Hyaluronic Acid in the Treatment of Osteoarthritis

            Hyaluronic Acid in the Treatment of Osteoarthritis

                                       DR KS Dhillon

What is hyaluronic acid?

Hyaluronate is a high molecular weight molecule that is naturally present within the cartilage as well as the synovial fluid. It is made of alternating d-glucuronic acid and N-acetyl-d-glucosamine residues attached by β(1–4) and β(1–3) bonds. Its molecular mass ranges from 6500 to 10,900 kDa [1]. 

Its main function in the synovial fluid is to serve as a lubricant, scavenger for free radicals as well as for the regulation of cellular activities such as binding of proteins [1]. Its function in the joint is to lubricate the joint, serve as a space filler to allow the joint to stay open. 

During osteoarthritis (OA) progression the endogenous hyaluronic acid (HA) in the joint is depolymerized from high molecular weight (6500–10,900 kDa) to lower molecular weight (2700–4500 kDa). This depolymerization diminishes the mechanical and viscoelastic properties of the synovial fluid in the affected joint [1,2].

Hence, exogenous HA injections are used to mitigate the macerated functions of the depolymerized endogenous HA in patients with OA [2]. 

The exogenous HA does not replace and restore the full properties and activities of the depolymerized endogenous HA of the synovial fluid. It may, however, induce pain relief via several mechanisms [2]. These mechanisms include the synthesis of glycosaminoglycan and/or proteoglycan, anti-inflammatory effect, and maintenance of viscoelasticity [2]. There is, however, heterogeneity in the therapeutic trajectory for OA patients following HA injections, since some studies report an overall beneficial effect while others report that there is only a small benefit [3].

One of the reasons for the variable effect of HA in the treatment of OA patients is the variable levels of hyaluronidases in a patient’s synovial fluid. Hyaluronidases are enzymes that degrade hyaluronic acid through cleaving the β(1–4) linkages of HA and thereby fracturing the large molecule into smaller pieces before degrading it [4].

Hyaluronic acid preparations for treatment of OA

Two forms of HA are available. One is oral form and the other is the injection form. The injectable form includes several preparations. These include Orthovisc, Euflexxa, Monovisc Gel-Syn, Synvisc, Synvisc-One, Gel-One, Hyalgan, and Supartz FX [5]. 

Each product has several varying characteristics, including the source (animal versus bacterial bio-fermentation), average molecular weight ranging from 500 to 6000 kDa, molecular structure (linear, cross-linked, or both), concentration (0.8–30 mg/mL), the volume of injection (0.5–6.0 mL), as well as dosing [6]. 

Animal source of HA obtained from rooster combs was traditionally used for many years. Now many source HA from bio-fermentation using genetically modified organisms. This modified bacterial source is cheaper and has fewer side effects [7,8].

There is no significant difference in the long-term outcome regardless of the preparation used for injection [9]. There are a variety of different mechanisms for relief of symptoms after injection of HA into the joint [10]. 

These include maintaining cartilage thickness, area, and surface smoothness, reducing the motility of lymphocytes, enhancing the synthesis of extracellular matrix proteins, altering inflammatory mediators in order to shift away from degradation [11].

When oral HA is taken the body absorbs the high molecular weight polymer in the form of decomposed 2–6 membered polysaccharide. The ingested HA binds to Toll-like receptor-4 and thereby promotes the expressions of interleukin-10 and cytokine signaling, both of which lead to reduction of inflammation in the arthritic joint [12]. 

Both the oral HA and the injection HA appear to be effective in combating OA symptoms. Oral HA appears to extend the benefits of injection HA when the two treatments are combined [13].

Mechanism of action of hyaluronic acid (HA)

The predominant mechanism of action of HA for the treatment of pain associated with knee osteoarthritis (OA) remains unknown. 

Studies demonstrate various physiological effects of HA [14]. 

• Reduction of nerve impulses and nerve sensitivity leading to reduction of pain.
• Glycosaminoglycan has protective effects on cartilage. This is mediated by its molecular and cellular effects. 
• Exogenous HA increases the chondrocyte HA and enhances proteoglycan synthesis. It reduces the production and activity of pro-inflammatory mediators and matrix metalloproteinases. It also alters the behavior of immune cells.

