Wisdom tooth developmental timing and eruption are independent of first molar size in a study of contemporary Canadian children

Background/Objective Third molar (M3) impaction is a common dental health concern worldwide. Classically, M3 impaction risk is higher with reduced jaw space for M3 eruption. Recently, we have shown that late‐developing M3s are also at higher risk of impaction. The factors that influence M3 developmental timing remain enigmatic. Past work has indicated that the length of the developing first molar (M1) exerts an inhibitory effect, resulting in a smaller and later‐forming M2 and M3. Here we test the hypothesis that M1 crown size predicts M3 developmental timing (advanced, or delayed) and M3 eruption (vs. impaction) in a group of young Canadian dental patients. Methods We sampled retrospective cone beam computed tomography scans and panoramic radiographs taken for clinical diagnostic purposes of multi‐ethnic patients aged 6–24 years residing in Western Canada. Post‐exclusion criteria, we studied 99 patients, sampling 327 quadrants from 57 girls and 42 boys, at two distinct times that allowed us to know if an M3 had properly erupted. Using Horos and Xoran i‐Cat software, we scored permanent molar development using a 9‐stage classification after Demirjian’s system. We measured mesiodistal lengths of M1, M2 and M3 crowns. In SPSS v.25, we calculated a Generalized Estimating Equation model through a multivariable linear regression to test if M1 crown length predicted M3 developmental timing and eruption status. We also tested other variables including patient sex and jaw type (upper; lower). Significance was set to p<0.05. Results Crown length ratios (M3:M1 by M2:M1) did not predict M3 impaction (Wald Chi‐Square [Wald X 2 ]=0.124, p=0.73) or M3 developmental stage (M2:M1, Wald X 2 =0.228, p=0.63; M3:M1, Wald X 2 =3.253, p=0.07). Patient sex was not associated with either M3 developmental timing or eruption. Jaw type was significantly associated with M3 eruption status (Wald X 2 =13.573, p=0.000) but not with M3 developmental timing (Wald X 2 =0.644, p=0.42). Among the 12 different patterns of M1:M2:M3 ratios that we observed, 3 were significantly more prevalent in the mandible versus the maxilla. Relative to M1 crown length, if M2 crown length increased by 10%, the adjacent M3 crown lengthened by 7% (±3%), indicating that M2 increased “faster” than M3. Absolute M2 crown length significantly predicted M3 crown length (Wald X 2 =14.04, p=0.000) while absolute M1 crown length did not (Wald X 2 =0.701, p=0.40). Conclusions M1 crown length does not predict early versus late M3 development or M3 eruption status, thus M1 does not appear to predispose a given M3 towards impaction. However, whether an M3 forms in the lower jaw or upper jaw does appear to increase impaction risk, but not by delaying M3 development. Instead of a local variable, some systemic factor likely regulates timing of human permanent molar development. Compared to past work in non‐human primates and other mammals, modern human children show a wider range of molar crown size ratios and thus greater phenotypic variation within the molar row, of which M2 crown length appears most labile. Support or Funding Information University of Saskatchewan Colleges of Dentistry & Medicine, and the Natural Sciences & Engineering Research Council of Canada.

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