Osteoarthritis pathophysiology: Difference between revisions

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[[File:Oa-pth.jpg|thumb|What is going on in involved joint?!]]__NOTOC__
{{Osteoarthritis}}
{{Osteoarthritis}}
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{{CMG}} {{AE}}[[User:DrMars|Mohammadmain Rezazadehsaatlou]] [2], [[User:Irfan Dotani|Irfan Dotani]] [3]


==Overview==
==Overview==
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Osteoarthritis (OA) is a well-known [[degenerative joint disease]] influencing millions of people worldwide. Osteoarthritis is a complex disease caused by changes in the tissues' homeostasis of articular cartilages and subchondral bones. The cell/extracellular matrix (ECM) and their interactions play an important role in the pathophysiology of articular cartilage and the occurrence of Osteoarthritis. Consequently, the main feature of OA is that after this process is involved, the articular cartilages of the involved joint no longer will have a normal acting system because the destruction of the articular cartilages can no longer act as shock absorber.  


==Pathophysiology==
==Pathophysiology==
Osteoarthritis (OA) is a well-known degenerative joint disease influencing  millions of peopleworldwide. Osteoarthritis count as a complex disease caused by changes in the tissue homeostasis of articular cartilages and subchondral bones. The cell/extra-cellular matrix (ECM) and their interactions play an important role in the pathophysiology of articular cartilage and the occurrence of Osteoarthritis; consequently, the main feature of OA is that after this process in involved joint the articular cartilages of involved joint no longer have a normal acting system and for example  because of the the extracellular matrix  destruction this articular cartilages cannot act as a shock absorber. Different pathogenic mechanisms have been proposed to be responsible for the occurrence of OA. Heredity, obesity, hypoxia, synovitis–capsulitissubchondral bone overload, joint instability (mechanical integrity disturbances) are the most important underlying causes in this regard. In the current pathogenesis of osteoarthritis (OA) all joint tissues including cartilage, bone, synovium, ligamentous capsular structures, and surrounding muscle are involved. OA characterized by structural changes such as: active bone remodeling, synovial inflammation, and articular cartilage degradation leading to the loss of joint function and angular deformity or malalignment. Also, a variety of bio markers in synovial fluid helped to create more clear insight about the biological response of joints to injury but no biomarker have been declared to be reliable for monitoring the development,  progression, and response to therapy of OA. Its been reported that certain factors can increase the risk of the OA development such as: hereditary elements, trauma and mechanical stress, joint injury, age, obesity, physical activity, bone mineral density (BMD), congenital anomalies. and, during the last years signaling pathways mad a lot of attention and its been proven that these pathways play important rolls in inflammation in the remodeling subchondral bone, synovium, enzyme activation, and extracellular matrix degradation in articular cartilage.                                      
