Hyaluronic acid (HA) can be injected into the knee to restore the joint's ability to move and glide easily. The knee already has some of its own HA. By injecting more, the HA already in the joint improves the flow of the synovial fluid. Injected HA also helps keep the naturally present HA from breaking down.

Overall, studies show that HA is both safe and effective. It seems to work better for younger patients (under 65 years) and for patients with only mild to moderate disease. Patients with severe osteoarthritis may not have any natural lubrication left in the joint. It seems without some HA, the injection doesn't work as well.