Abstract
Children affected by orofacial disorders mix functional alterations with morphological problems, and suitable techniques should be devised for their analysis. Stereophotogrammetry and 3D-3D facial superimposition have already proven to reliably assess morphological differences even between twin siblings, separating the effect of genetic and environmental factors. However, little information is available about twin babies. We longitudinally analyzed a couple of healthy monozygotic twin sisters aged 6 months to 5 years (height time points). The entire 3D facial models of the two sisters were registered according to the least point-to-point distance, and the relevant RMS (root mean square) distance between the facial models was calculated at each time and compared with reference data recorded from adult twins (Mann-Whitney test, p < 0.05). RMS values in the twin sisters were on average 1.18 ± 0.21 mm, and 1.86 ± 0.53 mm in adults, with a significant difference (p < 0.01). Results showed that twins are more similar in early childhood when environmental factors are supposed to have not influenced facial morphology sufficiently. Additionally, the technique seems adequate to detect even small differences: the faces of the twin sisters were not fully identical. 3D-3D facial superimposition techniques can objectively quantify facial dissimilarity even in monozygotic twins. The method may be applied to the faces of twins discordant for some orofacial and maxillofacial pathology and potentially separate genetic and environmental factors.
Highlights
Morphological analysis of face has importance in several fields of research, including the diagnosis of genetic and congenital diseases, assessment of results after aesthetic and maxillofacial surgery, and evaluation of facial modifications due to orthognathic procedures [1,2,3,4]
The concepts of similarity and dissimilarity can be very useful for the analysis of facial modifications, for instance, to detect changes due to malformations or produced by a surgical procedure; to track facial changes related to growth; to correlate facial features to peculiar genetic and congenital diseases; and in the field of personal identification as, for example, for identifying kidnapped children released after years or main suspects recorded from video surveillance systems, just to mention some [26]
In the assessment of monozygotic twins, 3D-3D registration and superimposition procedures may help in analyzing differences among and within subjects, allowing quantification, or at least to approximation, of the impact of environmental factors in the construction of the “definitive” adult facial morphology, as the genetic component is minimized in twins who share 100% identical genetic materials
Summary
Morphological analysis of face has importance in several fields of research, including the diagnosis of genetic and congenital diseases, assessment of results after aesthetic and maxillofacial surgery, and evaluation of facial modifications due to orthognathic procedures [1,2,3,4]. A challenging field of application of facial analysis concerns the examination of monozygotic twins: they share the same genetic alleles, but can be influenced by different environmental determinants (i.e., discordance for some malformation in the dental and maxillofacial areas) which might produce instead dissimilarities at all levels, including facial morphology [3,10,11]. Hormones and environmental determinants play an important role in the facial growth and its final appearance mainly during the childhood and puberal periods [12,13,14]
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