local wound healing.1,2 Today, it has been well established that platelet concentrates act as a potent mitogen capable of the following (Fig 1-1):
Speeding the revascularization of tissues (angiogenesis)
Recruiting various cells including stem cells
Inducing the prompt multiplication of various cell types found in the human body (proliferation)
Fig 1-1 The three main GFs that are released from PRF include VEGF, a known inducer of angiogenesis; PDGF, a known inducer of cell recruitment; and TGF-β1, a known stimulator of cell proliferation. MSC, mesenchymal stem cell.
Wound healing is a complex biologic process whereby many cell types interact with one another as well as their local extracellular matrix (ECM) in order to repair and regenerate damaged tissues.3–6 While many regenerative agents currently exist on the market to help speed tissue regeneration, it is important to note that the majority are derived from other human sources (allografts) and animal byproducts. These naturally create a foreign body reaction when implanted into host tissues. While the majority of such biomaterials do certainly favor improved healing, it has generally been recognized and accepted that the gold standard for the majority of tissue-regenerative procedures in basically every field of medicine has been the use of autogenous tissues.
Specifically in dentistry, platelet concentrates were introduced over 20 years ago by Robert E. Marx and colleagues with the aim of concentrating blood proteins as a natural source of GFs that would stimulate vascularization (angiogenesis) and tissue ingrowth based on the fact that blood supply is pivotal for tissue regeneration of all tissues.7 Wound healing has been described as a four-step process that includes (1) hemostasis, (2) inflammation, (3) proliferation, and (4) maturation8–10 (Fig 1-2). Each phase overlaps one another and encompasses various microenvironments, including different cell types that assist in wound healing. Noteworthy are the implications of immune cells during biomaterial integration. In a study titled “OsteoMacs: Key players around bone biomaterials,” osteal macrophages were discussed as being key and pivotal cells during the wound healing process.11 Thus, as tissue biology has continued to evolve, platelet concentrates have also seen significant advancement with respect to their ability to favor healing by incorporating immune cells (leukocytes). Various systematic reviews from multiple fields of medicine have now demonstrated their ability to support tissue regeneration across many tissue types and cell types. This chapter reviews the evolution of platelet concentrates.
Fig 1-2 Four phases of wound healing: (1) hemostasis, (2) inflammation, (3) proliferation, and (4) maturation. Noteworthy are the overlaps between each of the phases and the population of cells found in each category. Whereas lymphocytes typically arise at 7 days, the ability of PRF to introduce a high number at day 0 acts to speed the regenerative phase during this process.
PRP (1990s)
The use of platelet concentrates has slowly and gradually gained popularity over time, with a dramatic increase being observed in the past 5 to 10 years. This parallels precisely the massive increase in research articles being published on the topic. Despite this, it is important to review and highlight the pioneering work conducted by Marx and colleagues over 20 years ago, without which none of this textbook would exist.12–14
Platelet-rich plasma (PRP), as its name implies, was designed to accumulate platelets in supraphysiologic doses within the plasma layer following centrifugation. The main aim of PRP was to isolate and further concentrate the highest quantity of platelets and their associated GFs for regenerative purposes, thereafter reimplanting this specialized supraconcentrate at sites of local injury. This concept has been the basis of thousands of research articles, with their protocols being utilized to favor wound healing in millions of patients.
Initial protocols typically ranged in duration from 30 minutes to 1 hour based on the centrifugation/collection systems and protocols utilized. The original concept was pioneered by Harvest Technology, where it was shown that over 95% platelet concentration could be accumulated, having the potential to help favor the regenerative phase of many cell types including soft tissues, epithelial cells, periodontal ligament cells, and bone cells.15,16 Because these initial protocols were lengthy, anticoagulants were added to the blood collection tubes. These typically were various forms of concentrated bovine thrombin or sodium citrate.
Despite its growing success and continued use after its discovery, several reported limitations existed with these initial formulations of PRP. The 30-minute or longer technique was generally considered lengthy for routine dental or medical practice, and more importantly, the use of anticoagulants was shown to limit wound healing from reaching its maximum potential. Simply put, when injury is created following an open wound, a blood clot is one of the first steps that occurs in order for healing to take place. Shortly thereafter, cells and GFs get trapped within this newly formed ECM, and the wound healing process/cascade begins. By limiting the body’s ability to form a stable clot, wound healing is limited. Several studies have now demonstrated the superior outcomes of platelet-rich fibrin (PRF) when compared to PRP simply by removing anticoagulants from their formulations.17–21 Even the pioneering research team behind the plasma rich in growth factors (PRGF) concept (Anitua et al) have since demonstrated more physiologic healing ability with anticoagulant removal.17
Another drawback of PRP was the fact that it remained liquid by nature (due to the use of anticoagulants), so when it was combined with biomaterials, a much faster delivery of GFs was observed (Fig 1-3). While an initial burst of GFs is typical of PRP therapy, a slower release of GFs over an extended period of time has been shown to better stimulate cell growth and tissue regeneration.22,23
Fig 1-3 (a and b) GF release from PRP and PRF at each time point of PDGF-AA over a 10-day period. Notice that while PRP has significantly higher GF release at early time points, over a 10-day period, significantly higher levels are most commonly found with A-PRF due to the slow and gradual release of GFs utilizing slower centrifugation speeds. (Adapted from Kobayashi et al.19)
Much advancement related to PRP therapy has been made over the past 20 years, and two excellent textbooks have been written by its pioneers—Dental and Craniofacial Applications of Platelet-Rich Plasma by Robert E. Marx and Arun K. Garg (Quintessence, 2005), and Autologous Blood Concentrates by Arun K. Garg (2018). Its breakthrough features include the novel ability to concentrate platelets to supraphysiologic doses and further stimulate tissue regeneration across virtually all tissue types. For these reasons, PRP has not surprisingly been utilized in practically every field of medicine.
Snapshot of PRP
Marx was the first to show that a concentration of platelets could favor tissue regeneration in the oral cavity.
A subsequent device was brought to market thanks to these breakthrough research projects conducted at the University of Miami (Harvest system).
PRP is credited for having exponentially grown the entire field of platelet concentrates, including its subcategories such as PRF.
L-PRF (2000–2010)
Because the anticoagulants utilized in PRP prevented clotting, pioneering work performed by Dr Joseph Choukroun and Dr David Dohan Ehrenfest led