Introduction
This study aimed to determine a measurement plane that could represent the maximum cross-sectional area (MCSA) of masseter muscle using an artificial intelligence model for patients with skeletal Class III malocclusion.
Methods
The study included 197 patients, divided into subgroups according to sex, mandibular symmetry, and mandibular plane angle. The volume, MCSA, and the cross-sectional area (CSA) at different levels were calculated automatically. The vertical distance between MCSA and mandibular foramen, along with the ratio of the masseter CSA at different levels to the MCSA (R), were also calculated.
Results
The MCSA and volume showed a strong correlation in the total sample and each subgroup ( P <0.001). The correlation between the CSA at each level and MCSA was statistically significant ( P <0.001). The peak of the r and the correlation coefficient between the CSA at different levels and MCSA were mostly present 5-10 mm above the mandibular foramen for the total sample and the subgroups. The mean of R A5 to R A10 was >0.93, whereas the corresponding correlation coefficient was >0.96, both for the entire sample and for the subgroups.
Conclusions
MCSA could be used as an indicator for masseter muscle size. For patients with skeletal Class III malocclusion, the CSA 5-10 mm above the mandibular foramen, parallel to the Frankfort plane, could be used to estimate the masseter muscle MCSA.
Highlights
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The automatic masseter muscle segmentation model was used.
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The model outputted the cross-sectional area at different levels of the masseter.
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The larger sample size facilitated subgroup analysis.
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The cross-sectional area 5-10 mm above the mandibular foramen can be measured.
Skeletal Class III malocclusion can result from pure mandibular prognathism, maxillary hypoplasia and retrognathism, or a combination of the two. A higher prevalence of skeletal Class III malocclusion has been demonstrated in Mongolian patients than in whites. As a result, a greater number of Asian patients seek orthodontic treatment for skeletal Class III malocclusion. Patients with severe skeletal Class III malocclusion require surgical-orthodontic treatment, which improves not only facial esthetics but also masticatory function.
Previous studies have assessed the effect of surgical-orthodontic treatment on masticatory function by analyzing occlusal forces, occlusal contact, and masticatory efficiency. However, the measurement of these parameters is less practical for retrospective studies. , Masticatory muscles, particularly the masseter muscle, are essential for occlusal force generation. The masseter muscle originates from the zygomatic arch and terminates at the surface of the ramus and the angle of the mandible. Compared with other masticatory muscles, it is more clearly demarcated and easier to distinguish. It is also the primary source of bite force. As a result, it is the most extensively studied masticatory muscle.
The masseter muscle’s volume is a direct indicator of its size and morphology. Although some studies have used artificial intelligence to achieve automatic segmentation of the masseter muscle volume, most studies rely on manual segmentation of dozens or even hundreds of layers of images to evaluate the volume of the masseter muscle. This process is time-consuming and error-prone. Even for studies that have devised a simple method for volume estimation of the masseter muscle, the scope of application requires further verification in different participant groups. ,
The cross-sectional area (CSA) of masseter muscle correlates positively with its volume , , , and is, therefore, another commonly used indicator for its size. Studies have demonstrated that CSA is correlated with the occlusal force , , and can be used as an indicator of the magnitude of occlusal force, masticatory function, and the size of the masseter muscle. Determination of the measurement plane for masseter muscle CSA requires precise localization and horizontal reference planes, several of which have been used in previous studies. The maximum cross-sectional area (MCSA) has been used as a measurement plane, but this requires either manual segmentation of the entire masseter muscle, , which is cumbersome, or an estimation of its size, which is imprecise. The use of the CSA ≥8 mm in the mandibular foramen, parallel to the Frankfort plane, has been advocated in a previous study. However, because of the limited sample size and technical difficulties in imaging, this method did not gain acceptance. A few studies that have investigated the CSA of masseter muscle at different levels found that the size of midbelly CSAs remained relatively constant over a range of 12 mm. , In a study by Lee et al, the mandibular cone-beam computer tomography (CBCT) images were digitally cut into 5.0-mm thick sections parallel to the mandibular plane from the gonion inferius point to gonion inferius point >30 mm, and then the CSA of the masseter muscle was measured at each cut level. In this study, the mandibular plane was defined as the plane passing the menton point and bilateral gonion inferius point according to the Downs analysis. However, they did not recommend any specific planes for CSA measurement of the masseter. Differences in the measurement planes for the CSA may account for the inconsistent results of previous studies. , Therefore, it is necessary to investigate masseter muscle CSAs at different levels and establish a representative measurement plane reflecting the masseter morphology and function for optimal measurements to simplify the process and improve comparability between studies.
Previous studies have commonly used ultrasound (UTS), magnetic resonance imaging (MRI), and computed tomography (CT) as imaging modalities for the masseter muscle. , , However, these methods are less commonly used in oral diagnosis and treatment, which is an important reason for the limited sample sizes of previous studies. Although CBCT is inferior to CT for soft-tissue imaging, it is widely used in the field of oral medicine because of its low radiation dose and high resolution of teeth. Advancements in CBCT technology have led to improvement in soft-tissue image resolution, allowing masseter muscle segmentation using CBCT images. ,
This study aimed to (1) investigate the correlation between masseter volume and CSA at different levels using an artificial intelligence model for autosegmentation and postprocessing of the CBCT images of patients with skeletal Class III malocclusion and (2) determine the optimal plane for measurement that is comparable with the MCSA and reflects the masseter morphology and function, facilitating future studies of masseter morphology and function after surgical-orthodontic treatment in patients with skeletal Class III malocclusion.
