I have a pacemaker and my husband has both hips replaced. We both have cards from our doctors but have never had to show them. For my husband, they take him to the side and use the wand instead of going thru the scanner. For me, I have to go to the side also but I get the full pat-down since I can't be scanned or wanded. Hope this help!
I've had 2 hip replacements and I always set off the metal detector but it's never been a big issue. When I walk through and it beeps I tell them right away and they just do a full scan with the wand. I also have a card but have never been asked to show it.
Only once in Switzerland I got pulled behind the curtain for a full pat down, but I didn't care too much.
Just be up front and tell them about your knee - you will be fine!
I'm so glad I saw this thread. My mother had a total knee replacement, and during the surgery, the doctor accidently cracked her patella, so she has a couple of permanent screws through the bone. This is her first time out of the country and doesn't have a 'card' so I hope a verbal explanation will be enough.
My husband had a knee replacement a few years ago. Everytime he goes through security he sets it off. They always pull him off to the side and scan him. Thats all they will do. He does have a card that shows he had a replacement but they never want to see that. You will have no problem, just will get a better scan.
Since my wife's knee replacement always set's off the alarm, we make sure I go through the scanner first so I am able to retrieve her belongings going through the X-Ray machine while they do a hand scan. A doctor's card or letter is useless because they will scan anyway.