An update on the pathophysiology of joint disease (Sponsored by Iams)

Joints are highly specialized organs allowing repetitive pain-free and largely frictionless movements.1 A joint consists of bony epiphyses covered with cartilage and surrounded by a synovial membrane and the adjacent soft tissues. The surrounding ligaments, fibrous joint capsule, muscles, and tendons contribute to the integrity and movement of the joint. Muscle mass is also present to provide concussive cushioning and to limit joint movement so that it does not exceed safe anatomical boundaries. The ligaments, tendons, and muscles contain mechanoreceptors that provide proprioceptive input to allow fine motor control and prevent injury.2 Any disruption to the mechanoreceptor input, for example a deep bruise causing some bleeding into a neighboring muscle, will decrease fine motor control and predispose the joint to overload.3

Inside the joint itself, the synovium is a complex structure, with the inner lining of cells producing viscous joint fluid. Interspersed throughout the synovium are cells that have immune function, and so the synovium contributes to the inflammation that occurs in joint disease.4

Traumatic joint disease can occur following trauma from an abnormal load on a normal joint1 (a single event or repetitive microtrauma in an obese animal) or because a joint is abnormal and cannot withstand normal loads.1 An abnormal joint can arise from developmental factors (e.g. elbow incongruency), genetic factors (e.g. slope of the tibia and femur causing an upright knee prone to more concussive forces), infection (e.g. Lyme disease), immune-mediated arthropathies, and biomechanical factors such as poor muscle support.

Basic structure of articular cartilage

The architect of cartilage is the chondrocyte, which produces the extracellular matrix. The matrix is composed of glycosaminoglycans (hyaluronan and proteoglycan) and collagens (mainly type II). The collagen forms a dense network that retains the proteoglycan. The proteoglycan is highly charged and attracts water into the tissue. Thus cartilage is 75% water.

In normal cartilage, there is a very slow turnover of collagens but the proteoglycan is constantly being renewed. The proteoglycans are aggregated into large molecules ("aggrecan") with a protein core and many side chains of keratan sulfate and chondroitin sulfate. This core is in turn bound to hyaluronan chains, with each chain containing many proteoglycan molecules. Aggrecan and water provide the compressive stiffness to the tissue whereas collagen provides the tensile strength.

What is osteoarthritis?

Osteoarthritis (OA) is one of the most common diseases of dogs. Estimates suggest that 20% of the canine population is affected by OA,5 and therefore the disease's impact is very large. OA is a disease of the whole joint — the articular cartilage, bone, and synovium. The relationship between the pathology in each of these tissues is poorly understood. The articular cartilage has received most attention from researchers. Although joint biomechanics undoubtedly play an important role in disease initiation and progression, biochemical changes occur in all joint tissues and contribute to joint failure.

OA is a heterogeneous disease and assessment of the disorder is difficult. The poor correlation between radiographic and clinical data highlights this difficulty. In dogs, a typical example of this is the dysplastic hip with secondary OA — severe radiographic changes are present in a clinically silent joint. Expression of different facets of the disease seems to vary between individuals and even between different joints in the same individual. In small-animal medicine this is exemplified by differences in osteophyte expression, which clearly do not tally with the clinical picture. Most orthopedic experts agree with a model of OA that incorporates the heterogenic nature of OA and how various contributing factors interact in its development and progression. It is helpful to think of OA as a disease process rather than a disease entity.

Disease mechanisms of OA

One major target in OA therapy is effective disease-modifying agents that might slow the degradation of articular cartilage. With this in mind, the following section highlights some of the advances in the understanding of the pathogenesis of OA. In gaining a handle on the cellular and intercellular processes at work, we can understand how drugs might be designed to interrupt or slow down the destructive processes or stimulate repair mechanisms. Cartilage is the main tissue affected in OA, but the subchondral bone and synovium are also affected and may be important in disease progression.