Different pathogenic mechanisms have been proposed to be responsible for the occurrence of OA. Heredity, obesity, hypoxia, synovitis–capsulitissubchondral bone overload, joint instability (mechanical integrity disturbances) are the most important underlying causes in this regard. In the current pathogenesis of osteoarthritis (OA), all joint tissues including cartilage, bone, synovium, ligamentous capsular structures, and surrounding muscle are involved. OA is characterized by structural changes such as active bone remodeling, synovial inflammation, and articular cartilage degradation leading to the loss of joint function and angular deformity or malalignment. Also, a variety of biomarkers in synovial fluid have helped to create more clear insight into the biological response of joints to injury. However, no biomarker has been declared to be reliable for monitoring the development,  progression, and response to therapy of OA. Its been reported that certain factors can increase the risk of the OA development such as hereditary elements, trauma and mechanical stress, joint injury, age, obesity, physical activity, bone mineral density (BMD), and congenital anomalies. During the last years, signaling pathways have drawn a lot of attention and proven that these pathways play important roles in inflammation and in the remodeling of the subchondral bone, synovium, enzyme activation, and extracellular matrix degradation in articular cartilage <ref name="pmid22632700">{{cite journal |vauthors=Vincent KR, Conrad BP, Fregly BJ, Vincent HK |title=The pathophysiology of osteoarthritis: a mechanical perspective on the knee joint |journal=PM R |volume=4 |issue=5 Suppl |pages=S3–9 |date=May 2012 |pmid=22632700 |pmc=3635670 |doi=10.1016/j.pmrj.2012.01.020 |url=}}</ref> <ref name="pmid22238208">{{cite journal |vauthors=Wise BL, Niu J, Yang M, Lane NE, Harvey W, Felson DT, Hietpas J, Nevitt M, Sharma L, Torner J, Lewis CE, Zhang Y |title=Patterns of compartment involvement in tibiofemoral osteoarthritis in men and women and in whites and African Americans |journal=Arthritis Care Res (Hoboken) |volume=64 |issue=6 |pages=847–52 |date=June 2012 |pmid=22238208 |pmc=3340516 |doi=10.1002/acr.21606 |url=}}</ref> <ref name="pmid19182033">{{cite journal |vauthors=Andriacchi TP, Koo S, Scanlan SF |title=Gait mechanics influence healthy cartilage morphology and osteoarthritis of the knee |journal=J Bone Joint Surg Am |volume=91 Suppl 1 |issue= |pages=95–101 |date=February 2009 |pmid=19182033 |pmc=2663350 |doi=10.2106/JBJS.H.01408 |url=}}</ref><ref name="pmid12897215">{{cite journal |vauthors=Haq I, Murphy E, Dacre J |title=Osteoarthritis |journal=Postgrad Med J |volume=79 |issue=933 |pages=377–83 |date=July 2003 |pmid=12897215 |pmc=1742743 |doi= |url=}}</ref>.