Material and methods
The CBCT scans in this study (NewTom VGi, Verona, Italy; field of view, 24 × 19 cm; 90 kV; 6.0 mA; scan time, 15 seconds; and voxel size, 0.3 mm) were obtained from the Department of Oral Maxillofacial Surgery and Department of Orthodontics, which were used for cephalometric analysis and pretreatment planning. Before the surgical-orthodontic treatment, the subjects were fully informed of the purpose and risk of the CBCT scans, and informed consent was provided for the use of CBCT images in this scientific study. This study was approved by the Biomedical Ethics Committee of Peking University School and Hospital of Stomatology (PKUSSIRB-201944062).
The inclusion criteria were as follows: (1) aged ≥16 years; (2) skeletal Class III malocclusion (ANB angle <0°); (3) CBCT performed before preoperative orthodontic treatment, including the entire masseter muscle; and (4) Mongoloid race.
The exclusion criteria were as follows: (1) severe posterior crowding, (2) severe periodontitis, (3) cleft lip or palate, and (4) temporomandibular joint disease.
Based on the inclusion and exclusion criteria, a total of 197 patients with skeletal Class III malocclusion were included ( Table I ). The patients were divided into 2 subgroups on the basis of sex (male group = 67; female group = 130). The patients were also divided into 3 subgroups on the basis of the Frankfort-mandibular plane angle (FMA): (1) low angle group (n = 41), FMA <22°; (2) normal angle group (n = 110), 22°≤ FMA ≤32°; and (3) high angle group (n = 46), FMA >32°. The mandibular plane in FMA was defined as the tangent line to the lower margin of the mandible through the menton point. The patients were further divided into 2 subgroups according to the degree of mandibular deviation from the midsagittal plane, measured using 3-dimensional CBCT images: (1) asymmetrical group (n = 104) having mandibular deviation >3.5 mm and (2) symmetrical group (n = 93) having mandibular deviation <2 mm. The mandibular deviation was evaluated on the basis of the distance of menton deviation compared with the midsagittal plane. The deviated side was defined as the side on which the menton shifted toward the midsagittal plane, whereas the other side was defined as the nondeviated side.
| Subgroups | n | Age (y) | ANB (°) | Male/female (n/n) | Mp-FH (°) | Distance of menton deviation (mm) |
|---|---|---|---|---|---|---|
| Sex | ||||||
| Male | 67 | 21.03 ± 3.71 | −4.70 ± 2.86 | 67/0 | 26.91 ± 6.12 | 4.71 ± 4.28 |
| Female | 130 | 23.07 ± 5.63 | −3.01 ± 2.43 | 0/130 | 26.78 ± 5.85 | 4.22 ± 3.65 |
| Vertical skeletal morphology | ||||||
| Low angle | 41 | 24.46 ± 5.07 | −4.95 ± 3.20 | 14/27 | 18.37 ± 2.76 | 3.72 ± 3.68 |
| Normal angle | 110 | 22.10 ± 5.38 | −3.68 ± 2.43 | 37/73 | 26.91 ± 2.75 | 4.63 ± 4.01 |
| High angle | 46 | 21.21 ± 4.08 | −2.15 ± 2.15 | 16/30 | 34.14 ± 2.85 | 4.40 ± 3.72 |
| Mandibular deviation | ||||||
| Asymmetry | 104 | 22.18 ± 4.78 | −3.20 ± 2.64 | 33/60 | 27.00 ± 5.58 | 7.23 ± 3.28 |
| Symmetry | 93 | 22.61 ± 5.54 | −4.02 ± 2.72 | 34/70 | 26.62 ± 6.32 | 1.20 ± 0.73 |
Sample size calculation was based on a previous study of CT imaging data from 65 subjects, which showed that the correlation coefficient between the MCSA parallel to the Frankfort plane and volume was 0.903, and the 2-sided confidence interval width can be calculated to be 0.0946. To investigate the correlation between masseter volume and CSA, a minimum sample size of 30 was required in each subgroup to achieve statistical significance on the basis of a significance level of 0.05 and a statistical power of 80%. The sample size was calculated using the PASS software (version 11; NCSS, Kaysville, Utah).
To standardize the orientation of craniofacial structures, the following 3-dimensional reference planes were set for the CBCT images performed before preoperative orthodontic treatment: (1) Frankfort plane was defined as the horizontal plane; (2) the midsagittal plane was perpendicular to the Frankfort plane and passed through nasion and basion; and (3) the coronal plane was perpendicular to these 2 planes, passing through basion ( Fig 1 ). Reorientation of the images was performed using Dolphin imaging (version 11.8; Dolphin Imaging and Management Solutions, Chatsworth, Calif).
