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Natl J Maxillofac Surg
Natl J Maxillofac Surg
NJMS
Natl J Maxillofac Surg
National Journal of Maxillofacial Surgery
0975-5950
2229-3418
Wolters Kluwer - Medknow India

NJMS-15-208
10.4103/njms.njms_145_22
Original Article
Morphometric analysis of the supraorbital region for sexual dimorphism: A study on Brazilian adult dry skulls
De Lima Allana Muniz
Júnior Silvio Antônio Garbelotti 1
Serrano Rafael Verardi 2
Palma Luiz Felipe 3
De Moraes Luís Otávio Carvalho 1
Biomedical Sciences Degree Course, Department of Morphology and Genetics, São Paulo, SP, Brazil
1 Discipline of Descriptive and Topographic Anatomy, Department of Morphology and Genetics, São Paulo, SP, Brazil
2 Graduate Dentistry Program, Ibirapuera University, São Paulo, SP, Brazil
3 Department of Pathology, Federal University of São Paulo, Brazil
Address for correspondence: Prof. Luiz Felipe Palma, Federal University of São Paulo - R. Botucatu, 720 - Vila Clementino, São Paulo - SP, 04023-062, Brazil. E-mail: luizfelipep@hotmail.com
May-Aug 2024
24 7 2024
15 2 208213
13 8 2022
02 11 2022
10 1 2023
Copyright: © 2024 National Journal of Maxillofacial Surgery
2024
https://creativecommons.org/licenses/by-nc-sa/4.0/ This is an open access journal, and articles are distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 License, which allows others to remix, tweak, and build upon the work non-commercially, as long as appropriate credit is given and the new creations are licensed under the identical terms.
Introduction:

Pelvis, long bones, and skull are good indicators of sexual dimorphism. In the skull, the supraorbital region is considered a highly sexually dimorphic part. Thus, the present study aimed to analyze the sexual dimorphism of Brazilian adult dry skulls using conventional and geometric morphometry.

Materials and Methods:

Conventional morphometry was performed on 179 skulls, through the analysis of six linear measurements. For geometric morphometry, 89 skulls (right side) were selected and seven landmarks were considered. Generalized procrustes analysis, principal component analysis, and linear discriminant analysis were then carried out.

Results:

All linear measurements presented differences between both sexes. Geometric morphometry showed that 77.05% of the sample variation could be explained by the first three principal components. Moreover, considering the centroid size, there was a difference in shape between the sexes. Geometric morphometry classified sex correctly in 77.32% of the skulls and conventional morphometry from 60.89% to 73.74%.

Conclusions:

According to the analyses, the supraorbital region presents significant sexual dimorphism in Brazilian adult dry skulls. Moreover, it can be analyzed efficiently by both conventional and geometric morphometry, although the latter seems to be slightly more accurate.

Frontal bone
sex characteristics
skull
==== Body
pmcINTRODUCTION

Pelvis, skull, and long bones are good indicators of sexual dimorphism. In cases in which the pelvis and long bones cannot be located or there is not enough conservation for analysis, skull bones are the most efficient ones for sex determination.[1,2,3] The external surface of the inferior (facial) portion of the frontal bone is shaped mainly by the supraorbital region, which presents a complex morphology[4] and can be considered an important highly sexually dimorphic part of the skull.[2,5,6,7]

Male skulls generally have larger and more robust superciliary arches, whereas female ones present mild-to-moderate prominence. Moreover, the glabella shows greater variation between the sexes, being more prominent in males.[8] The supraorbital region is not only valuable as an indicator of sexual dimorphism but also to predict individual development since it is related to hormonal production, more specifically androgens.[9]

Considering that sexually dimorphic cranial features are traditionally assessed visually and scored using an ordinal scale, a highly subjective method,[10] and that the Brazilian population has unique characteristics due to marked miscegenation,[11] conducting anthropology studies is not a simple task. Although the supraorbital region is considered useful for investigating postmortem remains of multi-ethnic populations,[7,12,13,14] other techniques such as geometric morphometry are of paramount importance since they allow efficient analyses of shape.[15]

In light of these facts, the present study aims to conduct both conventional and geometric morphometric analyses of the supraorbital region of Brazilian dry skulls considering sexual dimorphism.

