Cutting off your arm isn't necessarily illegal (didn't look into that specifically, and it may well be), but from my quick googling is, legally required to be reported by most if not all medical professionals. That report will almost certainly fall well within the rules of what will put you into a 72 hour involuntarily psychiatric hold.

So... Cutting your own arm off, regardless of legality, is likely to lead 72 or more hours of imprisonment.

I don't know where exactly the line between cutting your own arm off for fun vs. to study it vs. self treating with homemade cancer drugs falls on the danger to self and others scale. My line would likely be a lot closer to 'do what you want to do' than most judges, but I do think cutting off your arm for most any reason is a reasonable bar for some outside inquiry, from a mental health standpoint.

Just noodling here as well.