The effect of sex and age on facial shape directional asymmetry in adults: A 3D landmarks-based method study

Abstract

Objectives: Facial directional asymmetry research, including age-related changes, is crucial for the evaluation of treatment of craniofacial malformations/trauma in orthodontics, facial surgery and forensic sciences. The aim was to describe facial directional asymmetry (DA) in different age categories of adults using 3D methods. According to our hypothesis, facial shape DA (1) depends on sex; (2) differs among age groups; and (3) has wider variability in older age.

Material and methods: A cross-sectional sample of healthy Czech adults without craniofacial trauma or anomalies consisted of 300 3D facial models (151 females). The age-range in the study was between 20-80 years. The shape asymmetry of 28 3D landmarks was evaluated using geometric morphometrics and multivariate statistics.

Results: The manifestation of DA was similar in both sexes and in each age category; however, there were some statistical differences. In contrast to the ideal symmetrical face, the mean asymmetrical faces tended to create a slightly bent "C" shape of the midline. Therefore, the upper face was rotated slightly clockwise and the lower face counter-clockwise. The right eye was located slightly higher, with the nasal tip and mandibular region tilting to the left. Sex differences in facial DA were significant before the age of 40. DA was more significant in the youngest males than in the oldest, while the women's DA did not change.

Conclusions: The DA patterns were similar in both sexes and in all age categories (a slightly bent C shape of the midline); however, some significant local differences between male age groups were found. A significantly more pronounced asymmetry compared to other age groups was found only in the youngest males from 20 to 40 years. Moreover, significant sexual dimorphism of DA rapidly decreased after middle age, likely caused by the same age-related changes of the face during aging.

Fig 1. Shows the localization of the 28 used 3D landmarks.

Fig 2. Mean facial asymmetry from the frontal view for males (blue wireframe graph) and females (red wireframe graph).

The grey wireframe graph shows the mean symmetrical shape for the whole group. The blue and red wireframe graphs show the specific directional asymmetry in shape beside the group-specific symmetrical shape. The green arrows show the direction and range of the asymmetrical shifts from the mean facial symmetry. Age groups: A (20–39.99 y), B (40–59.99 y), and C (60–79.99 y). Numbers 1 to 28 represent 3D landmark numbers. The scale factor for mean facial asymmetry was set at 5 (the default scale factor was 1).

Fig 3. A PCA scatter plot visualizing the variability of shape directional asymmetry in the space of the PC1 and PC2 for males with percentage of variance.

The 95% confidence ellipses represent age groups (A, B, C) in years of age. The wireframe graphs show shape asymmetry (blue lines) versus shape symmetrical mean (grey lines) for PC1 (from negative value -0.03 to positive value 0.04) and PC2 (from negative value -0.04 to positive value 0.04).

Fig 4. A PCA scatter plot visualizing the variability of shape directional asymmetry in the space of PC1 and PC2 for females with percentage of variance.

The 95% confidence ellipses represent age groups (A, B, C) in years of age. The wireframe graphs show shape asymmetry (red lines) versus shape symmetrical mean (grey lines) for PC1 (from negative value -0.06 to positive value 0.04) and PC2 (from negative value -0.04 to positive value 0.04).