Subjects
This was a retrospective longitudinal study. The study was approved by the Ethics Committee of Peking University School and Hospital of Stomatology (PKUSSIRB-201948110). The sample size of this preliminary study was determined according to historical studies (Solem RC et al. [11]: 24 subjects; Jung J et al. [12]: 32 subjects; Li H et al. [13]: 50 subjects).
The forty-two subjects (25.2 ± 1.9 years) in the treatment group were recruited from consecutive patients during their initial visit to the Department of Orthodontics, Peking University, School and Hospital of Stomatology, Beijing, China. In the treatment group, all the patients were treated using the same fixed appliances (0.022 × 0.028-inch bracket slot, Roth prescription; Xinya, Hangzhou, China) for at least 24 months.
The inclusion criteria were as follows: (1) Chinese female adults with lip protrusion who had achieved better sagittal profiles (judged by the consensus of 2 orthodontists) with treatments including extraction of the four first premolars (PM1) without mini-implants; (2) age from 18 to 30 years; (3) ANB > 0°; (4) overall good health; (5) body mass index (BMI) in the range of 18 to 24 kg/m2; and (6) no obvious facial asymmetry.
Twenty female subjects (25.5 ± 2.1 years) from among undergraduate students from Peking University, School and Hospital of Stomatology, Beijing, China, were enrolled as the non-treatment group.
The inclusion criteria were as follows: (1) Chinese female adults aged from 18 to 30 years; (2) mild crowding or spacing (<4 mm); (3) ANB > 0°; (4) overall good health; (5) BMI in the range of 18 to 24 kg/m2; and (6) no obvious facial asymmetry.
The exclusion criteria for all subjects were as follows: (1) previous orthodontic treatment; (2) anterior or posterior crossbite; (3) cleft lip and palate or other craniofacial syndromes; and (4) defects of dentition.
Three-dimensional facial scans (Figure 1) were available for each subject and were acquired using a structured light-scanning system (accuracy: ±0.05 mm; 3D CaMega; Boweihengxin Technology Inc., Beijing, China). To achieve the rest position of the lips, subjects were asked to relax their lips with full relaxation for 1 or 2 seconds in a natural head position and then pronounce the word “Emma” during facial scans [14]. All subjects had T1 and T2 facial scans. In the treatment group, T1 was the pre-treatment time point, and T2 was the post-treatment time point. In the non-treatment group, the duration between T2 and T1 was at least 12 months.
Reconstruction and analysis of morphological changes in the lip vermilion
Coordinate system construction
The coordinate system (Figures 2A and 2B) was constructed using the method described by Alqattan et al. [15] using the software Geomagic Qualify 12 (3D Systems, Rock Hill, South Carolina). The point located halfway between the inner canthi of the eyes was taken as the origin of the coordinate system at which three planes coincided. The transverse plane (XZ) was constructed to contain the right and left pupils and the soft tissue nasion (N). The sagittal plane (YZ) was set to be the perpendicular plane that contained the N-subnasal (Sn) line as the plane of symmetry of the original mirror face structure. The coronal plane (XY) was perpendicular to the sagittal and transverse planes.
Superimposition and visual analysis of morphological changes in the lip vermilion
The two selected digital facial scans (T1 and T2) in the coordinate system were superimposed according to the “best-fit alignment” algorithm [16]; thus, a superimposed 3D image with a unified system was obtained for each subject (Figure 2C). Superimposed colour maps and spectra were constructed for visual analysis (Figures 2D).
Quantitative analysis for landmark identification and linear measurements
Table 1 provides abbreviations and definitions of each lip vermilion measurement [17]. Six facial landmarks (Figure 3 and Table 1) were marked on the 3D image, including two landmarks in the middle line (Ls, Li) and two bilateral landmarks (R.Chp, L.Chp, R.Ch, L.Ch). Three straight line distances (Figure 3 and Table 1) as the linear measurements (mouth height, philtrum width, and mouth width) were measured from 3D images.
Table 1. Definition of lip vermilion measurements
Variable
|
Definition
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Landmarks in the midline
|
|
Ls
|
Labiale superius
|
Li
|
Labrale inferius
|
Bilateral landmarks
|
|
R.Chp (right chresta philtri)
|
Most prominent point of the vermilion border of right chresta philtri of the upper lip
|
L.Chp (left chresta philtri)
|
Most prominent point of the vermilion border of left chresta philtri of the upper lip
|
R.Ch (right cheilion)
|
Most lateral extent of the outline of the lip on the right side
|
L.Ch (left cheilion)
|
Most lateral extent of the outline of the lip on the left side
|
Linear measurements (mm)
|
|
Mouth height
|
Vertical distance between labrale superius (Ls) and labrale inferius (Li)
|
Philtrum width
|
Horizontal distance between right and left chresta philtri (Chp)
|
Mouth width
|
Horizontal distance between right and left cheilion (Ch)
|
Area measurements (mm2)
|
|
Upper vermilion area
|
Area of the upper vermilion along the surface
|
Lower vermilion area
|
Area of the lower vermilion along the surface
|
Total vermilion area
|
Area of the upper and lower vermilion along the surface
|
Set-up of the measuring planes and quantitative analysis for area and volumetric measurements
The measuring plane was defined (Figures 4A and 4B) for quantitative analysis. In the unified coordinate system of the superimposed 3D image, the measurement plane (Figures 4C and 4D) was perpendicular to the YZ sagittal plane, which passed through the bilateral inner canthi point. The 3D surfaces were marginated manually according to vermilion anatomical morphology (Figure 5A–5C), the other non-vermilion part was erased [18], and the remainder was projected onto the measurement planes, enabling three area measurements (upper, lower, and total vermilion area along the 3D surface) and three volumetric measurements (upper, lower, and total vermilion volumes projected onto the measurement planes) to be measured (Figures 5D–5F). The space deviations for volumetric changes (T2 measurement −T1 measurement = space deviation) in the upper, lower, and total vermilion were constructed and used for quantitative analysis.
Statistical analysis
Data were analysed using SPSS software (version 23.0; IBM Corp., Armonk, NY, USA) and G power (version 3.1.9.4; Germany). All measurements were repeated by the same operator. The degree of intra-observer error was assessed by comparing each measurement with measurements on 20 subjects chosen randomly in an interval longer than two weeks. The threshold for acceptable intra-observer error for each measurement was 0.90 with the calculation of the intraclass correlation coefficient (ICC). The normality of the data from 3D facial scans was confirmed using the Shapiro–Wilk test. A t-test was performed to evaluate changes during orthodontic treatment. A p-value of <0.05 was considered statistically significant.