== Subchondral Bone ==
== Subchondral Bone ==
Osteoarthritis influencing both articular cartilage and underlying bone structures. One of the most common findings in is the subchondral bone plate thickening. The diseased bone becomes brittle and sclerotic; and the frequent turnovers affecting bone quality. There is still a big question about this fact that does the subchondral bone changes happens simultaneously with the changes in articular cartilage or not. Macroscopic changes of the subchondral bone especially in load-bearing areas such as: increased osteogenetic reactions, increased stiffness, and increased density.
OA leads to the sub-chondral bone remodeling. This event is often along with the sub-chondral cysts formation as a result of focal resorption. OA can alter chondrocyte metabolism in bony cells. Osteoarthritis, influencing whole joint systems, includes both articular cartilage and underlying bone structures. One of the most common findings in OA is the subchondral bone plate thickening. The diseased bone becomes brittle and sclerotic, and the frequent turnovers affect bone quality. There is still controversy about whether the subchondral bone change happens simultaneously with the changes in articular cartilage or not. Articular overgrowths such as subchondral bone lead to microtrauma, hardening, remodeling, and displacement of the osteochondral line. Consequently, the energy-dissipation capacity and elasticity of the articular cartilage decrease. Macroscopic changes of the subchondral bone especially in load-bearing areas are increased osteogenetic reactions, increased stiffness, increased density, and excessive formation of bone and cartilage (called osteochondrophytes). OA is also capable of influencing the non–weight-bearing joints, such as hands, spine, shoulders, and temporomandibular joints. The osteochondrophytes usually can be found in intra-articular, marginal, extraarticular, insertional, or enthesiophytes. The osteochondrophytes frequently involves the joint space, and with synovial metaplastic fragments or flaps of cartilage, they lead to the articular ‘joint mice’ formation. On the other hand, the bone remodeling caused by microfractures within the superficial bone trabeculae with the formation of subchondral bone cysts (known as erosive alterations). Bony changes such as sclerosis of the subchondral bone plate, alterations in trabecular structure, osteophytes and bone marrow lesions are associated with the initiation and progression of OA. It’s been reported that the subchondral bone changes prior to the articular cartilage changes. Meanwhile, it’s been found that the molecular pathways (for example, cytokines such as IL-1, TNF-α, fibrinolytic system including plasminogen, tissue plasminogen activators, urokinase plasminogen activators, and plasmin) have in subchondral play important roles in the disbalance between the physiological connection of bone deposition and remodeling and resorption potential. Higher osteoblastic activity results in an exaggerated reparative response. In contrast, an increased osteoclastic degradative activity results in a predominantly erosive bony condition<ref name="pmid19037464">{{cite journal |vauthors=Stone L |title=Aches, pains and osteoarthritis |journal=Aust Fam Physician |volume=37 |issue=11 |pages=912–7 |date=November 2008 |pmid=19037464 |doi= |url=}}</ref><ref name="pmid15766999">{{cite journal |vauthors=Dieppe PA, Lohmander LS |title=Pathogenesis and management of pain in osteoarthritis |journal=Lancet |volume=365 |issue=9463 |pages=965–73 |date=2005 |pmid=15766999 |doi=10.1016/S0140-6736(05)71086-2 |url=}}</ref><ref name="pmid22730047">{{cite journal |vauthors=Witt KL, Vilensky JA |title=The anatomy of osteoarthritic joint pain |journal=Clin Anat |volume=27 |issue=3 |pages=451–4 |date=April 2014 |pmid=22730047 |doi=10.1002/ca.22120 |url=}}</ref><ref name="pmid24641343">{{cite journal |vauthors=Hassan H, Walsh DA |title=Central pain processing in osteoarthritis: implications for treatment |journal=Pain Manag |volume=4 |issue=1 |pages=45–56 |date=January 2014 |pmid=24641343 |doi=10.2217/pmt.13.64 |url=}}</ref><ref name="pmid24928208">{{cite journal |vauthors=Dimitroulas T, Duarte RV, Behura A, Kitas GD, Raphael JH |title=Neuropathic pain in osteoarthritis: a review of pathophysiological mechanisms and implications for treatment |journal=Semin. Arthritis Rheum. |volume=44 |issue=2 |pages=145–54 |date=October 2014 |pmid=24928208 |doi=10.1016/j.semarthrit.2014.05.011 |url=}}</ref><ref name="pmid24938198">{{cite journal |vauthors=Salaffi F, Ciapetti A, Carotti M |title=The sources of pain in osteoarthritis: a pathophysiological review |journal=Reumatismo |volume=66 |issue=1 |pages=57–71 |date=June 2014 |pmid=24938198 |doi= |url=}}</ref><ref name="pmid28381830">{{cite journal |vauthors=Mobasheri A, Rayman MP, Gualillo O, Sellam J, van der Kraan P, Fearon U |title=The role of metabolism in the pathogenesis of osteoarthritis |journal=Nat Rev Rheumatol |volume=13 |issue=5 |pages=302–311 |date=May 2017 |pmid=28381830 |doi=10.1038/nrrheum.2017.50 |url=}}</ref>.


