Three-dimensional analysis of lip soft tissue changes and related jaw changes in patients with skeletal class III malocclusion and facial asymmetry

Abstract

OBJECTIVE:

To investigate three-dimensional soft tissue changes of lips and related jaw changes in patients with skeletal class III malocclusion and facial asymmetry using cone-beam computerized tomography (CBCT).

METHOD:

This study included 46 CBCT scans of patients with aforementioned character before (T1) and 6–12 months after orthognathic surgery (T2). Subjects were divided into 2 groups according to two types of orthognathic surgery namely, the one-jaw surgery group who underwent bilateral sagittal splint ramus osteotomy (BSSRO) and the two-jaw surgery group who underwent Le Fort I osteotomy and BSSRO. Mimics 19.0 software are used for model reconstruction, landmark location and three-dimensional cephalometric analysis. Descriptive statistics and correlation analyses are used to investigate jaw hard tissue and lip soft tissue changes.

RESULTS:

In one-jaw group, the mandible shows changes in contour and position (p < 0.05), and the surgery causes changes of lip structure on the deviated side. While in two-jaw group, jaws only show changes in spatial position, and surgery changes contour of bilateral lips and nasolabial angle (p < 0.05). At the same time, lip symmetry increases significantly in both groups postoperatively.

CONCLUSIONS:

Orthognathic surgery can improve lip aesthetics in patients with skeletal class III malocclusion and facial asymmetry. However, changes induced by two surgical approaches are different. Surgeons should have a clear acquaintance with this difference to deal with different situations.

Keywords

CBCT 3D cephalometric analyses class III malocclusion facial asymmetry lip profile

1 Introduction

Lip profile plays an increasingly important role in facial aesthetics. Symmetrical, harmonious and delicate lips greatly enhance the beauty of the face [1 –3]. The character of the lips in patients with skeletal class III malocclusion and facial asymmetry often show as collapsed and thin upper lips, valgus and thickened lower lips, and accompanied by asymmetry of bilateral ones, which seriously affects the facial appearance of the patients [4, 5].

Orthognathic surgery is an established procedure to improve facial aesthetics of asymmetric deformity [6, 7]. Bilateral sagittal splint ramus osteotomy (BSSRO) and Le Fort I osteotomy have been used alone or in combination to correct skeletal class III malocclusion with asymmetry. When patients have only the deformity of the mandible, then the BSSRO procedure will be performed, namely one-jaw surgery. Correspondingly, when the skeletal deformity involves both the maxilla and mandible, Le Fort I osteotomy is supplemented as well, which called two-jaw surgery. Aoyama et al. [8] reported that the lip morphology in patients with facial asymmetry can be corrected by two-jaw surgery, but the samples they used was patients with skeletal class I malocclusion, which were less deformed than this study. Yamashita et al. [9] studied the asymmetry of the lips in orthognathic surgery patients and concluded that lip asymmetry can be almost completely corrected by the BSSRO surgery, but the data they used was frontal facial photographs, which providing limited information for assessment and diagnosis compared with CBCT. Some scholars [10 –13] also showed that two-jaw procedure can effectively correct the lip line cant. However, the lip line cant can only present the deviation on the horizontal plane, which was inadequate to evaluate the three-dimensional changes of the lips. Admittedly, as a pivotal structure for facial aesthetics, the three-dimensional overall changes of the lip deserve our more attention.

Clinically, patients diagnosed with skeletal class III malocclusion and facial asymmetry have overall greater improvement in beauty of the face after orthodontic-orthognathic treatment. However, some patients still complain of mild asymmetry of the inferior of the face, especially in the structure of the Cupid’s bow and the corner of the mouth, which asymmetry is easily perceived by patients and can tremendously impair the aesthetics of the face [14, 15]. Regrettably, no relevant research was found to systematically and completely studied the three-dimensional changes of the lips caused by the orthognathic surgery and the related jaw changes in such patients.

Therefore, this study aims to explore how one and two-jaw orthognathic surgery can change the morphology of the lips in patients with skeletal class III malocclusion and facial asymmetry, as well as to compare the corresponding effect of each surgery procedure.

2 Materials and methods

2.1 Subjects and surgery

This retrospective research was conducted with the approval of the ethical committee of the Stomatological Hospital of Chongqing Medical University. A total of 46 patients with skeletal class III malocclusion and facial asymmetry, and who underwent BSSRO with or without Le Fort I osteotomy by a same surgeon in the abovementioned hospital from 2017 to 2020 were recruited for the study. The inclusion criteria are as follows: (1) skeletal class III malocclusion (ANB < 0°); (2) facial asymmetry (hard tissue Me point deviation > 4 mm); (3) combined orthognathic-orthodontic sequential treatment, brackets are present preoperatively and postoperatively; (4) complete CBCT scans before (T1) and 6–12 months after orthognathic surgery (T2); (5) digital-assisted surgery; (6) rigid fixation methods of bone segments. The exclusion criteria were (1) systematic diseases or congenital syndromes; (2) cleft lip or palate; (3) temporomandibular joint disorder; (4) facial trauma or deformity. Subjects were then divided into 2 groups according to the type of orthognathic surgery: the one-jaw surgery group who underwent BSSRO procedure and the two-jaw surgery group who underwent Le Fort I osteotomy and BSSRO. The basic information of the subjects are shown in Table 1.

