Functional crown lengthening is usually performed to improve the health of the gum tissue and prepare the surrounding tissue for a restorative procedure. When a tooth is fractured or decay extends below the gum line, this area must be uncovered before it can be restored. 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 crown and the crest of the bone which supports the tooth. This allows space for proper attachment of the gum to the tooth.