Osteoarthritis (OA), the most common form of arthritis, is a chronic disease characterized by the slow degradation of cartilage, pain, and increasing disability. The disease can have an impact on several aspects of a patient's life, including functional and social activities, relationships, socioeconomic status, body image, and emotional well-being 1. Currently available pharmacological therapies target palliation of pain and include analgesics (i.e. acetaminophen, cyclooxygenase-2-specific inhibitors, nonselective nonsteroidal anti-inflammatory drugs, tramadol, opioids), intra-articular therapies (glucocorticoids and hyaluronan [hyaluronic acid] [HA]), and topical treatments (i.e. capsaicin, methylsalicylate) 2.

Intra-articular treatment with HA and hylans (see Table 1 for definitions) has recently become more widely accepted in the armamentarium of therapies for OA pain 2. HA is responsible for the viscoelastic properties of synovial fluid. This fluid contains a lower concentration and molecular weight (MW) of HA in osteoarthritic joints than in healthy ones 3. Thus, the goal of intra-articular therapy with HA is to help replace synovial fluid that has lost its viscoelastic properties. The efficacy and tolerability of intra-articular HA for the treatment of pain associated with OA of the knee have been demonstrated in several clinical trials 4567891011121314. Three (hylan G-F 20) to five (sodium hyaluronate) injections can provide relief of knee pain from OA for up to 6 months 6711. Intra-articular hylan or HA is also generally well tolerated, with a low incidence of local adverse events (from 0% to 13% of patients) 5681112 that was similar to that found with placebo 611.

Because the residence time of exogenously administered HA in the joint is relatively short, HA probably has physiological effects in the joint that contribute to its effects in the joint over longer periods. The exact mechanism(s) by which intra-articular HA or hylans relieve pain is currently unknown. Improvements in OA with administration of HA have been shown in both electrophysiology and animal pain model studies 151617; Gomis A, Pawlak M, Schmidt RF, Belmonte C: Effects of elastoviscous substances on the mechanosensitivity of articular pain receptors. Presented at the Osteoarthritis Research Society International World Congress on Osteoarthritis, September 2001, Washington, DC, USA]. HA treatment has also been shown to have protective effects on cartilage in experimental models of OA 181920. In vitro studies also show that HA has beneficial effects on the extracellular matrix, immune cells, and inflammatory mediators 212223242526. This article provides a brief introduction to the pathophysiology of OA and reviews the current scientific literature regarding the physiological effects of HA and hylans, focusing on antinociceptive effects, possible protective effects on cartilage, and effects on molecular and cellular factors involved in OA disease progression. The effects of HA and hylans on these factors may provide insight into the mechanism by which HA and hylans elicit their clinical benefits.

Relevant literature was identified by searching MEDLINE from 1966 through July 2002. The following search words were used alone and in combination when appropriate: hyaluronan, hyaluronic acid, sodium hyaluronate, hylan, OA, knee, cartilage, synovium, pathophysiology, extracellular matrix, proteoglycans (PGs), aggrecanase, inflammation, immunology, proteases, matrix metalloproteinases (MMPs), cytokines, proinflammatory mediators, nitric oxide (NO), prostaglandins, lymphocytes, nociceptors, and mechanoreceptors. Additional references were located by consulting the bibliographies of MEDLINE sources.

OA is characterized by a slow degradation of cartilage over several years. In normal cartilage, a delicate balance exists between matrix synthesis and degradation; in OA, however, cartilage degradation exceeds synthesis. The balance between synthesis and degradation is affected by age and is regulated by several factors produced by the synovium and chondrocytes, including cytokines, growth factors, aggrecanases, and MMPs 272829303132 (Fig. 1).

In addition to water, the extracellular matrix is composed of PGs entrapped within a collagenous framework or fibrillary matrix (Fig. 2) 33. PGs are made up of glycosaminoglycans attached to a backbone made of HA 33. In OA, the collagen turnover rate increases, the PG content decreases, the PG composition changes, and the water content increases 33. The size of HA molecules 3 and their concentration 34 in synovial fluid also decrease in OA. A significant PG in articular cartilage is aggrecan, which binds to HA and helps provide the compressibility and elasticity of cartilage 32. Aggrecan is cleaved by aggrecanases, leading to its degradation and to subsequent erosion of cartilage 3435. The loss of aggrecan from the cartilage matrix is one of the first pathophysiological changes observed in OA 32.

