**1. Introduction**

Despite joint degeneration, caused mainly by osteoarthritis (OA), not being a threat to life, it meets conditions that make it a real problem for both patients and health systems. This pathology is one of the leading causes of disability in the middle-aged and elderly population, and although any joint can be affected, the hip and knee are the most affected ones. This high prevalence, with 250 million people with knee OA throughout the world, represents up to 2.5% of gross domestic product for developed countries [1]. In the coming years, prevalence and costs will increase because the risk factors that favor OA are inherent to today's society such as the aging of the population, overweight, or an uncontrolled sports practice, both by excess and by default.

Patients with OA are characterized by pain, stiffness, and limitation of function, becoming disabling in the most advanced stages [2]. Initial conservative treatments include physiotherapeutic work, nutritional supplements, and oral administration of analgesic and anti-inflammatories. In the next phases, patients can be treated with intra-articular injections of hyaluronic acid. Regardless of the success of these treatments, all of them focus on symptomatic relief without stopping or slowing the progression of the disease, and the only solution for patients with the most severe degrees of OA is total knee arthroplasty [3]. This surgical intervention not only entails the risks derived from surgery, which may be unacceptable by some patients, but also involves the majority of the cost of health systems [4, 5]. Therefore, it is necessary to develop new therapies that improve the current ones in order not only to alleviate the symptoms but also to modify the course of the pathology to slow its progression or even reverse it. This would improve the quality of life of patients, delaying or avoiding a large number of surgical interventions as well as the expense they entail.

These therapies must be based on two main pillars that sustain a new approach in joint degeneration: first, treatments based on regenerative medicine which can act on tissue biology and modify the pathophysiology of OA such as gene therapy, Platelet-Rich Plasma (PRP), or mesenchymal stem cells (MSCs). Among these treatments, PRP is currently the most widely used due to its greater ease of regulating, obtaining, and applying as well as its low cost [6, 7]. However, it is necessary to deepen their knowledge and standardize products and protocols to optimize clinical results. The second cornerstone is to understand the joint as a whole organ, taking into consideration all its elements [8]. Knowing the relationships between the different tissues that form and define the joint is key for the correct application of treatments and address degenerative pathology completely. Thus, this chapter is intended to explain the role of PRP in joint degeneration, highlighting the therapeutic potential of PRP in all the components of the joint and its clinical translation.

### **2. The joint as an organ**

#### **2.1 Joint components and homeostasis maintenance**

All joint structures present a unique molecular and cellular composition as well as specific biomechanical properties; consequently, each element of the joint performs characteristic functions. However, they are all coordinated and related to create the biological machinery that allows the joint to have dynamic stability (**Figure 1**) [9]. This gives the joint a great adaptability to maintain a mechanical and biological balance, supporting and confronting physical forces or physiologic disorders. In a short look at the components of the joint, the periarticular muscles appear in the outermost section. This tissue presents vascular irrigation, many neuronal terminals, and high plasticity. The configuration of its extracellular matrix in a network of muscle fibers provides muscle elasticity and allows the mechanical forces generated by the muscle cells to be transmitted to the tendons, which will translate them into joint mobility [10]. However, its stability capacity is even more important than mobility in order to maintain joint homeostasis, the quadriceps muscle being key in knee anteroposterior steadiness. In addition, muscle tissue is essential in shock-absorbing, and together with the subchondral bone and ligaments, it accounts for 30–50% of the total absorbing energy [11]. Ligament composition is characterized by a high-water content and an extracellular matrix with

**69**

**Figure 1.**

*Isolation, Activation, and Mechanism of Action of Platelet-Rich Plasma and Its Applications...*

a small number of fibroblasts. Collagen is the most predominant protein, mainly organized in type I collagen fibers that adopt many directions and orientations due to several forces these structures are subjected to [12]. Apart from their stabilizing function due to their biomechanical and viscoelastic properties, they are also responsible for detecting and controlling the position and movement of the knee. In this way, the joint has a balanced biomechanical behavior that prevents the origin of mechanical problems that lead to degeneration. The meniscus plays a fundamental role in functions of mechanical nature such as stability and shock-absorbing. It is a fibrocartilaginous tissue with an abundant extracellular matrix where cells such as fibroblasts and fibrochondrocytes are dispersed and where type I collagen is the predominant molecule. The presence of vascularization and nerve terminals is limited to the external zone or meniscal wall. These intra-articular elements located between the femoral condyles and the tibial plateau help stabilize the joint and withstand compression and sharing forces. In addition, they participate in the

*maintain a gene expression that promotes the optimal maintenance of the extracellular matrix.*

*Joint as an organ. All the elements of the joint participate in its correct function and in the maintenance of the homeostasis. Although they all contribute to mechanical and biological stabilization, ligament and meniscus muscles play a mainly mechanical role, whereas the synovium, cartilage, and subchondral bone have a more biological action. Correct mechanical adaptation and a favorable biological environment allow the cells to* 

lubrication of the joint with the synovial membrane or synovium [13].