Table 1
Basic information of study subjects

One-jaw group two-jaw group

Sample sizes (n) 23 23

Age (mean±SD) 22.18±4.08 24.33±4.13

Male/female (n) 8/15 7/16

	One-jaw group	two-jaw group
Sample sizes (n)	23	23
Age (mean±SD)	22.18±4.08	24.33±4.13
Male/female (n)	8/15	7/16

Abbreviations: n, number; SD, standard deviation.

2.2 Study data and models

The data of CBCT scans (KaVo Dental Gmb H, USA; 80 mA, 80 kVp, and 8.9-second scan time) were imported into Mimics 19.0 software (Materialise, Leuven, Belgium) in Digital Imaging and Communications in Medicine (DICOM) format and the segmentation process was performed. The threshold values were set as ranging from 226 to 3071 Hounsfield units (HU) for the hard tissue and from –700 to 225 HU for the soft tissue. After smoothing and calculation, 3D hard and soft tissue models were reconstructed respectively (as shown in Fig. 1).

Fig. 1

Reconstruction process of hard and soft tissue models. A, Jaw hard tissue model. B, Lip soft tissue model.

2.3 Measurement contents

The 3D hard and soft tissue models were then imported into the Measure and Analysis Module of Mimics for the location of landmarks, and 3D cephalometric analysis method was used to measure changes of lip soft tissue and jaw hard tissue. The jaw hard tissue changes included changes of the morphology and position of the maxilla and mandible, and changes in jaw position can be decomposed into translational and rotational movement. Consequently, we defined three reference planes and two measurement planes to measure the rotational movements, and the translational movement of the jaws were measured by the three-dimensional coordinate changes of the midpoint of the alveolar margin of the maxillary and mandibular central incisors. The BSSRO procedure can induced morphology changes of the mandible, which was represented by the changes of the length of the mandibular body on both sides and mandibular prognathism. The detailed jaw hard tissue landmarks and measurement items were established as shown in Figs. 2 4 and Table 2.

Fig. 2

Hard tissue landmarks. 1) Ba, midpoint of anterior margin of foramen magnum; 2) Or (L/R), the lowest point of infraorbital margin (left/right); 3) Po (L/R), the highest point of external auditory canal (left/right); 4) ZT (L/R), midpoint of zygomatic temporal suture (left/right); 5) Go (L/R), the most lateral and posterior point of curvature along the angle of the mandible (left/right); 6) Spr, alveolar marginal midpoints of upper central incisors; 7) Id, alveolar marginal midpoints of lower central incisors; 8) U6(L/R), middle point of alveolar margin of maxillary first molar(left/right); 9) L6(L/R), middle point of alveolar margin of mandibular first molar(left/right); 10) Pog, the most convex point of chin; 11) Me, the most inferior midpoint on the symphysis.

Fig. 3

Hard tissue reference planes. a. sagittal plane (SP), the plane passing through Ba and perpendicular to CP and HP; b. coronal plane (CP), the plane passing through ZTL, ZTR and perpendicular to HP; c. horizontal plane (HP), the plane passing through PoL, PoR and OrR; d. Maxilla plane (MxP), the plane passing through U6L, Spr and U6R; e. mandible plane (MdP), the plane passing through L6L, Id and L6R.

Fig. 4

Jaw hard tissue measurement items: 1, Spr-X; 2, Spr-Y; 3, Id-X; 4, Id-Y; 5, Spr-Z; 6, Id-Z; 7, length of mandibular body(D); 8, length of mandibular body(ND); 9, protrusion of mandibular body; 10, transverse inclination of maxilla; 11, coronal inclination of maxilla; 12, transverse inclination of mandible; 13, coronal inclination of mandible.

Table 2

Jaw hard tissue measurement variables

Variable	Definition
Spr-X(mm)	Distance from Spr to SP
Spr-Y(mm)	Distance from Spr to HP
Spr-Z(mm)	Distance from Spr to CP
Id-X(mm)	Distance from Id to SP
Id-Y(mm)	Distance from Id to HP
Id-Z(mm)	Distance from Id to CP
Transverse inclination of maxilla (°)	Angle between MxP and SP in the horizontal direction(Small angle)
Coronal inclination of maxilla (°)	Angle between MxP and CP in the sagittal direction (Small angle)
Transverse inclination of mandible (°)	Angle between MdP and SP in the horizontal direction(Small angle)
Coronal inclination of mandible (°)	Angle between MdP and CP in the sagittal direction (Small angle)
Protrusion of mandibular body (°)	Angle formed by GoL-Pog-GoR
Length of mandibular body (D, mm)	Distance from Go to Me on the deviated side
Length of mandibular body (ND, mm)	Distance from Go to Me on the non-deviated side

Abbreviations: SP, the sagittal plane; HP, the horizontal plane; CP, the coronal plane; MxP, the maxillary plane; MdP, the mandibular plane D: deviated side; ND: non-deviated side.

In addition, changes in lip soft tissue symmetry were measured and compared by line distance and angles of the lips on both sides. Landmarks and measurement variables were established as follows (Figs. 5 6, and Table 3).