Most of the physiological effects of exogenous HA are a function of its molecular weight. 

Role of hyaluronan in the synovial fluid

The viscoelastic quality of synovial fluid that acts as a lubricant and shock absorber is due to HA in the synovial fluid [15]. The concentration of HA in synovial fluid of the knee joint is 2-3mg/ml. The Ha coats the surface of the articular cartilage and also shares space deeper in the cartilage among the collagen fibrils and sulfated PGs [15]. 

The HA probably protects the articular cartilage and blocks the loss of PGs from the cartilage matrix into the synovial space, thereby maintaining the normal cartilage matrix [15]. The HA may also help prevent penetration of inflammatory cells into the joint space.

In patients with joint inflammation, the size of HA molecules decreases, and the number of cells in the joint space increases [15]. The concentrations of HA, glycosaminoglycans and keratan sulfate in the synovial fluid from patients with OA are lower than in synovial fluid from normal knee joints [16]. 

Experiments in rabbits showed that the pro-inflammatory cytokines IL-1 and TNF-α stimulate the expression of HA synthetase [17], which contributes to the fragmentation of HA when there is inflammation in the joint.

Exogenous hyaluronic acid may facilitate the production of newly synthesized HA. Smith and Ghosh [18] cultured synovial fibroblasts obtained from patients with knee OA with HA formulations of various MWs. They found that the amount of newly synthesized HA in response to the exogenous HA was both concentration and MW-dependent. Higher MW agents stimulated the synthesis of more HA than lower MW formulations. and an optimal concentration was noted for each MW [18].

Synovial fluid HA binds to chondrocytes via the CD44 receptor, thereby playing a supporting role in healthy cartilage. 

The CD44 HA receptor helps in the retention and anchoring of PG aggregates to chondrocytes. Adhesion of CD44 to HA also mediates chondrocyte proliferation and function [19].

Hyaluronan and pain relief

HA has an effect on nerve sensitivity and nerve impulses and this is possibly the mechanism of pain relief in patients with knee OA.

Inflammation of the knee joint is known to influence excitability of nociceptors of nerves in the joint [20]. In cats with experimental OA, there was hyperalgesia in intraarticular nerves with spontaneous discharge, and the nerves were sensitive to normal joint movements [20]. Ongoing nerve activity and movement-evoked nerve activity is decreased with HA administration to isolated medial articular nerves in an experimental model of OA [20]. 

HA improved the abnormal gait in rats in whom OA was induced in the laboratory. This reflects the antinociceptive effect of HA [21]. 

HA may also have a direct or indirect effect on substance P, which is involved in pain generation [22]. HA can inhibit an increased vascular permeability induced by substance P [22].

Effects of hyaluronan on the extracellular matrix

In vitro laboratory, experiments show that HA administration can enhance the synthesis of extracellular matrix proteins such as keratin sulphate and chondroitin as well as PGs. Experiments in rabbits show that HA increased the synthesis glycosaminoglycan in rabbit chondrocytes cultured on collagen gels [23]. Release of keratan sulfate into synovial fluid is also suppressed by HA in an ovine model [24].

A clinical study by Creamer et al [25] showed that in patients treated with HA the levels of keratin sulfate in the synovial fluid were lower in more knees as compared to those treated with saline. The difference, however, did not reach clinical significance.

HA has been shown to increase PG synthesis in equine articular cartilage [26], rabbit chondrocytes [27], and bovine articular cartilage [28].

Experiments in rabbits also show that intra-articular administration of higher molecular weight HA is more effective than lower molecular weight HA in inhibiting cartilage degeneration in early OA [27].

In human osteoarthritic chondrocytes, HA alone decreased PG production. HA, however, countered the reduction of PG production induced by IL-1αb  [29]. 

HA suppresses the release of PGs from rabbit chondrocytes [30] and bovine articular cartilage [31] in the absence as well as in the presence of IL-1. 

Experiments show that Hylan inhibits the resorption of PGs from cartilage. High viscosity Hylan is more effective than a low-viscosity Hylan [32]. HA can also suppress the reduction in collagen gene expression induced by IL-1β in rabbit articular chondrocytes [33]. The amount of glycosaminoglycan released was reduced in hyaluronate-treated canine OA joints and there was an increased release in untreated joints [34].

