I have a crown (porcelain over metal) on a back molar on the top.  It
was put on four years ago, while I was pregnant.  Recently, the back
part of it broke off.  I am not experiencing any pain or discomfort.
I'm wondering if it is common for crowns to break so soon.  Do I have
to have it replaced if the tooth is not exposed?  It was very costly
when I had it put on and our dental insurance does not cover crowns.  I
really don't want to have to replace it if at all possible.  Should I
be responsible for the replacement cost or is there some type of
warranty on them?  Before I make an appointment to be seen, I'd like to
know if it "should" have broken after four years and if I should be
responsible for all of the costs involved.

Thank you in advance for any information you can provide.