Differences in soft-tissue thickness changes after bimaxillary surgery between patients with vertically high angle and normal angle

Introduction

We evaluated soft-tissue thickness changes after bimaxillary surgery according to vertical facial patterns in patients with skeletal Class III malocclusion with mandibular prognathism.

Methods

Forty-three Korean patients (16 men and 27 women; mean age, 22.6 ± 4.1 years) with skeletal Class III malocclusion who underwent bimaxillary surgery were divided into 2 groups: normal-angle group (N group) and high-angle group (H group), on the basis of the presurgical angle of the mandibular plane relative to the sella-nasion plane (SN-MP). Changes in hard-tissue landmarks and soft-tissue thickness before and after surgery were analyzed from reconstructed 3-dimensional cone-beam computed tomography images. Postoperative soft-tissue thickness in both groups was compared with that in 40 patients with normal skeletal Class I malocclusion in the reference group.

Results

Group N (27°-37°) and group H (>37°) did not differ significantly in terms of sex and age before surgery. Preoperative pogonion (Pog) thickness was significantly less in group H (9.7 ± 1.6 mm) than in group N (10.8 ± 1.9 mm) ( P = 0.042). Adjusted multiple linear regression analysis showed a weak positive linear relationship between the SN-MP before surgery and soft-tissue Pog thickness change ( R 2 of 0.361; P = 0.001) after surgery, but the area below the lower lips was not completely normalized despite surgery.

Conclusions

The thickness of the soft-tissue Pog may increase slightly after surgery in patients with skeletal Class III malocclusion with a higher preoperative mandibular plane angle, but normalization in the area cannot be completely achieved despite surgery.

Highlights

  • Pogonion was thinner in patients with a high-angle Class III than in those with normal angle.

  • Thickness was increased after surgery in patients with a larger mandibular plane angle.

  • Despite surgery, normalization cannot be completely achieved.

Bimaxillary surgery with mandibular setback is used to achieve functional and esthetic improvement in patients with skeletal Class III malocclusion with mandibular prognathism. Because soft-tissue changes after surgery directly affect patients’ satisfaction, , many studies have estimated the ratios of soft-tissue to hard-tissue movements at each measurement point, and various simulation software have been developed to predict postoperative facial appearance. Previous studies have mostly compared and analyzed the ratios for the 2-dimensional linear distance between hard-tissue and soft-tissue points. , However, this ratio varies greatly within studies and remains difficult to predict, because changes in soft tissues are affected by various factors such as soft-tissue thickness and muscle elongation, which in turn depend on sex and sagittal and vertical facial patterns. Therefore, it may not be accurate to predict postoperative facial appearance by applying the mean ratios of soft-tissue to hard-tissue movements as a whole without considering differences in the preoperative skeletal and soft-tissue characteristics.

In addition, most previous studies and programs for predicting postoperative soft-tissue changes have considered the outcomes of maxillary advancement and have focused mainly on changes in the midfacial area. , However, in Asian patients, because of the high incidence of a prognathic mandible, mandibular setback surgery with maxillary posterior impaction is performed more often than maxillary advancement surgery.

Several studies have reported that the thickness of the soft tissue varies according to the vertical facial pattern. , , Celikoglu et al reported that, for women, the soft-tissue thickness values at the labrale superius, labrale inferius, and pogonion (Pog) were significantly smaller in the high-angle group than in any other group. Macari et al also reported that the soft-tissue thickness values were smaller at the gnathion and menton areas in the high-angle group than in any other group.

To date, few studies have compared changes in soft-tissue thickness before and after bimaxillary surgery with mandibular setback according to the patients’ vertical facial pattern. Thus, this study aimed to evaluate whether soft-tissue thickness changes after bimaxillary surgery (LeFort I osteotomy and bilateral intraoral vertical ramus osteotomy [IVRO]) differ according to vertical facial patterns (high and normal mandibular plane angles) in Korean patients with skeletal Class III malocclusion with mandibular prognathism. The null hypothesis was that there would be no difference in soft-tissue thickness changes after bimaxillary surgery between the normal-angle group (N group) and the high-angle group (H group).