Fragments of fibronectin can bind and penetrate articular cartilage and increase levels of MMPs and suppress PG synthesis [35]. HA can block PG depletion induced by fibronectin fragments [36]. 

Effects of hyaluronan on inflammatory mediators

HA has significant effects on inflammatory mediators such as prostaglandins, cytokines, and proteases. HA alters the profile of inflammatory mediators, resulting in a shift away from cartilage degradation, by altering the balance between cell-matrix synthesis and degradation. 

HA has a chondrostabilizing influence on articular cartilage by down-regulating TNF-alpha. HA exerts its inhibitory influence on TNF-alpha, as well as stromelysin and TNF receptors [37]. 

The cytokine TNF-α and its receptor were not present in canine atrophied articular cartilage treated with HA but were seen in untreated cartilage [37]. 

HA also reduced the expression of IL-1β and stromelysin (MMP-3) in the synovium of rabbits with early OA [38]. These two mediators are known to play a role in cartilage degradation. 

High-MW HA stimulates the production of TIMP-1 in bovine articular chondrocytes. TIMP-1 inhibits MMP [39]. High-MW HA reduced the stromelysin/TIMP-1 ratio leading to a cartilage protective effect [39]. 

HA can also reduce the secreted antigen and activity urokinase plasminogen activator and its receptor in synovial fibroblast in OA and RA patients [40].

Intra-articular administration of HA decreases the urokinase plasminogen activator activity in the synovial fluid of patients who showed improvement in clinical parameters. Hence, a decrease of fibrinolytic activity in synovial fluid is associated with improvement of clinical parameters in patients with OA who are treated with HA [41].

Prostaglandins and other metabolites of arachidonic acid mediate inflammatory responses. HA can reduce arachidonic acid release [42] and IL-1α-induced PGE2 production [43]. These actions of HA are dose and MW-dependent. The higher the MW and concentration, the more potent is the inhibition. 

HA injections reduced levels of prostaglandin F2α, 6-keto-prostaglandin F1α, and leukotriene C4 [44] and also reduced PGE2 levels and stimulated cAMP, thereby producing its anti-inflammatory effect [45,46]. 

HA also has antioxidant effects which were both MW- and dose-dependent [47].

High-MW HA protects against damage to articular chondrocytes by oxygen-derived free radicals, which play a role in the pathogenesis of arthritic disorders [48]. 

Nitric acid (NO) is well known for its role in inflammation. Production of NO is significantly reduced by HA in some tissues [49].

Cartilage effects of HA

There is no strong clinical trial data on the effects of HA and hylans on cartilage histology, although there is well-documented data in experimental animal studies. The protective effect of HA on cartilage has been shown in several animal models of experimental OA. HA has been shown to reduce the severity of OA and also to maintain cartilage thickness and surface smoothness [4]. 

Beneficial effects of HA on cartilage has been shown in rabbits with experimental OA of the knee induced after anterior cruciate ligament transection. The grade of cartilage damage 9 weeks after treatment with HA was less severe in animals treated with HA as compared to those who were not treated with HA [50]. Surface roughness was also significantly less in HA-treated animals when compared to non-HA treated rabbits [50].

Beneficial effects have been seen after 21 weeks [50] and after 6 months [51].

Another study in dogs showed that HA treatment significantly reduced OA progression [52]. 

Clinical studies involving hyaluronic acid use in humans

Over the last several decades there have been clinical trials that investigated the efficacy and safety of HA in the treatment of OA. 

Dan Xing et al [53] carried out a systematic review of overlapping meta-analysis that compared HA and placebo for knee OA. They found that HA is an effective intervention in treating knee OA without an increased risk of adverse events. The authors, however, admitted that there were limitations in their study. There could be variances in study design, publication bias, and clinical heterogeneity in the articles reviewed.

There are, however, a larger number of meta-analyses on this topic that have published conflicting results.

Bellamy et al [54] conducted a Cochrane meta-analysis on the effectiveness of viscosupplementation in the treatment of OA of the knee. They reported that HA was an effective treatment for knee OA at different post-injection periods. They found few adverse events with the use of HA.   A critical review of the study reveals that there was ‘considerable between-product, between-variable and time-dependent variability in the clinical response’ [53]. Doubts have been raised regarding the effectiveness of HA in the treatment of knee OA [53]. 

