Hyaluronic acid (HA) - also known as hyaluronan- is found in all tissues and body fluids. What this article will describe is what HA does in the joints, particularly the knee.

HA is a major component of synovial fluid. Synovial fluid is the naturally-occurring lubricant found in joints.

HA has a variety of functions within the joint including lubricating effects, water balance, and stress distribution through what are termed "viscoelastic properties." What this means is that at low levels of impact (standing and walking), HA has mostly lubricating properties. At high levels of impact (running and jumping), HA functions to cushion the joint.

Hyaluronic acid (HA) is often used as a treatment for osteoarthritis (OA), particularly in the knee. HA therapy supplements HA in the knee, restoring the synovial fluid's ability to cushion, lubricate, and protect the knee joint.

A number of different mechanisms of action are thought to be responsible for the beneficial effects of HA. These include fluid replacement, inducement of the body's own HA production by the knee joint, increase in cartilage synthesis, reduction in cartilage degradation, inhibition of inflammation, protection of the cells that make cartilage, and decrease in pain through inhibition of pain stimulus on nerve fibers within the joint.

Osteoarthritis is the most common form of arthritis. Its hallmark is the progressive breakdown of cartilage. As cartilage becomes progressively more damaged, changes in the HA within the joint also occur. Both the concentration as well as the molecular weight of HA undergo change.

The reduced concentration and molecular weight of HA lead to worsening of cartilage damage, increase in inflammation within the joint, and further reduction of function of synovial fluid.

Synovial fluid in patients with OA no longer performs its normal viscoelastic function. This perpetuates a cycle of reduced cushioning and lubrication leading to more damage and inflammation, and so on.

Both the American College of Rheumatology and the American Academy of Orthopedic Surgeons list HA therapy as an alternative and supplement to oral drugs for patients in whom non-drug therapy and treatment with acetaminophen (Tylenol) has failed, especially if non-steroidal anti-inflammatory drugs (NSAIDS) and COX-2 inhibitors are contraindicated.

The advantage of HA is not limited to just restoration of the synovial fluid's ability to cushion, lubricate, and protect the knee joint. HA also leads to pain relief and improvement in knee function and health-related quality of life. Finally, it is a localized therapy with no systemic side-effects.

There is also some data that HA therapy may actually postpone the need for knee replacement surgery in some cases.

Side effects of HA are minimal. The most common are local injection site reactions. Different forms of HA are available and these include Hyalgan, Synvisc, Supartz, Orthovisc, and Euflexxa. Most HA products are derived from chicken sources.

The exception is Euflexxa which is bioengineered from a different source.

Different injection schedules are required for the different preparations. Some require weekly injections for 3 weeks; others are weekly injections for 5 weeks.

The important things to remember are that HA should be administered by an experienced rheumatologist or orthopedic surgeon. Either ultrasound or fluroscopy should be used to ensure proper location of the needle and accuracy of HA injection.

Contraindications include active infection of the joint or skin near the joint, history of sensitivity to chicken prodcuts (except for Euflexxa), and prior allergic reaction to HA.