Material and methods

This study was retrospective. The study sample was composed of Asian Korean patients who presented to the Department of Orthodontics and Department of Oral and Maxillofacial Surgery, Yonsei University Dental Hospital, Seoul, South Korea, from 2016 to 2018, for evaluation and management of skeletal Class III malocclusion with either of 2 vertical facial patterns. On the basis of a preliminary study, a minimum sample size of 15 was required within each group, with a P value <0.05 indicating statistical significance, a power of 80%, and an effect size of 0.8 for detecting differences in soft-tissue changes between before surgery (T1) and after surgery (T2) (G∗Power, version 3; Heinrich Heine University Düsseldorf, Düsseldorf, Germany).

The study included patients who, 1 month before surgery, (1) were aged ≥18 years; (2) had no loss of teeth except extraction of third molars; (3) had skeletal Class III malocclusion, with the angle formed by point A, the nasion, and point B (ANB) being smaller than 0°; (4) required conventional orthognathic bimaxillary surgery with presurgical orthodontics (1-piece LeFort I osteotomy and bilateral IVRO); and (5) had no severe dentofacial anomalies, such as a cleft lip or palate. In addition, to be eligible, the patient had to be in good general health.

The exclusion criteria were as follows: (1) existing serious medical conditions for which the patient had been hospitalized in the previous 3 months; (2) history of orthodontic treatment or orthognathic surgery; (3) history of trauma or cosmetic surgical procedures, such as genioplasty or zygomatic enhancements; (4) requirement for single-jaw surgery or a surgery-first approach; (5) menton deviation >4 mm from the facial midline; and (6) loss of or incomplete series of identifiable cone-beam computed tomography (CBCT) records.

From 2016 to 2018, 70 consecutive patients underwent orthognathic bimaxillary surgery (1-piece LeFort I osteotomy and bilateral IVRO) without genioplasty, which was performed by a single surgeon for the management of skeletal Class III malocclusion. Among these patients, 43 (16 men and 27 women; mean age, 22.6 ± 4.1 years) met the inclusion criteria of the study.

To determine whether the facial soft-tissue contours normalized after surgery, we compared postoperative soft-tissue thickness in experimental groups with that in subjects with normal skeletal relationships in the reference group. These subjects were selected from Asian Korean patients who visited Yonsei University Dental Hospital and had undergone CBCT for orthodontic diagnosis during 2007-2017. The inclusion criteria for the reference subjects were as follows: (1) age ≥18 years; (2) no loss of teeth except extraction of third molars; (3) well-balanced facial profile with ANB between 1°-4° and an angle of the mandibular plane to sella-nasionplane (SN-MP), including subjects with a normal SN-MP, ranging from 27° to 37° , , ; (4) without significant mandible asymmetry (menton deviation <3 mm); and (5) an Angle Class I molar relationship. The exclusion criteria were as follows: (1) any inherited deformation or acquired trauma of the face and (2) history of previous orthodontic treatment or orthognathic or cosmetic surgery. The reference group was composed of 40 Korean patients with skeletal Class I malocclusion (20 men and 20 women; mean age, 24.7 years). The mean ANB and SN-MP values were 2.0 ± 0.8° and 29.2 ± 4.6°, respectively, and these values did not differ according to sex in the reference group.

This study was approved by the Institutional Review Board of Yonsei University Dental Hospital (approval no. 2-2019-0003). This study conformed to the Declaration of Helsinki. Written informed consent was obtained from all patients before the initiation of treatment and included permission for the use of the data for research purposes.

All patients underwent conventional bimaxillary surgery, including maxillary LeFort I osteotomy with posterior impaction and bilateral IVRO for the mandibular setback. The same protocol was used for all surgeries, which were performed by a single surgeon (H.-D.J.). In addition, all patients underwent pre- and postoperative orthodontic treatment at the Department of Orthodontics, Yonsei University Dental Hospital, Seoul, South Korea.

All patients underwent CBCT examinations (Alphard, version 3030; Asahi Roentgen, Kyoto, Japan) at T1 and T2. CBCT scanning of the maxillofacial regions was performed (10 mA, 80 kV; 0.39-mm voxel size; scan time, 17 seconds; and a field view of no more than 200 mm in height × 200 mm in depth). During CBCT, the patient was asked to be seated in an upright position with lips relaxed and their mandibular jaw in a resting position, because lip strain causes deformation of the soft tissues, particularly at the Pog.

