According to the ADA, an old filling may need to be replaced because the surrounding tooth structure has become decayed. It can also chip, crack, or fall out due to constant stress from chewing.
When a filling is worn or has pulled away from the tooth, decay can leak under the filling and cause bacteria. If decay is left untreated it can infect the dental pulp, which can result in the need for root canal treatment.
We encourage patients to replace old fillings before the decay begins to avoid more complicated and expensive treatment.