Joost Peerbooms

27 Concentrations of blood components in PRP separation systems low concentration of leukocytes. 11,15,50 The majority of separation systems in the current literature yield leukocyte-rich PRP. As also shown in this review, the ACP, Cascade, and Endoret systems are known to produce leukocyte-poor PRP. Currently, the inclusion of leukocytes in PRP is subject to debate, as both beneficial and adverse effects of leukocyte inclusion have been suggested. 50 Potential beneficial effects of leukocyte inclusion include their role in tissue remodelling and their increased antibacterial and immunological resistance. 12,44 Furthermore, the presence of leukocytes in PRP is associated with an increased concentration of growth factors, especially VEGF. 9,10,28,64 On the other hand, the inclusion of leukocytes might have catabolic and inflammatory effects on the targeted tissue as a result of the release of proinflammatory cytokines by leukocytes, which is associated with decreased proliferation and increased apoptosis. 2,4,5,8,38,49,59-62 As the aim of this reviewwas to evaluate the differences between the concentrations of blood components in PRP produced by the various PRP separation systems, no definitive answer can be provided on whether leukocyte-rich or leukocyte- poor PRP is best based on the results of this review. There is, however, increasing evidence that the type of PRP (leukocyte-rich or leukocyte-poor) should be matched to the specific clinical field of application. In the treatment of knee osteoarthritis, for example, the use of leukocyte-poor PRP seems to be more beneficial than leukocyte-rich PRP. 48 In the treatment of chronic tendinopathy, in contrast, the use of leukocyte-rich PRP is superior to leukocyte-poor PRP. 20 To gain more insight in the specific indications for the different types of PRP, future research should focus on which type of PRP is most suitable for the specific fields of application. Concentrations of Growth Factors A wide variation was found regarding the concentrations of growth factors both between different systems as well as within systems. These differences can partly be explained by the use of the specific enzyme-linked immunosorbent assay kits. The assays of growth factors contained in the platelets may be influenced by the incomplete removal of platelets and red blood cells and therefore give variable results. 36 Data within the studies are comparable, but a comparison between studies is less reliable, which limits the relevance of these findings. In this review, it seemed, however, that the concentration of VEGF tended to be higher in PRP produced by systems with higher concentrations of platelets and leukocytes. Higher amounts of growth factors have indeed been correlated with higher amounts of platelets and leukocytes. 55,63 Although evidence about the role of the specific growth factors is scarce, in vitro studies have suggested that PDGF and TGF-B are the 2 most important growth factors in PRP. 1,6,35,45 In contrast to the platelet and leukocyte concentrations, there is no evidence about ideal concentrations of growth factors in PRP for tissue regeneration. Therefore, future studies are necessary to reveal the exact mechanisms of growth factors in PRP and their role in tissue regeneration. 2