3rd Molar Status Predicts Incident Disease of Adjacent 2nd Molars

Objectives: The purpose of this study was to investigate the potential risk of incident caries, bone loss, and/or periodontal pocket on the distal of a 2^nd molar when both 1^st and 2^nd molars are present, depending on whether a 3^rd molar is (1) absent, (2) erupted, (3) soft-tissue impaction, or (4) full bony impaction.

Methods: Study subjects were participants in the US Department of Veteran Affairs (VA) Dental Longitudinal Study (DLS). For inclusion in the study, both 1^st and 2^nd molars needed to be present at baseline.  A total of 416 subjects were included in the study with the following distribution of 3^rd molars: 460 missing, 230 erupted, 25 soft-tissue impactions, 89 full bony impactions. Risk of incident distal caries, increased distal bone loss, and increased distal probing depth of 2^nd molars over time according to 3^rd molar status at baseline was compared using multivariate survival analysis.

Results: 2^nd molars adjacent to erupted 3^rd molars were at greatest risk for incident caries at 2.5 times the risk of incident distal caries compared to 2^nd molars adjacent to missing 3^rd molars. 2^nd molars adjacent to soft-tissue impacted 3^rd molars were at greatest risk for incident bone loss at 9.2 times the risk of >40% distal bone loss over time compared to 2^nd molars adjacent to missing 3^rd molars. 2^nd molars adjacent to soft-tissue impacted 3^rd molars were at greatest risk for increased distal pocket depth over time at 6.4 times the risk of >3 mm increased distal pocket depth compared to 2^nd molars adjacent to missing 3^rd molars.

Conclusions: Soft-tissue impacted 3^rd molars increase the risk of periodontal disease of adjacent 2^nd molars. Evidence supports extraction of soft-tissue impacted 3^rd molars, although it does not support routine extraction of bony impacted 3^rd molars.