Teeth that have large amalgam (silver) restorations are prone to fracturing over time, especially if you are a person who grinds their teeth. The way that amalgam is placed in large cavities creates areas of stress when pressure is applied from chewing or grinding. Eventually part of the tooth, or even the whole tooth can fracture leading to pain, infection and much more.
In dentistry one always aims to be conservative where possible, meaning that we aim to remove as little natural tooth structure when performing fillings and restorations. In the case of largely restored teeth, treatment generally entails the removal of the amalgam filling, the taking away of any weakened tooth structure, the cleaning out of underlying decay or bacteria and then the re-filling of the tooth with a white composite filling material.
The tooth then needs to be prepared for a crown which is designed to bind all of the natural tooth structure together and make the tooth resistant to further damage. Crown materials such as Zirconia can be up to four times as strong as natural teeth and their long-term success rate is high.