My wife has a replacement hip which always sets off the machines. She explains and the staff are always familiar with this and understanding - sometimes they (immediately) ask before she has explained. Most of the time she is scanned with a hand held device, sometimes not. In the UK she is usually (but rarely elsewhere) religiously frisked with her shoes off in addition to the handheld scanning and I think once or twice she has been swabbed for chemical traces as well. She has no letter or X-ray and I very much doubt if the staff would be interested in one (except out of politeness), these people are trained to be (a bit variously from country to country) pedantic and independent of persuasion.
We never avail ourselves of the opportunity presented at every single airport in the UK and abroad (and infuriatingly unavoidable) of taking aboard aircraft a potentially very offensive weapon, namely a "duty free" glass bottle purchased in the post-inspection shopping areas.