Temporomandibular Joint Osteoarthritis: Pathogenic Mechanisms Involving the Cartilage and Subchondral Bone, and Potential Therapeutic Strategies for Joint Regeneration

The temporomandibular joint (TMJ) is a specialized synovial joint that is crucial for the movement and function of the jaw. TMJ osteoarthritis (TMJ OA) is the result of disc dislocation, trauma, functional overburden, and developmental anomalies. TMJ OA affects all joint structures, including the articular cartilage, synovium, subchondral bone, capsule, ligaments, periarticular muscles, and sensory nerves that innervate the tissues. The present review aimed to illustrate the main pathomechanisms involving cartilage and bone changes in TMJ OA and some therapeutic options that have shown potential restorative properties regarding these joint structures in vivo. Chondrocyte loss, extracellular matrix (ECM) degradation, and subchondral bone remodeling are important factors in TMJ OA. The subchondral bone actively participates in TMJ OA through an abnormal bone remodeling initially characterized by a loss of bone mass, followed by reparative mechanisms that lead to stiffness and thickening of the condylar osteochondral interface. In recent years, such therapies as intraarticular platelet-rich plasma (PRP), hyaluronic acid (HA), and mesenchymal stem cell-based treatment (MSCs) have shown promising results with respect to the regeneration of joint structures or the protection against further damage in TMJ OA. Nevertheless, PRP and MSCs are more frequently associated with cartilage and/or bone repair than HA. According to recent findings, the latter could enhance the restorative potential of other therapies (PRP, MSCs) when used in combination, rather than repair TMJ structures by itself. TMJ OA is a complex disease in which degenerative changes in the cartilage and bone develop through intricate mechanisms. The regenerative potential of such therapies as PRP, MSCs, and HA regarding the cartilage and subchondral bone (alone or in various combinations) in TMJ OA remains a matter of further research, with studies sometimes obtaining discrepant results.


Introduction
The temporomandibular joint (TMJ) is a synovial joint that allows mandibular motion relative to the cranial base and distributes normal function (chewing and speaking) and parafunction stresses (clenching and bruxism). The ability of condylar cartilage to rebuild in response to changes in condylar repositioning, articular function, and particular mechanical loading is the most remarkable biological characteristic that distinguishes it from other types of cartilage [1][2][3][4][5][6][7][8].

Chondrocyte Apoptosis
Multiple studies have shown that an increase in chondrocyte turnover initiates the deterioration of condylar cartilage [27]. Apoptosis, autophagy, and necroptosis are important in chondrocyte death. Apoptosis and necroptosis promote articular cartilage degeneration [14]. Chondrocyte apoptosis creates space for neovascularization, and it is believed that the apoptotic bodies created by this process constitute the origin of cartilage mineralization [25].
Calcium is essential for chondrocyte death, and mechanical strain can raise calcium concentration in chondrocytes [28]. Endoplasmic reticulum stress (ERS) induced by calcium influx can cause death in chondrocytes; this phenomenon is called ERS-mediated apoptosis [29]. A high concentration of intracellular calcium can activate inducible nitric oxide synthase (iNOS). By releasing cytochrome C (Cyt C) and caspase-9, nitric oxide (NO) generated by iNOS limits mitochondrial respiration and leads to chondrocyte death [30].
In addition, tumor necrosis factor (TNF) and fibroblast growth factor receptor 1 (FGFR1) can promote chondrocyte apoptosis by working on the death receptor pathway [31]. Necroptosis is an important phenomenon in OA and is caused by oxidative stress. The breakdown of the cartilage is accelerated by necroptosis, which is mediated by TNF and receptor-interacting proteins 1 and 3 (RIP1/RIP3). According to studies, inhibiting apoptosis promotes the necroptosis pathway [32].
Chondrocytes recycle or reuse large macromolecules through a process known as autophagy, which is believed to be a self-protective mechanism. In most cases, aberrant mortality of chondrocytes not only results in a decrease in the total number of chondrocytes but also initiates the degeneration of cartilage and the breakdown of subchondral bone [33].
Autophagy is a crucial survival strategy for chondrocytes in OA [33]. The primary step of autophagy is the production of autophagosomes, which sequester organelles or macromolecules that have been eliminated. Eventually, autophagosomes merge with lysosomes to generate autolysosomes, which destroy the stored materials and release tiny molecules that can be reused. In the early stage of TMJ OA, the autophagy markers beclin 1 and light chain 3 beta (LC3B) rise, but they decrease considerably in the late phase [24]. Early autophagy increases and protects chondrocytes against environmental alterations. Cartilage destruction is connected with the suppression of autophagy and cell death [24,34].
Furthermore, the endoplasmic reticulum-associated proteins ERN1, mTORC1, and EIF2AK3 trigger apoptosis and limit autophagy. The ERN1-MTORC1-EIF2AK3 signaling axis is the name given to this pathway [24]. As a result, controlling autophagy may be a valuable method for treating TMJ OA.

