When you say knee fusion I'm assuming that means that your knee will no longer be able to bend. Am I correct?
I have psoriatic arthritis and I have a lot of joints that are involved (feet, knees, fingers, wrists, & elbows). My right wrist is so bad it fused on its own and I can longer bend it (my left is almost as bad). My knees are so bad that I cannot straighten them out.
What kind of arthritis do you have and how bad is it? How old are you if you don't mind me asking? I'm not a doctor so I cannot tell you if knee fusion is your best choice but I think I'd get a second opinion.