A dental crown lengthening procedure is done to adjusts the gum and bone level to expose more of the tooth so that it can be restored.
The procedure doesn't actually lengthen the crown of the tooth but instead remodels the contour of the gum line. The gum line is lowered, which gives the dentist more tooth to work with when it's time to attach the crown.
Crown lengthening can be the only option available when the dentist discovers that there isn't enough tooth structure for a crown to be placed on a tooth. On occasions when a tooth has been broken below the gum line, crown lengthening is often used to successfully expose more of the tooth structure. This gives the dentist more tooth to work with when attaching the crown.