Old fillings may need to be replaced when they start to fail or develop recurrent decay. No matter how good today’s technology is, the seal between the tooth and the filling material is not perfect. Over time, this seal breaks down because of attrition, erosion, grinding and chewing, etc. The filling material starts to pull away from the tooth, leaving gaps at the filling edges and creating an entry point for food particles and decay-causing bacteria. When decay develops along the edges of a filling or underneath a filling, it is called “recurrent decay”.