I think it's possible that the teeth have a necrosys (You can't see with rx,) cused by a pulp infiltration. so not depending from Diamond Crown or from an amalgama. It's possible also a bad rooth therapy_ Diamond Crown is a good materials, here in Italy the distributor is DL Medica (Milan) . I'm a Dentist of Tourin, I've seen in my research that Diamond Crown (Pex is the materials) have a few Bibliography, instead of tha other Composites that have a lot of reserche, generally sponsorized from the big :::