i had a full meniscus tear from a freak knee-bending incident on christmas eve over 3 years ago. it was some of the worst pain i've ever known. the torn part of my meniscus got double over on itself and locked the joint at a 30 degree angle. my doctor likened it to shoving a 2x4 into a door jamb to hold it open. i had to have full-blown reconstructive knee surgery. i was in physical therapy for 2 months relearning how to walk, go up and down stairs, things like that. part of the PT was also riding a stationary bike to get low impact motion back into the joint. through that whole process, i got back into cycling after a roughly 10 year hiatus. i talked to my doctor about riding my bike to work to help make the knee stronger and he told me to go for it. i'm now on my 4th year of bike commuting, and overall my knee feels great. there are some days when it gets tight (though some people say that can be weather related), and occasional soreness after really long rides, but those also can affect my non-surgery knee as well.