When all the cartilage in your knee is gone, the only effective treatment is to replace the whole knee.
The ends of bones are soft, so they must be covered with a thick white gristle called cartilage. Once damaged, cartilage can never heal or re-grow. Small holes in cartilage can be repaired, but usually once cartilage is damaged the person spends the rest of his or her life losing cartilage until it is completely gone and the knee hurts 24 hours a day. The only effective treatment at that time is to replace the knee. The artificial knee joints being used today can be expected to last 20 years or more. However, the artificial knee can become dislodged from the bones and may have to be replaced. The area around the knee is so full of scar tissue that the second surgery is far more difficult than the first.
Partial knee replacements may be appropriate for some people, but they are more fragile and do not last as long as total joint replacements.
If you have damaged cartilage in your knee, you have to protect that knee for the rest of your life. Running 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 probably need a knee replacement.
If you have knee replacement surgery, follow the rehabilitation instructions carefully and get back into an exercise program as soon as possible. Strengthening the muscles around the site of the surgery helps to make the bones strong and stabilize the joint. Choose exercises that use smooth motions and do not pound on the joint, such as cycling, rowing or swimming.