Sometimes a tooth needs more than just a filling. When there is decay under an existing filling or when a tooth cusp is weakened, a crown will help distribute the force of the bite over the entire surface of the tooth. A crown is a protective covering over a tooth that has been weakened - either by decay, a large filling or a root canal. The crown is precisely fitted to the trimmed tooth and permanently cemented in place. Modern crowns are made of very durable ceramic materials.