I have a 'blown' left knee from my early 20's. It required arthroscopic surgery. I now have a chunk of my anterior medial meniscus gone, and 10% reduction in strength in my anterior cruciate ligament. My doctor recommended bike riding as therapy, since it has very little torque under load.

Riding fixed on a fairly spinny ratio (45x17) in somewhat hilly terrain has made my knees feel great. Stronger than ever.

The only thing that makes me feel any knee discomfort whatsoever is trying to backpedal to slow down. If I do that a lot on a ride, I feel it for several days afterwards.