Human Premolar Crown Sizes are Highly Variable Even in the Same Mouth, with Strong Crown Size Correlations among Premolars and All Molars Except the Third Molar

Abstract

The Inhibitory Cascade Model (ICM) predicts in mice that a larger permanent first molar (M1) results in a smaller second molar (M2) and an even smaller third molar (M3). Counter to the ICM, our lab found in contemporary humans that instead of M1 size, only M2 size predicts M3 size. Our lab also identified in humans 13 different molar size ratio patterns instead of one classic ICM pattern (M1>M2>M3). Further, other work in nonhuman primates reported a Premolar Molar Module (PMM) where sizes of the fourth premolar (P4) and all three molars are highly correlated. However, premolar:molar size correlation in contemporary humans is yet to be explored. This exploration can help define how the development of these two different classes of teeth influence each other’s sizes. In humans, M1 initiates near birth, followed ~24 months later by P3 and P4 (almost simultaneously with each other), then ~12 months later by M2, and ~6 years later by M3. Our first aim was to define variation in premolar size ratio pattern. We hypothesized that all three mathematically possible premolar size ratio patterns (P3=P4, P3>P4, P3<P4) will be observed in our sample. Our second aim was to test whether premolar crown size and size ratio predict molar absolute sizes and molar size ratio patterns (e.g., M1>M2>M3). We hypothesized that absolute sizes as well as size ratios of earlier initiating teeth will predict the absolute sizes and size ratios of later initiating teeth. Maximum mesiodistal lengths of P3, P4, M1, M2, and M3 crowns were measured in millimeters to represent absolute tooth sizes using retrospective Cone Beam Computed Tomography scans of 91 dental patients (54 female, 37 male) aged 13‐23 years. A frequency analysis revealed all three mathematically possible premolar size ratio patterns in our sample (P3>P4: 49.83%, P3<P4: 34.13%, and P3=P4: 16.04%, n=293 oral quadrants). Also, 57% (n=49) of the subjects with data collected from all four quadrants showed all three premolar size ratio patterns within the same mouth. A linear mixed model analysis (95% confidence level, alpha = 0.05) showed that P3 and P4 predicted each other’s crown size, as well as that of M2, but not M3. M1 size predicts P4 size only. In sum, we found that premolar size ratio patterns are highly variable even within the same oral cavity. We also found strong correlations among P3, P4 and M2 sizes, and between M1 and P4 sizes. Our results found variability in premolar size ratio patterns that is similar to the variability that we previously found in molar size ratio patterns. Our study in contemporary humans does not support the ICM prediction of a single classic tooth size ratio pattern. However, our study outcomes supported the PMM findings with the exception that P4 was not correlated with M3.