MATERIALS AND METHODS

Ethical issues and sample selection

This study was conducted using 403 Brazilian adult dry skulls from the anatomical collection of skulls of the Federal University of São Paulo (São Paulo, Brazil), ”Museu de Crânios,” after approval by the local Research Ethics Committee (Ref. No. 36246420.2.0000.5505, dated 23 October 2020). Those with deformities from pathology, trauma, cranial surgery, or damage to the supraorbital region were excluded from analyses. So, 179 skulls were actually evaluated (112 males and 67 females older than 18 years; mean age = 38.40 years, standard deviation = 16.44 years).

Conventional morphometry

All the skulls were evaluated by only one experienced observer by using a digital caliper (Digimess, 100.174BL, Digimess Instrumentos de Precisão Ltda, São Paulo, SP, Brazil; accuracy ±0.03 mm). Each measurement was performed three times but only the mean value from them was considered.

Initially, two virtual points were determined in every skull. These points were defined by placing the measuring arms of the digital caliper concomitantly on the landmarks frontotemporale (the most anterior point on the temporal line of the frontal bone—ft) and frontomalaretemporale (the most posterolateral point of the frontozygomatic suture—fmt) and the distance halfway between them was then considered point A on the left side and point B on the right side.[10,16] Next, the following morphometric analyses[10,17] were carried out bilaterally in all specimens: [Figure 1]

Figure 1 Points and measurements used for conventional morphometric analysis. Note that some landmarks are well-established craniometrics points and others were determined according to anatomic structures and planes

The distance from A or B to the outermost projection of the glabella (point G);

The distance from A or B to the outermost projection of the superciliary arch (point SA);

The distance between the temporal lines of the frontal bone, i.e., from A to B;

Considering a parasagittal plane (a plane virtually situated parallel to the sagittal plane), the maximum superoinferior height of the superciliary arch, i.e., the distance from point M (the outermost projection of the medial region of the superciliary arch) to the supraorbital margin—“medial supraorbital height”;

Considering a parasagittal plane (a plane virtually situated parallel to the sagittal plane), the maximum superoinferior height of the supraorbital trigone, i.e., the distance from point L (the outermost projection of the lateral region of the superciliary arch) to the supraorbital margin—“lateral supraorbital height”;

Considering a parasagittal plane (a plane virtually situated parallel to the sagittal plane), the distance from the point I (located halfway between points L and M) to the supraorbital margin—“intermediate supraorbital height”.

The data were tabulated into Microsoft Office Excel™ spreadsheets (Microsoft Corporation, Santa Rosa, California, USA) and analyzed descriptively and inferentially in the Statistics Package for Social Sciences—version 22.0™ (IBM, Armonk, USA) and Past™—version 4.03.[18] When applicable, P values of less than 0.05 were regarded as statistically significant.

Geometric morphometry

Among the skulls previously evaluated, the more representative ones (i.e., those without fractures or damages to any region) were selected for geometric morphometry analysis, totalizing 89 specimens (49 males and 40 females).

Firstly, the specimens were placed on a sheet of graph paper and then photographed. Image deformation obtained from the lens of a Finepix S1800 camera (Fujifilm Holdings Corporation, Tokyo, Japan) could then be identified by digital measurements in the TPSDig2™—version 2.30 (the State University of New York at Stony Brook, New York, USA). Following, the skulls were positioned with the right side facing the camera, 24 cm away from the camera lens, and with the mastoid process and the alveolar maxillary processes positioned perpendicularly to a straight line in relation to the camera lens. The points depicted in Figure 2 were used for analysis in the TPSDig2™: 1) Glabella (craniometric landmark); 2) Nasion (craniometric landmark); 3) Frontomalare orbitale (craniometric landmark); 4) Frontomalare temporale craniometric landmark); 5) Frontotemporale (craniometric landmark)[16] [Figure 2a]; 6) 30° angle between the glabella and the frontotemporale (a semilandmark determinated by the authors) [Figure 2b]; 7) 60° angle between the glabella and the frontotemporale (a semilandmark determinated by the authors) [Figure 2c].

Figure 2 Points used for geometric morphometry analysis. (a) Well-established craniometrics points as landmarks. (b) A semilandmark determinated by the authors: 30° angle between the glabella and the frontotemporale. (c) A semilandmark determinated by the authors: 60° angle between the glabella and the frontotemporale

After digitizing the landmarks, a generalized procrustes analysis was performed. Then, the covariance matrix was generated and principal component analysis (PCA) was performed using MorphoJ™—version 1.07a.[19] Simultaneously, relative warps were generated by TPSRelw™—version 1.53[20] for evaluating size variation, and centroid size was used for evaluating the size of each region.[19,21] Linear discriminant analysis was conducted using Past™—version 4.03.[18] When applicable, P values of less than 0.05 were regarded as statistically significant.

