The easiest way to think of hyaluronic acid injections is that they help replace the joint's natural fluid, which is otherwise lost in osteoarthritis. Hyaluronic acid injections don't "cure" osteoarthritis. It is sort of like changing the oil in your car. Or, another way to think of hyaluronic acid injections is that they pave the pot-holes in the joint.
When hyaluronic acid injections are effective, the pain reduction typically lasts for six months to a year. If the pain recurs, the injections can be repeated. However, as with other injections and medications, it is best to think of hyaluronic acid injections for osteoarthritis as a window of opportunity during which the pain from osteoarthritis is significantly reduced and the patient can take advantage of this time to stretch and strengthen the surrounding muscles so that the pain never returns.
There are currently five FDA-approved injectable preparations of hyaluronic acid: Synvisc, Hyalgan, Supartz, Orthovisc, and Euflexxa. Some hyaluronic acid injections are given as a series of five injections (once weekly), and others come in a series of three injections. The entire series is given because the effect can be cumulative. Sometimes, the benefits from the injections do not occur until a few weeks after the last injection, and sometimes the effects are immediate.
Synvisc was the first hyaluronic acid preparation available in the United States and it continues to be the most popular. However, a disadvantage of Synvisc is that it has a slight increased risk of causing increased inflammation in the joint. If this happens, a steroid injection is needed to counteract the effects. None of the preparations have been conclusively proven to be superior to another and clinicians generally use their clinical judgment about which preparation to use.
Hyaluronic acid injections is currently only approved for knee osteoarthritis. However, research is underway to evaluate its effectiveness for osteoarthritis of other joints, including hips and shoulders. It makes sense, though it is yet to be proven, that if hyaluronic acid injections are effective for osteoarthritis of the knee, they should also be effective for other joints with the same problem. This is because each mobile joint in the body has the same basic structure. There is an outside joint capsule that is made of tough fibrous tissue and articulating bones inside. Lining the tough fibrous joint capsule is a layer of synvoium. Synovial cells secrete a thick, viscous synovial fluid. Synovial fluid plus water equals the fluid in the joint. The viscous nature of the joint fluid is due in large part to the presence of hyaluronic acid. Hyaluronic acid gives nourishment to the cartilage, helps eliminate waste products from within the joint, and lubricates the joint to cushion shear (side-to-side) stresses as well as acting as a shock absorber for the joint. In addition to all this, hyaluronic acid is thought to potentially offer a shield to the pain receptors that line the joint. In osteoarthritis, hyaluronic acid is lost from the joint, which clearly creates a problem for the joint. The injections are designed to replace this lost fluid.
Hyaluronic acid injections are not appropriate for everyone. There are risks with these as with other injections. Only your doctor can let you know if hyaluronic acid injections might be appropriate for you.