Hyaluronic acid (HA) is a substance already in the fluid of your knee. It helps lubricate the joint. When yo have osteoarthritis (OA), the HA in your joint becomes thinner and less effective. Euflexxa is a hyaluronic acid that is injected directly into the knee by your healthcare provider. It replenishes some of the HA in your knee, helping with the lubrication and shock absorption you need within the knee joint.