It is prudent to place a restoration that protects the biting surface of a
back tooth when the tooth is heavily restored.  If the tooth has had a root
canal in addition to large fillings, it becomes considerably more important,
in the you-better-do-it-or-yer-gonna-lose-the-tooth range.

Leaving the root canal out of the equation, though, most of the back teeth
with large fillings that break can still be restored once they do so.
Usually it's a corner of the tooth that fractures off.

A small percent will fracture vertically and have to be extracted.  My wife
lost a tooth with a little, dinky filling in it this way.  No one would have
looked at it prior to the break and thought it needed a cusp-protecting
restoration.  So, sh--t happens, no?  I'm still trying to get her to do an
implant.

So, the best restoration for a heavily filled back tooth is something that
protects the biting surface.  Crown, 3/4 crown, onlay.  Crowns aren't the
most conservative - sometimes they're the best choice, but they're not the
only choice in many instances.  My brother recently had an onlay after a
minor fracture on a molar following the advice that it would be better than a
refill.  'Course he's well off, can afford the best.  He had a Cerec onlay
which is an all-ceramic, tooth-colored restoration that protects the biting
surfaces.

A lot of folks do not need a cleaning every 6 months.  My patients who fall
in this category are offered a longer interval - usually a year.  Most of
them still prefer every 6 months.

I'm in my 40's and have quite a few heavy fillings in most of my molars. I
recently attended a dental clinic for the first time and they advised me
that I need 6 crowns. Apparently if I don't get the crowns I run the risk of
breaking the fillings and the underlying tooth. Just how true is this, and
do crowns last longer than fillings? I asked about the longevity of crowns
and one dentist said they could do me my lifetime, whereas another one (in
the same clinic!) said 10 - 15 years. Asuming I live longer than 15 years,
if I get 15 years from a crown can I simply get it recrowned?

One last unconnected question... is cleaning of teeth by a dentist a
necessary procedure? This clinic have advised that they clean my teeth, even
though they have inspected them and classed them as being in fair condition.
It has been about 3 years since my last cleaning. What gives me some concern
is that they want CAN$150 for the cleaning.

I guess dentists are just like any other business. As soon as you walk in
the door they try and sell you all sorts of stuff. Some worthwhile, some
not. The problem is knowing which is which.