ECM Degeneration
The cartilage ECM is mainly made up of collagen fibers and big proteoglycans. It not only serves as a protective structure for cartilage against elastic and shear pressures, but it also governs chondrocyte behavior via matrix-cell interactions [35].
MMPs and a disintegrin and metalloproteinase production with thrombospondin motifs (ADAMTS) initiate ECM breakdown in TMJ OA. Through the bone morphogenetic protein (BMP) pathway, the degradation of type II collagen (Col2A1) promotes the hypertrophy of chondrocytes, hence accelerating the course of TMJ OA [36]. In addition, cartilage mineralization has been demonstrated to contribute to the development of TMJ OA [25].
Activation of 2A-adrenergic receptor signals through the extracellular regulated protein kinases 1 and 2 (ERK1/2) and protein kinase A (PKA) pathways increases the synthesis of matrix degradation-associated enzymes, such as MMP-3 and MMP-13. Osteopontin, an inflammatory agent, stimulates the production of MMPs through the NF-κB signaling pathway [37].
The HTRA1-DDR2-MMP-13 axis is essential for ECM breakdown. This process starts with the overexpression of high-temperature requirement A1 (HTRA1) and the breakdown of pericellular matrix components, including type VI collagen. Col2A1 can activate the transmembrane protein Discoidin Domain Receptor Tyrosine Kinase 2 (DDR2) in the absence of a pericellular matrix. DDR2 ultimately triggers MMP-13 and hastens TMJ OA [38]. The Wnt/β-catenin signaling pathway has been researched for decades in stem cell self-renewal, cell proliferation and differentiation throughout embryonic development, and adult tissue homeostasis. It is a conserved cellular communication system that influences the onset of OA and other forms of arthritis [9,39].
Intracellular β-catenin is stabilized and translocated to the nucleus thanks to a complex formed by Frizzled and low-density lipoprotein receptor-related proteins 5 or 6 (LRP5/6). Signaling through the Wnt/β-catenin pathway begins at the cell surface when the Wnt glycoprotein binds to specific receptors. After attaching to nuclear transcription factors, β-catenin protein allows for expressing Wnt-targeted proteins [9].
Furthermore, lower cell proliferation and higher cell death were found in these mice's condylar cartilage. This evidence suggests that β-catenin plays a crucial role in TMJ pathophysiology and that Wnt/β-catenin signaling is a possible therapeutic target for treating TMJ OA [40].

TGF-β and BMP Signaling
Transforming growth factor β (TGF-β)/BMP signaling has been widely investigated concerning bone formation. It performs a variety of functions throughout life. The TGF-β superfamily consists of more than forty members, including TGF-βs, BMPs, and activin. They are embedded in the bone matrix and govern bone remodeling or influence the production of bone and cartilage [9,41].
The TGF-β signal pathway initiates intracellular signaling following the creation and activation of a heteromeric complex of types II and I serine/threonine kinase receptors, followed by the phosphorylation of particular Smad proteins, R-Smads. The phosphorylated R-Smads can heterodimerize with co-Smad, Smad4, ultimately translocating to the nucleus and activating the transcription of target genes [42].
In the TGF-β/Smad3 signaling pathway, sphingosine 1-phosphate (S1P), a bioactive lipid, is produced to function as an intracellular mediator or extracellular ligand for different receptors, resulting in inflammation, cell migration, and angiogenesis. The interaction between TGF-β/Smad3 and S1P/S1P3 and Smad3/S1P3 signaling in chondrocytes may play a role in the development of TMJ OA [43].
Additionally, it has been documented that overexpressing TGF-β1 causes aberrant subchondral bone remodeling that causes mice to develop TMJ OA and degrade mandibular condylar cartilage [9].

