Introduction
This study aimed to analyze adults with mandibular characteristics of skeletal Class II malocclusion with chin deviation.
Methods
Seventy-five adult patients aged from 18 to 35 years were included and divided into 3 groups on the basis of sagittal skeletal pattern and chin deviation: skeletal Class I symmetry group, skeletal Class II symmetry group, and skeletal Class II asymmetry group (25 patients per group). Mandibular measurements on cone-beam computed tomography images were performed, and the differences between 2 sides in each group and the differences among the 3 groups were investigated.
Results
Compared with the contralateral side, the deviated side of patients in the Class II asymmetry group showed significantly smaller condyle angle to midsagittal plane, condylar height, ramal length, and length of the mandibular body, whereas it showed a significantly larger distance from condylion to the midsagittal plane, ramus angle to the horizontal plane, and distance from gonion to the midsagittal plane. Most linear measurements in the Class II symmetry group were significantly smaller than those in the Class I symmetry group. These linear measurements on the contralateral side of the Class II asymmetry group showed no significant difference with the Class I symmetry group, and these measurements on the deviated side of the Class II asymmetry group showed no significant difference with the Class II symmetry group.
Conclusions
Length of the mandible, rotation of condyle, the inclination of the ramus, and position of gonion should be considered in subjects with skeletal Class II asymmetry when making diagnosis and treatment planning.
Highlights
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We compared deviated and contralateral sides and patients with and without deviation.
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Chin deviated to the side with a shorter condyle, ramus, and mandibular body.
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Length difference might be more related to underdevelopment of the deviated side.
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Condyle rotates more inwardly, and ramus inclines more laterally.
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Condyle and gonion locate more laterally when compared with the contralateral side.
Facial symmetry is very important in esthetic evaluation. Mild facial asymmetry is usually difficult to detect, but when asymmetry increases, clinically observable facial deviation occurs. According to the literature, the incidence of facial asymmetry ranges from 11% to 37%, and even up to 40%-50% if strictly defined. ,
The mandible grows and develops for a long time and is not directly connected with the skull; thus, the most important manifestation of facial asymmetry is the asymmetry of the mandible, particularly chin deviation.
Two-dimensional radiographic images were used to study the characteristics of the mandible in patients with chin deviation. However, 2-dimensional radiographic images have limitations such as distortion, elongation, and superimposition of the anatomic structures. , In recent years, cone-beam computed tomography (CBCT) with the advantages of high resolution and no magnification have overcome the limitations of traditional images and widely used for analyzing chin deviation. ,
Previous studies on chin deviation focused on patients with skeletal Class III malocclusion and skeletal Class I malocclusion. , Few studies on skeletal Class II asymmetry were available. Kim et al compared chin-deviated patients with skeletal Class II and Class III malocclusion. They focused on the measurements of the ramus and mandibular body but not included condyle in their study. Thiesen et al found that bilateral differences in gonion position and ramal height were significantly correlated with chin deviation in patients with Class II malocclusion. However, no comparison with patients with Class I malocclusion was performed in their study. In addition, the majority of mandibular measurements in their study were linear measurements. For a more meaningful clinical interpretation, we believe that analyzing with angular and linear measurements and comparing them with subjects with Class I malocclusion are helpful to figure out mandibular morphologic characteristics of patients with Class II malocclusion with chin deviation.
This study aimed to analyze mandibular characteristics of adults with skeletal Class II malocclusion with chin deviation. In this study, mandibular measurements on CBCT images were performed in patients with skeletal Class II asymmetry and patients with skeletal Class I and Class II symmetry. Then the differences between 2 sides in each group and the differences among the 3 groups were evaluated to analyze mandibular characteristics of patients with skeletal Class II malocclusion with chin deviation.
Material and methods
Seventy-five adult patients seeking treatment at the Department of Orthodontics and Orthognathic Surgery in Peking University School and Hospital of Stomatology from 2015 to 2018 were enrolled in the study. CBCT images were taken for routine record of diagnosis and treatment planning. The study was approved by the Biomedical Ethics Committee of Peking University School and Hospital of Stomatology (PKUSSIRB-201946087).