Cytokines produced by the synovium and chondrocytes, especially IL-1 and tumor necrosis factor alpha (TNF-α), are also key players in the degradation of cartilage 29. IL-1β is spontaneously released from cartilage of OA but not normal cartilage 36. Both IL-1β and TNF-α stimulate their own production and the production of other cytokines (e.g. IL-8, IL-6, and leukotriene inhibitory factor), proteases, and prostaglandin E₂ (PGE₂) 30. Synthesis of the inflammatory cytokines IL-1 and TNF-α and expression of their receptors are enhanced in OA 293031. Both cytokines have been shown to potently induce degradation of cartilage in vitro 31. Other proinflammatory cytokines overexpressed in OA include IL-6, IL-8, IL-11, and IL-17, as well as leukotriene inhibitory factor 30. The production of the chemokine RANTES (regulated upon activation, normal T-cell expressed and secreted), is also high in OA cartilage compared with normal cartilage, is stimulated by IL-1, and increases the release of PGs from cartilage 37.

Prostaglandins and leukotrienes may also be involved in cartilage destruction in OA. PGE₂ is spontaneously produced by OA cartilage 38 and leukotriene B4 is elevated in the synovial fluid of OA 36. Although IL-1β stimulates the release of PGE₂ 39, the role of PGE in cartilage biology is unclear, since studies show both anabolic and catabolic effects of PGE on cartilage 38.

The extracellular matrix in cartilage is degraded by locally produced MMPs. Elevated levels of stromelysin (MMP-3), collagenases (MMP-1, -8, and -13), and gelatinases (MMP-2 and -9) have also been found in chondrocytes or the articular cartilage surface in OA 2931. The activity of many MMPs increases in OA by either an increase in their own synthesis, an increased activation by their proenzymes, or decreased activity of their inhibitors 29. Proinflammatory cytokines, including IL-1, TNF-α, IL-17, and IL-18, increase synthesis of MMPs, decrease MMP enzyme inhibitors, and decrease extracellular matrix synthesis 29. To further exacerbate the degradative activity in OA, expression levels of tissue inhibitor of metalloproteinases (TIMP)-1 are reduced 29.

In an attempt to reverse the breakdown of the extracellular matrix, the chondrocytes increase synthesis of matrix components including PGs 29. Even though this activity increases, a net loss of PG in the upper cartilage layer is seen, because the increased activity has been observed only in the middle and deeper layers of cartilage 29. Elevated anti-inflammatory cytokines found in the synovial fluid of OA include IL-4, IL-10, and IL-13 30. Their role is to reduce production of IL-1, TNF-α, and MMPs, increase TIMP-1, and inhibit prostaglandin release 3240. Local production of growth and differentiation factors such as insulin-like growth factor 1, transforming growth factors, fibroblastic growth factors, and bone morphogenetic proteins also stimulate matrix synthesis 2941.

The production of NO, another inflammatory mediator synthesized by the cartilage in OA and well documented in experimental OA, is stimulated by the proinflammatory cytokines IL-1 and TNF-α 29303136. NO may be involved in cartilage catabolism by inhibiting the synthesis of collagen and PG, enhancing MMP activity, reducing the synthesis of an IL-1 receptor antagonist by chondrocytes, and increasing susceptibility to cell injury (i.e. apoptosis) 303642. NO can also inhibit the attachment of fibronectin to chondrocytes, thus enhancing PG synthesis 42.

Additionally, NO can induce apoptosis of chondrocytes in OA 30. Chondrocyte apoptosis occurs in both human and experimental OA and is correlated with the severity of cartilage destruction 42. Apoptosis of chondrocytes in OA has been shown to have a higher incidence in OA than in normal cartilage, to be present close to the articular surface, and to be significantly correlated with OA grade 4344. Death of chondrocytes could easily lead to reduced matrix production, since chondrocytes are the only source of matrix components and their population is not renewed 29. Depletion of PGs was observed in cartilage areas that contained apoptotic chondrocytes 43. Cellular products of apoptosis may also contribute to the pathophysiology of OA, because apoptotic cells are not effectively removed from cartilage 29 due to its avascular nature and can cause pathogenetic events such as abnormal cartilage calcification or extracellular matrix degradation 43.

HA is responsible for the viscoelastic quality of synovial fluid that acts as both a lubricant and shock absorber 3. In synovial fluid, HA coats the surface of the articular cartilage and shares space deeper in the cartilage among collagen fibrils and sulfated PGs 3. In this respect, HA probably protects the cartilage and blocks the loss of PGs from the cartilage matrix into the synovial space, maintaining the normal cartilage matrix 3. Similarly, HA may also help prevent invasion of inflammatory cells into the joint space.

In acute and chronic inflammatory processes of the joint, the size of HA molecules decreases at the same time as the number of cells in the joint space increases 3. In synovial fluid from knee joints in OA, concentrations of HA, glycosaminoglycans, and keratan sulfate are lower than in synovial fluid from normal knee joints 34. Additionally, experiments using rabbit synovial cells showed that the proinflammatory cytokines IL-1 and TNF-α stimulate the expression of HA synthetase 45, which may contribute to the fragmentation of HA under inflammatory conditions.