RESULTS

Conventional morphometry

All the measurements showed statistically significant differences between male and female skulls, according to the independent samples t test [Table 1]. The linear discriminant analysis for each analysis (both antimeres) is shown in Table 2, presenting values from 60.89% to 73.74% according to the correct sex classification.

Table 1 Results, in millimeters, from conventional morphometric analysis regarding sexual dimorphism

Measurements	Skulls	P	
Male	Female	
Mean	Standard deviation	Mean	Standard deviation	
1—right	54.64	2.81	52.87	2.54	<0.001*	
1—left	53.64	2.91	51.46	2.74	<0.001*	
2—right	39.37	3.27	37.14	2.84	<0.001*	
2—right	37.99	3.08	35.72	2.90	<0.001*	
3—right	100.57	4.78	96.95	4.07	<0.001*	
4—right	11.94	1.72	10.36	1.72	<0.001*	
4—left	12.27	1.73	10.73	1.67	<0.001*	
5—right	5.15	0.94	4.70	0.87	0.002*	
5—left	5.04	0.92	4.50	0.78	<0.001*	
6—right	5.83	1.32	4.80	0.93	<0.001*	
6—left	5.65	1.26	4.58	0.94	<0.001*	
*Statistically significant value—independent samples t test

Table 2 Using linear discriminant analysis for conventional morphometry, rates of correct sex classification

Measurements	Accuracy (%)	
1—right/left	65.36	
2—right/left	71.5	
3—right/left	64.25	
4—right/left	73.74	
5—right/left	60.89	
6—right/left	69.27	

Geometric morphometry

Multivariate analysis of variance (MANOVA) of the first ten principal components (PCs) showed a statistically significant difference between the shape of the supraorbital region of male and female skulls (Wilks’ lambda = 0.57, p = 2.47e-7). Figure 3 shows that, although the distribution of skulls of both sexes was homogeneous on the PC1 and PC2 axes, the averages from females and males diverge. Moreover, female skulls were positioned lower than male ones on the vertical axis (PC2). Lastly, the first three PCs [Figure 4] were responsible for 77.05% of the observed variance: PC1, eigenvalue = 0.014, variance = 42.62%; PC2, eigenvalue = 0.007, variance = 21.22%; PC3, eigenvalue = 0.004, variance = 13.20%.

Figure 3 Scatter plot of individuals between the PC1 and PC2 axes and ellipses of the means (confidence interval = 95%). In grey are the male skulls and in black are the female ones. PC: principal component

Figure 4 Graphical representation of the variation in the shape of the supraorbital region concerning the first three principal components of male and female skulls. In grey are the male skulls and in black are the female ones. PC: principal component

The size of the region was analyzed using the independent samples t test and showed a statistically significant difference between the size of the centroids between male and female individuals (p < 0.001).

Figures 5 and 6 graphically illustrate the differences in the shape of the supraorbital region of the skulls. Figure 5 represents the first two relative warps, which explained 68.91% of the total sample variation. By means of vectorial estimation of the morphological variation for the extremities of the axes, the possible amplitude of the difference in shape could be identified concerning the sample and taking into account the first two relative warps. Figure 6 summarizes the difference in shape, in which male skulls had a more projected glabella and a sharper curvature of the frontal bone.

Figure 5 Representation of the first (horizontal axis) and second relative warps (vertical axis). Shape estimation of the skulls that would be in the extremity of the axes

Figure 6 Shape differences in the configuration between mean female and male skulls. In grey are the male skulls and in black are the female ones

The linear discriminant analysis resulted in a value of 77.32% concerning the correct sex classification.