Fig. 5

Landmarks of lip soft tissue. 1) Cm, the uppermost point of the nasal columella; 2/3) Sbal(L/R), apex point of left/right philtral column; 4) Sn, subnasale; 5/6) Cph(L/R), the highest point of the left/right lip arches; 7) Ls, midpoint of the vermilion border of the upper lip; 8/9) Ch(L/R), the outermost point of the left/right mouth corner; 10) Li, midpoint of the vermilion border of the lower lip; 11/12) Stms/Stmi, midpoint of the upper/ lower oral cleft line.

Fig. 6

Lip soft tissue measurement variables. A, Distance; B, angle.

Table 3

Lip soft tissue measurement variables

Variable	Definition
Protrusion angle of UL (°)	Angle formed by ChL-Ls-ChR
Protrusion angle of LL (°)	Angle formed by ChL-Li-ChR
Angle of UMC (D/ND°)	Angle formed by Cph-Ch-Stms(D/ND)
Angle of LMC (D/ND°)	Angle formed by Cph-Ch-Stmi(D/ND)
Angle of CB (D/ND°)	Angle formed by Ch-Cph-Ls(D/ND)
Central angle of CB (°)	Angle formed by CphL-Ls-CphR
Nasolabial angle (°)	Angle formed by Cm-Sn-Ls
Length of PC (D/ND, mm)	Distance from Sbal to Cph(D/ND)
Vermilion thickness of UL (mm)	Distance from Ls to Stms
Vermilion thickness of LL (mm)	Distance from Li to Stmi
Width of the mouth (mm)	Distance from ChL to ChR

Abbreviations: UL, the upper lip; LL, the lower lip; UMC, the upper mouth corner; LMC, the lower mouth corner; CB, the Cupid’s bow; PC, the philtral column; D, the deviated side; ND, the non-deviated side.

In order to avoid the bias of researchers, all variables were evaluated manually by the same operator (Wenjie Xu) twice in two weeks, and the internal reliability of observers was analyzed by using the intra-class correlation coefficient. All variables were measured twice in two-week interval by the same operator, and the mean of the two measured values was used for final statistical analysis.

2.4 Statistical analysis

The basic information and measurement data of the patients were stored in Microsoft Excel 2016 (Microsoft, Redmond, WA) database, and then imported into SPSS 26.0 (IBM Co., Armonk, NY, USA) for statistical analysis. The paired-samples T test was used to compare the differences between structures on deviated and non-deviated side in each group before and after surgery and the changes of the same measurement index after the surgery. Pearson correlation coefficient was used to analyze the correlations among the changes in jaw hard and lip soft variables. The significance level was set at 5%.

3 Results

The high intraclass coefficient for correlation value (95%confidence interval ranging from 0.891 to 0.912) indicated the good intra-observer reliability in this study.

3.1 Jaw hard tissue changes

Table 4 shows changes of jaw hard tissue before and after surgery. In the one-jaw group, there were no significant changes in the maxilla (P > 0.05 for all relating to the maxilla). The mandible exhibited three-dimensional movements, which were vertical upward translation, sagittal backward translation and rotation and horizontal translation to the non-deviated side (P < 0.05 for all). The protrusion of mandible decreased significantly after surgery (P = 0.004). In the two-jaw group, the maxilla and mandible showed horizontal and sagittal translation (P < 0.05 for all), but no significant vertical movement (P > 0.05). Meanwhile, the maxilla and mandible rotated horizontally and coronally (P < 0.05 for all), and the mandibular protrusion did not change significantly (P = 0.364).

Table 4
Changes in the jaw measurements between T1 and T2 within each group

Measurements One-jaw group Two-jaw group

T1 T2 P value T1 T2 P value

Id-X(mm) 5.01±2.74 1.96±1.03 0.000^ 5.20±2.79 2.46±1.46 0.000^

Id-Y(mm) 63.54±3.98 61.95±4.34 0.000^ 60.35±5.25 59.46±4.26 0.229

Id-Z(mm) 44.59±5.57 40.51±4.98 0.000^ 46.23±6.87 43.57±5.03 0.014^*

Spr-X(mm) 1.75±1.59 1.51±1.20 0.535 2.31±1.17 1.45±0.69 0.000^

Spr-Y(mm) 43.19±3.62 43.30±3.77 0.651 40.66±3.75 40.71±3.95 0.923

Spr-Z(mm) 42.26±3.97 41.94±3.78 0.083 42.07±4.57 44.63±4.48 0.000^

Coronal inclination of maxilla (°) 83.22±3.47 83.39±4.27 0.745 83.76±3.64 85.86±3.31 0.003^**

Transverse inclination of maxilla (°) 88.46±1.17 88.59±0.92 0.496 87.88±2.09 88.75±0.98 0.048^*

Protrusion of mandibular body (°) 65.98±4.23 68.15±3.85 0.004^ 65.18±4.19 65.74±3.60 0.364

Coronal inclination of mandible (°) 76.61±5.59 78.66±5.35 0.007^ 79.30±6.71 82.66±4.30 0.007^

Transverse inclination of mandible (°) 88.05±1.81 88.47±1.40 0.299 86.87±2.72 88.68±1.15 0.001^

Length of mandibular body (D, mm) 83.57±5.07 86.97±5.68 0.004^** 83.95±6.77 86.41±5.33 0.017^*