The synovium, together with the cartilage and subchondral bone, forms an important biological triangle to maintain homeostasis of the knee. Both nerve fibers and blood vessels are abundant in the synovium, which provides nutrients not only to this structure but also to the adjacent avascular cartilage. Its cellular composition stands out mainly for synoviocytes (macrophagic cells or type A and fibroblast-like cells or type B), although immune system cells and even MSCs are also present, the synovium being a source of stem cells of increasing interest [14]. Its main function is the production of synovial fluid, which is produced by type B synoviocytes. It soaks the intra-articular space and structures, being essential in the lubrication of the joint due to its hyaluronic acid and lubricin content. The synovial fluid is also an important source of nutrients, biomolecules, and cellular signals, so it is essential for the biological balance of the joint [15]. The second element of this biological triangle

*DOI: http://dx.doi.org/10.5772/intechopen.90543*

*Isolation, Activation, and Mechanism of Action of Platelet-Rich Plasma and Its Applications... DOI: http://dx.doi.org/10.5772/intechopen.90543*

#### **Figure 1.**

*Regenerative Medicine*

excess and by default.

they entail.

**2. The joint as an organ**

**2.1 Joint components and homeostasis maintenance**

the aging of the population, overweight, or an uncontrolled sports practice, both by

Patients with OA are characterized by pain, stiffness, and limitation of function, becoming disabling in the most advanced stages [2]. Initial conservative treatments include physiotherapeutic work, nutritional supplements, and oral administration of analgesic and anti-inflammatories. In the next phases, patients can be treated with intra-articular injections of hyaluronic acid. Regardless of the success of these treatments, all of them focus on symptomatic relief without stopping or slowing the progression of the disease, and the only solution for patients with the most severe degrees of OA is total knee arthroplasty [3]. This surgical intervention not only entails the risks derived from surgery, which may be unacceptable by some patients, but also involves the majority of the cost of health systems [4, 5]. Therefore, it is necessary to develop new therapies that improve the current ones in order not only to alleviate the symptoms but also to modify the course of the pathology to slow its progression or even reverse it. This would improve the quality of life of patients, delaying or avoiding a large number of surgical interventions as well as the expense

These therapies must be based on two main pillars that sustain a new approach in joint degeneration: first, treatments based on regenerative medicine which can act on tissue biology and modify the pathophysiology of OA such as gene therapy, Platelet-Rich Plasma (PRP), or mesenchymal stem cells (MSCs). Among these treatments, PRP is currently the most widely used due to its greater ease of regulating, obtaining, and applying as well as its low cost [6, 7]. However, it is necessary to deepen their knowledge and standardize products and protocols to optimize clinical results. The second cornerstone is to understand the joint as a whole organ, taking into consideration all its elements [8]. Knowing the relationships between the different tissues that form and define the joint is key for the correct application of treatments and address degenerative pathology completely. Thus, this chapter is intended to explain the role of PRP in joint degeneration, highlighting the therapeutic potential of PRP in all the components of the joint and its clinical translation.

All joint structures present a unique molecular and cellular composition as well as specific biomechanical properties; consequently, each element of the joint performs characteristic functions. However, they are all coordinated and related to create the biological machinery that allows the joint to have dynamic stability (**Figure 1**) [9]. This gives the joint a great adaptability to maintain a mechanical and biological balance, supporting and confronting physical forces or physiologic disorders. In a short look at the components of the joint, the periarticular muscles appear in the outermost section. This tissue presents vascular irrigation, many neuronal terminals, and high plasticity. The configuration of its extracellular matrix in a network of muscle fibers provides muscle elasticity and allows the mechanical forces generated by the muscle cells to be transmitted to the tendons, which will translate them into joint mobility [10]. However, its stability capacity is even more important than mobility in order to maintain joint homeostasis, the quadriceps muscle being key in knee anteroposterior steadiness. In addition, muscle tissue is essential in shock-absorbing, and together with the subchondral bone and ligaments, it accounts for 30–50% of the total absorbing energy [11]. Ligament composition is characterized by a high-water content and an extracellular matrix with

