Here's the deal: "bionic" people like you routinely set off metal detectors at airports and cruise terminals. It is absolutely no big deal. Lots of people set them off for various reasons. There's no cause for concern.
My wife has had both her knees replaced and there's not a metal detector on earth that she doesn't trip. When she had her knees done, she got a note from her surgeon (he wrote it on a prescription form). She had it laminated and carried it with her for a while. Eventually she stopped carrying it because nobody at the security checkpoints cares about them.
If you set off the machine, you're going to get wanded, note or no note, so there's no real need to have one, even though it sounds like such a nice idea. When the buzzer goes off, just tell the constabulary where your metal parts are, and do what they tell you to do. All will be well.
My wife and I have a set routine when we fly or sail together. She buzzes and goes off for her wanding. I don't buzz, so I gather up all our hand luggage from the belt and move off to the side to wait for her to be finished. It's so routine that we don't even think about it.
There's certainly no reason to feel ashamed in any way. Zillions of people get wanded for bionic parts.
Get a note from your doctor if it makes you feel better, but you absolutely don't need one.