Rutjes et al [55] carried out a high-quality systematic review and meta-analysis on the use of viscosupplementation in OA of the knee. Their study used effect size statistics and they demonstrated that HA use was associated with a small and clinically irrelevant benefit. There was also an increased risk for serious adverse events.   

Richette et al [56] performed a systematic review and meta-analysis by only including low bias and high-quality RCTs. They showed that HA provided a moderate but real benefit for patients with knee OA. 

Strand et al [57] conducted a systematic review and meta-analysis to investigate the safety and efficacy of HA for knee OA. They found that  US-approved HA is safe and efficacious through 26 weeks in treating knee OA.

The American Academy of Orthopaedic Surgeons (AAOS) evidence-based clinical practice guidelines for the treatment of OA of the knee (2013) do not recommend the use of hyaluronic acid for patients with symptomatic osteoarthritis of the knee [58]. The strength of recommendation is strong, which means the quality of the supporting evidence is high.

Orthopedic surgeons are urged to follow a strong recommendation unless there is a clear and compelling rationale for an alternative approach.

The authors of the guideline evaluated 14 studies (three high-strength studies and 11 moderate-strength studies) to assess intraarticular hyaluronic acid (HA) injections. 

Analysis of the studies showed that there was a low likelihood that an appreciable number of patients achieved clinically important benefits in the outcomes from the use of HA. 

Although the studies showed statistically significant treatment effects as far as the WOMAC pain, function, and stiffness subscales scores were concerned, none of the improvements met the minimum clinically important improvement thresholds. 

The authors of the guidelines concluded that there was a lack of efficacy of HA in the treatment of OA of the knee and therefore they strongly recommend against the use of HA for OA of the knee.

Intraarticular hyaluronic acid injections are conditionally recommended against in patients with knee OA by the 2019 American College of Rheumatology/Arthritis Foundation Guideline for the management of osteoarthritis of the hand, hip, and knee [59].

The Osteoarthritis Research Society International (OARSI) guidelines

conditionally recommended HA (with degree of uncertainty) in individuals with knee OA [60]. They believe that HA may have benefits in the treatment of knee OA and there is a degree of uncertainty.

Generally, the current clinical guidelines on the use of HA makes mention of poor study quality, publication bias, conflicting results, industry sponsorship, as well as unclear clinical significance for their inconclusive recommendations [61].

Conclusion

There is more data now on how HA works when injected into joints. There is no strong clinical trial data on the effects of HA and hylans on cartilage histology, although there is well-documented data in experimental animal studies. The protective effect of HA on cartilage has been shown in several animal models of experimental OA. HA has been shown to reduce the severity of OA and also to maintain cartilage thickness and surface smoothness.

Over the last several decades there have been clinical trials that investigated the efficacy and safety of HA in the treatment of OA in humans. 

Although HA is widely used in clinical practice, there are reservations about its use and effectiveness. The American Academy of Orthopaedic Surgeons (AAOS) evidence-based clinical practice guidelines for the treatment of OA of the knee (2013) do not recommend the use of hyaluronic acid for patients with symptomatic osteoarthritis of the knee. The strength of recommendation is strong, which means the quality of the supporting evidence is high.

The American College of Rheumatology has conditionally recommended against the use of intraarticular hyaluronic acid injections. The Osteoarthritis Research Society International (OARSI) guidelines

conditionally recommended HA in individuals with knee OA. They found that patients may benefit from the use of HA and that there is uncertainty about the benefits in the literature.

Generally, the current clinical guidelines on the use of HA makes mention of poor quality of clinical studies, publication bias, conflicting results, industry sponsorship, as well as unclear clinical significance for their inconclusive recommendations. 

The treatment with HA also does not provide immediate relief to most patients, as studies have shown that it takes about 5 weeks before patients feel the full effect of the treatment.

There are some side effects associated with the use of HA including local pain and swelling of the joint with frequent injections. More randomized controlled trials with a large sample are required to test the efficacy of HA versus the other established therapies of OA.

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