Currently only indicated for knee pain, this is the injection of a naturally occurring substance called hyaluronic acid. Hyaluronic acid is a gel-like substance that the cartilage cells make in the knee and other joints. This medication decreases knee pain by blocking some chemicals that cause pain and inflammation. Hyaluronic acid also provides some cushioning and lubrication due to its thick viscosity. These injections are like an oil change for the knee.