Asymmetry was defined by the deviation of gnathion from the midsagittal plane. Patients with a gnathion deviation <2 mm were categorized as relative symmetry. In contrast, those with a deviation >4 mm were categorized as asymmetry.
Patients with skeletal Class II malocclusion were included with the following inclusion criteria: (1) aged from 18 to 35 years; (2) Mongolian; (3) permanent dentition, no missing teeth except the third molar; (4) ANB angle, >4.7°, Wits appraisal, >2.8; and (5) no prior orthodontic or orthognathic treatment. Exclusion criteria included: (1) retained deciduous teeth and ectopic teeth; (2) crowns or significant restorations in posterior teeth; (3) severe periodontitis; (4) systemic diseases; (5) cleft lip or palate, osteoarthritis of temporomandibular; and (6) trauma or tumor.
Patients with skeletal Class I malocclusion were included with the following inclusion criteria: (1) 0° < ANB angle < 4° and −2 < Wits appraisal < 0, (2) relatively symmetrical mandible, (3) crowding less than 2 mm and no obvious gaps, and (4) other inclusion criteria the same as that for subjects with skeletal Class II malocclusion. Exclusion criteria included: (1) crossbite or scissors-bite in posterior teeth, (2) crossbite or edge-to-edge position in anterior teeth, and (3) other exclusion criteria the same as that for patients with skeletal Class II malocclusion.
The condylar height was considered to be one of the core indexes. In the preliminary experiment, the condylar height was reported as 20.67 ± 3.41 mm on the nondeviated side of patients with skeletal Class II malocclusion with chin deviation and was 18.14 ± 2.10 mm on patients with skeletal Class II malocclusion without chin deviation. On the basis of these results, according to the formula proposed by Chow et al, at least 22 subjects are needed per group to reject the null hypothesis that patients with chin deviation were the same as patients without chin deviation, with a power of 0.8. The probability of type I error associated with this hypothesis test was 0.05 http://powerandsamplesize.com/Calculators/ .
Patients information in each group was summarized as follows: (1) Class I symmetry group included 25 patients with skeletal Class I malocclusion (6 males, 19 females; average age, 23.46 ± 3.99 years) with gnathion deviation <2 mm; (2) patients with skeletal Class II malocclusion were further divided into 2 subgroups according to the degree of gnathion deviation from midsagittal plane measured on 3-dimensional (3D) CBCT images; (3) Class II symmetry group included 25 patients with skeletal Class II malocclusion (3 males, 22 females; average age, 25.57 ± 4.55 years) with gnathion deviation <2 mm, crowding <4 mm, and no obvious gaps. Patients with crossbite or scissors-bite in posterior teeth or crossbite or edge-to-edge position in anterior teeth were excluded; and (4) Class II asymmetry group included 25 patients with skeletal Class II malocclusion (5 males, 20 females; average age, 25.08 ± 3.59 years) with gnathion deviation >4 mm.
No significant difference was noted among the 3 groups regarding gender and age ( Table I ).
| Group | Class I symmetry group | Class II symmetry group | Class II asymmetry group | P value |
|---|---|---|---|---|
| Sex (n) | 0.541 ∗ | |||
| Female | 19 | 22 | 20 | |
| Male | 6 | 3 | 5 | |
| Age, y (mean ± SD) | 23.46 ± 3.99 | 25.57 ± 4.55 | 25.08 ± 3.59 | 0.120 † |
All images were taken with a NewTom Scanner (NewTom G, Marburg, Germany) at these settings: 2.81 mA, 110 kV, exposure time of 3.6 seconds, isotropic voxels, axial slice thickness of 0.3 mm, and scanning area of 15 × 15 cm. Patients underwent CBCT in natural head posture and maximum dental intercuspation.
