The perspective of head height in this study we evaluated that the growth proportions were greatest for the head height in both sexes. The growth of the lower head height was threefold higher than that of the upper head height in both sexes (Table 3). The pattern of the total head height was the same as that for the average of the upper and lower head heights (Fig. 2).
The lower head dimensions are significantly affected by bone deposition on the inferior border of the zygomatic bone and surrounding temporal bone that occurs for vertical craniofacial growth2,8. It is particularly interesting that the measurements of the upper and lower head differed significantly with sex. Throughout the adolescent period, the lower head height in males was greater than the upper head height, whereas the upper head in females remained slightly larger than the lower head, and even larger than the upper head height in males. In general, sexual dimorphism begins to become prominent at an age of 12 years. Song et al.9 found that the head height was approximately 5% greater in males than in females. However, the peripubertal development period typically occurs 2 to 5 years earlier in females than in males2. It is reported that the growth of the bone underneath the sphenoid bone is sexually determined by 14 years in females and 17 years in males10. This sex-related difference might be attributable to the hormonally induced growth system, which occurs earlier and is more rapid in girls during puberty11.
In terms of face height, the total face height was overall larger in males than females (Table 2). The growth of the midface and lower face heights dominated in the present study (Table 3), which is consistent with previous reports of these two dimensions being crucially responsible for increased measurements irrespective of race and sex12,13.
In Koreans, the upper face height is larger than the midface and lower face heights in both sexes. In terms of face growth proportions, the midface, lower face, and upper face heights increased by 22.0%, 23.5%, and 4.2%, respectively, in males, and by 19.6%, 14.7%, and 0% in females (Table 3). These findings for the midface and lower face heights probably involved the growth of orbital contents and the surrounding wall of the orbital cavity and mandible acting via masticatory movements8. However, the growth of the upper face was less active in adolescents in the present study, which might be related to the brain size, which would increase steadily to 95% of the adult size during the first 10 years of life, with the growth rate decreasing thereafter14.
Previous comparative studies of the growth proportions for the maxillary and mandibular heights in Caucasian females during the adolescent period found that the increase in the mandibular height on the lower face was nearly double that of the maxillary height on the midface15. However, the present study found similar growth proportions of 10.7% and 11.8% in the midface height and lower face height, respectively, during the same period (Table 3). The extent of the anterior cranial base reportedly differs by race, indicating differences in morphological features and growth proportions of the maxillomandibular relationships between races16.
Meanwhile, the growth of the midface height was the most noticeable in both sexes in the present study. The midface height increased by 22.0% in males and 19.6% in females, which led to the midface height becoming close to the upper face height (Table 2 and Figs. 3,4). Thus, the growth pattern of the total face height was the most similar to the growth pattern of the midface height among all face height measurements (Fig. 3). It was particularly interesting that the proportional growth between the midface and lower face differed with sex: the lower face and midface heights increased by 23.5% and 22.0%, respectively, in males, in contrast to by 14.7% and 19.6% in females. These findings resemble the reports of Farkas et al.5 and Battagel17 that the height of the mandible on the lower face in females increased by 5 mm, compared with by about 8 mm in the midface up to 18 years of age.
Another point of view for head and face length we investigated that the face length increased more than the head length in both sexes. The head and face are overall longer in males than in females. Farkas et al.13 reported that the face length of Caucasian males increased by about 15 mm up to 16 years of age, whereas the increase was about 9 mm in the present study during the same period.
There was a typical morphological difference between the skull shapes of Caucasians and Koreans. Caucasians tend to have a long oval-shaped head (dolichocephalic) while Koreans tend to have a short more-rounded head shape (brachycephalic). It could therefore be assumed that dimensions in the anteroposterior direction would generally be longer in Caucasians than in Koreans. Anatomically, the lateral surface growth in the zygomaticotemporal region makes the largest contribution to the anteroposterior length18. Also, the morphological growth of the ventral surface of the sphenoid causes bone expansion of the cranial base up to the age of 19 years19,20. Battagel17 determined that the face length increased by 1.0 ~ 1.7 mm per year from 6 to 16 years of age. Sequential increases in face length would contribute to changes in the adjacent breadth components in the craniofacial region6.
In the meantime, as seen our findings of the head and face breadth comparing with other races and sex, breadth-related dimensions generally differ significantly between the sexes21. The overall breadth tends to be around 20% larger in males than in females due to difference in the facial musculature and bone growth3. The horizontal dimensions of the face have visually noticeable racial differences5, with the faces of Asians typically being transversely wider and flatter than Caucasian faces22. Farkas et al.13 reported that the face breadth of Caucasian females was an average of 117.3 mm at 8 years of age and increased to an average of 130.6 mm at 16 years of age; the corresponding breadths were 124.1 and 133.1 mm in the present study, which indicates that the face is wider in Koreans than in Caucasians. Moreover, the head is wider than the face in Caucasians23, whereas the opposite is true in East Asian females24.
Meanwhile, the difference in head breadth and face breadth between the sexes was smaller than the differences in the other measurements in the present study. The breadth differences between the head and face were 9 ~ 10 mm for all development periods in the present study (Table 2). This can be attributed to the growth of the skull having similar effects on the euryon and zygion. The head breadth (as measured from euryon to euryon) increases vertically and horizontally as the brain size increases, which affects the entire cranial shape and the face breadth6.
While many similarities were found in the comparative analysis of breadth measurements between Koreans and Caucasians, direct comparisons with other races are made difficult by the use of different types of statistical measurements1. Also, it should be remembered that breadth measurements are affected by the morphology varying between different geographical regions25.
The specific relationships between the growth proportions of the head and face and bone maturation remain unclear. Many authors have hypothesized that these relationships are most likely driven by diverse growth factors in craniofacial regions, such as the amount of masticatory movements, the dietary intake of meat, and the physiological activity level26.
The present study was subject to a few limitations. First, the mandibular breadth was not measured. Ritz-Timme et al.27 considered the average mandibular breadth to be the most critical horizontal dimension for growth proportions in craniofacial regions. Second, the body height is related to craniofacial growth, and the proportional growth change between the head and face could vary with race according to the body height1,28. Therefore, further investigations should assess diverse measurements in the zygomandibular region as well as statistical correlations between craniofacial growth and body growth.
In conclusion, this study has investigated the craniofacial growth proportions in Koreans aged 8 to 24 years by analyzing data obtained in 2003 survey of the Size Korea project. The study included large quantitative samples in the morphological analysis of a single (Korean) population during the development period. Sex-related differences during the adolescent period present as clear sexual dimorphism, with there being statistically significant differences in measurements and changes in relative growth proportions morphologically and proportionally during development. The findings of this study for craniofacial growth proportions could be useful in various fields related to human morphology.