Geometeric Morphometric Assessment of Mandibular Shape in 6.0-8.0 Year Olds

Objectives: We employ geometric morphometrics methods to explore three-dimensional data in an effort to delineate idiosyncratic and progressive growth variation in 6.0-8.0 year old mandibles.

Methods: We used a Microscribe digitizer to collected 31 three-dimensional landmarks from 31 mandibles (Institute for Craniofacial Study, UOP). To assess developmental age we; CT scanned the mandibles, visualized teeth as isosurfaces/volume renderings, and assigned ages based on the tooth calcification sequence of Schour and Massler. Age groups were constrained to ages 6.0-8.0 years. 3D coordinate data were analyzed with Morphologika. Shape was explored using Principle Components Analysis on Procrustes-aligned shape variables.

Results: Principle components (PC) 1-2 explain 18.5 and 12.0% of the variance, respectively. PC2-PC3 explain a further 9.6 and 8.0% of the variance, respectively. PC1 drifts to more positive scores between ages 6.0-6.5 years, but stabilizes after 7.0 years. This PC indicates that the corpus-to-ramus angular relationship is becoming more obtuse in ages 6-7.0 years. The dimensions comprising PC1 are stable between groups but idiosyncratic variation increases in ages 7-8.0 years. PC2 scores indicate little growth between groups but significant change in the range of idiosyncratic variation related to corpus width. PC3 shows substantial laterally-directed growth of the coronoid process region and a concomitant anterior progression of the chin related to idiosyncratic variation but little in the way of progressive age differences.

Conclusions: Geometric morphometric methods allows visualization of the range of idiosyncratic variation in specific age groups but also the type and degree of shape change between minor increments of age.