Length of mandibular body (ND, mm) 89.70±5.33 86.45±4.98 0.008^** 89.97±6.28 86.22±5.58 0.002^*

Measurements	One-jaw group	Two-jaw group
Id-X(mm)	5.01±2.74	1.96±1.03	0.000^**	5.20±2.79	2.46±1.46	0.000^**
Id-Y(mm)	63.54±3.98	61.95±4.34	0.000^**	60.35±5.25	59.46±4.26	0.229
Id-Z(mm)	44.59±5.57	40.51±4.98	0.000^**	46.23±6.87	43.57±5.03	0.014^*
Spr-X(mm)	1.75±1.59	1.51±1.20	0.535	2.31±1.17	1.45±0.69	0.000^**
Spr-Y(mm)	43.19±3.62	43.30±3.77	0.651	40.66±3.75	40.71±3.95	0.923
Spr-Z(mm)	42.26±3.97	41.94±3.78	0.083	42.07±4.57	44.63±4.48	0.000^**
Coronal inclination of maxilla (°)	83.22±3.47	83.39±4.27	0.745	83.76±3.64	85.86±3.31	0.003^**
Transverse inclination of maxilla (°)	88.46±1.17	88.59±0.92	0.496	87.88±2.09	88.75±0.98	0.048^*
Protrusion of mandibular body (°)	65.98±4.23	68.15±3.85	0.004^**	65.18±4.19	65.74±3.60	0.364
Coronal inclination of mandible (°)	76.61±5.59	78.66±5.35	0.007^**	79.30±6.71	82.66±4.30	0.007^**
Transverse inclination of mandible (°)	88.05±1.81	88.47±1.40	0.299	86.87±2.72	88.68±1.15	0.001^**
Length of mandibular body (D, mm)	83.57±5.07	86.97±5.68	0.004^**	83.95±6.77	86.41±5.33	0.017^*
Length of mandibular body (ND, mm)	89.70±5.33	86.45±4.98	0.008^**	89.97±6.28	86.22±5.58	0.002^*

Abbreviations: D: deviated side; ND: non-deviated side; ^*P < 0.05; ^**P < 0.01.

3.2 Lip soft tissue changes

Changes of lip soft tissue before and after surgery are exhibited in Table 5. In the one-jaw group, the BSSRO procedure induced changes on the deviated side of the bilateral symmetric structure (P < 0.05 for all, except for the angle of the upper mouth). Nevertheless, the two-jaw surgery adds to increased impact on nasolabial angle on this basis (P < 0.01), meanwhile the bilaterally symmetrical structures are altered in both the deviated and non-deviated sides (P < 0.05 for all, except for the angle of the lower mouth).

Table 5
Changes in the lip measurements between T1 and T2 within each group

Measurements One-jaw group Two-jaw group

T1 T2 P value T1 T2 P value

Protrusion angle of UL (°) 112.00±6.38 108.20±5.00 0.012^* 113.08±6.98 104.26±6.59 0.000^

Protrusion angle of LL (°) 114.40±8.03 121.68±6.30 0.000^ 114.66±6.08 118.86±6.82 0.002^

Angle of UMC (D°) 25.83±5.90 26.97±4.57 0.091 24.26±3.14 28.59±2.85 0.000^

Angle of UMC (ND°) 28.29±5.72 27.59±4.33 0.339 26.08±3.38 28.87±2.89 0.001^**

Angle of LMC (D°) 23.20±3.40 21.77±2.58 0.025^* 22.40±3.42 21.54±2.73 0.247

Angle of LMC (ND°) 22.61±2.96 21.95±2.20 0.256 22.23±2.72 23.06±6.63 0.536

Angle of CB (D°) 129.91±6.64 126.69±5.74 0.004^ 134.70±8.97 127.74±5.69 0.000^

Angle of CB (ND°) 125.37±6.45 124.88±6.27 0.612 129.58±9.49 126.33±4.81 0.035^*

Central angle of CB (°) 138.66±10.13 135.91±11.22 0.284 138.53±9.63 139.85±8.94 0.524

Nasolabial angle (°) 91.30±12.68 93.74±13.43 0.065 84.75±15.34 94.44±12.58 0.000^

Vermilion thickness of UL (mm) 8.28±1.96 9.03±1.80 0.006^ 8.52±1.53 9.98±1.33 0.000^**

Vermilion thickness of LL (mm) 10.81±1.41 10.31±1.14 0.043^* 10.96±1.47 10.01±1.36 0.003^**

Width of the mouth (mm) 46.78±4.02 48.47±3.41 0.027^* 48.55±3.64 46.26±3.38 0.001^**

Length of PC (D, mm) 12.91±2.18 13.34±2.08 0.029^* 11.32±1.84 12.16±1.85 0.000^

Length of PC (ND, mm) (mm) 13.00±2.06 13.42±2.01 0.062 11.23±1.88 12.15±1.89 0.000^

