I have also had a few ops on my knee and was concerned about getting a class 1 - you obviously have to declare the details in the medical history part of your initial medical but there were no problems for me. I was asked whether I had any problems walking and running and how I found the knee on things like stairs and they were fine with my answers. They checked my range of motion and strength very briefly and that was it. I would suggest though if you have had the op recently, do the physio and rehab first so that you can demonstrate that you are fit to fly when you go. Hope that helps