Indian Hedgehog Signaling
Indian hedgehog (Ihh), a signaling molecule, is essential for controlling the formation of the skeleton. During endochondral ossification, it is mainly expressed in prehypertrophic and hypertrophic chondrocytes. The production of parathyroid hormonerelated protein (PTHrP) in periarticular tissue is one of the processes it controls during cartilage growth [44].
In TMJ osteoarthritic cartilage stimulated by unilateral anterior cross-bite (UAC), enhanced Ihh signaling promotes the terminal differentiation of deep zone chondrocytes. In contrast, the OA-like lesions and UAC-promoted chondrocyte terminal differentiation were rescued by the deletion of Smo in mice [9]. Another research found that Ihh facilitated OA development by controlling genes involved in cartilage deterioration and that Ihh inhibition mitigated the disease. Activation of Ihh, Smo, and Gli1 was detected in adjuvantinduced TMJ OA in mice [45], suggesting that Ihh signaling may exacerbate TMJ OA by encouraging chondrocyte hypertrophy.

FGF Signaling
Articular cartilage and the control of skeletal development are two of the essential functions of the FGF signaling system. There are 22 ligands in the FGF family, all of which bind to one of four different FGFRs to perform various tasks [46]. The binding of FGFs to the FGFRs' extracellular domain is the usual starting point for FGF/FGFRs signaling. After the phosphorylation of the cytoplasmic tail of FGFRs and the recruitment of the corresponding target proteins, several different signaling processes are triggered.
Several signaling pathways are involved in the downstream signaling activities of FGF [47]. These include the phosphoinositide 3-kinase(PI3K)/Akt pathway, the phospholipase C (PLC) pathway, the mitogen-activated protein kinase (MAPK) pathway, and the signal transducers and activators of transcription (STAT) 1/p21 pathway. In addition, the FGF signaling pathway is connected with the development of OA via MEK/ERK, a critical downstream signaling molecule of FGFR1 [9].
FGF signaling may be increased in TMJ OA, as evidenced by the finding that ablation of FGRF1 in TMJ chondrocytes reduced TMJ OA development in particular OA models [48].

NF-κB Signaling
RelA, RelB, c-Rel, NF-κB1, and NF-κB2 are the five proteins that make up the NF-κB family. They interact with NF-κB inhibitors, form active complexes, move into the nucleus, bind to DNA, and control the expression of NF-κB-target genes [9]. The immune system, the inflammatory process, stress responses, cell proliferation, and cell death are all thought to be partially mediated by NF-κB.
The activation of the IKK-α/IKK-β/IKK-γ-NEMO complex is mediated by the TNF receptor (TNF-R), Toll-like receptor (TLR), or T-cell receptor (TCR) in the classical NF-κB signaling pathway. Inactive NF-κB dimers are linked to inhibitory NF-κB (I-κB) proteins in the cytoplasm. When mechanical and chemical cues stimulate cells, I-κBs are phosphorylated by I-κB kinases and degraded by the ubiquitin-proteasome system, allowing NF-κB heterodimers to translocate into the nucleus and promote the expression of target genes [9,49].

Notch Signaling
Necessary for cell differentiation and death, the Notch receptor is a single-pass transmembrane receptor at the cell surface. The highly conserved Notch signaling system consists of many components, including Notch ligands, Notch receptors, transcriptional effectors, and target genes [50]. Notch ligands bind to Notch receptors to commence the process, after which Notch receptors are cleaved, the intracellular domain of Notch receptors translocates to the nucleus, and target genes are activated [51]. The Notch signaling system controls the molecules involved in cartilage production and breakdown and hence plays a dual function in cartilage maintenance [52]. It is well known that Notch signaling is critical in the angiogenesis of condylar cartilage and disc, which is required to form TMJ OA [53]. Recent research has shown that changing Notch signaling may cause TMJ OA.

