Viscosupplementation is a relatively new injection procedure that helps to relieve the pain of a degenerative joint. This involves the injection of a preparation of hyaluronic acid into the knee joint. Hyaluronic acid is a naturally occurring substance found in joint (synovial) fluid. It acts as a lubricant to diminish friction and enable bones to move smoothly over one other. Osteoarthritic joints tend to have a lower-than-normal concentration of hyaluronic acid. Viscosupplementation injections are a concentrated preparation of hyaluronic acid. The technique has been used in Europe and Asia for a number of years years, but the U.S. Food and Drug Administration did not approve it until 1997, and then only for treating osteoarthritis of the knee. Several preparations of hyaluronic acid are now commercially available. Each vary in certain ways, such as their molecular weight and procurement methods. Examples of viscosupplementation preparations include Synvisc, Hyalgan, Euflexxa, Supartz, Orthovisc and others.

The exact mechanism of viscosupplementation is debatable. In theory, it replenishes low levels of hyaluronic acid in the joint and acts as a lubricant to lower the friction of degenerative surfaces.  It does not repair cartilage or act as a spacer between the joint surfaces. Its therapeutic goal is to restore the visco-elasticity of synovial hyaluronic acid, thereby decreasing pain, improving mobility and restoring the natural protective functions of hyaluronic acid in the joint. The short-term mode of action of viscosupplementation is believed to be based on the pain relieving effect of the elastoviscous fluid in the affected joint. In the long term, the restoration of the joint mobility due to relief of pain triggers a sequence of events, which restores the trans-synovial flow and subsequently the metabolic and rheological homeostasis of the joint.  Hyaluronic acid does seem to have anti-inflammatory and pain-relieving properties. The injections may also stimulate the body to produce more of its own hyaluronic acid.  Effects may last for months.

Viscosupplementation is a pain-relieving therapeutic option for individuals with osteoarthritis of the knee.  Other indications, such as the arthritic shoulder, hip and ankle are being investigated and will likely to show equal beneficial effects.  Many insurance programs require that other treatment regimens have been attempted (http://www.aetna.com/cpb/medical/data/100_199/0179.html);

• Conservative therapy (including physical therapy, pharmacotherapy (e.g., non-steroidal anti-inflammatory drugs (NSAIDs), acetaminophen (up to 1 g four times per day) and/or topical capsaicin cream)) has been attempted in each joint to be treated with viscosupplements and has not resulted in functional improvement after at least 3 months or the member is unable to tolerate conservative therapy because of adverse side effects; and
• The member has documented symptomatic osteoarthritis of the knee; and
• The member has failed to adequately respond to aspiration and injection of intra-articular steroids; and
• The member reports pain which interferes with functional activities (e.g., ambulation, prolonged standing); and
• The pain cannot be attributed to other forms of joint disease; and there are no contraindications to the injections (e.g., active joint infection, bleeding disorder).

Viscosupplementation is injected into the joint. This may be in a single injection preparation or multiple weekly injections, up to 5.

Viscosupplementation has been shown to relieve pain in many patients who cannot get relief from non-medicinal measures or analgesic drugs. According to a review of the literature in the journal Clinical Evidence (Scott & Kowalczyk, 2006), compared with placebo, intra-articular hyaluronic acid and hyaluronan derivatives may improve pain and function compared with placebo at up to 13 weeks after injection, but may have no longer-term benefits. The review stated that this conclusion is based upon low-quality evidence. The assessment also found that, compared with intra-articular corticosteroids, hyaluronic acid may be more effective than intra-articular corticosteroids at reducing pain at 5–13 weeks, although they may be as effective as each other in the shorter term. According to the review, this conclusion is based upon very low-quality evidence. The assessment also noted that there is no evidence on the effectiveness of subsequent courses of hyaluronic acid, and if diminishing returns exist.

Kirwan (1997) reviewed 10 clinical trials of hyaluronic acid of the knee joint. The review found slightly greater benefit from the injections versus placebo at 1 to 6 months after treatment. Of four subsequently published randomized controlled trials, three (Lohmander, 1996; Corrado, et al, 1995; Formiguera, 1995) found no significant difference versus placebo at 2 to 5 months after treatment, but both active and placebo groups improved compared with baseline. One of the trials (240 people) included a subgroup analysis of people aged over 60 years with moderate to severe symptoms; these benefited more with active treatment than placebo (Lohmander, 1996). The fourth subsequent randomized controlled trial, involving 100 people, found significant benefit on a standardized pain assessment tool (the Lequense index) with hyaluronic acid versus placebo, both at 5 weeks and four months (Huskisson, 1999). Another randomized controlled clinical trial also found a trend toward greater pain relief and functional recovery in patients treated with intra-articular hyaluronan versus placebo injection, but the differences between the two groups were not statistically significant (Tamir, 2001).

Bellamy (2002) viewed the evidence comparing viscosupplementation to steroid injections.
One randomized controlled clinical trial reviewed by Bellamy (2002) found a benefit of hyaluronan at 5 and 8 weeks against steroids, but no difference in effect between steroid and hyaluronan injections was found in two other randomized controlled clinical trials.
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Hyaluronic acid may not have an immediate pain-relieving effect.  One may notice a local reaction, such as pain, warmth, and slight swelling immediately after the injection. These symptoms generally do not last long.  Longer term pain relief is expected, generally 6 months.  The treatment may be repeated very 6 months if its effectiveness continues.  The long-term efficacy of viscosupplementation is not yet known and research continues in this area.

Head to head studies to determine which commercial preparation of hyaluronic acid have not been done, so the effectiveness of one product over the other has not been established.