Crown lengthening is done when a tooth needs to be fixed. Sometimes, not enough of the tooth sticks 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. Some people have a lot of gum tissue around their upper teeth. Dentists call this “gummy smile.” This also can be treated with crown lengthening.