you haven't said how old you are, but has your ortho talked to you about replacement? I had knee replacement four years ago and my only regret is that I didn't do it sooner. I'm 58 and my right knee is now starting to give me grief. Cycling actually helps, but I'm seriously thinking about getting the other one replaced.
If you do go that route, make sure your doc has done tons of them. The guy who did mine does nothing else.