DISCUSSION

The “Museu de Crânios” of the Federal University of São Paulo presents the largest number of identified skulls in Brazil, a country with about 200 million inhabitants marked by intense miscegenation due to immigration.[11,22] Although there is a consensus in the literature that human skull morphology is related to the population origin,[4,5,19] very few studies have addressed Brazilians. Moreover, most of them have used conventional morphometry.[23,24,25,26] To the best of the authors’ knowledge, the present study is the first one combining both morphometry methods for studying the supraorbital region of Brazilian individuals, an important highly sexually dimorphic part of the skull.[8,10]

Conventional morphometry can evaluate only partially the shape of the objects since linear angles and distances do not effectively represent the real anatomy (e.g., the existence or not of bone projections).[27] On the other hand, geometric morphometry provides analyses of shape and size separately.[28]

By using conventional morphometry, all the linear measurements evaluated showed statistically significant differences between male and female skulls, the same reported by a study in the USA.[10] Through geometric morphometry, the shape and size of the supraorbital region also presented a significant difference between both sexes. PCA demonstrated that Brazilian skulls present about 77% of the shape variation explained by the first three PCs. A study in Colombia[13] and another one in the USA[7] reported that this variation could be explained mainly by the first two PCs (82% and 84%, respectively) while the others were less important for shape variation. Therefore, as each PC can be considered as a vector that summarizes the variation in the data,[29] Brazilian skulls probably present a more complex variation in shape concerning the supraorbital region.

By evaluating the relative warps, the first two explained about 70% of the anatomical variation. The figures resulting from this technique depicted the trends of variation through deformation in shape.[20] Thus, the results confirmed that there is a variation in the curvature of the frontal bone, as well as in the glabella, of male and female Brazilian skulls.

Discriminant function analysis showed that geometric morphometry is better to classify the Brazilian skulls into sex than linear measurements from conventional morphometry. However, both morphometry analyses presented a relatively high success rate in relation to sex classification. Studies in Colombia, USA, and Germany also assessed the supraorbital region by geometric morphometry and reported higher degrees of accuracy in classifying different samples into sexes: 84.31%,[13] 79.8%,[7] and 79.1%.[14]

Although animal studies suggest that geometric morphometry is considered a convenient, low-cost, and quick-to-perform technique,[30,31,32] it requires sophisticated statistical approaches, which would make it a time-consuming procedure and could discourage its use routinely.[33] Mikery et al.[34] showed similar results when both conventional and geometric morphometry were compared but the latter required more time and effort. On the other hand, given the present study, the authors can affirm that analyses of conventional morphometry were much easier and quicker to perform, with no need for special and complex equipment.

CONCLUSION

According to the analyses, the supraorbital region presents significant sexual dimorphism in Brazilian adult dry skulls. Moreover, it can be analyzed efficiently by both conventional and geometric morphometry, although the latter seems to be slightly more accurate.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.
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REFERENCES

