There are many different types of dental fillings, including silver amalgam, gold, porcelain, and composite resin, each having their own advantages and disadvantages.
While silver fillings (called amalgams) are less expensive, they often break down over time as a result of micro leakage and need to be replaced.
Porcelain fillings bond to the tooth enamel and enhance the strength of the surrounding tooth enamel. By creating a bonded interface, the white filling can last longer from less micro leakage and breaking down over time.
Another reason old fillings fail is because the metal filling and tooth enamel contract and expand over time, creating stress on the filling material and enamel. It is very common for a tooth with an old amalgam to eventually break, requiring a crown.