Interest in structural remodeling, vascular biology, and osteoblast cytokine expression of subchondral bone in OA has been stimulated by a large number of studies suggestively associating a role for subchondral bone changes in the pathogenesis of OA. Active bone remodeling is associated with the initiation and progression of OA including sclerosis of the subchondral bone plate, alterations in trabecular structure, osteophytes and bone marrow lesions.2-5 Some studies with a guinea pig OA model suggest that subchondral bone changes precede degradation of articular cartilage.6, 7 Additionally, several cytokines have been found in subchondral bone that play major signaling roles associated with cartilage degradation including IL-1, TNF-α and those of the fibrinolytic system including plasminogen, tissue and urokinase plasminogen activators (tPA, uPA), and plasmin.8
== Articular Cartilage ==
The articular cartilage damage is one of the most important pathological causes of OA. It is not clear whether this pathological event originates from the cartilage or subchondral bone, loss and/or damage to articular cartilage or both of them are responsible for the development and progression of OA. Human articular cartilage system, acting as a shock absorber, consists of a hydrated extracellular matrix (as the functional elements of the tissue) with few numbers of chondrocytes within. 70–80% of cartilage consists of water and collagens and proteoglycans are the major organic components. Collagen type II builds a network of fibers containing molecules within. Collagen type XI helps collagen type II in fibril network formation and also limiting the fiber diameter. Collagen type IX make crosslinks the whole collagen network. Heparan sulfate proteoglycans such as perlecan, have important roles (such as interactions with heparin-binding growth factors like fibroblast growth factors, heparin binding forms of vascular endothelial growth factor (VEGF), and bone morphogenetic proteins (BMP)) in chondrogenesis. Higher demolition of heparan sulfate proteoglycans by glycosidases and matrix metalloproteinases are known to responsible for OA. Calcification and ossification in articular cartilage during OA and aging occurs due to the differentiation of chondrocytes. During the degenerative changes in involved joints, calcification happens simultaneously with to increasing alkaline phosphates and pyrophosphate levels.  Since the Articular cartilage has no internal vascular or lymphatic supply system so it is dependent on near tissues including subchondral bone and synovial membrane in receiving nutrients elements and excretion of products of made by articular matrix turnover and chondrocyte metabolism <ref name="pmid15480079">{{cite journal |vauthors=Carter DR, Beaupré GS, Wong M, Smith RL, Andriacchi TP, Schurman DJ |title=The mechanobiology of articular cartilage development and degeneration |journal=Clin. Orthop. Relat. Res. |volume= |issue=427 Suppl |pages=S69–77 |date=October 2004 |pmid=15480079 |doi= |url=}}</ref><ref name="pmid18723377">{{cite journal |vauthors=Krasnokutsky S, Attur M, Palmer G, Samuels J, Abramson SB |title=Current concepts in the pathogenesis of osteoarthritis |journal=Osteoarthr. Cartil. |volume=16 Suppl 3 |issue= |pages=S1–3 |date=2008 |pmid=18723377 |doi=10.1016/j.joca.2008.06.025 |url=}}</ref><ref name="pmid22632700">{{cite journal |vauthors=Vincent KR, Conrad BP, Fregly BJ, Vincent HK |title=The pathophysiology of osteoarthritis: a mechanical perspective on the knee joint |journal=PM R |volume=4 |issue=5 Suppl |pages=S3–9 |date=May 2012 |pmid=22632700 |pmc=3635670 |doi=10.1016/j.pmrj.2012.01.020 |url=}}</ref><ref name="pmid24928208">{{cite journal |vauthors=Dimitroulas T, Duarte RV, Behura A, Kitas GD, Raphael JH |title=Neuropathic pain in osteoarthritis: a review of pathophysiological mechanisms and implications for treatment |journal=Semin. Arthritis Rheum. |volume=44 |issue=2 |pages=145–54 |date=October 2014 |pmid=24928208 |doi=10.1016/j.semarthrit.2014.05.011 |url=}}</ref><ref name="pmid14698638">{{cite journal |vauthors=Martel-Pelletier J |title=Pathophysiology of osteoarthritis |journal=Osteoarthr. Cartil. |volume=12 Suppl A |issue= |pages=S31–3 |date=2004 |pmid=14698638 |doi= |url=}}</ref><ref name="pmid24072604">{{cite journal |vauthors=Houard X, Goldring MB, Berenbaum F |title=Homeostatic mechanisms in articular cartilage and role of inflammation in osteoarthritis |journal=Curr Rheumatol Rep |volume=15 |issue=11 |pages=375 |date=November 2013 |pmid=24072604 |pmc=3989071 |doi=10.1007/s11926-013-0375-6 |url=}}</ref><ref name="pmid26094910">{{cite journal |vauthors=Scotece M, Mobasheri A |title=Leptin in osteoarthritis: Focus on articular cartilage and chondrocytes |journal=Life Sci. |volume=140 |issue= |pages=75–8 |date=November 2015 |pmid=26094910 |doi=10.1016/j.lfs.2015.05.025 |url=}}</ref>.