**68**

*Joint as an organ. All the elements of the joint participate in its correct function and in the maintenance of the homeostasis. Although they all contribute to mechanical and biological stabilization, ligament and meniscus muscles play a mainly mechanical role, whereas the synovium, cartilage, and subchondral bone have a more biological action. Correct mechanical adaptation and a favorable biological environment allow the cells to maintain a gene expression that promotes the optimal maintenance of the extracellular matrix.*

a small number of fibroblasts. Collagen is the most predominant protein, mainly organized in type I collagen fibers that adopt many directions and orientations due to several forces these structures are subjected to [12]. Apart from their stabilizing function due to their biomechanical and viscoelastic properties, they are also responsible for detecting and controlling the position and movement of the knee. In this way, the joint has a balanced biomechanical behavior that prevents the origin of mechanical problems that lead to degeneration. The meniscus plays a fundamental role in functions of mechanical nature such as stability and shock-absorbing. It is a fibrocartilaginous tissue with an abundant extracellular matrix where cells such as fibroblasts and fibrochondrocytes are dispersed and where type I collagen is the predominant molecule. The presence of vascularization and nerve terminals is limited to the external zone or meniscal wall. These intra-articular elements located between the femoral condyles and the tibial plateau help stabilize the joint and withstand compression and sharing forces. In addition, they participate in the lubrication of the joint with the synovial membrane or synovium [13].

The synovium, together with the cartilage and subchondral bone, forms an important biological triangle to maintain homeostasis of the knee. Both nerve fibers and blood vessels are abundant in the synovium, which provides nutrients not only to this structure but also to the adjacent avascular cartilage. Its cellular composition stands out mainly for synoviocytes (macrophagic cells or type A and fibroblast-like cells or type B), although immune system cells and even MSCs are also present, the synovium being a source of stem cells of increasing interest [14]. Its main function is the production of synovial fluid, which is produced by type B synoviocytes. It soaks the intra-articular space and structures, being essential in the lubrication of the joint due to its hyaluronic acid and lubricin content. The synovial fluid is also an important source of nutrients, biomolecules, and cellular signals, so it is essential for the biological balance of the joint [15]. The second element of this biological triangle

is the hyaline articular cartilage. It has a very low coefficient of friction that resists compression and shear forces and absorbs only 1–3% of the total energy. The main cellular element of this tissue is a low population of chondrocytes that is distributed along the extracellular matrix composed principally of type 2 collagen, in addition to other molecules such as proteoglycans or aggrecans. Its functions of lubrication and transmission of mechanical forces are performed thanks to a stratified tissue in different zones, from the most superficial, with a higher water content and chondrocytes, to a deeper area of calcified cartilage that is over the subchondral bone [16].

Subchondral bone, together with the osteochondral unit, completes the triad of elements with a predominant role in the biological maintenance of the joint. This structure consists of a plate of cortical bone from where the bone marrow and trabecular bone areas emerge. The importance of the subchondral bone lies in its communication with the cartilage, providing this tissue with at least 50% of the oxygen and glucose requirements. This communication not only is limited to the nutritional contribution but also covers the cellular and molecular signaling that participates in the cartilage homeostasis. Besides this, it is also a source of MSCs and participates in absorbing joint loads along with the other elements mentioned above [17].

The joint adaptability is both mechanical and biological, and it is in this last component where the action of regenerative medicine could positively influence. All the structures and tissues described above participate in joint stability by adapting to the different alterations and stimuli received, ultimately maintaining a healthy cartilage. Because of the "mechanical stabilizers" of the joint, the mechanical loads and forces that it receives become molecular and cellular stimuli that are maintained at physiological levels. These stimuli activate the chondrocyte gene expression, allowing them to synthesize proteins, such as proteoglycans, collagen, and metalloproteases, that ensure the integrity and renovation of the articular cartilage [18]. The continuous adaptation of the cells to the mechanical stimuli they receive in order to maintain the adequate extracellular matrix is based on a very delicate anabolic/catabolic balance, and any mechanical or biological alteration can break it resulting in joint degeneration [19].

#### **2.2 Joint degeneration process**

The balance present in the joint may be broken because of multiple causes (**Figure 2**). For example, injuries of the tissues involved in the mechanical stabilization of the knee could entail an abnormal load distribution. This would cause an unsatisfactory shock absorption into the joint, and the stimuli generated would exceed the physiological level [20]. Lifestyle can also have an impact on the generation of pathological stimuli. Both uncontrolled physical activity and sedentary lifestyle lead to an excess or defect of stimuli, respectively. The result is a biological and cellular malfunction and, consequently, a defective tissue renewal. In addition, pathologies and biological disorders such as inflammatory processes or those affecting the structures responsible for maintaining and nourishing the cartilage could also cause cellular failures that lead to imbalance and joint degeneration.

