Before a tooth-coloured filling
Amalgam fillings do not bond to teeth - rather, they simply occupy the cavity prepared for them. This can result in stress and cracking, as can be seen clearly on the outside of this discoloured molar.
After a tooth-coloured filling
Bonding a tooth-coloured, mercury-free filling to the enamel has actually strengthened this tooth and will arrest the progress of the crack. Mercury fillings are at the very least extremely old-fashioned and, for me, it goes without saying that these composite restorations are far more desirable in all respects.