A frontal sphere

bulut-et-al-2016So far, the majority of new quantitative methods and approaches to investigate sexual dimorphism have focused primarily on the morphology of the five most commonly studied sexually dimorphic traits of the skull (glabella, orbital margins, mastoid process, nuchal crest, and mental eminence), while other cranial traits are still being evaluated in terms of simple subjective descriptions. One of the cranial regions showing great potential for further development of sex estimation techniques is the frontal region. Recently, Bulut and colleagues quantified the shape differences between male and female frontal bones using a novel and landmark-free 3D modeling method. Their new finding that the male frontal bone is actually more spherical than the female is in opposition to the common perception. In their study, CT scans of 80 male and 80 female Caucasian frontal bones from a Turkish population between the age of 25 and 40 years were obtained. The frontal bone was isolated by carrying out the “selection tool” in the GOM Inspect software using STL models. The frontal bone model is aligned to the CAD sphere model, using the best-fit registration method in the GOM Inspect software. Next, the difference in surface morphology between the frontal bone data and the CAD sphere was quantified, using the sphere model as the reference surface. Also, color maps were generated to show the deviations between frontal bone surface and the CAD sphere surface. Deviations of ±1mm were calculated as the overlapping areas. Color maps show that, for males, the areas exhibiting the largest discrepancy between frontal bone and CAD sphere surface are glabella, the supraorbital margins, the zygomatic processes, the superciliary arc, and the temporal face.

The area displaying most overlap with the sphere is the upper frontal region, including the frontal squama and the frontal eminences. For females, the frontal squama showed the main congruence with the sphere surface, while the largest deviations were observed for glabella, the supraorbital margins, the zygomatic processes, the superciliary arch, the frontal eminences, and the temporal face. The amount of frontal bone overlapping with the sphere was found to range from 30.1% to 56.1% for males, and from 19.6% to 48.3% for females. The difference in average values between males (43.2 ± 6.5%) and females (33.9 ± 6.6%) was found to be statistically significant, i.e. p < 0.0001, using the double-sided version of student’s t-test. This finding is in opposition to the common perception that the male frontal bone is more inclined than the female, which is described as more vertical and rounded, convex, smooth, and broad. Using the overlapping surface parameter to develop linear discriminant functions, sex was accurately predicted for 61 of 80 females (76.3%) and 63 of 80 males (78.8%) after leave-one-out cross-validation, yielding an average of 77.5% correct classifications.

Yameng Zhang

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