I got crowns on my upper teeth about a year ago. They are made of some
kind of metal and I think are covered with porcelain. I just noticed a
few very small, very light brown spots near the lower edge of both
front teeth.

What could have caused these? Do crowns need to be brushed very
carefully? Would acidic fruits eat into them? Could they have been
harmed while having my teeth cleaned? I don't smoke. It isn't likely
they came from the factory with spots, is it?

If I leave them alone, will the spots probably get worse? If the
dentist offers to remove the spots by polishing the crowns, will it
make them weaker or more likely to get stained (or pitted) again? I
would like to avoid replacing the crowns.because of pain and possible
trauma to the teeth.

It never occurred to me to ask before getting them, but how long
should crowns hold up and look good?