Is the lower molar eruption adapting to mandibular displacement?

Objectives: To better understand the adaptive mechanisms involved, our goal was to investigate the relationships between mandibular molar eruption and inferior mandibular molar growth displacement, controlling for maxillary molar vertical movements. Methods: A mixed-longitudinal sample of French-Canadians, including of 124 males and 122 females, was evaluated between 10 and 15 years of age. Cephalometric tracing were superimposed on the mandibular stable structures to quantify lower molar eruption; they were superimposed on stable cranial/cranial base structures to quantify inferior mandibular molar displacement and inferior maxillary molar movement (displacement + eruption). Iterative generalized-least squares procedures were used to estimate sex-specific growth curves for the each of the measures. Results: For males, the mandible underwent 16.8±2.9 mm of inferior displacement, the lower molar erupted 5.7±1.2 mm, and the upper molar moved 11.3±1.7 mm downward; females showed 2.9±0.8 mm of eruption,9.4±1.4 mm of mandibular displacement and 6.6±1.2 mm of upper molar downward movement. The growth curves ranged from relatively simple quadratic polynomials (male molar eruption) to more complex quartic polynomials (male molar displacement). Maximum growth velocities were attained at approximately 14 and 12 years for males and females, respectively. Velocities of dental eruption ranged from 0.4-3.6 mm/yr and 0.5-2.6 mm/yr for males and females, respectively. Controlling for upper molar inferior movements, the total amount of lower molar eruption that occurred between 10-15 years was closely correlated with total inferior mandibular molar displacement (r=-.731 and -.668 for males and females, respectively). Relative variability in lower molar eruption was 23.6% (males) and 86.5% (females) higher than variability in inferior mandibular molar displacement. Conclusions: Because lower molar eruption is closely associated with inferior mandibular molar displacement and variability of eruption is greater than displacement, it appears that eruption is adapting to mandibular displacement, rather than vice-versa.