One mechanism behind OA is repetitive biomechanical overload, as can occur in the obese patient. A study in people showed a correlation between high body mass index and the probability of sustaining an injury.6 Overweight people7 and dogs8-10 are significantly more likely to develop OA than are individuals of normal weight. Possible causes are not only the increased load on joints but the relative inactivity in overweight patients causing muscle loss and decreased joint protection. Adipose tissue is also pro-inflammatory and can release inflammatory cytokines into the circulation.11

In human OA, it has been shown that obesity is a major contributor to progression of knee OA, but this is not the case for hip OA. The variability of risk factors for canine OA at different sites is not known. However, anecdotally, most obese dogs seem to improve if weight is reduced. There is limited published information on the effect of weight loss on clinical signs of OA, but that which is available suggests a positive effect.12 There is great potential for targeting this population with improved nutrition and physical rehabilitation. In fact, exercise training has been shown to inhibit inflammation in adipose tissue.13

Pathologic changes of OA

The morphologic changes seen in OA include:

• Cartilage loss, especially in areas of increased load

• Subchondral bone remodeling (loss of bone initially followed by sclerosis)

• Marginal osteophytosis

• Variable synovial inflammation.

The biochemical changes in the cartilage include:

• Loss of proteoglycan

• Upregulation in the degradative and synthetic activities of chondrocytes

• Disruption of the collagen network

• Increase in water content.

These changes reduce the elasticity of the cartilage, leading to fibrillation and fissuring of the cartilage and eventual loss of tissue. If this continues, eburnation of subchondral bone may result.

Intercellular events in OA

In cases of OA, the chondrocytes themselves are upregulated and the rates of proteoglycan synthesis and degradation are increased with the overall balance towards matrix depletion. It seems likely that the chondrocyte activity is increased following the binding of cytokines to the cell surface. Cytokines are cellular messengers produced locally in the tissues in response to various biologic stimuli, such as inflammation. It is proposed that the cytokines responsible for stimulating cartilage degradation in OA are interleukins 1 and 6 (IL-1 and IL-6), tumor necrosis factor-α (TNF-α), and oncostatin M.

Binding of these cytokines to the chondrocyte stimulates the production of enzymes that we know can degrade all the components of the cartilage matrix. Synovial cells also release natural inhibitors of these cytokines, such as IL-1 receptor antagonist (IL-1ra). There are also cytokines that stimulate synthesis of matrix; likely candidates for this include the insulin-like growth factors I and II (IGF-I and IGF-II) and transforming growth factor-ß (TGFß). Studies have demonstrated that the normal availability of IGF-I and IGF-II may be limited in patients with OA.14

Researchers have demonstrated an association between lipid composition and tissue pathology in OA articular cartilage.15 Specifically, this study indicated that the severity of OA cartilage lesions is linked to a higher proportion of the n-6 fatty acid, arachidonic acid. Two of the local eicosanoid hormones produced from n-6 fatty acids — prostaglandin E2 (PGE2) and leukotriene B4 (LTB4) — are considered key mediators of inflammation in arthritis. In addition, consistent with the previous study, another study showed that PGE2 levels produced spontaneously by OA cartilage are highly elevated.16 The levels were found to be elevated even relative to healthy cartilage stimulated directly by the pro-inflammatory cytokines that promote cartilage degradation. This latter study also revealed that COX-2, which is not expressed constitutively is upregulated in chondrocytes from cartilage affected by OA as well as cartilage affected by rheumatoid arthritis, lending further support to the hypothesis that OA has a strong inflammatory component.

Other studies17,18 have provided direct evidence that n-3 fatty acid supplementation can reduce or abrogate the inflammatory and matrix degradative response elicited by chondrocytes during OA progression. As n-3 and n-6 fatty acids become incorporated into cell membranes, their ratio within the membrane determines whether the cell is primed for a pro-inflammatory response (high n-6) or not. By preincubation of chondrocytes with different fatty acids, researchers17 showed that this membrane ratio could be modified to render the cell less pro-inflammatory, concomitant with downregulation of COX-2, chondrocyte-derived proinflammatory cytokine mRNA and cytokine-induced matrix degradation. This study was extended18 to show that OA cartilage proteoglycan and collagen degradation and inflammation could also be reduced by exposure to n-3 fatty acids.

The abrogation of expression of key mediators of cartilage degradation in OA (such as MMP-3, MMP-13, COX-2, 5-lipoxygenase, TNF-α, and IL-1) suggests that n-3 fatty acid supplementation, resulting from dietary intake of fish oil, may be an extremely powerful approach to alleviating clinical signs and progression of OA. Thus, taking a systemic approach, the imbalance of fatty acid intake may place an individual in a proinflammatory state. Not only are n-6 polyunsaturated fatty acids (PUFAs) like arachidonic acid preferentially metabolized by COX, but n-6 PUFA-derived eicosanoids are more efficacious receptor agonists.