The multifactorial nature of this pathology makes it difficult to know the exact origin of the triggered processes as well as their sequence and timing. These events take place with special importance in the interaction between the synovial membrane, cartilage, and subchondral bone. Regardless of the original cause, one of the main consequences of this imbalance is the deterioration of the extracellular matrix and the generation of degradation products that are released to the synovial fluid [21]. The cells of the different joint tissues such as chondrocytes, synovial macrophages, osteoblasts, or fibroblast interact with these molecules, which act as Toll-like-receptors (TLRs) and damage-associated molecular patterns (DAMPs).

**71**

**Figure 2.**

*Isolation, Activation, and Mechanism of Action of Platelet-Rich Plasma and Its Applications...*

As a consequence of these interactions, the intracellular pathway of the nuclear factor kappa β (NF-kB) is activated, connecting the mechanobiological program and the inflammatory response. The gene expression of the affected cells shifts to an inflammatory pattern synthesizing molecules, namely, interleukins (IL-1b, IL-6, IL10), prostaglandins (PEG-2) and other pro-inflammatory biomolecules, and cytokines (necrosis factor alpha (TNF-α), interferon gamma, or nerve growth factor (NGF)). Pathological levels of these molecules also interfere in physiological repairing responses. For instance, the action of MSCs from the bone marrow is altered by high levels of transforming growth factor beta (TGF-β), compromising

*Joint degeneration processes. Different causes such as abnormal mechanical loads, injuries of stabilizing structures, or pathologies and biological disorders cause nonphysiological stimuli that modify the gene expression of cells. As a consequence, the extracellular matrix degenerates, activating pro-inflammatory pathways that* 

All the harmful biological environment generated by this event cascade leads

Moreover, the negative effects arising from joint degeneration can affect other tissues as well [8]. For example, studies conducted in the meniscus of patients with arthrosis showed a tissue with increased vascularization and nerve terminals, with the unstructured extracellular matrix, abnormal cell organization, and cell death [25]. Likewise, ligaments with osteoarthritic patients also showed calcifications and

to pathological outcomes in the cartilage, synovium, and subchondral bone. Chondrocytes of cartilage turn into a much more active state, forming cell clusters and increasing their proliferation. They also increase the synthesis of both extracellular matrix proteins and enzymes, causing an altered remodeling of the matrix with hypertrophy and calcifications [19]. Concerning the synovium, inflammation occurs in the early stages together with macrophage infiltrates and an increased synovitis in the advanced stages [23]. Communications between the cartilage and subchondral bone are increased due to the presence of fissures and microcraks, in addition to the remodeling of this tissue with fibroneuroangiogenesis because of the overexpression of molecules like TGF-β and vascular endothelial

their modulating and repairing functions [22].

*create a harmful environment and joint degeneration.*

growth factor (VEGF) [24].

*DOI: http://dx.doi.org/10.5772/intechopen.90543*

*Isolation, Activation, and Mechanism of Action of Platelet-Rich Plasma and Its Applications... DOI: http://dx.doi.org/10.5772/intechopen.90543*

**Figure 2.**

*Regenerative Medicine*

is the hyaline articular cartilage. It has a very low coefficient of friction that resists compression and shear forces and absorbs only 1–3% of the total energy. The main cellular element of this tissue is a low population of chondrocytes that is distributed along the extracellular matrix composed principally of type 2 collagen, in addition to other molecules such as proteoglycans or aggrecans. Its functions of lubrication and transmission of mechanical forces are performed thanks to a stratified tissue in different zones, from the most superficial, with a higher water content and chondrocytes, to a deeper area of calcified cartilage that is over the subchondral bone [16]. Subchondral bone, together with the osteochondral unit, completes the triad of elements with a predominant role in the biological maintenance of the joint. This structure consists of a plate of cortical bone from where the bone marrow and trabecular bone areas emerge. The importance of the subchondral bone lies in its communication with the cartilage, providing this tissue with at least 50% of the oxygen and glucose requirements. This communication not only is limited to the nutritional contribution but also covers the cellular and molecular signaling that participates in the cartilage homeostasis. Besides this, it is also a source of MSCs and participates in

absorbing joint loads along with the other elements mentioned above [17].

break it resulting in joint degeneration [19].