Angiogenesis in TMJ Osteoarthritis
Angiogenesis has been shown to enhance the development of OA [54]. Wang and colleagues discovered that mice with TMJ OA-like alterations increased the number of newly created blood vessels at the osteochondral junction [53]. In TMJ OA, these new blood vessels can carry inflammatory mediators and prolong inflammation [55]. When new blood arteries enter the cartilage, they stimulate chondrocyte enlargement and mineral deposition in the matrix. Through endochondral osteogenesis, osteophytes can integrate with newly created vessels on the joint's surface to enhance complex tissue creation [14].
VEGF is generated by chondrocytes in articular cartilage and modulates autocrine levels of MMP-13 and tissue inhibitor of metalloproteinase-1 (TIMP-1). Reduced TIMP concentration and increased MMP expression disrupt the circulation of ECM components, collagen, and proteoglycans, which is shown by an increased breakdown. VEGF may increase articular cartilage deterioration by activating osteoclasts and allowing blood vessels to penetrate the cartilage [15,17].
Due to the breakdown of hyaluronic acid (HA) and the increased activity of free radicals, joint hydration is also reduced. When the pressure within a joint begins to surpass the capillary pressure, transient hypoxia and joint degeneration ensue. Reoxygenation is detected when the stress on the joint is reduced, and joint degeneration is halted.
During hypoxia and reperfusion cycles, free radicals are released. Free radicals hinder production and accelerate HA breakdown, lowering synovial fluid viscosity and increasing friction between joint surfaces [59]. Increased friction during TMJ movement causes permanent joint structure damage, internal articular disc derangement, and degenerative alterations [15,60].

Subchondral Bone Changes in Temporomandibular Joint Osteoarthritis
The main pathogenic mechanism highlighted at the level of the subchondral bone in TMJ OA is abnormal bone remodeling. This is due to complex mechanisms that include mechanical loading, inflammation, and degradation of the articular cartilage [9,13,61,62]. The onset of degenerative changes is characterized by a loss of bone mass which acts as a trigger for the development of TMJ OA and which participates in the degradation of articular cartilage [63]. Then follows slow bone repair mechanisms that cause an increase in bone density at the subchondral level, which leads to increased thickening and stiffness of the condylar osteochondral interface [64,65]. Erosions, osteophytes, the appearance of cyst-like lesions, and subchondral sclerosis represent the main radiographic changes evident in TMJ OA [20,61,66,67]. Clinically, patients present a decrease in mandibular mobility, pain during mastication and with the opening of the oral cavity, as well as joint sounds (cracking) during TMJ mobilization [68].

Bone Cells in Temporomandibular Osteoarthritis
Bone remodeling in TMJ OA is associated with a decrease in the number and activity of osteoblasts, as well as an increase in osteoclast activity [63,64]. At the level of osteoblasts, bone-forming cells, a multitude of metabolic mechanisms with increased activity have been observed that favor angiogenesis, osteoclastogenesis, and subchondral bone sclerosis [69,70]. Thus, an increase in alkaline phosphatase (ALP) activity and an elevated expression of receptor activator of nuclear factor kappa-B ligand (RANKL), transforming TGF-β1, VEGF, and insulin-like growth factor-1 (IGF-1) were highlighted in TMJ OA [71][72][73][74]. Moreover, osteoblasts participate directly in the formation of subchondral sclerosis characterized by an increase in bone density and volume, but associated with deficient mineralization [75,76]. This is due to the particular phenotype of osteoblasts that secrete abnormal type I collagen and that show increased expression of TGF-β which inhibits mineralization by stimulating the secretion of DKK2 [77,78].
Osteoclasts, the cells involved in bone resorption, present an increased number and intense activity in TMJ OA, the main activating mechanism of osteoclastogenesis being the binding of RANK with its ligand (RANKL) [79,80]. The most important degradative enzyme produced by osteoclasts that favors bone resorption is cathepsin K (CTSK), animal studies highlight its important role in the development of OA [81]. In addition, the migration and differentiation of osteoclast precursors are achieved through the WNT5A/receptor tyrosine kinase-like orphan receptor 2 (Ror2) pathway [82]. The activation of adrenergic receptors (β2 and α2A) via neurotransmitters determines, also through the RANKL pathway, the maturation of osteoclasts, and bone destruction [83]. Last but not least, osteoclast activity is stimulated by increased TGF-β1 secretion [84].
Although the data are limited, it seems that estrogen and progesterone hormones have a role in the occurrence of TMJ OA by direct action on bone cells [85,86]. Thus, at the onset of the disease, an increased level of estrogen has a protective effect by inhibiting the Wnt pathway and the activity of osteoclasts [85]. Moreover, the increased level of progesterone, by inhibiting NF-kB activity, has a beneficial effect on the subchondral bone, decreasing bone resorption [86].
Osteocytes, the most abundant bone cells, have an important role in osteoclastogenesis, being sensitive to joint mechanical loading and producing RANKL [87,88]. By secreting degradative enzymes such as CTS and MMPs, osteocytes react to different mechanical forces and resorb the bone matrix, leading to perilacunar/canalicular remodeling [89]. Studies have shown that a decrease in this bone remodeling determined by osteocytes can favor the onset of OA [90]. Figure 1 summarizes the role and the main metabolic changes occurring at the level of bone cells that have been highlighted in TMJ OA. esis, being sensitive to joint mechanical loading and producing RANKL [87,88]. By secreting degradative enzymes such as CTS and MMPs, osteocytes react to different mechanical forces and resorb the bone matrix, leading to perilacunar/canalicular remodeling [89]. Studies have shown that a decrease in this bone remodeling determined by osteocytes can favor the onset of OA [90]. Figure 1 summarizes the role and the main metabolic changes occurring at the level of bone cells that have been highlighted in TMJ OA. RANKL-receptor activator of nuclear factor kappa-B ligand; RANK-receptor activator of nuclear factor kappa-B; TGFβ1-transforming growth factor β1; VEGF-vascular endothelial factor; IGF1insulin-like growth factor 1; DKK2-dickkopf-2; CTSK-cathepsin K; Wnt5A/Ror2-Wnt5A/receptor tyrosine kinase-like orphan receptor 2; MMPs-matrix metalloproteinases, ERS-endoplasmic reticulum stress; TNF-tumor necrosis factor; FGFR1-fibroblast growth factor receptor 1; RIP1/RIP3-receptor-interacting proteins 1 and 3; LC3B-light chain 3 beta; ADAMTS-a disintegrin and metalloproteinase with thrombospondin motifs; Col2A1-type II collagen A1; HTRA1high-temperature requirement A1.