Exogenous HA may facilitate the production of newly synthesized HA. When synovial fibroblasts from OA knees were cultured with HA formulations of various MWs (3.4 × 10⁵ to 4.7 × 10⁶), the amount of newly synthesized HA in response to the exogenous HA was both concentration- and MW-dependent 21. Higher-MW agents stimulated the synthesis of HA more than lower-MW formulations and an optimal concentration was noted for each MW 21.

HA in the synovial fluid binds to chondrocytes via the CD44 receptor 4647, supporting a role for HA in healthy cartilage. The primary means of retention and anchoring of PG aggregates to chondrocytes is the CD44 HA receptor 48. When expression of CD44 was suppressed in bovine articular cartilage slices, a near-complete loss of PG staining was observed 48. A similar decrease in PG staining was found when very small HA molecules were used to block the binding of HA to the CD44 receptor 47. CD44 adhesion to HA has also been shown to mediate chondrocyte proliferation and function 49.

Relief of knee pain from OA with HA in clinical studies may be due to the effects of HA on nerve impulses and nerve sensitivity. Inflammation of the knee joint influences excitability of nociceptors of articular nerves 15. In experimental OA, these nerves become hyperalgesic, spontaneously discharge, and are sensitive to non-noxious joint movements 15. Administration of HA to isolated medial articular nerves from an experimental model of OA significantly decreased ongoing nerve activity as well as movement-evoked nerve activity 15. In another model, nerve impulses evoked by movement of an inflamed knee were significantly reduced with hylan G-F 20 to about 60% of that of the controls (Gomis A, Pawlak M, Schmidt RF, Belmonte C: Effects of elastoviscous substances on the mechanosensitivity of articular pain receptors. Presented at the Osteoarthritis Research Society International World Congress on Osteoarthritis, September 2001, Washington, DC, USA). These authors reported that HAs with lower MWs had either less of an effect or no effect on nerve impulse frequency. Impulse discharge and firing frequency of activated nerve sensory fibers decreased to 65% and 45% of that of control values, respectively, when hylan was administered 50. Mechanical forces on stretch-activated ion channels are involved in depolarization of the articular nerve terminal. In the presence of hylan, these ion channels also have decreased mechanical sensitivity (de la Peña E, Pecson B, Schmidt RF, Belmonte C: Effects of hylans on the response characteristics of mechanosensitive ion channels. Presented at the 9th World Congress on Pain, Vienna, Austria 1999).

In a rat model, HA improved the abnormal gait of rats with experimentally induced OA in a dose-dependent manner, indicating an antinociceptive effect of HA 16. This effect may be mediated through the attenuation of prostaglandin E₂ (PGE₂) and bradykinin synthesis, since HA inhibited their synthesis in a MW-dependent manner 16. Further, HA has been shown to induce analgesia in a bradykinin-induced model of joint pain in rats 17. This analgesic action was also MW-dependent, as significant effects were observed at lower concentrations with a higher-MW formulation than with lower-MW HAs 17.

Lastly, HA may have direct or indirect effects on substance P, which can be involved in pain 51. Since substance P interacts with excitatory amino acids, prostaglandins, and NO, the effects of HA on these factors can indirectly affect the pharmacology of substance P 51. Additionally, HA has been shown to inhibit an increased vascular permeability induced by substance P 51.

Many effects of exogenous HA on the extracellular matrix, inflammatory mediators, and immune cells have been reported in in vitro studies. The influence of HA on these factors may contribute to cartilage protection in OA.

The effects of HA and hylans on cartilage histology are well documented in experimental animal studies, but strong clinical trial data is lacking (Table 5).

The clinical effects of HA on pain associated with OA of the knee are probably mediated by several factors. In vitro and in vivo studies indicate that HA can enhance PG synthesis and prevent its release from the cell matrix. Regarding inflammation, HA suppresses the production and activity of proinflammatory mediators and proteases as well as altering the function of certain immune cells. Histological evidence shows that HA prevents the degradation of cartilage and may promote its regeneration. Collectively, the physiological effects of intra-articular HA reviewed here support a multifactorial mechanism for HA and hylans in the treatment of pain from knee OA. Future studies investigating the effects of HA and hylans on cartilage in well-controlled clinical studies may help determine whether intra-articular HA and hylans aid in chondroprotection and slowing the progression of disease, and whether the MW of the HA formulation contributes to its efficacy.

LWM has been a paid consultant for Wyeth, Genzyme and Sanofi-Synthelab, companies that market intra-articular hyaluronan products.

HA = hyaluronan (hyaluronic acid); IL = interleukin; MMP = matrix metalloproteinases; MP = methylprednisolone; MW = molecular weight; NO = nitric oxide; OA = osteoarthritis; PG = proteoglycan; PGE₂ = prostaglandin E₂; PMN = polymorphonuclear; RA = rheumatoid arthritis; TIMP = tissue inhibitor of metalloproteinases; TNF-α = tumor necrosis factor alpha.