Measurements	One-jaw group	Two-jaw group
Protrusion angle of UL (°)	112.00±6.38	108.20±5.00	0.012^*	113.08±6.98	104.26±6.59	0.000^**
Protrusion angle of LL (°)	114.40±8.03	121.68±6.30	0.000^**	114.66±6.08	118.86±6.82	0.002^**
Angle of UMC (D°)	25.83±5.90	26.97±4.57	0.091	24.26±3.14	28.59±2.85	0.000^**
Angle of UMC (ND°)	28.29±5.72	27.59±4.33	0.339	26.08±3.38	28.87±2.89	0.001^**
Angle of LMC (D°)	23.20±3.40	21.77±2.58	0.025^*	22.40±3.42	21.54±2.73	0.247
Angle of LMC (ND°)	22.61±2.96	21.95±2.20	0.256	22.23±2.72	23.06±6.63	0.536
Angle of CB (D°)	129.91±6.64	126.69±5.74	0.004^**	134.70±8.97	127.74±5.69	0.000^**
Angle of CB (ND°)	125.37±6.45	124.88±6.27	0.612	129.58±9.49	126.33±4.81	0.035^*
Central angle of CB (°)	138.66±10.13	135.91±11.22	0.284	138.53±9.63	139.85±8.94	0.524
Nasolabial angle (°)	91.30±12.68	93.74±13.43	0.065	84.75±15.34	94.44±12.58	0.000^**
Vermilion thickness of UL (mm)	8.28±1.96	9.03±1.80	0.006^**	8.52±1.53	9.98±1.33	0.000^**
Vermilion thickness of LL (mm)	10.81±1.41	10.31±1.14	0.043^*	10.96±1.47	10.01±1.36	0.003^**
Width of the mouth (mm)	46.78±4.02	48.47±3.41	0.027^*	48.55±3.64	46.26±3.38	0.001^**
Length of PC (D, mm)	12.91±2.18	13.34±2.08	0.029^*	11.32±1.84	12.16±1.85	0.000^**
Length of PC (ND, mm) (mm)	13.00±2.06	13.42±2.01	0.062	11.23±1.88	12.15±1.89	0.000^**

Abbreviations: UL, the upper lip; LL, the lower lip; UMC, the upper mouth corner; LMC, the lower mouth corner; CB, the Cupid^′s bow; PC, the philtral column; D, the deviated side; ND, the non-deviated side; ^*P < 0.05; ^**P < 0.01.

3.3 Difference between bilateral symmetrical structures of the lips

Table 6 exhibits contrasts of bilateral symmetrical structures of the lips before and after surgery in each group. Before the orthognathic surgery, both one- and two-jaw group showed differences in the angle of upper mouth and the angle of Cupid’s bow between the deviated and non-deviated side (P < 0.05 for both). However, these differences disappeared after the surgery (P > 0.05 for both).

Table 6
Pre-and post-operative differences in bilateral symmetrical structures of the lips in each group

Measurements One-jaw group Two-jaw group

D ND P value D ND P value

Pre-

Angle of UMC (°) 25.83±5.90 28.29±5.72 0.009^ 24.26±3.14 26.08±3.38 0.000^

Angle of LMC (°) 23.20±3.40 22.61±2.96 0.269 22.40±3.42 22.23±2.72 0.658

Length of PC (mm) 12.91±2.18 13.00±2.06 0.538 11.32±1.84 11.23±1.88 0.352

Angle of CB (°) 129.91±6.64 125.37±6.45 0.005^ 134.70±8.97 129.58±9.49 0.000^

Post-

Angle of UMC (°) 26.97±4.57 27.59±4.33 0.372 28.59±2.85 28.87±2.89 0.606

Angle of LMC (°) 21.77±2.58 21.95±2.20 0.722 21.54±2.73 23.06±6.63 0.199

Length of PC (mm) 13.34±2.08 13.42±2.01 0.546 12.16±1.85 12.15±1.89 0.940

Angle of CB (°) 126.69±5.74 124.88±6.27 0.157 127.74±5.69 126.33±4.81 0.097

Measurements	One-jaw group	Two-jaw group
Pre-
Angle of UMC (°)	25.83±5.90	28.29±5.72	0.009^**	24.26±3.14	26.08±3.38	0.000^**
Angle of LMC (°)	23.20±3.40	22.61±2.96	0.269	22.40±3.42	22.23±2.72	0.658
Length of PC (mm)	12.91±2.18	13.00±2.06	0.538	11.32±1.84	11.23±1.88	0.352
Angle of CB (°)	129.91±6.64	125.37±6.45	0.005^**	134.70±8.97	129.58±9.49	0.000^**
Post-
Angle of UMC (°)	26.97±4.57	27.59±4.33	0.372	28.59±2.85	28.87±2.89	0.606
Angle of LMC (°)	21.77±2.58	21.95±2.20	0.722	21.54±2.73	23.06±6.63	0.199
Length of PC (mm)	13.34±2.08	13.42±2.01	0.546	12.16±1.85	12.15±1.89	0.940
Angle of CB (°)	126.69±5.74	124.88±6.27	0.157	127.74±5.69	126.33±4.81	0.097

Abbreviations: UMC, the upper mouth corner; LMC, the lower mouth corner; CB, the Cupid^′s bow; PC, the philtral column; D, the deviated side; ND, the non-deviated side; ^*P < 0.05; ^**P < 0.01.