A typical finding in horses that exercise at speed is subchondral sclerosis of bone in joints subjected to high weight-bearing impact and shear forces (e.g., carpus and fetlock).3,5 It has been postulated that articular overload, especially of subchondral bone, produces microtrauma, remodeling, hardening, and displacement of the osteochondral line.5 These changes reduce the elasticity and energy-dissipation capacity of the articular cartilage during locomotion.3 Furthermore, the injured tissue fails to heal because of the combined effects of high-impact exercise protocols, a lack of adequate warm-ups and post-exercise stretching, inadequate development of proprioception, working musculoskeletal tissue while it is fatigued, poor neuromuscular training, and inadequate rest intervals.8 The results of these forces are mechanical lesions that affect the joint tissue and its extracellular matrix (ECM),3 which may account for the common finding of OA in fetlock joints of performance horses or in knee joints of human athletes.5 However, OA also affects non–weight-bearing joints, such as those in the hands, spine, shoulders, and temporomandibular joints in humans and other mammals. Consequently, this theory does not completely explain the origin of these lesions, although either misalignment of articular surfaces or abnormalities of deep ligamentous components in the spinal and temporomandibular joints9 may result in abnormal load distribution.
== Synovial Membrane ==
 
The main task of Synovial Membrane is repairing any defects found in joint. The cellular compartment of the synovial membrane of the SM is a major source of synovial fluid. These components are responsible for management of chondrocyte activities and maintaining the integrity of articular cartilage surfaces (using lubricin and hyaluronic acid molecules) in diarthrodial joints. After a joint injury, the concentration of this molecular system changes. During the progression of OA, this synovial membrane changes into the main origin of proinflammatory and catabolic products such as metalloproteinases and aggrecanases. Thus, any damage to the synovial membrane can result in reducing of cartilage-protecting factors, and also an increase in production of articular matrix degradation factors. A normal synovial membrane has full control on the transmitted molecules in and out of the joint space. During some conditions such as trauma, inflammation, and OA this permeability of synovial membrane disrupts leading to reduced concentrations of lubricin and hyaluronic acid<ref name="pmid20042798">{{cite journal |vauthors=Henrotin Y, Pesesse L, Sanchez C |title=Subchondral bone in osteoarthritis physiopathology: state-of-the art and perspectives |journal=Biomed Mater Eng |volume=19 |issue=4-5 |pages=311–6 |date=2009 |pmid=20042798 |doi=10.3233/BME-2009-0596 |url=}}</ref><ref name="pmid14535372">{{cite journal |vauthors=Mortellaro CM |title=Pathophysiology of osteoarthritis |journal=Vet. Res. Commun. |volume=27 Suppl 1 |issue= |pages=75–8 |date=September 2003 |pmid=14535372 |doi= |url=}}</ref><ref name="pmid10343769">{{cite journal |vauthors=Martel-Pelletier J |title=Pathophysiology of osteoarthritis |journal=Osteoarthr. Cartil. |volume=6 |issue=6 |pages=374–6 |date=November 1998 |pmid=10343769 |doi=10.1053/joca.1998.0140 |url=}}</ref><ref name="pmid25179389">{{cite journal |vauthors=Onuora S |title=Osteoarthritis: a role for CXCR2 signalling in cartilage homeostasis |journal=Nat Rev Rheumatol |volume=10 |issue=10 |pages=576 |date=October 2014 |pmid=25179389 |doi=10.1038/nrrheum.2014.148 |url=}}</ref>. [[File:Model of Toll-like Receptor (a) and complement activation (b) in the joint leading to synovitis and potentiation of cartilage erosion in OA.jpg|center|thumb|500x500px|'''A model of Toll-like Receptor (a) and complement activation (b) in the joint leading to synovitis and potentiation of cartilage erosion in OA''']] .
It is still a matter for debate, however, whether the subchondral bone changes occur at the same time as changes in articular cartilage, and thus are causative, or are the consequence of cartilage degradation. Because bone adapts to changes in mechanical forces (Wolff ’s law), subchondral stiffening could be due to normal bone adaptation [20], because loss of articular cartilage would mean that an increased load is transmitted to bone. On the other hand, several researchers [15,47,48] have suggested that bone sclerosis precedes cartilage degradation and that enhanced bone remodeling by abnormal OA osteoblasts is the initiating event that triggers cartilage damage. Evidence in support of that sequence of events is based on in vitro studies of osteoblasts or of isolated chondrocytes [76]. It is not clear how factors released from osteoblasts can act on chondrocytes in vivo, inasmuch as the mineralized bone matrix constitutes a barrier to diffusion. Microdamage or microfracture could initiate vascular invasion [15,81], but healthy articular cartilage contains antiangiogenic factors [87], is resistant to vascular invasion, and is likely capable of repelling any invasion from the subchondral bone. However, OA cartilage has lost its antiangiogenic factors [39] and its resistance to vascular invasion [30,84]. This suggests that changes in the cartilage itself permit vascular invasion to take place. On balance, although the evidence for an association between OA and changes in osteoblasts and the subchondral matrix is strong, the inference that these are causes of OA is as yet controversial (for further details, see Chapter 2).


