Third molar outcomes from age 18 to 26: findings from a population-based New Zealand longitudinal study

The purpose of this study was to describe the presence and impaction status of people's third molars at age 18 years, as well as the observed changes in their clinical status between ages 18 and 26 years.

Eight hundred twenty-one participants in a prospective cohort study were dentally examined at ages 18 and 26 years, and panoramic radiographs were taken at age 18 years but not at 26 years. For each tooth, its radiographic impaction status at age 18 years was compared with the clinical status by age 26 years.

Of the 2857 third molars assessed at age 18 years, 92.8% were able to be followed up clinically at age 26 years. Approximately 54.9% of the teeth that were not impacted by age 18 had erupted by 26 years. Of the teeth that were impacted by age 18, 33.7% had fully erupted by age 26, 31.4% had been extracted, and 13.1% remained unerupted. Of the maxillary teeth that were categorized as "impacted" at age 18 years, 36.2% had fully erupted by age 26, whereas only 25.6% of the mandibular teeth had done so (P <.01). Fewer mandibular teeth than maxillary teeth remained unerupted by the time the patient was 26 years old (27.4% and 41.4%, respectively; P <.01), but there was no significant difference between the jaws in the proportion of impacted teeth at age 18 years that had been extracted by age 26 years (29.6% and 30.3%, respectively). For mesioangularly impacted third molars, 39.3% of maxillary teeth and 20.4% of mandibular teeth had fully erupted by age 26. Of the third molars that were mesioangularly impacted at age 18 years, 39.3% of maxillary teeth and 20.4% of mandibular teeth had fully erupted by age 26 years, whereas almost one-third of each had been extracted. Of the distoangularly impacted third molars, 20.4% of the maxillary teeth and one-third of the mandibular teeth had erupted by age 26, with 21.6% of the maxillary teeth and 31.6% of the mandibular teeth having been extracted.

Other than horizontally impacted third molars, a substantial proportion of other impaction types do erupt fully, and radiographically apparent impaction in late adolescence should not be sufficient grounds for their prophylactic removal in the absence of other clinical indications.