If a tooth has a large filling, or as a cavity gets closer to the nerve on the inside a tooth, a tooth may start to get more and more sensitive and ache as bacteria infect the tooth. In such cases fillings only trap the infection inside the tooth not helping the situation and making things worse. If the tooth is too important to lose, Root Canal treatment clears the infection and seals the inside of the tooth to relieve pain and prevent re-infection allowing the tooth to be saved.