The CBCT scan data were converted into Digital Imaging and Communications in Medicine format. Craniofacial 3-dimensional (3D) images were reconstructed from the Digital Imaging and Communications in Medicine data using the Invivo dental software program (version 5.1; Anatomage, San Jose, Calif). The horizontal reference plane was set to the Frankfort horizontal plane, which was constructed on both sides of the porion and left of the orbitale. The midsagittal plane was drawn perpendicular to the Frankfort horizontal plane and passed through the nasion and midpoint of both orbitale points. The coronal plane was constructed at right angles to the horizontal and midsagittal planes while passing through the nasion. The nasion was set to the zero point (0, 0, 0), and coordinates of other landmarks were acquired on the basis of this system. Positive coordinate values indicated values to the left, posterior, and superior to the zero point, whereas negative values indicated values to the right, anterior, and inferior to the zero point ( Fig 1 ).

Fig 1
Reference plane and coordinate system. Nasion was set to the zero point. x-axis: (+) left, (−) right; y-axis: (+) posterior, (−) anterior; z-axis: (+) superior, (−) inferior.

The vertical facial pattern was the primary predictor variable in this study. The study sample was divided into 2 groups according to the SN-MP angle before surgery: a normal-angle group (group N), including patients with an SN-MP ranging from 27° to 37°, and a high-angle group (group H), including patients with an SN-MP exceeding 37°, on the basis of previous studies. , ,

On the basis of a previous study, an observer (H.L.), who was blinded to the subject’s identity and clinical status, labeled 9 hard-tissue structures and their corresponding soft tissue, including 5 midsagittal structures (A/subnasale, maxillary central incisor/upper lip, mandibular central incisor/lower lip, point B/labiomental sulcus [B/Si], Pog), and 4 corresponding lateral landmarks (the canine tip of each maxilla and mandible to corresponding soft tissue) ( Fig 2 ). The 9 distances mentioned above were identified to determine soft-tissue thickness. The Invivo program (version 5; Anatomage) calculated the distance between the 2 points using Euclidean distance for the 5 midsagittal structures and their corresponding soft tissue, and the distance between the 2 points along an anteroposterior (AP) line parallel to the horizontal plane for the 4 corresponding lateral landmarks.

Fig 2
Landmarks and 3D distances. Five midsagittal landmarks (A/Sn, U1/U lip, L1/L lip, B/Si, Pog) were identified, and Euclidean distances were calculated as the distance between the 2 points on the midsagittal plane. Four lateral landmarks (tips of the maxillary and mandibular canines to the corresponding soft tissue) were identified as the distance between the 2 points along an anteroposterior line parallel to the horizontal plane. UR , maxillary right; A, point A; Sn, subnasale; U1, maxillary central incisor; U, upper; L, lower; B, point B; Si, labiomental sulcus.

Intraobserver reliability was determined by comparing the measurements obtained from the original examinations with those obtained from repeated examinations. All measurements were repeated by the same examiner after 2 weeks. The method error was calculated using the intraclass correlation coefficient, which was 0.85-0.90 for all linear and angular measurements.

Statistical analysis

All statistical analyses were performed using SPSS (version 21.0; IBM, Armonk, NY). To verify the normality of the data distribution, we applied the Shapiro-Wilk test. Descriptive statistics, such as the mean and standard deviation (SD), were used to describe the distribution of each variable in the study. Differences in demographic characteristics, including sex and age, between groups N and H were analyzed using the chi-square test and the Mann-Whitney U test. The independent t test was used to determine the statistical significance of differences in soft-tissue thickness between men and women in each group, and between the 2 groups at preoperative and postoperative periods. In addition, hard-tissue landmarks and soft-tissue thickness changes before and after surgery within each group, as well as between the 2 groups, were analyzed using the paired t test and the independent t test, respectively. Analysis of variance and a post-hoc Tukey test were used to compare postoperative soft-tissue thickness in groups N and H with the soft-tissue thickness in the reference group.

We analyzed the correlation between the 3D hard-tissue landmark movements (the AP, vertical, and mediolateral components) and their corresponding soft-tissue thickness change, using Pearson correlation analysis. To investigate factors affecting the soft-tissue thickness changes at B/Si and Pog before and after surgery, we performed multiple linear regression analyses using sex, age, SN-MP at T1, and 3D skeletal landmark movements of point B and Pog before and after surgery as independent variables. A P value <0.05 was considered statistically significant.

Results

Group N included 20 patients (9 men and 11 women) with a mean age of 22.2 years (SD, 3.7 years), and group H included 23 patients (7 men and 16 women) with a mean age of 23.5 years (SD, 4.5 years). There were no significant between-group differences in demographic characteristics before surgery. The mean SN-MP angle was 33.3° (SD, 2.3°) in group N and 39.9° (SD, 1.9°) in group H ( Table I ). There was a significant difference in the SN-MP angle between the 2 groups ( P <0.001).

