Hyaluronic Acid, also called Hyalgan, is similar to the fluid that naturally occurs in the joint. Simply stated, a joint is comprised of an outer sturdy fibrous tissue, which is lined with synovial cells that secrete a thick, viscous synovial fluid. Hyaluronic acid acts as this fluid by naturally nourishing the cartilage, eliminating waste products, lubricating the joint, and acts as an absorber to decrease shock inside the joint.
Summed up, hyluronic acid injections help to replace the joint's natural fluid, which is lost with Osteoarthritis. Think of this, like changing the oil in the car. If a car were running low on oil, you would need to replace it, in order to prevent wear and tear on the engine. Just as the knee joint fluid needs to be replaced when low.