4.1. PRP
PRP is obtained via whole blood centrifugation. This orthobiologic product is known to contain a variety of cells and biomolecules with a primary focus on the high concentration of platelets above baseline [
30,
31,
32]. PRP can be processed manually or with the assistance of commercial kits; however, since there are many different protocols in the literature there is a lack of consensus regarding an idealistic preparation of PRP. This also creates variance in content among PRP products and, therefore, different terminologies [
33,
34,
35].
Out of the three platelet granules (alpha, delta and lambda), the α-granules found in PRP are the most abundant [
36]. The number of α-granules per thrombocyte is estimated to lie around 50-80, constituting approximately 10% of platelet volume [
37]. Platelet granules are known to contain a large number of bioactive molecules which, upon activation, are released and subsequently stimulate the natural healing cascade [
33,
38]. The dense delta granules carry molecules such as magnesium, calcium, adenosine, serotine and histamine, which stimulate clotting [
36]. Lambda granules are often regarded as lysosomes because, much like these cellular organelles, they carry enzymes involved in protein, lipid and carbohydrate degradation. Therefore, they are also responsible for the removal of debris and infectious agents from injured tissue [
39]. The α-granule proteins, in turn, are involved in crucial biological events including inflammation, clotting, host defense, cell adhesion and cell growth [
36].
A treatment plan with PRP ensures accelerated neovascularization, increasing blood supply and nourishment of nearby cells. This is essential for cellular regeneration and the restoration of damaged tissue. Additionally, PRP can improve other biological events including recruitment, proliferation and differentiation of cells, contributing to the proper healing of complicated wounds and tissue injury [
40]. In fact, there are multiple studies in the literature which have long shown the benefits of PRP therapy for many musculoskeletal diseases, especially knee OA. Recent randomized clinical trials (RCT), meta-analyses and systematic reviews [
41,
42,
43,
44,
45] have yet again shown that when compared with conventional alternatives such as HA and NSAIDs, both leukocyte-poor and leukocyte-rich PRP have superior results. In the majority of the studies found, PRP had a more significant effect regarding improvement of pain and function in symptomatic knee OA with safety and efficacy. Only one of the most recent RCTs [
46] (published in November 2022) showed that a single intra-articular injection of leukocyte-poor PRP with HA (Artz or HYAJOINT Plus) is effective and safe for 6 months in knee OA patients. Patients showed significant improvements in visual analogue scale (VAS) pain, Western Ontario and McMaster Universities Osteoarthritis (WOMAC) index, Lequesne index and Single leg Stance (SLS) test at 1, 3 and 6 months post intervention.
There has been much investigation aiming to unveil the regenerative mechanisms of platelet concentrates. The benefits associated with PRP therapy, for example, were long believed to be only a result of the relevant abundance of growth factors and their individual biological roles (
Table 2). However, further research showed that PRP also promoted important secondary effects, including inflammatory modulation [
30,
47,
48,
49,
50], anti-catabolic activity [
51], normalization of integral autophagy [
52,
53], cytokine regulation [
54,
55,
56], and pro-anabolic stimuli [
57,
58]. More importantly, it is also directly involved in fibrinolytic reactions, which are a key step in the resolution of tissue injury [
59]. This is of particular biological value as the fibrinolytic system, as a whole, is necessary for the recruitment of mesenchymal stem cells (MSCs), which play an indispensable role in tissue repair [
33]. Lastly, another beneficial effect associated with PRP therapy is mononuclear cell recruitment. Thrombin and platelet factor 4 (PF4) released by platelets promote recruitment of monocytes and their subsequent differentiation into macrophages [
60,
61,
62]. The role of these cells has been much appreciated by researchers due to their plasticity. They have the inherent ability to switch phenotypes (polarization) and also transdifferentiate into other cell types like endothelial cells in order to display additional functions in response to biological cues in the wound microenvironment [
63,
64,
65]. Macrophages express two major phenotypes: M1 and M2. M1 is induced by microbial agents, therefore assuming a more pro-inflammatory role associated with wound debridement. M2 is generally produced by type 2 responses, conveying anti-inflammatory properties characterized by the upregulated expression of IL-4, IL-5, IL-9 and IL-13 [
64]. Macrophage polarization is mostly driven by the final stages of wound healing, as M1 macrophages trigger neutrophils apoptosis, initiating clearance [
66]. Once neutrophils are phagocyted, pro-inflammatory cytokine production is switched off and macrophages are then allowed to undergo polarization and release TGF-β1. This molecule plays a pivotal role in regulating myofibroblast differentiation for wound contraction, thus allowing resolution of inflammation and initiation of the proliferative phase in healing [
59].