Digital imaging and communications in medicine data of obtained images were imported into the Dolphin 3D Imaging software (version 11.8; Dolphin Imaging and Management Solutions, Chatsworth, Calif) to reconstruct 3D images. The plane passing through 2 orbitals, and the right porion was the Frankfort horizontal (FH) plane, and the plane perpendicular to the horizontal plane and passing through nasion and basion was the midsagittal plane (MSP).
Landmarks were located in 3D reconstructions and multiplanar reconstruction view, with measurement scales of 0.01 mm and 0.01°.
Landmarks used in the study are defined in Table II and illustrated in Figure 1 . Measurements used to evaluate the morphology of the mandible , , are defined in Table III and illustrated in Figure 2 . The side that the gnathion deviate toward was defined as a deviated side, and the opposite side was defined as the contralateral side.
| Landmark | Abbreviation | Definition |
|---|---|---|
| Condylion superius | Co | Most superior point of the condyle |
| Condylion lateralis | C-lat | Most lateral point of the condyle head |
| Condylion medialis | C-med | Most medial point of the condyle head |
| Gonion | Go | Most inferior and posterior point on the contour of the gonial angle |
| Condylion anterius | C-ant | Most anterior point of the condyle head |
| Condylion posterius | C-pos | Most posterior point of the condyle head |
| Sigmoid notch | Sn | Most inferior point of the sigmoid notch |
| Gnathion | Gn | Most anterior inferior point of the contour of the bony menton |
| Variable | Measurement | Definition |
|---|---|---|
| Joint space | ||
| Superior (mm) | Superior joint space (mm) | Distance perpendicular to the FH plane between Co and the articular fossa outline |
| Anterior (mm) | Anterior joint space (mm) | Distance parallel to the FH plane between C-ant and the articular eminence outline |
| Posterior (mm) | Posterior joint space | Distance parallel to the FH plane between C-pos and the articular fossa outline |
| Condyle | ||
| ∠C-MSP (°) | Condyle angle to midsagittal plane | Angle between the midsagittal plane and the line connected C-lat and C-med |
| ∠C-FH (°) | Condyle angle to horizontal plane | Angle between the horizontal plane and the line connected C-lat and C-med |
| ⊥Co-Sn (mm) | Condylar height | Vertical distance from Co to Sn (horizontal plane passing through Sn) |
| C-lat-C-med (mm) | Mediolateral dimension of the condyle | Distance between C-lat and C-med |
| C-ant-C-pos (mm) | Anteroposterior dimension of the condyle | Distance between C-ant and C-pos |
| Co-MSP | Condylar position | Distance from Co to the midsagittal plane |
| ∠Co-Sn-FH (°) | Condylar neck angle to FH plane | Angle between the horizontal plane and the line connected Co and Sn |
| ∠C-lat-Go-FH (°) | Ramus angle to horizontal plane | Angle between the horizontal plane and the line connected C-lat and Go |
| Co-Go (mm) | Ramal length | Distance between Co and Go |
| Go-Gn (mm) | Length of mandibular body | Distance between Go and Gn |
| Go-MSP (mm) | Gonial position | Distance from Go to the midsagittal plane |
Statistical analysis
SPSS software (version 20.0; IBM, Armonk, NY) was used for statistical analysis. Two weeks after completion of all data measurements, 5 patients were randomly selected from each group for repeated measurements. The intraclass correlation coefficient was used to evaluate the observer’s reliability. All intraclass correlation coefficient values were >0.95, indicating good reliability in the group.
The Wilcoxon signed rank test was used for intragroup comparison between the deviated and the contralateral side. Mann-Whitney U test and Kruskal-Wallis 1-way analysis of variance was used for intergroup comparisons.
Results
Measurements on skeletal Class I and Class II symmetry groups showed no significant difference between 2 sides in each group except for ramal length (Co-Go) and gonial position (Go-MSP) in the Class II symmetry group ( P >0.05; Table IV ). So, the means of 2 sides in each group were used for the following analysis and comparison.
