Quantification of mandibular sexual dimorphism during adolescence

Abstract

The present study investigates how sexual dimorphism in the human mandible develops in three-dimensionally during adolescence. A cross-sectional sample of mandibular meshes of 268 males and 386 females, aged between 8.5 and 19.5 years of age, were derived from cone beam computed tomography and were analysed using geometric morphometric methods. Growth trajectories of the mandible in males and females were modelled separately using a recently developed non-linear kernel regression framework. Growth rate and direction at a dense array of points all over the mandibular surface were visualized within each group and compared between groups. We found that mandibular sexual dimorphism already exists at 9 years of age, but this is mostly in size not in shape. The differential growth rate and duration between the sexes during pubertal growth largely explained by adult sexual dimorphism: the growth direction in both males and females is similar but the male mandible changed more quickly and over a longer period than the female mandible, where the growth rate peaked and declined earlier. This results in increasing dimorphism in form, which is evident in both size and shape. The development of dimorphic features, concentrated in the chin and ramus, were further visualized. The dense morphometric approach provides detailed three-dimensional quantitative assessment of the development of sexual dimorphism of the mandible.

3D assessment of mandibular skeletal effects produced by the Herbst appliance

“…The morphological changes due to treatment for each patient were determined by comparing the growth observed for the patient to a model for normal mandibular growth, which consisted of population-based, healthy pre-treatment orthodontic patients with a range of occlusal classifications [13]. These data were derived from a sample of 782 subjects (268 males and 386 females) of predominantly European descent that was based on a cross-sectional mandibular normal growth study (8.5-19.5 years) conducted in Melbourne, Australia.…”

“…Subjects with a history of craniofacial anomalies, trauma or multiple missing teeth were excluded. Statistical outliers were excluded as previously described [13]. In brief, the study subjects and normal controls were drawn from the same general area.…”

“…Spatially-dense morphometric techniques cover the mandibular surface with a large number of points that capture the morphology of the entire mandible, including areas like the condyles and chin where traditional anatomical landmarks are poorly defined by local geometric features. An automatic template mapping strategy was used to ensure that each of the 17,415 defined points on one mandible corresponds with an anatomically similar point on the others [13,16]. This procedure ensures that the morphological changes were measured over the same points between different patients.…”

“…This method will take the threedimensional changes (sagittal, vertical and transverse) into account. The mandibles were iteratively aligned by estimating and applying: 1) the weighted Procrustes rotation and translation of each mandible onto the template, and 2) adjusting the 'weights' according to influence on transformation estimated by the next iteration to those points that were closest between the mandibles [13]. A weight of zero was imposed on the landmarks representing the teeth to eliminate the effect of teeth during mandibular superimposition.…”

“…The purpose of this study is to assess the orthopedic effect of the Herbst appliance on the mandible in 3D using geometric morphometrics. This is achieved by comparing changes following Herbst appliance treatment to the morphological changes estimated from normative mandibular growth curves [13].…”