I tore my ACL and messed up most of the cartilage around my knee (before I rode bikes every day). I never had surgery but went for phys therapy and yoga instead. I still get knee pain every now and then, but I am really careful and hyper-aware so that I take a break as soon as the pain gets serious. Whenever it hurts I just take a few days off, stretch and take it easy. And really, that knee sometimes hurts for no apparent reason so it's not always biking related.

I think it is becoming generally accepted that running a lower gear (so you spin more and have less pressure) is the best way to go until your leg gets stronger. Then after that just pay close attention to it and do everything in your power to keep your leg from twisting - ever. Straight leg motions are the best.

Good luck. Get riding!