When a tooth is fractured or decay extends below the gum line, this area must be uncovered before it can be restored, for a number of reasons.
In order for the gum to heal against the tooth in a healthy manner, there must be 3 millimeters of healthy tooth between the margin of a filling or cap and the crest of the bone which supports the tooth. This allows space for proper attachment of the gum to the tooth

A tooth which is broken or decayed below the gum is also difficult, if not impossible, for your dentist to restore correctly, as access and visualization of the area is compromised.  In addition, even the best filling or cap will trap bacteria where the restoration meets the tooth.  If this area is below the gum where you can not properly clean it, the bacterial accumulation results in both periodontal problems and the formation of a new cavity at the base of the filling.

A crown lengthening procedure, which reshapes the gum and supporting tissues, will solve these problems.  After treatment, the fractured or decayed area is uncovered and accessible to the dentist for restoration, and the needed space has been established between the supporting tissues and the final position of the restoration.  This will allow a final restoration to be placed in a healthy environment.