**2.2 Joint degeneration process**

The joint adaptability is both mechanical and biological, and it is in this last component where the action of regenerative medicine could positively influence. All the structures and tissues described above participate in joint stability by adapting to the different alterations and stimuli received, ultimately maintaining a healthy cartilage. Because of the "mechanical stabilizers" of the joint, the mechanical loads and forces that it receives become molecular and cellular stimuli that are maintained at physiological levels. These stimuli activate the chondrocyte gene expression, allowing them to synthesize proteins, such as proteoglycans, collagen, and metalloproteases, that ensure the integrity and renovation of the articular cartilage [18]. The continuous adaptation of the cells to the mechanical stimuli they receive in order to maintain the adequate extracellular matrix is based on a very delicate anabolic/catabolic balance, and any mechanical or biological alteration can

The balance present in the joint may be broken because of multiple causes (**Figure 2**). For example, injuries of the tissues involved in the mechanical stabilization of the knee could entail an abnormal load distribution. This would cause an unsatisfactory shock absorption into the joint, and the stimuli generated would exceed the physiological level [20]. Lifestyle can also have an impact on the generation of pathological stimuli. Both uncontrolled physical activity and sedentary lifestyle lead to an excess or defect of stimuli, respectively. The result is a biological and cellular malfunction and, consequently, a defective tissue renewal. In addition, pathologies and biological disorders such as inflammatory processes or those affecting the structures responsible for maintaining and nourishing the cartilage could

also cause cellular failures that lead to imbalance and joint degeneration.

The multifactorial nature of this pathology makes it difficult to know the exact origin of the triggered processes as well as their sequence and timing. These events take place with special importance in the interaction between the synovial membrane, cartilage, and subchondral bone. Regardless of the original cause, one of the main consequences of this imbalance is the deterioration of the extracellular matrix and the generation of degradation products that are released to the synovial fluid [21]. The cells of the different joint tissues such as chondrocytes, synovial macrophages, osteoblasts, or fibroblast interact with these molecules, which act as Toll-like-receptors (TLRs) and damage-associated molecular patterns (DAMPs).

**70**

*Joint degeneration processes. Different causes such as abnormal mechanical loads, injuries of stabilizing structures, or pathologies and biological disorders cause nonphysiological stimuli that modify the gene expression of cells. As a consequence, the extracellular matrix degenerates, activating pro-inflammatory pathways that create a harmful environment and joint degeneration.*

As a consequence of these interactions, the intracellular pathway of the nuclear factor kappa β (NF-kB) is activated, connecting the mechanobiological program and the inflammatory response. The gene expression of the affected cells shifts to an inflammatory pattern synthesizing molecules, namely, interleukins (IL-1b, IL-6, IL10), prostaglandins (PEG-2) and other pro-inflammatory biomolecules, and cytokines (necrosis factor alpha (TNF-α), interferon gamma, or nerve growth factor (NGF)). Pathological levels of these molecules also interfere in physiological repairing responses. For instance, the action of MSCs from the bone marrow is altered by high levels of transforming growth factor beta (TGF-β), compromising their modulating and repairing functions [22].

All the harmful biological environment generated by this event cascade leads to pathological outcomes in the cartilage, synovium, and subchondral bone. Chondrocytes of cartilage turn into a much more active state, forming cell clusters and increasing their proliferation. They also increase the synthesis of both extracellular matrix proteins and enzymes, causing an altered remodeling of the matrix with hypertrophy and calcifications [19]. Concerning the synovium, inflammation occurs in the early stages together with macrophage infiltrates and an increased synovitis in the advanced stages [23]. Communications between the cartilage and subchondral bone are increased due to the presence of fissures and microcraks, in addition to the remodeling of this tissue with fibroneuroangiogenesis because of the overexpression of molecules like TGF-β and vascular endothelial growth factor (VEGF) [24].

Moreover, the negative effects arising from joint degeneration can affect other tissues as well [8]. For example, studies conducted in the meniscus of patients with arthrosis showed a tissue with increased vascularization and nerve terminals, with the unstructured extracellular matrix, abnormal cell organization, and cell death [25]. Likewise, ligaments with osteoarthritic patients also showed calcifications and disorganized collagen fibers [26]. Finally, muscle tissue is also affected by inflammation produced in joint degeneration, showing fibrosis, collagen depositions, and muscle wasting [27]. Considering all this, it is clear that joint degeneration is not a sole cartilage disease. Instead, it affects all the elements present in the joint, and, therefore, it should be clinically tackled taking into consideration all of them in order to reverse or slow down the degenerative progression.
