Impaction risk increases significantly with each year of late third molar development in Western Canadian youths

Third molar (M3) impaction affects millions of patients annually worldwide yet its etiology is unclear. Here we study an underexplored risk factor for impaction, later versus earlier M3 development, towards calculating impaction risk as a resource for clinicians.

A total of 324 M3 areas (157 maxillary, 167 mandibular) were studied using retrospective CBCT scans and orthopantomograms of 99 Western Canadians (57 females, 42 males) aged 6-24 years. M3 crypt, crown and root development stage data were collected at an earlier timepoint T1 and then tested for association with M3 eruption status data documented at a later timepoint T2. This testing used Generalized Linear Mixed and Estimation modeling to explore relationships between M3 development stage and eruption status, and measure likelihood of versus functional emergence versus impaction risk (alpha level=0.05).

Statistically, each year of late M3 development at T1 nearly doubled an M3's risk of impaction at T2. Early-developing M3s had minimal impaction risk, as low as 5 %. This outcome was equivalent in upper and lower jaws and both sexes. Impacted M3s' development was delayed due to later developmental onset, not to slower developmental rate.

Late-developing M3s are statistically more likely to become impacted. Clinical monitoring of M3 development from preadolescence is suggested. As early-developing M3s present minimal impaction risk, they are less likely to require prophylactic removal. This work provides foundational new insights into molar odontogenesis alongside practical information to help develop guidelines for managing patients with developing M3s.