Both of my knees have been replaced. I have never gone through a screen that was not activated by my knees. I generally travel in a wheelchair. Whether I walk through the scanner is completely up to the individual airport. Some places ask me to walk through the scanner, even though I assure them that I will alert the scanner. In other airports, I by-pass the scanner and go directly to the pat-down area or the little machine that lets them see you without your clothing.
It is not a decision I ever make on my own.
Knee surgery is not as easy as hip surgery, but I felt better immediately after surgery than I did before receiving it. I would plan on about 4 months of discomfort. If you do your physical therapy faithfully you will recover more quickly than if you do not. I was out of bed 12 hours after both of my knee surgeries.
There are several appliances you can use post-surgery that help. One is a machine that automatically extends and retracts the leg. Another is a compression device that pumps cold water around your knee to reduce pain and swelling. With my second surgery I used the extension device almost all of the time when I was in bed. I thought my second recovery was easier, but it may have only been because I knew what to expect and what to do to speed my own recovery.
I can tell you that life post-surgery is significantly better than pre-surgery.