Table I
Sample characteristics (n = 43)
Variables Group N (n = 20) Group H (n = 23) Between-group comparison
SN-MP, ° 33.3 ± 2.3 39.9 ± 1.9 <0.001
ANB, ° −2.7 ± 3.4 −1.7 ± 2.5 0.300
Gender 0.361
Men 9 (45.0) 7 (30.0)
Women 11 (55.0) 16 (70.0)
Age, y 21 (19-22) 22 (20-25) 0.242

Note. Data are presented as mean ± SD or n (%). Age is presented as median and interquartile range. Normal SN-MP angle ranges from 27° to 37°, and high SN-MP angle is >37°.

P value calculated with independent t test.

P value calculated with chi-square test.

P value calculated with Mann-Whitney U test.

Table II shows the measurements of preoperative soft-tissue thickness by sex and vertical facial patterns. The thickness values of A/subnasale for men were significantly higher than those for women in both vertical facial groups ( P = 0.013 in group N and P = 0.021 in group H). In addition, in group N, men presented significantly greater soft-tissue thickness than women at the maxillary right canine and maxillary left canine ( P = 0.041 and P = 0.020, respectively). In terms of vertical facial pattern, soft-tissue thickness values at the lower anterior face (B/Si, Pog, mandibular right [LR] canine, mandibular left [LL] canine) were smaller in group H than in group N. Patients with a high angle presented significantly thinner Pog thickness than those with a normal angle ( P = 0.042).

Table II
Comparison of preoperative soft-tissue thickness between groups
Paired landmarks Group N, mm Group H, mm Between-group comparison
Men Women Total P value Men Women Total P value
A/subnasale 14.5 ± 1.5 12.5 ± 1.5 13.3 ± 1.7 0.013 13.8 ± 1.0 12.0 ± 1.7 12.6 ± 1.7 0.021 0.180
Maxillary central incisor/upper lip 14.9 ± 1.9 15.1 ± 1.9 15.0 ± 1.9 0.817 15.4 ± 2.6 15.6 ± 2.3 15.5 ± 2.4 0.842 0.491
Mandibular central incisor/lower lip 14.6 ± 2.6 14.5 ± 2.2 14.5 ± 2.3 0.912 14.8 ± 2.6 14.9 ± 2.0 14.9 ± 2.1 0.934 0.644
B/Si 12.5 ± 1.5 12.0 ± 2.1 12.2 ± 1.8 0.561 12.1 ± 1.1 10.9 ± 1.8 11.2 ± 1.7 0.109 0.071
Pog 10.3 ± 1.9 11.2 ± 1.9 10.8 ± 1.9 0.336 9.6 ± 1.4 9.7 ± 1.8 9.7 ± 1.6 0.955 0.042
Maxillary right canine 15.6 ± 1.5 14.1 ± 1.5 14.7 ± 1.6 0.041 15.5 ± 1.9 14.3 ± 1.5 14.7 ± 1.7 0.135 0.931
Maxillary left canine 16.5 ± 1.6 14.2 ± 2.0 15.2 ± 2.1 0.020 16.0 ± 2.1 14.6 ± 1.6 15.1 ± 1.8 0.117 0.868
Mandibular right canine 14.2 ± 2.2 14.8 ± 1.9 14.5 ± 2.0 0.527 15.1 ± 1.7 13.7 ± 1.4 14.1 ± 1.6 0.059 0.487
Mandibular left canine 15.4 ± 1.9 15.0 ± 1.5 15.2 ± 1.7 0.668 15.1 ± 1.4 14.1 ± 1.4 14.4 ± 1.5 0.130 0.097

Note. Data are presented as mean ± SD. Group comparisons were tested with an independent t test.

Comparisons between men and women in each group were tested with an independent t test.

After surgery, the maxilla showed a small amount of movement, whereas the mandible showed significant superior and posterior movement. There was no statistically significant difference in the amount of AP, superoinferior, and mediolateral movement of point A between the 2 groups during surgery ( Table III ).

Jan 30, 2021 | Posted by in Orthodontics | Comments Off on Differences in soft-tissue thickness changes after bimaxillary surgery between patients with vertically high angle and normal angle
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