Older fillings are often replaced, if necessary due to "leakage" around the margin. This replacement, if done in a timely fashion, will prevent the "leakage" from becoming larger decay. Decay is bacteria "eating away" at the enamel and underlying dentin. "Leakage" is due to the margin (or "edge") of the filling breaking down microscopically, allowing bacteria to make a pathway deeper below the surface of the tooth. This means that "leakage" is actually one of the ways that decay can begin. The surface is no longer intact and bacteria is traveling deeper into the tooth.