At present, most research methods used to measure the soft tissue morphology of the lips are based on 2D films and measurement indicators are not sufficiently comprehensive. Research into 3D measurements is scarce; most existing research into 3D measurements of 2D indicators does not fully reflect the morphological characteristics of the soft tissue of the lips and the changes that occur during orthodontic treatment. This is because facial soft tissue is an irregular curved surface. In many cases, soft tissue samples must reflect 3D features such as curved surfaces to enable a more accurate measure of morphology. In this study, 3D facial scanning technology was used to establish an appropriate measurement method, especially to measure changes in area and volume in the lip vermilion. The data collected using a 3D facial scanner was analyzed using Geomagic Qualify software; it was easy to quantify and describe morphological changes in the lip vermilion by establishing a coordinate system.
Adult female subjects who did not receive orthodontic treatment were enrolled in this study as the non-treatment group. In the present study, changes in cupid’s bow width and vermilion height in the treatment group (p < 0.01, p < 0.05, respectively) were consistent with those comparisons with the non-treatment group (p < 0.05, p < 0.05, respectively). Changes in lower and total vermilion area in the treatment group (p < 0.01, p < 0.05, respectively) were also statistically consistent with the non-treatment group (p < 0.05, p < 0.05, respectively). Masticatory muscle tension and physiological condition of lips were usually different at different times and might be possible influencing factors contributing to the accidental error of measurement and the variation in vermilion changes [10]. Those results in this study suggested that the changes of those measurements in the treatment group mainly due to the influence of extraction treatment, rather than being affected by the differences in time point for measurement.
In the present study, the statistical results of the linear and area measurements were considered in combination with the results of landmark identification. It was a preliminary study to the pattern of lip vermilion morphological change under extraction treatment. There was no significant change in mouth width, which may be related to the observation that R.Ch and L.Ch showed no significant changes in their horizontal dimension. The upper vermilion area was also not significantly difference, which may be associated with the fact that R.Cu, L.Cu, and Ls did not show significant changes in their sagittal dimension. Besides, changes in mouth width and upper vermilion area in the treatment group (p = 0.488, p = 752, respectively) were statistically consistent with those comparisons with the non-treatment group (p = 0.516, p = 0.139, respectively). Therefore, the accidental error of measurement could be considered to be negligible and mouth width and upper vermilion area appeared to be stable after the extraction treatment. A significant decrease in cupid’s bow width may be related to the significant changes in R.Cu and L.Cu in the horizontal dimension. The lower vermilion area was significantly reduced, which may be associated with significant changes in R.Ch, L.Ch, and Ll in the sagittal dimension. The significant decrease in vermilion height may be related to significant changes in Ll in the vertical dimension. Trisnawaty et al. reported that vermilion height decreased by 0.39 mm as the linear measurement of St–Me when the four first premolars were extracted [14]. However, Maltagliati et al. reported that lower lip height increased (4.61 ± 3.61 mm) significantly and the vertical dimension of the upper lip was not modified in the treated group with extraction [15], which indicates that vermilion height was increased in their study. The controversy presented by those articles requires further investigation.
A qualitative impression was obtained using the superimposed color maps and spectra. As presented in Fig. 6, the vermilion area in the treatment group retracted backwards after orthodontic treatment and differed from the superimposed results observed in the non-treatment group. A statistically significant degree of retraction occurred in the vermilion (p < 0.01) after orthodontic extraction in the present study, which can be used as quantitative evidence to support the qualitative results of the superimposed color maps and spectra. Both the upper (p < 0.05) and lower (p < 0.01) vermilion were retracted in the treatment group. These results are in accordance with the clinical observations of historical studies [4, 16]. Lee et al. reported that extraction of the four premolars led to significantly greater retraction of the upper/lower lip from Ricketts’ E plane [17]. Liu et al. reported a significantly smaller lip vermilion after extraction, indicating that the lip vermilion was more slender when the vermilion height–width ratio was decreased [10]. Nevertheless, contradicting conclusions drawn by Freitas et al. [18] and Basciftci et al. [19] reported that extraction of first premolars did not imply a greater degree of retraction in lip soft tissue. A more harmonious vermilion was considered attractive and had positive effects on lip esthetics [5], but the relationship between lip morphology and facial esthetics is complex. Ethnicity, skin color, age, and profile characteristics might all affect the quality of treatment [20]. According to Kocadereli et al., for the sake of retraction in the upper and lower lips, extraction caused a harmful effect on facial esthetics, flattening the facial profile [21]. However, in cases of nose or chin protrusion, compensatory lip protrusion improves the profile attractiveness [22]. Compared with the area and volume measurement of upper vermilion retraction (− 6.250 mm2 and − 1625.808 mm3, respectively) after extraction, the lower vermilion (− 51.005mm2 and − 1893.223mm3, respectively) had a greater degree of retraction, suggesting that the lower vermilion plays a major role in retraction. These results are consistent with historical articles that studied the responding patterns of soft tissue. A recent systematic review reported that the average lip retraction after extraction of the 4 premolars was 1.4 mm for the upper vermilion and 2.0 mm for the lower vermilion [23]. The difference between the upper and lower vermilions may vary considerably with incisor variables [24], the distance of the mandibular lip to the esthetic line [5], and other factors.
One limitation of the present study was the small sample size and there was a remarkable variation of lip morphology between different populations [20]. Large values of standard deviations among people can be an evidence and the standard deviations of upper, lower and total vermilion, as the area measurement, were larger than 80 mm2 of all calculations in this study. Previous 3D studies found that vermilion height of Asian adult females was in the range of 16.0 mm to 19.0 mm [10, 25, 26] and the results in the present study were larger both in pre- and post-treatment (21.887 mm, 20.806 mm, respectively) in the treatment group. Although samples were enrolled in an effort to eliminate bias, heterogenous intervention could not be avoided, which is a common problem in clinical research.
Thus, this study was a preliminary exploration and the results should not be generalized to clinical orthodontic treatment as a whole (e.g., in the context of different malocclusions or different treatment methods). A larger sample size would be needed in further studies to clarify the findings. Possible influencing factors that could contribute to changes in the vermilion (e.g., facial soft tissue conditions and skeletal patterns) also require further investigation or verification.