1 Spradley MK Jantz RL Sex estimation in forensic anthropology: Skull versus postcranial elements J Forensic Sci 2011 56 289 96 21210801
2 Shearer BM Sholts SB Garvin HM Wärmländer SKTS Sexual dimorphism in human browridge volume measured from 3D models of dry crania: A new digital morphometrics approach Forensic Sci Int 2012 222 400.e1 5
3 Goyal M Acharya AB Sattur AP Naikmasur VG Are frontal sinuses useful indicators of sex? J Forensic Leg Med 2013 20 91 4 23357393
4 Athreya S The frontal bone in the genus Homo: A survey of functional and phylogenetic sources of variation J Anthropol Sci 2012 90 59 80 22781585
5 Rogers TL Determining the sex of human remains through cranial morphology J Forensic Sci 2005 50 1 8
6 Williams BA Rogers TL Evaluating the accuracy and precision of cranial morphological traits for sex determination J Forensic Sci 2006 51 729 35 16882212
7 Garvin HM Ruff CB Sexual dimorphism in skeletal browridge and chin morphologies determined using a new quantitative method Am J Phys Anthropol 2012 147 661 70 22371071
8 Krogman WM The Human Skeleton in Forensic Medicine 1st Springfield, Illinois Charles C. Thomas 1962
9 Godinho RM Spikins P O’Higgins P Supraorbital morphology and social dynamics in human evolution Nat Ecol Evol 2018 2 956 61 29632349
10 Casado AM Quantifying sexual dimorphism in the human cranium: A preliminary analysis of a novel method J Forensic Sci 2017 62 1259 65 28547887
11 Cunha E Lopez-Capp TT Inojosa R Marques SR Moraes LOC Liberti E The Brazilian identified human osteological collections Forensic Sci Int 2018 289 449.e1 6
12 Çelbiş O İşcan MY Soysal Z Çaǧdir S Sexual diagnosis of the glabellar region Leg Med 2001 3 162 70
13 Perlaza NA Sex determination from the frontal bone: A geometric morphometric study J Forensic Sci 2014 59 1330 2 24611586
14 Koelzer SC Kuemmel IV Koelzer JT Ramsthaler F Holz F Gehl A Definitions of frontal bone inclination: Applicability and quantification Forensic Sci Int 2019 303 109929 31472305
15 Rohlf FJ Marcus LF A revolution morphometrics Trends Ecol Evol 1993 8 129 32 21236128
16 Pereira CB Alvim MC de M e Manual para estudos craniométricos e cranioscópicos Rev Acad Bras Odontol 2015 4 1 125
17 Vinyard CJ Smith FH Morphometric testing of structural hypotheses of the supraorbital region in modem humans Z Morphol Anthropol 2001 83 23 41 11372465
18 Hammer Ø Harper DAT Ryan PD Past: Paleontological statistics software package for education and data analysis Palaeontol Electron 2001 4 art4
19 Klingenberg CP MorphoJ: An integrated software package for geometric morphometrics Mol Ecol Resour 2011 11 353 7 21429143
20 Rohlf FJ Marcus L Bello E Garcia-Valdecasas A Relative warp analysis and an example of its application to mosquito wings Contributions to Morphometrics Madrid, Spain Museo Nacional de Ciencias Naturales 1993
21 Klingenberg CP Visualizations in geometric morphometrics: How to read and how to make graphs showing shape changes Hystrix 2013 24 15 24
22 Instituto Brasileiro de Geografia e Estatística (IBGE) Censo 2010 2010 Available from: https://censo2010.ibge.gov.br/. [Last accessed on 2022 Aug 16]
23 Sassi C Picapedra A Álvarez-Vaz R Martins Schmidt C Ulbricht V Daruge Júnior E Sex determination in a Brazilian sample from cranial morphometric parameters - A preliminary study J Forensic Odontostomatol 2020 1 8 17
24 Cantín López M Suazo Galdames IC Zavando Matamala DA Smith RL Sexual dimorphism determination by piriform aperture morphometric analysis in Brazilian human skulls Int J Morphol 2009 27 327 31
25 Manoel C Prado F Caria P Groppo F Morphometric analysis of the foramen magnum in human skulls of brazilian individuals: Its relation to gender J Morphol Sci 2009 26 104 8
26 Rossi AC de Souza Azevedo FH Freire AR Groppo FC Júnior ED Ferreira Caria PH Orbital aperture morphometry in Brazilian population by postero-anterior Caldwell radiographs J Forensic Leg Med 2012 19 470 3 23084311
27 Adams DC Rohlf FJ Slice DE Geometric morphometrics: Ten years of progress following the ‘revolution ’ Ital J Zool 2004 71 5 16
28 Dryden I Mardia K Statistical Shape Analysis: With Applications in R 2nd Chichester, UK; Hoboken, NJ John Wiley & Sons 2016
29 Jolliffe IT Principal Component Analysis 2nd New York Springer 2002
30 Chatpiyaphat K Sumruayphol S Dujardin J-P Samung Y Phayakkaphon A Cui L Geometric morphometrics to distinguish the cryptic species Anopheles minimus and An. harrisoni in malaria hot spot villages, western Thailand Med Vet Entomol 2021 35 293 301 33205850
31 Gómez GF Márquez EJ Gutiérrez LA Conn JE Correa MM Geometric morphometric analysis of Colombian Anopheles albimanus (Diptera: Culicidae) reveals significant effect of environmental factors on wing traits and presence of a metapopulation Acta Trop 2014 135 75 85 24704285
32 Virginio F Domingues V da Silva LCG Andrade L Braghetto KR Suesdek L WingBank: A wing image database of mosquitoes Front Ecol Evol 2021 9 660941
33 Garcovich D Albert Gasco L Alvarado Lorenzo A Aiuto R Adobes Martin M Sex estimation through geometric morphometric analysis of the frontal bone: An assessment in pre-pubertal and post-pubertal modern Spanish population Int J Legal Med 2022 136 319 28 34693478
34 Mikery OF Rebollar-Téllez EA Cruz-López LC Marina CF Castillo A Traditional and geometric morphometry analyses of lutzomyia cruciata (Diptera: Psychodidae: Phlebotominae) Populations of Chiapas, Mexico J Med Entomol 2019 56 697 707 30615180
