Recently I had a CEREC onlay done on my lower left first molar, as it
had a crack running through one of its rear cusps (*exactly through*
the cusp, not around it - it then appeared to curve around on the
cheek side until it ran parallel to the gumline, three or four
millimetres above the gum). The tooth had a large occlusal amalgam
filling.

The dentist said it just needed an onlay, and so only ground about
two-thirds of the tooth down, leaving a third above the gumline. He
used an intra-oral camera and pointed out to me that there was a crack
running through the floor of the filling cavity, probably a
continuation of the cuspal crack that I'd originally spotted. He said
it wasn't a good idea to 'chase' the crack by grinding more of the
tooth, so he just fitted the onlay (I was glad I'd found such a
'minimally-invasive' dentist, BTW!)

The restoration hurt on biting down for a few days (just like a
freshly-filled tooth, in fact), but it's fine now.

But what I'm wondering is: what sort of lifespan will this tooth have?
I always thought that a crack in the floor of a filling cavity meant
the tooth just had to be extracted - but is there more to it than
this? I also seem to remember him saying this was a crack along the
midline of the tooth - again, I thought this was the worst type, but
then, all I know is gleaned from the Web.

I asked the dentist about this and he just said that whereas I had a
filling that was causing a wedge effect, I now have a strongly bonded
onlay that's working to keep the tooth together. I understand that,
but my main worry is: how long?

I know it's impossible to comment on my case without seeing my teeth,
but has anyone here got experience of how such a crack behaves once a
CEREC onlay is in place?

Thanks in advance

Ken