What factors lead to tooth survival?

Patient: 72 year-old male with controlled hypertension (at recall examination in 2015)

Chief Complaint: the patient initially presented without symptoms

Dental History:

The patient initially presented to the author in 1993 with a request to have a maxillary left third molar treated which had been started by his referring dentist. Tooth #16 (the maxillary left third molar) had migrated into the #15 position with #15 having been previously extracted. The maxillary right second molar (#2) was subsequently treated in 1994 when he presented with a gross caries exposure. In 2000 he returned to have the maxillary right lateral incisor (#7) retreated due to an inadequate existing crown restoration with a poorly cemented post and coronoapical leakage. The maxillary left first molar (#14) and second bicuspid (#13) had been treated by a general practitioner in the 1970’s