Early Recognition of Mandibular Growth Pattern Using Geometric Morphometrics

Objectives: The objective of this study is to determine when the mandibular shape, indicative of growth direction, is discernable.
Methods: Sample Selection:
This study is a retrospective analysis of consecutive lateral cephalograms of brachycephalic and dolichocephalic subjects from the Forsyth Twin Study (#H-31945). Facial type was determined using Ricketts’ 5 component score, and subjects were included if their score indicated at least 1 standard deviation brachycephalic or dolichocephalic pattern. The cephalograms for each subject span 8-10 years time, with the final cephalogram ranging between 15-18 years old.

Landmark Selection and Identification:
Cephalometric radiographs were digitized using TPSDig. A single investigator identified 23 anatomical landmarks that outline the mandible. A matrix correlation analysis was then performed using MorphoJ to compare the Procrustes coordinates of the two groups at each age (6-17 years old). Two separate two-block PLS analyses were performed to determine the strength of age as a predictor of shape. The analyses used the brachycephalic or dolichocephalic sample as one block and the whole sample pooled by age as the second block. A principal components analysis of the whole sample was also performed.
Results: The matrix correlation shows that the two groups become much less similar after ages 8 and 9. The two-block PLS tests of brachycephalic subjects yielded a RV-coefficient of 0.9349, while dolichocephalic subjects yielded a RV-coefficient of 0.8957. The principal component analysis shows that the first 3 principal components account for 56.4% of the variability of the sample.

Conclusions: The results show that as the subjects age their dissimilarity in shape increases. At age 7 the mandibular form for the brachycephalic and dolichocephalic subjects show similarity as indicated by a .30 matrix correlation. By age 9 the correlation had already decreased by half (0.16). Matrix correlation scatter plots also suggest that the groups become more separate as age increases. Therefore, it is possible to assume that the growth pattern resulting in the final adult shape of a patient is established at 8-9 years of age. The two-block PLS test confirms that age is a predictor of shape, and that it is stronger for brachycephalic than dolichocephalic subjects.
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