The ends of bones are soft, so they must be covered with a thick white gristle called cartilage. Once damaged, cartilage can never heal. When knee cartilage is damaged, the person spends the rest of his life losing more cartilage until it is completely gone and the knee hurts 24 hours a day.
If only a small area of the cartilage is damaged, it may be treated with cartilage extracted from your own body. (If cartilage is taken from someone else, your immunity will try to kill it, but it does not try to kill your own cartilage.) Your extracted cartilage is grown in special culture dishes and then injected into a hole in your own cartilage and secured in place. This procedure works very well if you have only a small piece of cartilage missing. The doctors just fill the hole. However, unless they can enclose the entire area for the donor site, the cartilage will not stay where it was put and will not be beneficial. That is why this procedure cannot be used to treat a cracked cartilage in the knee. Remember they have to cover the articular surface that meets the cartilage from the other side of a joint.
When all the cartilage in your knee is gone, the only effective treatment is to replace the whole knee. Knee replacements are now lasting for twelve to twenty years or more, and most remain pain-free.
If you have damaged cartilage in your knee, you should protect that knee for the rest of your life. Running, fast walking and jumping cause further damage, while pedaling and swimming usually do not. When the knee hurts all the time, your doctor will check to see how much cartilage is left. If it's gone, you will probably be told that you need a knee replacement.