Crowns are indicated when there is inadequate tooth structure remaining after removing existing tooth decay. It is also indicated when the tooth is fractured enough that doing nothing will leave it vulnerable to further damage and a “filling” is not strong enough to last biting forces.
Crowns and Bridges have a finite life. If a crown starts to leak, it is important to replace them promptly. If caught early, the process usually involves simply removing the crown and cleaning the debris underneath. If this small problem is ignored, the bacterial leaking underneath the crown will erode more tooth structure, leaving an ever larger gap for more food and bacteria to leak in.
Since inner layers of our teeth are organic, once a bacteria gets inside your tooth, it does not necessarily need food debris to proliferate. Instead, bacteria will digest your tooth for its survival and multiplication. This process is usually asymptomatic except for the occasional sensation to cold, sweets, etc. Leaking crowns left untreated eventually progress to pain, root canal infection/treatment, and possibly even loss of the tooth.