3.4 Correlation between hard and soft tissue variables

In the one-jaw group, it revealed a significant negative correlation between the thickness of the upper lip vermilion and the sagittal movement of the mandible (r = –0.535, P < 0.05). While in two-jaw group, the thickness of the upper lip vermilion was negatively correlated with the sagittal protrusion of the mandible (r = –0.609, P < 0.01) and the transverse inclination of the maxilla (r = –0.541, P < 0.01) and mandible (r = –0.584, P < 0.01). As for the increment of the thickness of the upper lip vermilion in each group, the one-jaw surgery was 0.75 mm on average and the two-jaw surgery was 1.46 mm.

For the changes of protrusion of the lower lip, there were also different outcomes. In the one-jaw surgery group, the protrusion angle of the lower lip was negatively correlated with sagittal position of the mandible (r = –0.748, P < 0.01). Nevertheless, the two-jaw surgery group showed no correlation between hard tissue and the changes of protrusion of the lower lip. In addition to these two soft tissue changes occurred in both groups, the two-jaw surgery group exhibited much more correlations between skeletal hard and lip soft tissue, as is showed in Table 7 and Table 8.

Table 7
Pearson correlation analysis between hard and soft tissue in one-jaw group

Pearson correlation (r/p value) Protrusion angle of UL Vermilion thickness of UL Protrusion angle of LL Vermilion thickness of LL Width of the mouth Angle of LMC (D) Length of PC (D) Angle of CB (D)

Id-X –0.003 –0.051 0.106 0.045 –0.085 0.195 0.252 –0.250

0.990 0.841 0.676 0.858 0.737 0.438 0.341s 0.318

Id-Y 0.024 0.168 0.191 0.395 –0.078 0.376 0.143 –0.052

0.924 0.504 0.449 0.105 0.758 0.124 0.57 0.837

Id-Z 0.116 –0.535 –0.748 –0.078 –0.292 –0.131 –0.086 0.219

0.647 0.022^* 0.000^* 0.758 0.240 0.605 0.736 0.382

Protrusion of mandibular body 0.250 –0.276 0.125 –0.390 –0.048 –0.367 –0.196 0.118

0.318 0.267 0.622 0.110 0.849 0.134 0.436 0.640

Coronal inclination of mandible –0.016 –0.235 –0.240 0.308 0.008 –0.244 –0.053 0.288

0.949 0.374 0.338 0.213 0.974 0.330 0.836 0.246

Transverse inclination of mandible 0.202 –0.436 –0.236 0.022 0.071 –0.202 –0.131 –0.161

0.422 0.071 0.345 0.932 0.780 0.422 0.603 0.524

Length of mandibular body(D) –0.030 –0.229 –0.199 0.196 –0.185 0.179 0.156 0.092

0.907 0.361 0.429 0.436 0.461 0.477 0.537 0.717

Length of mandibular body (ND) 0.046 –0.133 0.016 0.536 0.169 –0.449 0.079 –0.462

0.856 0.598 0.950 0.015 0.502 0.061 0.755 0.053

Pearson correlation (r/p value)	Protrusion angle of UL	Vermilion thickness of UL	Protrusion angle of LL	Vermilion thickness of LL	Width of the mouth	Angle of LMC (D)	Length of PC (D)	Angle of CB (D)
Id-X	–0.003	–0.051	0.106	0.045	–0.085	0.195	0.252	–0.250
	0.990	0.841	0.676	0.858	0.737	0.438	0.341s	0.318
Id-Y	0.024	0.168	0.191	0.395	–0.078	0.376	0.143	–0.052
	0.924	0.504	0.449	0.105	0.758	0.124	0.57	0.837
Id-Z	0.116	–0.535	–0.748	–0.078	–0.292	–0.131	–0.086	0.219
	0.647	0.022^*	0.000^*	0.758	0.240	0.605	0.736	0.382
Protrusion of mandibular body	0.250	–0.276	0.125	–0.390	–0.048	–0.367	–0.196	0.118
	0.318	0.267	0.622	0.110	0.849	0.134	0.436	0.640
Coronal inclination of mandible	–0.016	–0.235	–0.240	0.308	0.008	–0.244	–0.053	0.288
	0.949	0.374	0.338	0.213	0.974	0.330	0.836	0.246
Transverse inclination of mandible	0.202	–0.436	–0.236	0.022	0.071	–0.202	–0.131	–0.161
	0.422	0.071	0.345	0.932	0.780	0.422	0.603	0.524
Length of mandibular body(D)	–0.030	–0.229	–0.199	0.196	–0.185	0.179	0.156	0.092
	0.907	0.361	0.429	0.436	0.461	0.477	0.537	0.717
Length of mandibular body (ND)	0.046	–0.133	0.016	0.536	0.169	–0.449	0.079	–0.462
	0.856	0.598	0.950	0.015	0.502	0.061	0.755	0.053

Abbreviations: UL, the upper lip; LL, the lower lip; LMC, the lower mouth corner; CB, the Cupid’s bow; PC, the philtral column; D, the deviated side; ND, the non-deviated side; ^*P < 0.05; ^**P < 0.01. “+” represents a positive correlation; “–” represents a negative correlation.