Cartilage degradation

Catabolic cytokines can stimulate the chondrocyte to produce and release degradative enzymes. The enzymes studied in most detail in this respect are the matrix metalloproteinases (MMPs) and the family of endopeptidases known as the ADAMTS-4 and -5 (A disintegrin and metalloproteinase with a thrombospondin motif). ADAMTS-4 and -5 are also known as aggrecanases. MMPs and aggrecanases can cleave the protein core of aggrecan so as to release most of the molecule from the matrix.19 Under normal circumstances, the chondrocyte also produces a natural inhibitor of these enzymes known as tissue inhibitor of metalloproteinase (TIMP). TIMP production appears to be decreased in OA.

Enzymatic degradation appears to play a central role during the OA disease process. The destruction of articular cartilage and the loss of its biomechanical function begin at the surface and the area that immediately surrounds the chondrocytes.20 These pathologic changes are caused by an imbalance between degradation and synthesis of matrix components by the chondrocytes.21 The fibrillar part of the matrix is composed mostly of type II collagen.22 The non-fibrillar component contains large aggregates of highly sulfated aggrecan monomers attached to hyaluronic acid via link protein.23 The tensile strength of the matrix comes from the collagen network, which limits expansion of the aggrecan component and so provides compressive stiffness of the tissue.24 The aggrecan-hyaluronan aggregates are responsible for the elasticity of the tissue by binding high amounts of intercellular water. Therefore, the cartilage matrix is compliant under compression, but rapidly expands upon unloading as water molecules are pulled back into the matrix.25 In the early stages of osteoarthritis, the cartilage surface shows roughening, which in the later stages evolves into overt collagen breakdown and matrix loss, until the subchondral bone is exposed. Degradation of molecular components occurs as well as destabilization of the collagen network.1

Characteristic for the early stages of cartilage degeneration is the loss of aggrecan and its fixed negative, hydrophilic charges. The general collagen content has been said to remain stable until the very end of the disease process.26 Studies involving molecular aging of cartilage reveal that the matrix components of arthritic joints undergo dynamic change and turnover and different protein pools exist in articular and meniscal cartilages turning over at different rates.27 It is unknown what happens first — the loss of proteoglycans or the loosening of the collagen network, as each affects the other. Loosening of the collagen network leads to a loss of proteoglycans and a loss of proteoglycans leads to a mechanical overload and, thus, damage and loosening of the collagen network.

During the OA disease process, the chondrocytes' cellular reaction patterns are altered. The number of chondrocytes changes, through cell death or proliferation to compensate for cell loss or to increase synthetic activity. Along with this, there is a great alteration in gene expression profile of the cells in the diseased tissue. By enhancing their anabolic activity, chondrocytes attempt to repair the damaged matrix. Type II collagen expression appears to be much more upregulated than aggrecan,28 which indicates a return to the characteristics of fetal cartilage.29 In fetal cartilage, chondrocytes have to synthesize and construct a new extracellular matrix consisting largely of collagen type II. In contrast, in normal articular cartilage, chondrocytes only need to control tissue homeostasis by maintaining a stable matrix. This mainly involves the control of proteoglycan turnover; collagen type II turnover is negligible.30 It is possible that cartilage stem cells could aid the production of type II collagen and thus some recovery.31 Despite the chondrocytes' increased biosynthesis in arthritic cartilage, proteoglycan content is still lost.32 Therefore we can assume that enzymatic degradation of matrix components is responsible for the changes and this gives us another therapeutic target.

Changes occurring in the synovial membrane are largely neglected as therapeutic targets. Instead, they are simply interpreted as secondary to the degeneration of the articular cartilage. However, the synovial capsule and lining cells represent critical joint components. As pointed out earlier, the mechanical stability is provided by the capsule and surrounding ligaments. In late-stage OA, thickening of the collagen network within the joint capsule significantly reduces joint movement, resulting in stiffness. Decreased joint motion inhibits circulation of synovial fluid and further decreases joint health.33 This represents a therapeutic target that can be treated with physical rehabilitation.