== Joint Instability ==
== Joint Instability ==
 
Joint instability occurs due to the ligament laxity enhancement, poor muscles conditions, or ligament tearing or strain in a ligament or abnormal muscles status. Joint instability increases the incidence of OA. Joint instability could be found as a result of synovitis produces excessive amounts of synovial fluid<ref name="pmid17185832">{{cite journal |vauthors=Goldring SR, Goldring MB |title=Clinical aspects, pathology and pathophysiology of osteoarthritis |journal=J Musculoskelet Neuronal Interact |volume=6 |issue=4 |pages=376–8 |date=2006 |pmid=17185832 |doi= |url=}}</ref><ref name="pmid26002035">{{cite journal |vauthors=Funck-Brentano T, Cohen-Solal M |title=Subchondral bone and osteoarthritis |journal=Curr Opin Rheumatol |volume=27 |issue=4 |pages=420–6 |date=July 2015 |pmid=26002035 |doi=10.1097/BOR.0000000000000181 |url=}}</ref>.
== Synovitis–Capsulitis ==


== Hypoxia ==
== Hypoxia ==
Neovascularization in synovial membrane, subchondral bone, and cartilage is a common finding in OA. Neovascularization in the injured area increases the nutrients delivery of to the stressed articular cartilage and subchondral and also could cause the synovitis development in bone. Hypoxia as a common pathophysiological element of OA and rheumatoid arthritis because during OA, cartilage thinning and cartilage erosion, ECM composition changes, and the cartilage fissures development are the most common findings in involved joint. These structural alteration influence the oxygen gradient near the articular cartilage. In OA and rheumatoid arthritis, the two important angiogenic peptides including vascular endothelial growth factor and platelet-derived cellular endothelial growth factor. The increases due to the excessive expression of nuclear hypoxia-inducible factors. These angiogenic peptides increase local neovascularization and increase vascular permeability, consequently causing inflammation, cartilage damage, edema, and protein vascular leak that worsen the joint involvement <ref name="pmid24069595">{{cite journal |vauthors=Maldonado M, Nam J |title=The role of changes in extracellular matrix of cartilage in the presence of inflammation on the pathology of osteoarthritis |journal=Biomed Res Int |volume=2013 |issue= |pages=284873 |date=2013 |pmid=24069595 |pmc=3771246 |doi=10.1155/2013/284873 |url=}}</ref><ref name="pmid10525481">{{cite journal |vauthors=Poole AR |title=An introduction to the pathophysiology of osteoarthritis |journal=Front. Biosci. |volume=4 |issue= |pages=D662–70 |date=October 1999 |pmid=10525481 |doi= |url=}}</ref>.