4.2. PRF
Furthermore, platelet concentrates are not only appreciated for their capacity to yield enormous quantities of growth factors and proteins. Fibrin is another essential component in such products but it is often misprized and therefore deserves its fair share of credit in tissue repair. PRF, in particular, is a major source of fibrin and is acknowledged as a second generation of platelet concentrates. Although somewhat similar, PRF has a few comparable improvements over the traditional PRP, which has minor drawbacks such as blood handling as well as the addition of anticoagulants [
67]. This alternative biomaterial represents a natural fibrin matrix that not only displays immunological and platelet concentrate properties but also holds all the hematological components that are naturally involved in healing and immune function [
67]. PRF works like an autologous cicatricial matrix that is simply the result of centrifuged peripheral blood without the addition of any external agents [
68]. In reality, this biograft is a fibrin-matrix polymer with a tetra molecular structure, incorporating platelets, leukocytes, cytokines, and even circulating stem cells [
69]. The adhesive properties of PRF [
70] can convey superior advantages as this material may last longer in target joints, thus sustaining prolonged growth factor delivery and stable cell adhesion and proliferation.
Research indicates that at least in comparison to PRP, PRF could still contribute to the healing process without triggering intense “flare-ups” that can occasionally happen with PRP injections, especially when using leukocyte-rich PRP [
7]. PRF offers some versatility in the sense that it can also be easily manipulated used as a membrane, assisting the closure of chronic wounds, or even as a platelet gel in conjunction with other biomaterials such as bone grafts, targeting the improvement of bone tissue repair [
71,
72,
73,
74].
Although there are various protocols describing the preparation of platelet concentrates, the preparation of PRF is very simple with minor differences, starting by the exclusion of anticoagulants, which are known suppressors of tissue regeneration [
75]. In simple terms, venipuncture is performed and blood is collected into plastic tubes, which may or may not be coated with clot activator (i.e. glass, silica, thrombin) to accelerate coagulation. The tubes are then immediately placed in a centrifuge and only one round of centrifugation is performed, usually at 400 g for 10 minutes [
76]. After centrifugation, a heterogeneous mixture is obtained: erythrocytes, which are the densest particles, are separated from the plasma and remain at the bottom layer of the suspension, whilst platelet-poor plasma (PPP) occupies the superior fraction (
Figure 1C). The fibrin clot that is formed between the PPP and erythrocytes layers is what makes PRF such an interesting orthobiologic tool. It entraps platelets, leukocytes, and growth factors, thus becoming a natural and autologous bioscaffold (
Figure 2) and a rich reservoir of bioactive molecules for tissue regeneration [
76,
77,
78]. PRF can then be easily removed and collected from the tube with a forceps for further applications.
The fundamental mechanism at play is the combination of fibrinogen with circulating thrombin as centrifugation occurs in order to form fibrin and, ultimately, the fibrin clot [
67]. Platelet activation and fibrin polymerization are two important events that take place. Platelets are activated immediately upon contact with the wall of the tubes, leading to the formation of the dense fibrin network which gives the PRF clot its typical characteristics [
76]. Time is of the essence: when working with PRF, the operator must be able to quickly collect blood and centrifuge it immediately in order to avoid premature coagulation. Ideally, this should be done within 2 minutes and 30 seconds [
76]. Any prolongation during this stage causes diffuse fibrin polymerization, making the obtained PRF sample unsuitable for clinical use [
76,
79]. Despite these observations and “technical complications”, it is interesting to note that an injectable version of PRF (i-PRF) also exists, offering more feasibility in treatment alternatives [
75]. Injectable PRF is typically prepared in vacuum tubes with no additives (
Figure 1) in order to delay coagulation. This can provide physicians with sufficient time to process the orthobiologic sample as needed and make an intra-articular application. It is worthy to note that PRP alone is known to be a quite diffusible biostimulator, which can sometimes hinder its therapeutic value depending on where it is being injected [
80]. Therefore, the association of HA, i-PRF and PRP (power mix) may produce enhanced biological effects, allowing them to remain for an extended amount of time in the target joint.
Clinical studies and other previous investigations have revealed positive effects regarding the administration of PRF either alone or in association with other products for OA of different joints. A recent pilot study [
80] comparing leukocyte and platelet-rich fibrin (L-PRF) versus PRP + HA in the treatment of patients affected by unilateral knee degenerative OA revealed that the fibrin counterpart promotes better pain control and longer pain relief in short-and-mid-term.
Recent findings from a double-blind clinical study [
81] revealed that the combination of high molecular weight HA with plasma fibrinogen conveys positive effects on knee OA symptoms for all assessed parameters, marked by a significant reduction in OA-associated pain. The fibrin-HA formulations used in this clinical trial (RegenoGel and RegenoGel-OSP) demonstrated significant improvements in VAS scores compared to placebo at three months after the first injection, and sustained for another three months after the second injection with. These results indicate that fibrin and HA conjugates are safe and potentially effective for at least six months in the treatment of knee OA (KOA) symptoms. Indeed, combining HA with fibrin can enhance biological effects. Scanning electron microscopy of HA + fibrin composites shows that this association promotes more robust fibrin polymerization from solid to porous structures [
82]. This can further contribute to cell survival, attachment, migration and proliferation [
83,
84].
