Understanding Platelet-Rich Fibrin. Richard J. Miron

Fig 2-1 The three stages of wound healing: (a) Inflammatory phase. (b) Proliferative phase. (c) Remodeling phase.

      In general, blood provides essential components to the healing process that comprise both cellular and protein products that essentially are the base components of wound healing. During the healing process, blood will undergo clotting within a few minutes to prevent further blood loss. This is an important step that will be later discussed in the PRF tube section, because in order for clotting to occur and even be improved in both speed and quality (in particular in patients taking anticoagulants), a proper understanding of the clotting cascade is required. In its simplest of forms, oxygen helps improve blood clotting, and for this reason, the simple removal of centrifugation tube lids following the spin process will lead to faster clotting of PRF and a superior fibrin mesh. One of the major roles of platelets is to assist during hemostasis through a fibrin clot formation.^1,2 Not surprisingly, the additional use of PRF for wound healing (for example, following tooth removal and extraction socket healing) in patients taking anticoagulants can drastically improve the healing outcomes simply by improving clotting. Because PRF contains many platelets and a fibrin nucleus is already formed, bleeding has been shown to be significantly reduced postoperative when PRF is utilized in patients on anticoagulant therapy.³

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      Platelets also release various GFs and cytokines that further lead to tissue regeneration but also attract macrophages and neutrophils to the defect site. These cells are responsible for clearing debris, replacing necrotic tissue, and removing bacteria from the wound site.

      The proliferative phase begins by day 3, where the blood clot within the wound is further supplied with a provisional matrix typically composed in part with fibrin, which facilitates cell migration, while the clot within the vessel lumen contributes to hemostasis.² Fibroblast cells are recruited to the wound site and begin producing new collagen in a random and somewhat disorganized order. Simulta-neously, new blood vessel formation leads to new angiogenesis, and the wound gradually begins to gain initial stability.

      During the third and final stage (the remodeling phase), disorganized collagen is replaced by newly organized collagen fibrils that provide enhanced stability and strength to the injured site, where tissue regeneration takes place⁴ (see Fig 2-1).

      Whole blood is comprised of four main components: blood plasma, red blood cells (RBCs), white blood cells (WBC), and platelets. Initially, platelets were reported as the major responsible component for the activation and release of crucial GFs for wound healing, including PDGF, coagulation factors, adhesion molecules, cytokines, and angiogenic factors. Their role has been extremely well described in the literature, so typically the entire field has been referred to as platelet concentrates or platelet-rich plasma/fibrin. Interestingly, however, over the years more attention has been placed on leukocytes, which are not only responsible for host defense but also highly implicated in the wound healing and regenerative phases.

Table 2-1 highlights the various cell types found in blood, including their density, frequency, and surface area. Note that while platelets are the lightest of the group, WBCs and RBCs are very similar in density. For these reasons they are also harder to separate in a centrifuge based on density. Noteworthy is the fact that per µL, there are 5,000,000 RBCs when compared to only 5,000 WBCs. Therefore, RBCs outnumber WBCs in a 1,000:1 ratio, which make them difficult to separate, especially on a fixed-angle centrifugation device as discussed later in this chapter (Video