blood supply, so it heals slower than other connective tissues after damage. Anitua et al. co-cultured PRP with human tendon cells and found that as tendon cells proliferated, VEGF and hepatocyte growth factor (HGF) increased in the culture medium, and VEGF had a role in promoting angiogenesis and HGF had antifibrotic effects. and HGF has an anti-fibrotic effect and reduces scar formation. Sánchez et al. treated 12 patients with Achilles tendon tears, 6 of whom were treated with PRP as an adjunct to surgery in the trial group and 6 in the control group. The results showed that the patients in the trial group recovered their range of motion earlier than the control group, and no related complications occurred, and Mishra et al. achieved satisfactory results in the treatment of chronic elbow tendon disease using ultrasound-guided PRP. 140 patients were first given physiotherapy and other non-surgical treatments, and 20 patients had no improvement in pain, 15 of whom were treated with subcutaneous PRP and 5 of the control group with bupivacaine injection, and were followed up. Among the patients with subcutaneous PRP injection, 60% had pain relief after 8 weeks, 81% had pain relief after 6 months, and 93% had pain relief after 25.6 months and were able to resume training activities in a shorter period of time. 3.5 Prevention of bone and joint infections Bone and joint infections are one of the problems faced in orthopedic surgery, and the common method of prevention is to apply antibiotics during the perioperative period. However, the long-term application of large amounts of antibiotics not only brings various systemic side effects, but also leads to the emergence of drug-resistant strains of bacteria. Therefore, it is necessary to explore a new approach to solve the problem of bone infection.PRP can release a large amount of growth factors due to the high concentration of platelets it contains, and when PRP is activated by thrombin it forms platelet-leukocyte gel (PLG), which contains high concentrations of platelets and leukocytes, and these cellular components play important roles in the innate immune defense response of the body, such as chemotaxis, phagocytosis and oxidative bactericidal. This multiple characteristic of PRP gives it an advantage that traditional antibiotics do not have. Bielecki et al. showed that PLG inhibited the growth of S. aureus and E. coli in vitro by paper diffusion method, and Moojen et al. also reported that PLG inhibited the growth of S. aureus and E. coli in vitro. Moojen et al. also reported that PLG inhibited the growth of Staphylococcus aureus in vitro. In addition, some clinical studies have shown that PLG can reduce the incidence of bleeding and infection after surgical procedures. PLG can not only inhibit the growth of S. aureus in vitro, but also inhibit the growth of bacteria when applied locally in vivo, and can cooperate with the body’s immune defense system to kill bacteria, thus preventing the occurrence of bone and joint infections. PRP is completely autologous, without disease transmission and immune rejection, which fundamentally solves the risk of disease transmission and the difficulty of graft survival that bone tissue engineering has been facing; PRP contains a variety of high concentration of growth factors, and the ratio of each growth factor is similar to the normal ratio in the body, and has the best synergistic effect. PRP contains a high concentration of multiple growth factors, the ratio of each growth factor is similar to the normal ratio in the body, and has an optimal synergistic effect, both the biological effect of a single factor and the interaction between various growth factors. To a certain extent, this makes up for the shortcomings of single growth factor stimulating poor osteogenesis and meets the need for growth factors required for early bone healing; PRP has a pro-coagulant effect and can stimulate soft tissue regeneration and promote early wound healing; the growth factors contained in PRP do not enter the intracellular or nucleus, which accelerates the normal healing process, has no teratogenic effect, and does not have the ability to induce tumor formation. PRP is easy to produce and less damaging to patients, as it can be produced by taking blood from patients’ veins only. Therefore, PRP is a safe, simple and inexpensive treatment for various orthopedic fields, and has a wide range of applications. However, there are still many unsolved problems in the clinical use of PRP, especially in the field of orthopedics, such as the lack of uniform standards for PRP preparation, large differences in the concentration of PRP growth factors prepared by different methods, and the number of growth factors contained in PRP and the mechanism of their interaction are still unclear. Therefore, the research on PRP should firstly establish a set of PRP preparation methods that are efficient and stable, with little damage to PLT and high purity and stability; secondly, try to avoid factors affecting the efficacy of PRP when applying PRP; select appropriate carriers to bind PRP to carriers to improve the bone regeneration ability of PRP, establish animal models, and design standardized tests to provide a basis for the clinical application of PRP.