They are (afaik) screwed into the bones of my spine. The operation consisted of taking bone chips from my ribs and using them to fuse my spine (well, most of it) in place so that the curves didn't go back to how they were after the doctors straightened it as much as they could. The metal rods were then screwed in place to hold everything there until the bones fused. Apparently after the first year after the operation the rods aren't needed any more; their purpose has been fulfilled and the spine isn't going anywhere anyway. The operation was about 8 hours, I think, and pretty damn invasive, so they don't take the rods out unless there are issues with them, like them breaking. Hopefully that never happens to me. I haven't set off any metal detectors yet (unfortunately), and I don't feel the screws when I go about my business but down the bottom of the fused part there isn't much padding over the whole deal so I can feel either the bottom screws or the bottom of the rods themselves.
This is why getting a tattoo is going to hurt like a motherfucker.