In similar fashion, a recent prospective study [
85] aimed at evaluating a 36-month survival analysis of conservative treatment using PRP enhanced with i-PRF in 368 knee OA patients. Results showed that the overall survival rate of knees that did not require surgical intervention during the 36-month follow-up was 80.18%, thus being considered satisfactory. This reveals a potentiating property for i-PRF, which in this case was able to enhance regenerative effects upon being combined with PRP.
In cranio-maxillofacial applications PRF has also demonstrated promising results in temporomandibular joint (TMJ) disorders, such as TMJ-OA. A recent RCT [
86] assessed the treatment outcomes of intra articular (IA) delivery of i-PRF after arthrocentesis in patients with TMJ-OA. In comparison to the control group (arthrocentesis alone), i-PRF demonstrated superior results in terms of pain reduction and improvement of functional jaw movements. Another similar clinical study [
87] evaluated the effects of IA i-PRF treatment in patients suffering from unilateral click due to TMJ disorders. The clicking completely disappeared in 70% of the patients in just 1 week after the first injection, and then in all patients 1 week after second injection. Taken together, evidence indicates PRF’s efficacy in the management of TMJ disorders as well, demonstrating satisfactory results in pain relief and functional gains.
There are many molecular mechanisms underpinning the efficacy of platelet and fibrin rich products, however, particular importance should be given to the fibrinolytic systems, which will be discussed in the subsequent sections of this manuscript.
4.3. Hyaluronic Acid
Hyaluronic acid is an anionic, non-sulfated glycosaminoglycan abundantly found in multiple organ systems [
88]. HA is also an essential component of articular cartilage in which it exists as a protective layer surrounding chondrocytes. Furthermore, it acts as a lubricant for tendons and joints, reducing extracellular matrix (ECM) degradation via inhibition of matrix metalloproteinase (MMP) synthesis. Its anti-inflammatory effects attenuate the activity of tumor necrosis factor-alpha (TNF-α) and interleukin-1 (IL-1), two major pro-inflammatory mediators.
HA by itself can be a potent agent in the management of OA, especially of the knee. Its benefits for orthopedic conditions have been well documented in the literature for decades. More recently, systematic reviews have yet again shown that IA applications of HA are safe and cost effective as they reduce pain and improve knee function in comparison to conservative treatments, including NSAIDs, corticosteroids, and analgesics [
89,
90]. IA-HA is regarded as a minimally invasive interventional strategy and there are no records of major systemic adverse events [
91]. This approach has shown beneficial effects in vitro. IA-HA has been shown to not only reduce chondrocyte apoptosis but also increase its proliferation [
92]. In humans, it is best to use formulations with medium to high molecular weight (MW) HA in order to closely emulate the conditions and biological properties of HA naturally produced in the body. Also, it is important to use HA derived from biological synthesis, to avoid undesired side effects [
93]. HA products with higher molecular weights normally sustain anti-inflammatory effects because they regulate immune cell recruitment. On the other hand, formulations with lower molecular weights have been reported to promote angiogenesis and tissue remodeling in wound healing, however they may also display a more pro-inflammatory effect in specific cell types such, especially chondrocytes [
94,
95].
Low molecular weight (LMW) HA binds to cell surface receptors less efficiently, therefore promoting weak HA biosynthesis. Medium molecular weight (MMW) HA allows stronger binding, stimulating a higher number of HA receptors, thus enhancing endogenous HA production. It is worthy to note that extremely large molecules present in high molecular weight (HMW) HA products may not always be convenient [
96]. These molecules will still bind to HA receptors, however, their large domains can limit the number of free binding sites on the cell surface, which logically implies a less efficient stimulation of HA biosynthesis [
96].
The molecular mechanisms of this modality are attributed to the ability of HA to bind to cluster of differentiation 44 (CD44) receptors. This blocks the expression of IL-1β, therefore downregulating the production of MMPs 1, 2, 3, 9, and 13 [
97,
98,
99], bypassing the activity of catabolic enzymes in musculoskeletal structures [
100]. After binding to its receptor, HA triggers intracellular signaling pathways associated with proliferation, differentiation, migration and degradation of HA itself [
101]. CD44 is the most widely studied HA receptor because it is expressed in nearly all human cell types. Affinity between CD44 and HA is a crucial factor that dictates the potential of HA as a signaling molecule. However, this also depends on HA concentration and MW, as well as glycosylation of extracellular domains and phosphorylation of serine [
102]. CD44 can form clusters with HMW HA polymers, allowing interaction with growth factors, ECM proteins, MMPs and cytokines [
103]. Another important HA surface receptor is CD168. It is expressed in multiple cells and controls migration by interacting with skeletal proteins, especially in the healing cascade [
102].