Total knee replacement after RNY?

Well the time has finally come to give in to the knee replacement. I thought losing weight would help, but it only seems to be worse. I had RNY surgery on August 28th, 2006. My starting weight was 335. Now I'm 245. WOW Still can't believe it. Anyway, has anyone gone through a knee replacement? What are my expectations? Also, I've lost my a$$....How do I get it back? I'm flatter than a pancake back there...LOL