Metal implants are routine, and usually the "best of the alternatives" in implant design. The body generally tries to isolate metal implants, making them mostly inert, although this does depend a bit on which metal.
The main problems with metal implants are that the can shed ions that accumulate in some people, and that wear particles can cause psuedo-tumors. This is mainly a problem with metal-on-metal joint surfaces (ie in knee or hip replacements). Some more info here: http://en.wikipedia.org/wiki/Hip_replacement#Risks_and_compl...
I actually would have thought that titanium would be the go-to for orthopeadic plates in the skull. I would have thought plastics aren't stiff enough and ceramics too brittle.
I wonder if the increased thermal conductivity of titanium over plastic was a consideration. I can see how having a skull plate that heats/cools much faster than surrounding bone could be a problem.
