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.

Keywords: cone beam computed tomography; geometric morphometrics; mandible; ontogeny; sexual dimorphism.

Conflict of interest statement

None.

© 2019 Anatomical Society.

Figures

Figure 1

Number of males and females at each age during adolescence.

Figure 2

The estimated size of the mandible as a function of age. In general, the size of the mandible is larger, and change is faster in males than in females.

Figure 3

The total growth from 9 to 19 years. Mandibular changes are more pronounced in males than in females. The changes are larger in sagittal and vertical dimensions than in the transverse dimension.

Figure 4

The velocity of the annual morphological changes in form. The colour bar indicates the millimetre of change per year for each quasi‐landmark on the mandible. The growth rate is similar at 9–10 years for both sexes, with active growth at the chin and the condyles. The growth rate becomes slower from age 9 and visually plateaus by 16 years of age in females. Males growth accelerates from age 9 to 14 and slows down thereafter.

Figure 5

The growth rate difference of the mandibles between males and females in form. The growth rate is similar at 9 years. Thereafter, the mandible changes faster and goes on for a longer period in males than females, whereas it becomes rapid earlier and slows down earlier in females than in males.

Figure 6

Sexual dimorphism in shape during adolescence is shown in the first row. Sexual dimorphism in shape is not evident until around age 11 (P < 0.05). The exaggerated mandibles for males and females are plotted at the same selected ages in the second and third rows. Females are characterized by a more obtuse gonial angle and a narrower chin compared with males. These two traits become more distinct during growth.