All 34 facial measurements were found to be larger in males compared to females. Moreover, all of these differences were found to be statistically significant, even after adjusting for multiple testing, except for the upper and lower vermilion heights. The overall larger dimensions of the face and the cranium are in line with previous findings in the hard tissue (Ingerslev and Solow, 1975; Bibby, 1979; Miyajima et al. 1996; Rosas and Bastir, 2002; Franklin et al. 2005; Veyre-Goulet et al. 2008; Dayal et al. 2008; Green and Curnoe, 2009) as well as the soft tissue (Budai et al. 2003; Evison et al. 2010; Ferrario et al. 1993, 1994, 1995, 1996, 1998; He et al. 2009; Hennessy et al. 2002; Lundstrom et al. 1992; Scheideman et al. 1980; Starck and Epker, 1996).
In the orbital region, significant differences in outercanthal width, intercanthal width, and palpebral fissure length are consistent with conclusions drawn by Sforza et al. and Ferrario et al. where males have larger dimensions (Sforza et al. 2009a, Ferrario et al. 2001). In both the oral and nasal regions, the overall larger dimensions in males also agree with the findings of Sforza et al. (2011). The only non-significant findings were that of the upper and lower vermilion heights. Sforza et al. (2010) investigated the vermilion to mouth width ratio and found that females had a longer vermilion in comparison to the mouth width. Our derived ratios of vermilion height to mouth width (males: 0.3259, females: 0.3321) were in accord with that of Sforza et al.’s in that females had a larger total vermilion height in relation to the mouth width. Although the upper and lower vermilion heights were absolutely larger for males, vermilion height in females was proportionally larger relative to the size of the mouth.
It could be the case that all of the craniofacial differences described above were simply due to difference in body size, since males are on average larger than females. However, when the effects of body size (height) were controlled for, all measurements were still found to be larger in males than females and most of these differences (27/34) remained statistically significant. This indicates that even after body size differences were taken into account, males still had larger heads for virtually every dimension and feature. This finding suggests that the craniofacial sexual dimorphism that we observe in human adults cannot simply be accounted by differences in general somatic growth. It is interesting to note that minimum frontal width, both palpebral fissure lengths, nasal protrusion, and nasal height differences between the sexes were no longer a significant finding once height was factored in. This may be consistent with Toma et al.’s finding that females tended to have more prominent eyes (Toma et al. 2008). Minimum frontal width is measured from frontotemporale (left) to frontotemporale (right) which are located on the temporal crest of the frontal bone where it is the most medial and superior to the superior orbital rims (Kolar and Salter 1997). Palpebral fissure length, in effect, measures the size of the globe. Therefore, it may be the case that since females have more prominent eyes, the minimum frontal width and both palpebral fissure lengths have lost their significance once height was factored in. Nasal protrusion is measured from the base of the nose to the tip of the nose, and nasal protrusion may have lost its significance in the ANCOVA because the male nose tends to tip downwards while the female nose tends to tip upwards (Enlow 1990, Ferrario et al. 1992). In the nose, He et al. and Sforza et al. concluded that males have a wider and shorter nose than females (He et al. 2009, Sforza et al. 2011), and nasal height may have also lost its significance once height was accounted for.
Ratio measurements are useful because that can provide at least rudimentary information about shape, by comparing dimensions within the same face. In considering the seven ratios calculated here, three were found to be larger in males, while four were larger in females. Moreover, according to the T-test, cephalic, facial, upper facial, and intercanthal indices were found to be non-significant, while upper-middle facial depth, middle-lower facial depth, and nasal indices were significant at the p < 0.001 level. Although non-significant, the larger cephalic index observed in females indicated a slight tendency toward brachycephaly compared with males. Males had a larger facial index while the females had a larger upper facial index which means that males tend to have a longer and/or narrower faces overall while the females tend to have longer and/or narrower upper faces, although these differences were not statistically significant. In dissecting the facial and upper facial indices, the result may be due to the fact that males tend to be longer in the lower face (de Freitas et al. 2007, Yamauchi et al. 1967, Ferrario et al. 1996) which would increase the total face height, thus making the facial index larger in males. Once the lower face was detracted in the upper facial index, females had a larger value than males. This may be due to the fact that males have a wider face, as proposed by Bigoni et al. via the geometric morphometric analysis which also equalizes size in comparing the shape (Bigoni et al. 2010). Wider faces in males would produce a lower upper facial index in males as observed in our data. In reconsidering the facial index, the wider facial dimension may be overpowered by such a long face height, owing to the longer lower third, that it produced a larger mean than in females. Hennessy et al. (2002) via geometric morphometric analysis came to a different conclusion of females having a wider face, which is in direct contradiction to Bigoni et al. (2010). Bulygina et al. (2006) agreed with Hennessy et al. (2002) that males have a wider face owing to a larger airway. Bastir et al. (2011) agreed with Bulygina and Bigoni as well. But Bastir et al. (2011) added that males have a larger airway and have taller piriform rims, may help to offset the increase in the width dimension by increasing the height dimension and therefore maintaining the ratio. The equivocality in the facial width dimension may be why the cephalic, facial, and upper facial index differences between the sexes were found to be insignificant. Lastly, the intercanthal index tells us that although males had a higher mean and hence an increased tendency for relative hypertelorism, this difference was also found to be non-significant.
