I would have to say that the most dedicated athletic patients that I come across are runners. Runners almost have a withdrawal sensation when they cannot run. Unfortunately, very often, runners develop knee pain and need to shut it down as they recover. Inevitably, when they are about to return to their activities, they often ask whether running will cause permanent long term damage to their knees. The answer is yes, and no.
First, the most common cause of knee pain in runners is "Runner's Knee". This is a grab bag of conditions which are essentially a combination of Iliotibial Band (IT Band) Friction Syndrome and Patella Femoral Pain Syndrome. Iliotibial Friction Syndrome is caused by excessive friction of the outer tendon rubbing against the outer portion of the knee joint. As a matter of fact, this can occur in any overuse circumstance but the most common seems to be running. The causes include sudden change in training intensity or surface, poor shoe support, imbalance of strength between the quadriceps (front muscles) and the hamstrings (back muscles), and anatomical variation of the hips, knees and/or feet.
This excessive tightness of the outer structures can cause an imbalance of the kneecap (patella) movement and that may lead to Patella Femoral Pain Syndrome. This condition is characterized by vague and deep pain in the front of the knee that can not be pinpointed. It causes pain with stairs, difficulty getting up from a seated position and pain with squatting or kneeling.
The most effective treatment of Runner's Knee is rest and physical therapy. The physical therapist focuses on stretching the IT Band and outer structures, deep massage, strengthening hip muscles, balance exercises and re-training. I routinely recommend to stop running during the rehabilitation period. After the course of treatment, I recommend very gradual return to running, with no sudden increases in mileage. Routinely, this is enough to treat the condition and it is rare that surgery is required. When patients return to running they commonly want to know if they are causing any long-term, permanent damage to their knees.
The answer is not the same for everyone. Studies have shown that, if there is no degenerative damage to the joint, running will not lead to osteoarthritis. But, if there is already some sort of damage to the joint (such as articular cartilage damage) then the possibility of progression to irreversible permanent damage is very real. Thus, if I find such damage in runners, then I am very honest with my patients; You can continue running if you love it very much, and I understand, but it may be leading to further damage. So, moderation is the best option. I recommend low impact activities for the bulk of the aerobics - such as elliptical trainer, bike or swimming. You can continue running on a moderated level but understand that direct pounding on the joints may very well be advancing the damage.