Table 8

Pearson correlation analysis between hard and soft tissue in two-jaw group

Pearson correlation (r/p value)	Protrusion angle of UL	Vermilion thickness of UL	Protrusion angle of LL	Vermilion thickness of LL	Width of the mouth	Angle of UMC (D)	Angle of LMC (ND)	Length of PC (D)	Length of PC (ND)	Angle of CB (D)	Angle of CB (ND)
Id-X	–0.149	0.266	0.143	0.206	–0.099	0.437	0.191	0.289	0.054	–0.152	–0.380
	0.451	0.171	0.469	0.293	0.616	0.020^*	0.329	0.136	0.786	0.440	0.046^*
Id-Z	0.551	–0.609	–0.253	0.050	0.122	–0.579	–0.553	–0.720	–0.546	0.458	0.592
	0.002^**	0.001^**	0.193	0.800	0.536	0.001^**	0.002^**	0.000^**	0.003^**	0.01^**	0.001^**
Spr-X	0.256	–0.075	–0.079	0.156	0.062	–0.050	–0.022	0.137	0.062	–0.033	0.079
	0.189	0.705	0.690	0.428	0.755	0.801	0.913	0.486	0.755	0.867	0.689
Spr-Z	–0.077	0.020	–0.044	–0.092	0.145	–0.051	–0.119	–0.271	–0.091	0.046	0.118
	0.697	0.919	0.825	0.643	0.461	0.797	0.545	0.163	0.644	0.817	0.549
Coronal inclination of maxilla	0.035	–0.235	0.099	–0.096	0.197	–0.141	–0.286	–0.529	–0.383	0.173	0.261
	0.861	0.229	0.617	0.628	0.315	0.473	0.140	0.004^**	0.044^*	0.379	0.180
Transverse inclination of maxilla	0.769	–0.541	–0.115	0.135	0.207	–0.524	–0.369	–0.114	–0.096	0.214	0.509
	0.000^**	0.003^**	0.559	0.494	0.292	0.004^**	0.053	0.563	0.627	0.275	0.006^**
Coronal inclination of mandible	0.154	–0.231	–0.166	0.024	0.216	–0.238	–0.311	–0.535	–0.271	0.238	0.488
	0.434	0.237	0.400	0.902	0.271	0.222	0.107	0.003^**	0.163	0.222	0.008^**
Transverse inclination of mandible	0.672	–0.584	0.146	–0.181	0.073	–0.432	–0.397	–0.343	0.042	–0.043	0.418
	0.000^**	0.001^**	0.460	0.356	0.712	0.022^*	0.036^*	0.074	0.831	0.865	0.027^*
Length of mandibular body (D)	0.471	–0.462	0.102	–0.119	0.232	–0.482	–0.534	–0.581	–0.373	0.447	0.610
	0.011^*	0.013^*	0.605	0.546	0.236	0.009^**	0.003^**	0.001^**	0.051	0.017^*	0.001^**
Length of mandibular body (ND)	0.123	–0.280	–0.020	–0.288	0.304	–0.428	–0.402	–0.427	–0.420	0.620	0.529
	0.534	0.150	0.920	0.137	0.116	0.023^*	0.034^*	0.023^*	0.026^*	0.000^**	0.004^**

4 Discussion

In this study, we used CBCT as the tool to study facial soft tissue changes, although many analogous studies now use the records of three-dimensional facial scanner as the perfect research data [16 –18]. As we known, due to the fact that three-dimensional facial scanner cannot acquire facial soft and hard tissue data simultaneously and registration of soft and hard tissues is required when investigating the correlations between soft and hard tissues, thus this would have corresponding errors as a result of the different sources of the two data. In contrast, CBCT overcomes this drawback, and soft tissue models reconstructed with CBCT also have clear tissue surface after smooth restoration and are almost indistinguishable from the three-dimensional facial scanner models removing the texture of facial skin [19]. Consequently, CBCT data were selected to study the changes of soft and hard tissues in this study.

Three-dimensional cephalometric technique using CBCT has recently become popular in clinics of orthodontics and orthognathic surgery, which has many advantages over conventional two-dimensional cephalometric techniques [20, 21]. Although a complete analysis system has not been established yet, it still has been widely used to compare pre- and post-treatment changes utilizing traditional two-dimensional measurement indexes [22, 23]. Measurements of soft and hard tissues in this study were all performed using three-dimensional cephalometric analysis with Mimics software, which can automatically generate measurements after defining landmarks and measurement variables, and thus avoiding the errors caused by importing the model into other software for manual measurements.

The main finding of our research beyond other studies of the lips was that, for the lip changes caused by the jaw movements, we found two surgical approaches had different results. For example, about the changes in bilateral symmetric structure of the lips, two groups showed great differences. The one-jaw surgery mainly caused changes of the lip structure on the deviated side through the translation, rotation and morphological changes of the mandible, so that the measured values of this side were close to the contralateral side after surgery, and thereby increasing the symmetry of the lip, for example, the changes of the angle of CB. In the preoperative stage, there were significant differences between the angle of CB(D) and the angle of CB (ND), as is showed in Table 6. Next, the result of the paired-samples T test showed that the angle of CB(D) changed significantly after the BSSRO surgery, but no significant changes were observed on the contralateral side (Table 5). Meanwhile, there was no significant difference in the angle of CB between two sides postoperatively (Table 6), which indicated that the lip symmetry increased significantly after the BSSRO surgery. However, in the two-jaw surgery group, significant changes occurred on both sides, and the deviated side changed more than the non-deviated side, which narrowed the difference between the two sides, and this significant change in the same trend made the two postoperative structures become more symmetrical, which can be exemplified by the changes in the angle of CB and UMC. On the other hand, even if there was no statistical difference between the two sides after the operation, the measured values of the two sides were not exactly the same, and this may be the reason why some patients still complained of asymmetry postoperatively.

