Variability in the Lower Facial Skeleton During Development

Objectives: The lower facial skeleton is a highly variable region of the craniofacial complex. Considerable attention has been given to lower facial skeletal morphology with regard to growth patterns, the influence of masticatory function, and the relationship between vertical facial dimensions and the development of skeletal and dental discrepancies. Despite this, little is known about the ontogenetic trends with regard to the magnitude of variability of the lower facial skeleton. Using the longitudinal Iowa Facial Growth Study, the present study examines whether the magnitude of lower facial variation is established early in development or if it exhibits significant changes during ontogeny.
Methods: We collected coordinate landmarks from lateral cephalograms of n=63 subjects at ages 4yrs, 11yrs, and adulthood. Landmarks were aligned using Procrustes superimposition separately for each age group. To control for the increasing effects of sexual dimorphism on skeletal shape variables, we removed allometric variation within each age group using multivariate regression. Using size-adjusted residuals, we tested for significant differences in the magnitude of shape variation using a test of morphological disparity (i.e., testing for significant differences in the trace of variance-covariance matrices). We then analyzed patterns of shape variation within age groups using principal components analysis.
Results: There was no significant difference in the magnitude of variation between the 4yr and 11yr age groups (P=0.965). However, variation significantly increased between 11yrs and adulthood (P<0.001). Patterns of shape variation along the first principal component generally distinguished between longer and shorter anterior vertical dimensions. The range of shape variation was relatively reduced in the 4yr and 11yr groups and became more exaggerated in adulthood.
Conclusions: The magnitude and pattern of lower facial variation is stable from 4yrs-11yrs in our sample. Between 11yrs-adulthood, the lower facial skeleton significantly increased in variation independent of the effects of sexual dimorphism.