Inflammation, Subchondral Bone and Temporomandibular Osteoarthritis
Although OA is not a systemic inflammatory condition, data support the role of various pro-inflammatory cytokines in TMJ OA both at the level of articular cartilage and of subchondral bone [91,92]. In the synovial fluid of these patients, an inflammatory environment characterized by an increased secretion of many molecules such as IL (IL-1β, -2, -12, -17, -18), TNF (TNFα and TNFβ) and interferon (IFN)-γ was highlighted. Among all these, the main pro-inflammatory cytokine secreted in TMJ OA is IL-12 [93].
IL-1β and TNFα actively participate in inflammation by increasing the expression of chemokines, eicosanoids, and various proteins [94][95][96]. Moreover, through the stimulation achieved by IL-1β, TMJ synoviocytes increase their production of monocyte chemoattractant protein-1 (MCP-1) [97]. Recently published data support that MCP-1 can be considered the trigger for the appearance, progression, and persistence of inflammation even in the absence of IL-1β [98]. In addition, the secretion of these cytokines correlates with the suppression of the synthesis of the articular cartilage matrix [99].
TNFα, IL-1β, and IL-17 directly modulate osteoclast production, as well as bone resorption, by increasing RANKL secretion at the level of osteoblasts and fibroblasts from the synovial membrane [100]. The results of a study on mice support the role of TLR4 in the occurrence of TMJ OA. Thus, through the MyD88/NF-kB activation pathway, TLR4 favors the appearance of OA changes, in particular, the degradation of both cartilage and subchondral bone [101].
Inflammation also plays an important role in the development of joint pain, a frequent symptom in these patients. Pro-inflammatory cytokines such as TNFα or IL-1β can directly stimulate nociceptive receptors and cause sensory neuron hyperexcitability [102,103]. Other molecules such as growth factors, proteoglycans, or proteases seem to be involved in the development of arthritic pain, MMP-3 being considered a hallmark for TMJ pain [104]. Moreover, promising results on murine studies were published, pointing to the important role of macrophage/microglia activation in the development of pain in these patients [105].
Cytokines such as IL-1β and IL-6 can facilitate the transcription of VEGF in the nucleus, thus leading to an increased expression of VEGF [58,106]. IL-1β has the ability to directly activate NF-kB, while IL-6, through ERK1/2, activates ERRγ [26,58,106,107].
VEGF is the main molecule responsible for angiogenesis, a pathogenic mechanism involved in the development of TMJ OA and characterized by the formation of new blood vessels at the osteochondral junction [53,54,108,109]. By invading the articular cartilage, it determines both the formation of mineral deposits at the level of the ECM as well as the hypertrophy of chondrocytes. In addition, it participates in endochondral osteogenesis by incorporating new blood vessels into the osteophytes [110]. In addition to angiogenesis, an important role is played by neurogenesis, both processes favoring the appearance of OA and being involved in the appearance of joint pain [111]. Angiogenesis and neurogenesis influence each other through vascular and nerve growth factors, leading to the emergence of neurovascular interaction involved in the TMJ OA progression [112,113]. Figure 2 shows the important role of inflammation in the changes at the level of the subchondral bone as well as its role in the occurrence of TMJ OA pain. the occurrence of TMJ OA. Thus, through the MyD88/NF-kB activation pathway, TLR4 favors the appearance of OA changes, in particular, the degradation of both cartilage and subchondral bone [101].
Inflammation also plays an important role in the development of joint pain, a frequent symptom in these patients. Pro-inflammatory cytokines such as TNFα or IL-1β can directly stimulate nociceptive receptors and cause sensory neuron hyperexcitability [102,103]. Other molecules such as growth factors, proteoglycans, or proteases seem to be involved in the development of arthritic pain, MMP-3 being considered a hallmark for TMJ pain [104]. Moreover, promising results on murine studies were published, pointing to the important role of macrophage/microglia activation in the development of pain in these patients [105].
Cytokines such as IL-1β and IL-6 can facilitate the transcription of VEGF in the nucleus, thus leading to an increased expression of VEGF [58,106]. IL-1β has the ability to directly activate NF-kB, while IL-6, through ERK1/2, activates ERRγ [26,58,106,107].
VEGF is the main molecule responsible for angiogenesis, a pathogenic mechanism involved in the development of TMJ OA and characterized by the formation of new blood vessels at the osteochondral junction [53,54,108,109]. By invading the articular cartilage, it determines both the formation of mineral deposits at the level of the ECM as well as the hypertrophy of chondrocytes. In addition, it participates in endochondral osteogenesis by incorporating new blood vessels into the osteophytes [110]. In addition to angiogenesis, an important role is played by neurogenesis, both processes favoring the appearance of OA and being involved in the appearance of joint pain [111]. Angiogenesis and neurogenesis influence each other through vascular and nerve growth factors, leading to the emergence of neurovascular interaction involved in the TMJ OA progression [112,113]. Figure 2 shows the important role of inflammation in the changes at the level of the subchondral bone as well as its role in the occurrence of TMJ OA pain.

