Crowns are usually needed when a tooth has a large existing fill or cavity and has broken. When you break a tooth or loose a filling the retention is less on a tooth. Therefor there is not enough of the tooth structure to retain a filling, so a crown is needed to restore the tooth. If a tooth has a root canal on it then it generally requires a crown on it. The crown is placed to support the remaining tooth structure and prevent the tooth from breaking. A bridge is more than one unit of a crown. It replaces missing teeth. A bridge can replace teeth as long as there is a tooth on either side of the missing teeth.