Now that you have seen how a normal knee functions, let's look at what happens when your joint wears out. As you get older, parts of the cartilage in your joint begin to wear out. As a result of this, the ends of your bones no longer have a smooth surface for gliding and they begin to rub directly on each other, which causes pain. Some people have infections, injuries, or diseases such as rheumatoid arthritis or lupus that may speed up this wearing-out process.
A response to this process is that your body begins to produce more fluid in the joint to protect and lubricate the knee, making it swell and feel warm to the touch. As the process progresses, your knee movement decreases, and the pain and swelling get worse. Your joint can even become deformed so that the knee seems to bend in abnormal directions.