Mechanical Loading and Temporomandibular Osteoarthritis
The mechanical loading distributed on the surface of a joint is very important to maintain joint integrity and functionality, the bone microarchitecture being correlated with the direction and magnitude of the applied load [114]. Thus, the structure of the subchondral bone changes, highlighting changes such as: the increase in bone volume, the decrease in mineralization, the thickening of plates, and the decrease in the trabecular rod/plate ratio [115]. This is mainly due to the osteocytes that participate in the remodeling of the ECM through the increased secretion of degradative enzymes, as well as through the increase in their metabolic activity that leads to canalicular remodeling [116][117][118][119].
Although the TMJ is not a joint on which high mechanical load forces act, it seems that small alterations of the mechanical loading cause the appearance of degenerative changes [120]. The anatomical and positional changes in the fibrocartilaginous disc between the condyles and the joint fossa can be considered a cause of TMJ OA [121].
Another mechanism involved in the occurrence of TMJ OA is the decrease in the sensitivity of chondrocytes to mechanical loading, this being due to the knockdown of high mobility group protein B2 (HMGB2) [122]. Along with these, an abnormal subchondral bone remodeling characterized by a decrease in type I collagen production and increased bone resorption is involved in the development of TMJ OA [63]. Secondary to the increased resorptive activity, the data obtained from experimental models support the formation of new bone at the level of the condyles in the initial stages of degenerative changes [123].

Therapeutic Strategies Impacting Cartilage and/or Bone Changes in TMJ OA: Platelet-Rich Plasma, Hyaluronic Acid, and Stem Cell-Based Therapy
Presently, the most widely used treatment strategies in TMJ OA involve the diminishment of symptoms, with a focus on pain relief and improvement of mobility [124][125][126][127][128][129]. Nevertheless, recent studies support the regenerative or protective potential of certain therapies such as platelet-rich plasma (PRP), HA, and mesenchymal stem cells (MSCs) in OA [130][131][132]. The currently available data regarding the regenerative or protective potential of PRP, HA, and MSC-based therapy results from studies that used various treatment regimens and different assessment methods, which may partly explain the sometimes diverging findings.

