Late-forming Second- and Third-Molars and Risk of Third Molar Impaction

Objectives: Studies on the etiology of third molar (M3) impaction are important to help determine whether a M3 can be indicated to, or safely spared from, prophylactic surgical extraction. While shortage of distal jaw space is the main risk factor of M3 impaction, it does not explain all impaction cases. Our study objective was to investigate a potential new impaction risk factor, and one that can be assessed at early ages (e.g., 10 years) in patients. Specifically, we aimed to test whether delayed M2 and M3 development was associated with less jaw space availability.
Methods: Using retrospective CBCT oral images of 150 Canadian subjects aged eight to thirteen years, we measured availability of distal jaw space and compared against status of M2 and M3 mineralization based on Demirjian’s classic stage system. To test for group differences in molar development Mann-Whitney U Tests were used at a=5%.
Results: For a given sex and age, in jaws with below-average space availability, M3 and M2 mineralization was delayed by 1-2 Demirjian stages (p<0.001). Conversely, in jaws with above-average space availability, M2 and M3 development was advanced (p<0.001). These results were seen in upper and lower jaws but were more frequently significant in the maxilla. Over 80% of the M3s adjacent to delayed-developing M2s were similarly delayed in development. Sex differences were not seen.
Conclusions: M2 and M3 developmental delays occurred most often in jaws with below-average availability of distal space, the main risk factor for M3 impaction. Delayed M2 and M3 mineralization suggests overall delayed dental development, not just of M3. Delayed-forming molars in pre-adolescents may therefore reveal desynchronizations between timing of skeletal and dental development that will decrease the likelihood of M3 eruption, providing thus more predictive clinical information for M3 impaction than previously thought.