== Body Mass Index/Leptin ==
== References ==
 
== Vascular Biology of Subchondral Bone in OA ==
 
== Osteoblasts Recognize and Respond to Altered Perfusion a ==
 
== Hereditary Osteoarthritis ==
 
==References==
{{reflist|2}}
{{reflist|2}}



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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Mohammadmain Rezazadehsaatlou [2], Irfan Dotani [3]

Overview

Osteoarthritis (OA) is a well-known degenerative joint disease influencing millions of people worldwide. Osteoarthritis is a complex disease caused by changes in the tissues' homeostasis of articular cartilages and subchondral bones. The cell/extracellular matrix (ECM) and their interactions play an important role in the pathophysiology of articular cartilage and the occurrence of Osteoarthritis. Consequently, the main feature of OA is that after this process is involved, the articular cartilages of the involved joint no longer will have a normal acting system because the destruction of the articular cartilages can no longer act as shock absorber.

Pathophysiology

Different pathogenic mechanisms have been proposed to be responsible for the occurrence of OA. Heredity, obesity, hypoxia, synovitis–capsulitissubchondral bone overload, joint instability (mechanical integrity disturbances) are the most important underlying causes in this regard. In the current pathogenesis of osteoarthritis (OA), all joint tissues including cartilage, bone, synovium, ligamentous capsular structures, and surrounding muscle are involved. OA is characterized by structural changes such as active bone remodeling, synovial inflammation, and articular cartilage degradation leading to the loss of joint function and angular deformity or malalignment. Also, a variety of biomarkers in synovial fluid have helped to create more clear insight into the biological response of joints to injury. However, no biomarker has been declared to be reliable for monitoring the development, progression, and response to therapy of OA. Its been reported that certain factors can increase the risk of the OA development such as hereditary elements, trauma and mechanical stress, joint injury, age, obesity, physical activity, bone mineral density (BMD), and congenital anomalies. During the last years, signaling pathways have drawn a lot of attention and proven that these pathways play important roles in inflammation and in the remodeling of the subchondral bone, synovium, enzyme activation, and extracellular matrix degradation in articular cartilage [1] [2] [3][4].

Subchondral Bone

OA leads to the sub-chondral bone remodeling. This event is often along with the sub-chondral cysts formation as a result of focal resorption. OA can alter chondrocyte metabolism in bony cells. Osteoarthritis, influencing whole joint systems, includes both articular cartilage and underlying bone structures. One of the most common findings in OA is the subchondral bone plate thickening. The diseased bone becomes brittle and sclerotic, and the frequent turnovers affect bone quality. There is still controversy about whether the subchondral bone change happens simultaneously with the changes in articular cartilage or not. Articular overgrowths such as subchondral bone lead to microtrauma, hardening, remodeling, and displacement of the osteochondral line. Consequently, the energy-dissipation capacity and elasticity of the articular cartilage decrease. Macroscopic changes of the subchondral bone especially in load-bearing areas are increased osteogenetic reactions, increased stiffness, increased density, and excessive formation of bone and cartilage (called osteochondrophytes). OA is also capable of influencing the non–weight-bearing joints, such as hands, spine, shoulders, and temporomandibular joints. The osteochondrophytes usually can be found in intra-articular, marginal, extraarticular, insertional, or enthesiophytes. The osteochondrophytes frequently involves the joint space, and with synovial metaplastic fragments or flaps of cartilage, they lead to the articular ‘joint mice’ formation. On the other hand, the bone remodeling caused by microfractures within the superficial bone trabeculae with the formation of subchondral bone cysts (known as erosive alterations). Bony changes such as sclerosis of the subchondral bone plate, alterations in trabecular structure, osteophytes and bone marrow lesions are associated with the initiation and progression of OA. It’s been reported that the subchondral bone changes prior to the articular cartilage changes. Meanwhile, it’s been found that the molecular pathways (for example, cytokines such as IL-1, TNF-α, fibrinolytic system including plasminogen, tissue plasminogen activators, urokinase plasminogen activators, and plasmin) have in subchondral play important roles in the disbalance between the physiological connection of bone deposition and remodeling and resorption potential. Higher osteoblastic activity results in an exaggerated reparative response. In contrast, an increased osteoclastic degradative activity results in a predominantly erosive bony condition[5][6][7][8][9][10][11].

