Dental Arch Asymmetry, Fluctuating Dental Asymmetry, and Dental Crowding: A Comparison of Tooth Position and Tooth Size Between Antimeres

Heritability in the narrow sense (h2) is the proportion of a trait's variation that, under ideal, simplified conditions, is attributed to additive genetic variation. This ratio of additive genetic to total (additive genetic plus environmental) variation does not take into account gene-to-gene interaction (dominance and epistasis) or gene-environment interaction. Heritability estimates of dental occlusal characteristics (position, rotation, and angulation) that take into account environmental covariance collectively suggest that the predominant source of occlusal variation is environmental. However, the ability of each organism to develop appropriate symmetry relies on complex genetic interactions to buffer differences in right and left symmetrical development that increase with environmental disturbances during development. The purpose of this pilot project was to determine the relationship between dental arch asymmetry (right and left tooth position relative to the median palatal raphe) and right and left tooth size asymmetry. Pretreatment dental study casts of 28 patients from the orthodontic residency clinic at the Indiana University School of Dentistry were analyzed in a single blind fashion. Measurements were made to determine the amount and direction of right to left asymmetry relative to the median palatal raphe. Transverse and sagittal measurements were made to record asymmetries in canine and molar positions. Furthermore, three sets of antimeric maxillary teeth, the central incisors, the canines, and the first molars, were measured for crown length and width. Fluctuating asymmetry, the difference between two sides of a bilateral trait that does not involve antisymmetry and is not directional, was present in all measurements. A composite measure of total weighted dental (fluctuating) asymmetry (TWDA) in antimeric teeth was calculated by summing the size-adjusted differences in measurements of individual antimeric pairs. Statistically significant correlations were demonstrated between the extent of fluctuating asymmetry of the teeth and the extent of transverse maxillary dental discrepancies as measured from the canines and first maxillary molars relative to the median palatal raphe. Individuals with above average TWDA values were more likely to manifest dental crowding as measured by three methods. These findings indicate a previously unreported association between decreased developmental stability (evident in increased fluctuating asymmetry), arch form discrepancies, and anterior maxillary dental crowding. Although h2 estimations that include environmental covariance for dental position, rotation, and angulation collectively suggest that the predominant source of occlusal variation is environmental, this study suggests that a variable component of occlusal variation may be the individual's relative ability to develop right and left mirror images, which has experimentally been associated with gene-to-gene interaction that h2 does not measure.