Platelet-Rich Plasma (PRP)
In vitro research indicates that PRP (different formulations of autologous blood derivatives with a high concentration of platelets-cells involved in tissue healing) may bolster the proliferation of chondrocytes and MSCs with the concomitant deposition of type II collagen, thus potentially leading to cartilage repair [133]. Intraarticular PRP was proven effective in OA involving various joints, with studies showing that the treatment may significantly improve symptoms over various follow-up periods [134][135][136][137].
Diverse types of management options including PRP have shown promising results in in vitro and animal studies. Constructs of PRP embedded in different types of scaffolds and placed at the site of a cartilage defect where it exhibited positive effects on chondrocyte disposition, metabolic activity, glycosaminoglycan deposition, and collagen type II expression, as well as anti-apoptotic properties. Moreover, PRP was also used as a carrier for mesenchymal stem cells [138,139]. The molecular mechanisms through which PRP may contribute to tissue regeneration include its bioactivity. In this respect, growth factors such as TGFβ, IGF, VEGF, PDGF (platelet-derived growth factor), and bFGF (basic fibroblast growth factor) are known to be abundant in PRP and may encourage the proliferation of chondrogenic cells and the secretion of cartilaginous matrix components [140,141]. The development of PRP-based formulations for OA is based on several hypotheses including the potential anti-inflammatory and anti-catabolic properties of chemokines, and the anabolic effects of the growth factors found in PRP [142]. PRP inhibits serine/threonine kinase 1 (AKT1), phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K), and NF-kB (PIK/AKT signaling) and hinders the release of pro-inflammatory cytokines such as TNFα and IL6 [143,144].
Recently published studies involving patients with TMJ OA suggest that PRP may induce condylar bone repair according to imaging findings (MRI-magnetic resonance imaging and/or CBCT-cone beam computed tomography) [145,146]. In animal models of TMJ OA, PRP therapy was associated with the improvement of the joint structure histological aspect (Table 1) [147][148][149]. Local injections of PRP have also shown beneficial effects on pain relief and improved maximal mouth opening in patients as well as animal models of TMJ OA [150][151][152][153]. Animal studies based on the histological assessment of the TMJ cartilage and bone showed that intraarticular PRP leads to a restoration of the fibrocartilage and an improved microarchitecture of the subchondral bone [147][148][149]. AbuBakr et al. also observed a decrease in MMP and pro-inflammatory cytokine values following treatment, with a concomitant upregulation of type II collagen gene expression in experimental animals [149]. Arafat and Kamel found that combined treatment with PRP + HA may be associated with superior results compared to PRP alone [147].
The imaging findings in patients with TMJ OA indicated that PRP could improve the aspect of the condylar bone [145,146]. Nevertheless, these studies did not use the same evaluation methods in all patients (either CBCT or MRI).

Hyaluronic Acid (HA)
Intraarticular HA injections have largely been used to reduce OA-related symptoms in larger joints such as the knees [154][155][156]. Apart from its viscoelastic behavior, HA has shown the ability to support cell growth and the chondrogenic differentiation of stem cells, as well as to provide binding sites for growth factors, favoring tissue healing. Certain HA formulations were associated with decreased pro-inflammatory cytokine levels and MMPs [157,158]. An important topic in cartilage tissue engineering has been designing HA hydrogel formulations to enhance regeneration. A sulforaphane-loaded hyaluronic acid hydrogel demonstrated protective effects against cartilage degradation by reducing the depletion of proteoglycans, increasing collagen type II, while also modulating NF-κB [159]. Nevertheless, a great number of recent studies focus largely on the biomaterials used as scaffolds or carriers, rather than on the properties of HA itself [160][161][162].
A significant improvement in pain levels and functionality was also seen in patients with TMJ OA following HA treatment [15,163,164]. HA therapy has also shown some beneficial effects regarding the regeneration of joint structures in TMJ OA in both patients and animal models (Table 2). However, some studies did not support the restorative effects of HA in TMJ OA [165][166][167][168][169][170].
While some animal research suggested that HA could exhibit certain restorative or protective properties, in patients with TMJ OA, imaging findings (CBCT or MRI) indicated that HA alone does not improve the aspect of the cartilage or condylar bone [165][166][167]. However, the methodology and the treatment regimens varied greatly between studies.
Notably, the combination treatment of HA and PRP in a murine model of TMJ OA exhibited better results with respect to the regeneration of joint structures compared to HA alone [147]. Moreover, the addition of HA to MSCs may improve cartilage repair according to the results obtained by Köhnke et al. in a rabbit model of TMJ OA [168]. These findings suggest that HA could enhance the regenerative potential of other therapies (PRP, MSCs) rather than restore normal TMJ structures by itself.  The HMW-HA treatment group demonstrated restoration of the normal histological aspect of the TMJ (with respect to the condyle, the subchondral bone, and the articular disc). Cen et al., 2022 [165] Retrospective comparison between HA injections and oral glucosamine + diclofenac in TMJ OA patients

