Dentists will make every effort to allow patients to keep their natural teeth. It's one of several reasons why a dentist will fill a cavity instead of pulling an infected tooth.
In some cases, dentists will help a patient keep an infected tooth by using a technique called crown lengthening. The procedure doesn't actually lengthen the crown of the tooth, but instead remodels the contour of the gum line. As the gum line is lowered, it gives the dentist more tooth to work with when it's time to attach a crown.
Crown lengthening may be the only option available to a dentist when it's discovered that the patient lacks the necessary tooth structure to anchor a crown. On occasions when a tooth has been broken below the gum line, crown lengthening can be used to successfully expose more tooth, which gives the dentist more tooth to work with when the crown is placed.