There once was a Rav who was giving a shiur about preparations for Pesach and was asked this question repeatadly. Finally in exasperation he told the questioner to either gargle with boiling water for 5 minutes or to put his head in the microwave in high for 3 minutes.
But seriously, you are asking an excellent question because fillings should be no different than any other metal utensil that we would not use on Pesach without a proper hagola which is of course impossible. However before you run to your local dentist to replace your fillings you should be aware that the Poskim advise that it is sufficient to refrain from eating hot chometz for 24 hours before Pesach begins and to brush and clean the teeth very well imediately before Pesach. Even though we are usually machmir on eino ben yomo on Pesach, but here there are other considerations that can be relyed on.