From April 2014 to January 2017, data from 52 adult female patients (24 with teeth extractions, 28 without teeth extractions) who underwent orthodontic treatments in the Department of Orthodontics, Fourth Military Medical University were collected. The average patient age was 25.09 ± 4.04 years in the extraction group and 24.83 ± 3.46 years in the non-extraction group. Inclusion criteria were: (1) Chinese, Han nationality; (2) Aged 18–35 years; (3) Angle I or Angle II malocclusion, 6° > subspinale (A)-nasion (N)-supramental (B) [ANB] angle > 0°; (4) Patients who required or did not require dental extractions; (5) Initial body mass index (BMI) between 18.5 ≤ BMI < 24, and the fluctuation range of body weight at each 3dMD image collection was within the normal range; (6) Subjects voluntarily participated in the study and provided written informed consent. Exclusion criteria were: (1) Severe osseous asymmetry, open and closed deformity or partial deformity; (2) Maxillofacial trauma, surgery, orthodontic or a tooth extraction history; (3) Maxillofacial soft tissue development defects or soft tissue asymmetry more than 5 mm; (4) Temporomandibular joint disorders with obvious clinical symptoms.

All procedures followed the guidelines of the Declaration of Helsinki (2013) and given approval by the ethics committee of Hospital of Stomatology, Fourth Military Medical University (approval number IRB-REV-2021044). Written informed consent was obtained from all the patients.

Orthodontic process

All patients were treated with a labial orthodontic system, with a bracket slot of 0.022 × 0.028 inches. For all tooth extraction cases, different anchorage requirements were selected according to the patient’s diagnosis and protrusion including microimplants or transpalatal arch (TPA) devices without any external anchorage devices. In all extraction patients, the time of tooth extraction was one week before the any bracket bonding. Four orthodontic teeth were extracted at one time. Only half of the brackets were affixed at the first time, and the bonding time of the lower half brackets was 1 month later. The second molars were treated in all cases at the later stage of orthodontic treatment.

3D measurements of facial morphology

Facial 3D soft tissue features were measured before (T0) and after treatment (T4). Soft tissue markers are illustrated in Fig. 1A. 3D line distance, angles and ratios were measured according to the marker points. The angles of right (Rt) Soft-tissue gonion (Go’)-soft-tissue pogonion (Pog’)-left (Lt) Go’ [RtGo’-Pog’-LtGo’], Rt cheilion (Ch)-upper lip point midline (ULPm)-LtCh [RtCh-ULPm-LtCh] and Rt zygomatic point (Zy)- pronasale (Pn)-LtZy [RtZy-Pn-LtZy] were measured after projection to the horizontal plane. The angles of soft-tissue nasion (N’)-Pnglabella (G)- Pog’ [N’-PnG-Pog’], Lt tragus (Tra)-Lt nasal ala (Al)LtGo’-soft-tissue menton (Me’) [LtTra-LtAlLtGo’-Me’], and [RtTra-RtAlRtGo’-Me’ were measured after projection to the sagittal plane [15,16,17,18,19].

Fig. 1
figure 1

Illustrations of fix points for 3D measurements of the facial morphology. A soft tissue markers, B face regions for separate analysis

Division of measurement areas and image measurements

Four horizontal planes were determined according to the exocanthion, subnasal point, lateral tragus point and cheilion. Four sagittal planes were identified according to the endocanthion and exocanthion on both sides. Three coronal planes were determined according to the subnasal point, bilateral lateral tragus point, and the midpoint of the line between the lateral tragus point and the subnasal point. Then, the faces were divided into 11 regions relevant to this study (Fig. 1B). In this manner, the face can be divided into 11 regions on one side and 22 regions on both sides, which will not only illustrate the soft tissue changes comprehensively, but also focus on the changes in each region.

