Recently, three crowns in my mouth have been coming loose.  Repeatedly.
 All three teeth are adjoining molars on the bottom right of the mouth.
 One of these had been in place for years.  Another for one year, and
the third for only a couple of months. Root canals had been done on all
the teeth, and the crowns were all secured with metal posts that went
down into the roots. The problem is the posts refuse to stay cemented in
the roots. My dentist cemented the posts back in several times. Free of
charge, I might add.  He's been very patient with my situation, and I
can't fault him for lack of effort. He's tried different substances to
secure the posts, and the bite always seems good after the work is
completed. After a few days to a few weeks the posts are loose again.

He feels that the best course of action at this time would be to have
titanium dental implants placed in my jaw, and then have crowns placed
on the implants.  This means having an oral surgeon remove roots and
place titanium posts in the jaw where the roots used to be.  My regular
dentist would do the crowns on top of the posts.

I had assumed that there would be one implant and crown for each tooth.
In my case that would be three implants and crowns. Does this
necessarily have to be the case?  Can a bridge be anchored to two
implants to reduce the number of implants?  If it can, what are the pros
and cons of a bridge versus multiple crowns?  Thank you.

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John