The synovium lines the outer edges of the joint, and a fold of synovium (plica) extends onto the surface of the articular cartilage. Synovium also surrounds intraarticular ligaments. This synovium contains mechanoreceptors important for proprioceptive input and this input is altered by inflammation, further predisposing the joint to injury and abnormal function.34 The synovium contains many nerve endings, some of which carry pain signals and can contribute to inflammation.35 When the synovium becomes thickened (hyperplastic) in joint disease, the synovial fold or plica can become trapped between the bone surfaces, which can cause significant pain.36 Firing of pain nerves causes neurogenic inflammation.37 Several studies have shown that cases of clinically significant osteoarthritic joint disease are almost always associated with some significant synovial pathology.38 Synoviocyte activation, proliferation, and synovial hyperplasia all represent reactive changes to elevated demands for clearance of molecular debris coming from cartilage degradations and floating in the joint's synovial fluid.39

The proliferation and activation of synovial lining cells can cause significant problems for the articular cartilage, as these cells can secrete matrix-degrading proteases (MMPs) and catabolic cytokines (IL-1, TNF-α).40,41 It can be speculated that the cartilage matrix catabolism might be induced or promoted by catabolic mediators, such as interleukin-1ß (IL-1ß) and tumor necrosis factor – α (TNF-α), secreted by the activated synoviocytes. These inflammatory cytokines are top candidates for therapeutic intervention, because both are not only able to downregulate matrix anabolism in articular chondrocytes,42 but also induce expression and secretion of matrixdegrading proteases.42 Therefore, the inflammatory and degradative activity of synoviocytes could be a therapeutic target for even early-stage disease.43

One basis for the use of nonsteroidal anti-inflammatory drugs (NSAIDs) in cases of OA is that synovial inflammation may be decreased. NSAIDs are commonly used postoperatively to treat joint inflammation, yet it is common for NSAID therapy to be stopped seven to 14 days after surgery. This is probably a mistake for orthopedic patients, since pain and inflammation persist during this period, despite the apparent clinical improvement.

Follow-up arthroscopic evaluation of the elbow, stifle, and hip has shown sustained synovitis in dogs having undergone surgery for elbow dysplasia, cranial cruciate ligament rupture, and triple pelvic osteotomy (Beale BS, Unpublished data). NSAID administration for six to eight weeks postoperatively reduces synovitis, leading to less pain and a patient that returns to function more quickly (Figure 1). Patients that receive NSAIDS for six to eight weeks after surgery return to activity sooner, and this improves muscle mass and joint range of motion. Patients feel better if muscle mass is preserved and if joints are not stiff. Physical rehabilitation is also an important part of this period for the same reasons. Physical rehabilitation improves the general attitude of the patient as well.

Figure 1. This endoscope image shows synovitis is present eight weeks after surgical stabilization of a torn cranial cruciate ligament, despite apparent clinical improvement. This patient would have benefitted from six to eight weeks of NSAID administration after surgery.

Bone changes

So far, it is unclear whether pathologic changes within the subchondral bone tissue (e.g. sclerosis) can precede changes in the articular cartilage. There is some evidence that subchondral bone morphometry predicts subsequent cartilage loss.44 Increased thickness of the subchondral bone plate and alterations in underlying trabeculae are already apparent in early stages of arthritis. In moderate to advanced lesions, the changes in the subchondral bone contribute to joint pain.45 Hence, the osteoarthritic bone represents an interesting target tissue for symptomatic treatment. Additionally, in earlier disease stages, modification of bone remodeling might prevent subchondral alterations, thereby slowing cartilage destruction and possibly inhibiting osteophyte formation.46

Conclusion

By appreciating the cellular and intercellular processes at work in arthritic joints, we can better understand how therapies might be designed to interrupt or slow down the destructive processes and stimulate repair mechanisms.

Brian S. Beale, DVM, DACVS, Gulf Coast Veterinary Specialists, Houston, TX 77027.

Julia Tomlinson, BVSc (hons), MS, PhD, DACVS, CCRP, CVSMT, Twin Cities Animal Rehabilitation, 12010 Riverwood Drive, Burnsville, MN 55337.

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