This is why my mom prefers the body scans rather than the metal detectors. She’s had both knees replaced, and has to be wanded when she goes through the standard metal detector. The body scanner lets her through without wanding.
Anyone can fake a letter; that seems utterly useless. I have a pacemaker and tell them, so they always wand me, but I’ve never been asked for a letter. Even with a wand, they apparently never find the small metal plate in my ankle.
Regarding the letters/cards from a doctor. My wife has such a card because of a plate in her arm and it is ignored.
Because of that I didn’t even bother asking for one when I had a hip replacement.