Our findings revealed that upper-middle facial depth, middle-lower face depth, and nasal indices showed significant sexual dimorphism (p < 0.001). Upper-middle facial depth index was greater in females, indicating that females have a more anterior projection of nasion and/or a more posterior projection of subnasale in relation to tragion compared to males. A more posterior projection of subnasale is consistent with Bulygina et al.’s (2006) evaluation of the hard tissue as well as Hennessy et al.’s (2002, 2005) evaluation of the soft tissue. However, a more anterior projection of nasion is inconsistent with a more posterior and less prominent nasal bone finding in females (Ingerslev and Solow, 1975; Inoue et al. 1992; Carels 1998; Rosas and Bastir, 2002) as well as Hennessy et al.’s (2002, 2005) more posterior location of nasion in females. This may be explained in two ways. Firstly, tragion’s position may vary vertically and hence disturb the ratio. Secondly, the posterior projection of subnasale in females may far outweigh the anterior projection of nasion resulting in a ratio greater in women than in men. In considering the middle-lower facial index, females had a statistically greater value than males inferring that females have a greater maxillary projection and/or mandibular retrusion. This is consistent to the previous hard tissue findings that males have a flatter upper face while females have a more convex face (Bigoni et al. 2010). Moreover, males were found to have a more prominent chin (Thayer and Dobson, 2010; Bass 1995; Byers 2002; Schwartz 2007). Hennessy et al. (2002, 2005) reverberates this notion that the female chin is displaced superiorly and posteriorly. In the soft tissue, this is also in accord with the finding that the females have a more convex facial profile while the males have a straighter profile (Skinazi et al. 1994). In terms of facial convexity, which is defined by glabella, subnasale, and soft tissue pogonion, a more posteriorly placed chin would give males a straighter and even concave profile while in females it would give them a more convex profile. Lastly, the nasal index was found to be larger in males suggesting that males have a relatively shorter and wider nose. This confirms Sforza et al.’s (2011) previous finding that the male nose has a larger width to height ratio.
There are many different intrinsic and extrinsic factors that might explain why male and females faces differ in size and shape. First and foremost, one effect of having Y-chromosome is a vastly different hormonal profile in males. Although the effects of hormones on bone development are complex, the presence of excess androgens does generally have an osteo-inductive effect, particularly on the mandible (Fujita et al, 2004). Functional matrix effects may also be at work. The larger masticatory muscles in males affect both the structure and robustness of the underlying craniofacial skeleton as well as the appearance of the facial surface. Capsular matrices as well such as the brain, eyes and tongue may also exert influences on head size via secondary displacement forces as they attain their full growth. Brain growth, for example, largely drives cranial vault growth and males have been shown to have relatively and absolutely larger brains compared to females (Goldstein et al. 2001).
Some of the findings here may have implications for the orthodontist. Although males had larger overall craniofacial dimensions, conspicuously there were no differences found in the upper and lower lip vermillion portion of the lips. Therefore orthodontists may opt for more vermilion show in females as opposed to males. In considering the ratios, females had a more anteriorly placed upper face in relation to the lower face. Moreover, the middle-lower face depth ratio adds to the evidence that females have a more anteriorly placed midface and/or a more posteriorly placed lower face. Therefore, orthodontists may choose to finish their female patients with a profile that is more convex as opposed to concave. According to the nasal index, males tended to have a wider and a shorter nose. Therefore in orthognathic cases, the team may consider an alar cinch in maxillary LeFort I advancement cases to minimize flaring of the alae especially in female patients.
Compared with the relatively straightforward size differences reported here, shape differences between males and females, as revealed by craniofacial indices, were far more subtle. The next logical step would be to complete a more comprehensive assessment of the craniofacial shape differences between males and females. The type of three dimensional landmark data used for the present study can be subjected to a variety of sophisticated multivariate shape analyses, such as geometric morphometrics. Another fundamental question not addressed in the present study relates to the ontology of these sex differences in facial form. When during development do they arise? Which facial components show evidence of dimorphism the earliest? These questions are beyond the scope of the present thesis, but are possible with large normative data repositories like the one used in the current analysis.
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