Although we know that it is not possible to achieve complete symmetry even in normal individuals, patients with facial asymmetry are more sensitive to asymmetric deformities, and thus have higher expectations for the surgery, especially in female patients, which are understandable by surgeons. Pearson’s correlation coefficient showed there were many hard tissue variables causing the changes of bilateral symmetric structures of the lips, and the mechanism was therefore more complex. Accordingly, surgeons are advised to precisely design jaw movement patterns and amounts for optimal esthetic and functional outcomes on the basis of patient’s personalized lip characteristics.

As for changes in upper and lower lip thickness, two procedures both caused thickening of the upper lip vermilion and thinning of the lower lip vermilion, and the changes in the two-jaw group were greater than those in the one-jaw group, which were consistent with findings by Kim et al. [24] but different with results from Hemmatpour et al. [25], who used 2D frontal photographs and found that bimaxillary orthognathic surgery could increase vermilion height of the lower lip. The thickening of the upper lip vermilion and thinning of the lower lip vermilion are overall positive esthetic changes for skeletal class III patients, but there are ethnic and gender differences. Thick lips are widely popular among westerners, but not in accordance with the esthetics of eastern women [4].

Both one and two-jaw procedures increased the upper lip protrusion and decreased the lower lip protrusion, but the reduction of lower lip protrusion caused by the one-jaw surgery was significantly greater than that in the two-jaw group, and the increase of upper lip protrusion caused by the two-jaw procedure was significantly greater than that in one-jaw group, nevertheless, there was no significant difference in preoperative upper and lower lip protrusion between the two groups, suggesting that BSSRO surgery can still cause a small increase in upper lip protrusion while substantially reducing lower lip protrusion, and this change is proportional to the amount of mandible retraction from surgery.

Choi et al. [26] reported that the width of the mouth had the tendency of narrowing when two-jaw procedure was implemented, which was consistent with our results. But Sforza et al. [27] showed the different result, which compared changes of 7 patients before and after the two-jaw surgery and suggesting that two-jaw surgery caused wider of the mouth. In addition, we also found that after the one-jaw surgery, the width of the mouth tended to be wider. Pearson correlation results showed there were no correlation between skeletal variables and the oral width, this might be due to the influence of the muscle encircling the mouth, especially the orbicularis oris muscle, which were not only connected to the jaw bone and largely influenced the width of the mouth [8]. In view of this, we told the patient to bite their posterior teeth gently and relax their lips when the CBCT was shot, and therefore reduced the error for the location of Ch point as much as possible. Preferences for mouth width also vary by race and sex, therefore, surgeons should understand this point and apply it to clinical treatment.

Changes in lip symmetry structures were the main and differentiating finding of this study from others. In other words, when other studies mainly investigated symmetry of certain landmarks and angulation of the inter-commissural line of the lips, this study explored the three-dimensional comprehensive lip changes caused by one-jaw and two-jaw surgery, and we found the lip changes caused by the two procedures were different. As the most important aesthetic structure of the lower part of the face, the symmetry of the lip is one of the important elements of its beauty, which was ignored by peers in the past. An overall understanding of the changes of the lip structure caused by the orthognathic surgery would allow surgeons to have a more precise control over the surgery, and thus to individually design the surgical plan according to the patient’s request for optimal treatment outcomes, which is the central clinical significance of this study.

In conclusion, the orthognathic surgery can greatly improve the lip aesthetics in patients with skeletal class III malocclusion and facial asymmetry, but the changes induced by the two surgical approaches are different. Surgeons should have a clear acquaintance with this difference to deal with different situations in order to achieve the best treatment effect.

Footnotes

Acknowledgments

This work was supported by Medical Research Key Project of Chongqing Science and Technology Commission and Health and Family Planning Commission Joint (Grant number: 2018ZDXM020) and Medical Scientific Research Project of Chongqing (Grant number: cstc2020jcyj-msxmX0307).

Competing interests

No.

Ethical approval

This research was conducted with the approval of the ethical committee of the Stomatological Hospital of Chongqing Medical University (ID: CQHS-REC-2021001).

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Three-dimensional analysis of lip soft tissue changes and related jaw changes in patients with skeletal class III malocclusion and facial asymmetry

Abstract

OBJECTIVE:

METHOD:

RESULTS:

CONCLUSIONS:

Keywords

1 Introduction

2 Materials and methods

2.1 Subjects and surgery

Table 1 Basic information of study subjects One-jaw group two-jaw group Sample sizes (n) 23 23 Age (mean±SD) 22.18±4.08 24.33±4.13 Male/female (n) 8/15 7/16

3 Results

3.1 Jaw hard tissue changes

Footnotes

Acknowledgments

Competing interests

Ethical approval

References

Table 1
Basic information of study subjects

One-jaw group two-jaw group

Sample sizes (n) 23 23

Age (mean±SD) 22.18±4.08 24.33±4.13

Male/female (n) 8/15 7/16