Amalgam fillings last from 8-10 years and will need to be replaced after this time period. Each time a filling is replaced some preparation of the tooth is required which leads to futher loss of tooth structure. Large fillings or repeat fillings can leave the remaining tooth structure vulnerable to excessive stress and the possibility of a fractured cusp or root. It is  therefore advisable to have a porcelain or gold crown on a tooth that has a large filling, or on a tooth that has had large fillings replaced previously.