Excluding the obvious defects of fractured or loose restorations, the most common problem necessitating replacement of a restoration is recurrent decay. Recurrent decay is decay that has occured in the tooth around or under an old restoration. The restoration itself may not have changed or deteriorated appreciably, but it must be removed and replaced because new decay has occured.
Recurrent decay can occur at the interface where the restoration meets the tooth due to the presence of a microscopic gap there. This gap is present because of all the materials used in restoring decayed teeth none of them totally seals the cavity forever. Saliva and bacteria may penetrate the margins of the restoration and progress down the walls of the filled cavity. This process is referred to as microleakage. Once inside the tooth and isolated from any means of removal, the bacteria may then cause recurrent decay that necessitates the removal of both the decay and the filling, and their replacement with a new filling.
One of the major areas of development in dental materials and technology is in bonding restorative materials to tooth structure. This bonding, when successful, strengthens the tooth by adhering the tooth walls to the restoration as well as minimizing eliminating microleakage. Bonding materials, however, can still breakdown over time, resulting in microleakage.