Osteoarthritis is the most common form of arthritis of the knee, and can affect people over the age of 55 years, but it's more common in people over 60. The severity varies greatly, and ranges from mild, where treatment may consist of just an exercise and/or weight loss program, to severe, where raw bone is in contact with raw bone across the joint surfaces, and may require surgical treatment - total joint replacement - for the best result.

Often anti-inflammatory drugs are prescribed, but these may have side effects that, in the USA, can cost an additional $500 million annually in excess medical care. Since surgical treatment is not suitable for the mild-to-moderate degrees of severity, new therapies are being developed to help the sufferer with this degree of involvement.

Normally a joint contains synovial fluid that nourishes the joint and acts as a "cushion" during motion and weight bearing. The main component of this synovial fluid is hyaluronic acid. In osteoarthritis of the knee, the amount of hyaluronic acid is reduced to one-half to one-third of normal, which results in less nourishment for the joint surfaces.

Two injection treatments for osteoarthritic knee joints (called 'viscosupplements') are approved for use in the USA and two others are approved in Canada. Viscosupplements are NOT the same as cortisone injections into the joint, which are very beneficial when used correctly. Viscosupplements are, in fact, preparations of hyaluronic acid.

The usual course of treatment in the USA consists of single injections 7 days apart for 3 weeks (Synvisc) or for 5 weeks (Hyalgan). In Canada, injections are given at the same interval for 3 weeks (Orthovisc and Neocisc).

A lot of studies have been done to assess the benefit of injected hyaluronic acid to such osteoarthritic patients, but unfortunately the best type of studies (those using a comparison group of patients given injections of an inactive substance, such as a weak salt solution) have produced conflicting results. There is no good evidence that these injections actually provide statistically significant better results than placebo injections.

The incidence of side effects is about 1% per injection; they usually consist of mild swelling of the joint, pain, and warmth. No generalized side effects are to be expected.

The cost of this treatment is not covered by Medicare payments at the time of this report, which means that the suggested cost of over $500 per knee (for the series of 3 or 5 injections) makes this treatment expensive.

It's apparent that more scientifically rigorous evidence is needed before these hyaluronic acid injections can be generally recommended as an effective treatment for osteoarthritis of the knee. To clarify the situation, further clinical trials are currently under way in North America, with results expected next year.