Abstract
Objective To resolve how the preferred chewing side (PCS) affects facial asymmetry in twins, whether there are differences between monozygotic (MZ) and dizygotic (DZ) twins, and whether the twins with PCS have more asymmetric faces compared to symmetrically chewing twins. Material and methods The study included 106 Lithuanian twin pairs of the same sex, 59 MZ and 47 DZ pairs. The data were analysed from facial 3D images and manually added landmarks. 3D images were analysed by Rapidform2006 software and statistical analyses were done by using the R software environment version 4.1.0. Results The contralateral effect of PCS and larger chin side was dominant among right and non-right side chewing twins. Being female increased the whole face symmetry. Conclusion The volume of the chin becomes larger on the side opposite to the twins’ habitual chewing side. As the results are quite similar in both twin types, functional factors are more prominent than heredity.
Highlights
Chewing is a complex sum of many combined horizontal and vertical movements of the jaw
We developed a new method for measuring chin asymmetry from the facial 3D model
CI: confidence interval; AD: average distance; SP: symmetry percentage; Chin volume asymmetry score (CVAS): chin volume asymmetry score; Pg: pogonion. Both MZ and DZ twins did not have pairwise correlation within preferred chewing side (PCS) and facial asymmetry scores in means of statistical significancy, except MZ twins had weak correlation (0.32) in SP measured from whole face (Table 3)
Summary
Chewing is a complex sum of many combined horizontal and vertical movements of the jaw. These are called chewing cycles [1,2]. Chewing activity is controlled by a central pattern generator which is located in the brainstem. Most people have a preferred chewing side (PCS) so that chewing occurs mainly on one side, left or right [6]. There is no universal definition of PCS so there are countless methods to study the subject [3,7,8]
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