A team of researchers at the University of Western Ontario in London, Ontario have shown that arthroscopic removal of loose cartilage and trimming of knee cartilage is no better than doing no surgery at all ( NEJM, September 11, 2008). The only other study that also used sham surgery was done at Baylor Medical School and showed the same results ( New England Journal of Medicine 2002;347:81-8). The procedure is done when a surgeon inserts small tubes through the skin into the knee joint and trims the edges of cartilage and removes loose pieces of cartilage from the joint.
You hear about many athletes returning to the athletic field after breaking cartilage in their knees and having surgery. However, almost all will have pain in their knees for the rest of their lives and most will eventually have their knees replaced. When you break cartilage in your knee, it will never heal.
If you hurt your knee and the pain persists, your doctor will probably order an MRI. If it shows that you have a crack in your cartilage, you should never run or jump again. When you run, the force of your foot striking the ground is transmitted up to your knee and can extend the existing cracks. Running 6-minute miles exerts a force exceeding three times body weight. Landing from a jump exerts even greater force on your knee joint. You can usually ride a bike safely because you pedal in a smooth rotary motion that exerts little force on your knee joint. Swimming is also usually safe for your knees.
If you extend the cracks in your knee cartilage, you can have pain all the time. Then it is probably time for you to have knee replacement surgery. Surgery is also indicated for a torn anterior cruciate ligament. Your knee is really two sticks held together by four bands called ligaments. You can tear any one of three of these bands and usually do quite well. However if you tear the anterior cruciate ligament, the cartilage of your lower leg is allowed to slip backwards against that of your upper leg and shear off additional cartilage, eventually necessitating a knee replacement. So almost always doctors recommend replacing a torn anterior cruciate ligament.
Contact sports that require running and jumping, such as football and soccer, are the ones that put you at increased risk for knee damage. Once you crack cartilage in your knees, you probably should avoid all sports that require running and jumping. Most people can ride or even race on bicycles and not extend their knee damage. More on arthroscopic knee surgery