The past couple of years dentists have been able to glue the tooth-colored fillings to both the dentin and the enamel of the cavity.  For a long time we could glue the filling to the enamel, but the dentin has been a big problem.  The enamel is dead, but the dentin is alive.  And it has been hard to do this without causing a good deal of sensitivity.

These fillings actually become a part of the tooth.  Re-enforcing the tooth from the inside. 

Old silver fillings don’t make the tooth stronger at all.  They leak, and expand, and are entirely dependent on the enamel of the tooth to stay in the tooth.  On the other hand bonded fillings work with the tooth to make it stronger, and don’t act as a wedge to break the tooth apart as silver/mercury fillings do over time.