Mandibular shape and skeletal divergency

Pre-treatment lateral cephalograms of 41 skeletal Class I girls aged 11 to 15 were divided according to MP-SN angle: lower than 28 degrees (hypodiverg ent, 10 girls), between 31 and 34 degrees (normodivergent, 18 girls), or la rger than 37 degrees (hyperdivergent, 13 girls). The mandibular outlines we re traced and digitized, and differences in shape were quantified using the elliptic Fourier series. Size differences were measured from the areas enc losed by the mandibular outlines. Shape differences were assessed by calcul ating a morphological distance (MD) between the size-independent mean mathe matical reconstructions of the mandibular outlines of the three divergency classes. Mandibular shape was different in the three classes: large variations were found in hyperdivergent girls versus normodivergent girls (MD = 4.61), whil e smaller differences were observed in hypodivergent girls (MD versus normo divergent 2.91). Mean size-independent mandibular shapes were superimposed on an axis passing through the centres of gravity of the condyle and of the chin. Normodivergent and hyperdivergent mandibles differed mostly at gonio n, the coronoid process, sigmoid notch, alveolar process, posterior border of the ramus, and along the mandibular plane. A significant size effect was also found, with smaller mandibles in the hyperdivergent girls.