Guest editorial: Clarifications on the use of the new classification of periodontitis.

This editorial provides clarifications on the application of the Stage and Grade classification of periodontitis. In particular it describes: (1) how to apply the extent criterion to the defined Stage of the disease; and (2) how to calculate tooth loss because of periodontitis in Stage III and IV cases presenting with evidently hopeless (irrational to treat) teeth with a clinical definition of such teeth.