3D data collection

The 3D facial data of the orthodontic patients were collected before bracket bonding (T0), 3 months after treatment initiation (T1), 6 months after treatment initiation (T2), 12 months after treatment initiation (T3), and at the end of treatment after bracket removal (T4). T0 and T1–T4 images were imported into Geomagic Qualify software (Geomagic 2013, Research Triangle Park, NC) to generate a 3D head model with high accuracy and color information. The obtained 3D data at T1, T2, T3 and T4 were overlapped with the images at T0 to calculate the facial changes at different time periods. A detailed description of face change data acquisitions is provided as Additional File 1: Fig. S1 and S2 [15].

Measurement of occlusal height change

Image measurement

Cephalometric lateral radiographs (T0 and T4) were imported into Onyx 2.6 in BMP format (Image Instruments, Chemnitz, Germany) and the brightness and contrast were adjusted to make the soft and hard tissues clearly visible. Some measurement items in Steiner analysis [20], Tweed analysis [21] and Coben analysis [22] were selected for cephalometric analysis. Mark points used in X-ray cephalometric analysis are shown in Fig. 2A. The orbital point (Or) and Porion point (Po) were connected for constructing the Frankfort horizontal plane (HP), from which a line perpendicular to the Frankfort horizontal (FH) plane was constructed through the nasion (N) as the vertical plane (VP) (Fig. 2A). The measurement items used in cephalometric analysis were: 1. Sella (S)-NA [SNA]; 2. SNB; 3. ANB; 4. SN-D-point (D) [SND]; 5. Upper incisor (UI)-NA [UI-NA] angle; 6. UI-NA distance; 7. Lower incisor (LI)-NB [LI-NB] angle; 8. LI-NB distance; 9. UI-LI angle; 10. Gonial (Go) menton (Me)-LI [GoMe-LI] angle; 11. N-Me (Anterior facial height); 12. Anterior nasal crest (ANS)-Me [ANS-Me] (Anterior lower facial height); 13. N-Go (Posterior facial height); 14. Articulare (Ar)-Go (Posterior lower facial height) and 15. GoMe- HP plane angle [23,24,25,26].

Fig. 2
figure 2

Cephalometric and digital maxillary model illustrations. A cephalometric marker points, B marker points and measurement items of the maxillary digital model

Measurement of dental arch width

Model measurement

Dental plaster models were collected before and after orthodontic treatment from all subjects who met the inclusion criteria. The plaster model was scanned using 3Shape R700 3D digital model scanner (3Shape, Copenhagen, DNK) to obtain the digital model data of patients before and after treatment.

The maxillary digital model data before and after orthodontic treatment were imported into 3-Matic STL 9.0 software (Materialise, Leuven, BEL) in STL format for the determination of markers and model measurement. The markers on the maxillary model before and after correction are shown in Fig. 2B. Measurement items included the maxillary model before and after orthodontic treatment, maxillary canine width (the distance between the cusp points of left and right maxillary canine) and maxillary first molar width (distance between central fossa of left and right first molars). Measurement items were the distance between the cusp points of the left and right maxillary canine (1–2) and the distance between the central fossa of left and right first molars (3–4) (Fig. 2B) [27, 28].

Statistical analysis

Point fixation and measurements were carried out once every week, three times in total, and the average was taken as the experimental result. SPSS ver. 22.0 software was used for descriptive statistics (mean ± SD) on BMI index, changes in the overall facial soft tissue area and changes in different facial soft tissue regions at different times. A paired t-test was used to compare the statistical differences between the left and right sides of the face in the same period (α = 0.05), to observe the facial and the trend of facial changes in female patients with or without tooth extraction at different times. A paired t-test was used for comparison of the initial ages between the non-extractive and extractive groups. The comparison of BMI at the same time, the general change of soft tissue and soft tissue change in different regions at the same time, were dependent on whether the variables were normally distributed. If the variables were normally distributed and the variances uniform, a t-test was used to analyze the differences between the non-extraction group and the extraction group, to determine whether there were differences in the overall facial changes, and in different regions during the same period of treatment. The obtained 3D facial morphology features (line distance, angle and ratio), occlusal height change, dental arch change and the 3D facial soft tissue change data of adult female were used for two-sided Spearman correlation analysis (α = 0.05). An error analysis description is provided as Additional file 1.

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