Crown lengthening is done when a tooth needs to be fixed. Sometimes, not enough of the tooth sticking out above the gum to support a filling or crown. This can happen when a tooth breaks off at the gum line. It also can happen when a crown or filling falls out of a tooth and there is decay underneath. To place a filling or crown, your dentist needs to expose more of the tooth. This is done by removing some gum tissue or bone.