Articular Cartilage

The articular cartilage damage is one of the most important pathological causes of OA. It is not clear whether this pathological event originates from the cartilage or subchondral bone, loss and/or damage to articular cartilage or both of them are responsible for the development and progression of OA. Human articular cartilage system, acting as a shock absorber, consists of a hydrated extracellular matrix (as the functional elements of the tissue) with few numbers of chondrocytes within. 70–80% of cartilage consists of water and collagens and proteoglycans are the major organic components. Collagen type II builds a network of fibers containing molecules within. Collagen type XI helps collagen type II in fibril network formation and also limiting the fiber diameter. Collagen type IX make crosslinks the whole collagen network. Heparan sulfate proteoglycans such as perlecan, have important roles (such as interactions with heparin-binding growth factors like fibroblast growth factors, heparin binding forms of vascular endothelial growth factor (VEGF), and bone morphogenetic proteins (BMP)) in chondrogenesis. Higher demolition of heparan sulfate proteoglycans by glycosidases and matrix metalloproteinases are known to responsible for OA. Calcification and ossification in articular cartilage during OA and aging occurs due to the differentiation of chondrocytes. During the degenerative changes in involved joints, calcification happens simultaneously with to increasing alkaline phosphates and pyrophosphate levels.  Since the Articular cartilage has no internal vascular or lymphatic supply system so it is dependent on near tissues including subchondral bone and synovial membrane in receiving nutrients elements and excretion of products of made by articular matrix turnover and chondrocyte metabolism [12][13][1][9][14][15][16].

Synovial Membrane

The main task of Synovial Membrane is repairing any defects found in joint. The cellular compartment of the synovial membrane of the SM is a major source of synovial fluid. These components are responsible for management of chondrocyte activities and maintaining the integrity of articular cartilage surfaces (using lubricin and hyaluronic acid molecules) in diarthrodial joints. After a joint injury, the concentration of this molecular system changes. During the progression of OA, this synovial membrane changes into the main origin of proinflammatory and catabolic products such as metalloproteinases and aggrecanases. Thus, any damage to the synovial membrane can result in reducing of cartilage-protecting factors, and also an increase in production of articular matrix degradation factors. A normal synovial membrane has full control on the transmitted molecules in and out of the joint space. During some conditions such as trauma, inflammation, and OA this permeability of synovial membrane disrupts leading to reduced concentrations of lubricin and hyaluronic acid[17][18][19][20].
A model of Toll-like Receptor (a) and complement activation (b) in the joint leading to synovitis and potentiation of cartilage erosion in OA
.

Joint Instability

Joint instability occurs due to the ligament laxity enhancement, poor muscles conditions, or ligament tearing or strain in a ligament or abnormal muscles status. Joint instability increases the incidence of OA. Joint instability could be found as a result of synovitis produces excessive amounts of synovial fluid[21][22].

Hypoxia

Neovascularization in synovial membrane, subchondral bone, and cartilage is a common finding in OA. Neovascularization in the injured area increases the nutrients delivery of to the stressed articular cartilage and subchondral and also could cause the synovitis development in bone. Hypoxia as a common pathophysiological element of OA and rheumatoid arthritis because during OA, cartilage thinning and cartilage erosion, ECM composition changes, and the cartilage fissures development are the most common findings in involved joint. These structural alteration influence the oxygen gradient near the articular cartilage. In OA and rheumatoid arthritis, the two important angiogenic peptides including vascular endothelial growth factor and platelet-derived cellular endothelial growth factor. The increases due to the excessive expression of nuclear hypoxia-inducible factors. These angiogenic peptides increase local neovascularization and increase vascular permeability, consequently causing inflammation, cartilage damage, edema, and protein vascular leak that worsen the joint involvement [23][24].

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