Mesenchymal Stem Cells (MSCs)
MSCs may derive from a wide array of tissues such as the synovium, the umbilical cord, adipose tissue, dental pulp, and bone marrow. In recent years, the promising findings with respect to the important regenerative potential of MSC-based therapies in OA have been gathering interest [171][172][173].
Several mechanisms through which transplanted MSCs could restore the normal aspect of the arthritic joint have been proposed. In vitro studies indicated that, under controlled conditions, MSCs may differentiate into cartilage, bone, ligament, and tendon structures. Moreover, the exosomes (extracellular vesicles) released by these cells may carry various bioactive molecules. It has been shown that MSCs may exhibit immunomodulatory and anti-inflammatory effects and may promote angiogenesis [174][175][176].MSCs could hinder the degradation of the cartilage extracellular matrix. Additionally, under the influence of MSCs, the macrophage-like synoviocytes may switch to the M2 phenotype, rather than the predominantly pro-inflammatory M1 by secreting prostaglandin-E2 and indoleamine 2,3-dioxygenase [177].
In TMJ OA, their use in patients and experimental animals was tied to numerous favorable effects including cartilage restoration and delayed degradation, chondrogenic and osteogenic differentiation, as well as the improvement in subchondral bone volume and structure. Moreover, MSCs have exhibited anti-inflammatory and trophic effects in TMJ OA in addition to potential pain reduction and improved functionality [178][179][180][181][182]. Research conducted in the last years describes histological changes and imaging findings suggesting joint structure regeneration following treatment (Table 3). However, studies differ in terms of the types of MSCs used, the treatment regimens and follow-up periods, as well as assessment methods [179][180][181][182][183][184][185][186][187][188].
Notable histological signs of cartilage regeneration were seen in animal models of TMJ OA (induced chemically, surgically, or mechanically or by a combination of chemical factors and mechanical stress). Moreover, some studies indicated that MSCs-based therapy could be superior to other treatment strategies for TMJ OA such as intra-articular injections of PRP and HA for the restoration of joint structures [149,168].
In patients with TMJ OA, imaging findings (CBCT, MRI) showed regenerative joint changes related to MSCs-based therapy [183]. In patients treated with MSCs, Carboni et al. described a better MRI aspect of the TMJ structures compared to controls receiving saline injections [184]. Based on CBCT evidence, Khairy et al. found that adipose-derived MSCs had better effects with respect to joint remodeling compared to HA injections [166].
However, in the study conducted by De Riu et al., there was no significant evidence of joint reparation following treatment with BMNC (bone marrow nucleated cell concentrate) according to the MRI scoring system [167].  The condylar structure was improved after treatment with BM-MSCs-MVs. Moreover, there was a decrease in inflammatory cytokine levels, as well as MMP3 and MMP13, and an increased type II collagen gene expression. BM-MSCs-MVs were found to be superior to PRP in some respects.

Conclusions
Several potential mechanisms of TMJ OA have been hypothesized in the literature. The majority of these purported processes are predicated on the assumption that joint overloading, excessive pressures, and trauma all have a role in disturbing the structural integrity of the TMJ structure. Understanding the chain of biological processes that culminate in TMJ cartilage and bone degeneration is the first step in developing a complete model of the pathophysiology of TMJ OA.
Certain treatment strategies are linked to positive outcomes regarding the regeneration of joint structures over time in TMJ OA. While studies used diverse treatment regimens and evaluation methods, more prominent findings were described in MSCs-based treatment and PRP compared to HA. According to recent findings, the latter could enhance the restorative potential of other therapies (PRP, MSCs) when used in combination, rather than repair TMJ structures by itself.
TMJ OA is a complex disease in which degenerative changes in the cartilage and bone develop through intricate mechanisms. The regenerative potential of such therapies as PRP, MSCs, and HA regarding the cartilage and subchondral bone (alone or in various combinations) in TMJ OA remains a matter of further research, with studies sometimes obtaining discrepant results.