In our previous study, a deep learning-based automatic approach was used to accurately segment the masseter muscle from CBCT, refined using high-quality paired CT. The reoriented CBCT images were imported into the deep learning-based model for autosegmentation of the deviated and nondeviated masseter muscles ( Fig 2 ).
Mesh volume was computed using Pyradiomics (version 3.0.1, Brigham and Women’s Hospital, Harvard Medical School, Boston, Mass; https://pyradiomics.readthedocs.io ), a library for radiomic feature extraction from medical imaging ( Table II ).
| Measurement | Definition |
|---|---|
| Volume | Mesh volume of the masseter muscle autosegmentation |
| MCSA | Maximum CSA in all cross-sections of autosegmentation parallel to the Frankfort plane |
| Distance | Vertical distance between the MCSA and the mandibular foramen |
| CSA Bn | The CSA of the masseter muscle at 20, 15, 10, and 5 mm below the mandibular foramen parallel to the Frankfort plane (n = 20, 15, 10, and 5) |
| CSA F | The CSA of the masseter muscle at the mandibular foramen parallel to the Frankfort plane |
| CSA An | The CSA of the masseter muscle at 25, 20, 15, 10, 9, 8, 7, 6, and 5 mm above the mandibular foramen parallel to the Frankfort plane (n = 25, 20, 15, 10, 9, 8, 7, 6, and 5) |
| R Bn | The ratio of masseter CSA at 20, 15, 10, and 5 mm below the mandibular foramen to the MCSA (n = 20, 15, 10, and 5) |
| R F | The ratio of the masseter CSA at the mandibular foramen to the MCSA |
| R An | The ratio of the masseter cross-sectional area at 25, 20, 15, 10, 9, 8, 7, 6, and 5 mm above the mandibular foramen to the MCSA (n = 25, 20, 15, 10, 9, 8, 7, 6, and 5) |
The oriented images were imported into the ITK-SNAP software (version 3.8.0, Penn Image Computing and Science Laboratory (PICSL), Pennsylvania, Pa; http://www.itksnap.org ), and the horizontal coordinates of bilateral mandibular foramen were obtained. Using the deep learning-based model, the voxels and horizontal coordinates for each autosegmentation cross-section of the masseter parallel to the Frankfort plane were obtained. Because the MCSA could be seen on several contiguous CSAs, the horizontal coordination of the MCSA was determined by averaging the horizontal coordinate values for the top and bottom layers of the MCSA. The vertical distance between the MCSA and mandibular foramen was defined as distance (D). MCSAs above and below the mandibular foramen were represented by plus (+) and minus (−) symbols, respectively. MCSA was calculated according to the maximum voxels ( Table II ).
The location of the foramen was defined as the starting point. Voxel values at 0.3-mm intervals, extending from the top and bottom of the masseter muscle of several patients selected randomly from the whole sample, were converted into CSA measurements. Voxel values at 5-mm intervals, extending from 20 mm below to 25 mm above the mandibular foramen, were converted into CSA measurements for the whole sample (CSA B20 to CSA A25 , in which CSA B20 represents the CSA at 20 mm below the mandibular foramen, whereas CSA A25 represents the CSA at 25 mm above the mandibular foramen; Table II ). Voxel values at 1-mm intervals, extending 6-9 mm above the mandibular foramen, were converted into CSA measurements (CSA A6 to CSA A9 , Table II ).
The ratio of the masseter CSA to the MCSA at different levels (R B20 to R A25 and R A6 to R A9 ) was calculated to compare the CSAs and MCSAs ( Table II ).
Statistical analysis
Descriptive statistics were used to analyze the morphometric measurements of the masseter muscle ( Figs 3-5 ; Tables III and IV ). An independent-sample t test was used to compare the parameters of the male and female groups ( Table V ) and between the asymmetrical and symmetrical groups ( Table VI ). One-way analysis of variance and Bonferroni post-hoc comparisons were used to compare subgroups with different vertical facial morphologies ( Table VII ). Pearson’s correlation was used to analyze correlations among masseter measurements ( Fig 6 ; Table VIII ). All statistical evaluations were performed using SPSS software (version 27.0; IBM, Armonk, NY), with the significance level set as P <0.05.
| Variables | Mean ± standard deviation | Standard error | Minimum | Maximum |
|---|---|---|---|---|
| Volume (cm 3 ) | ||||
| Deviated | 22.19 ± 5.58 | 0.40 | 1.07 | 3.81 |
| Nondeviated | 22.33 ± 5.42 | 0.39 | 1.14 | 4.37 |
| D (mm) | ||||
| Deviated | 6.98 ± 7.80 | 0.56 | −14.55 | 21.75 |
| Nondeviated | 8.04 ± 8.78 | 0.63 | −17.25 | 24.15 |
| MCSA (cm 2 ) | ||||
| Deviated | 4.48 ± 0.93 | 0.07 | 2.17 | 8.01 |
| Nondeviated | 4.47 ± 0.92 | 0.07 | 2.04 | 8.60 |
| CSA F (cm 2 ) | ||||
| Deviated | 4.20 ± 0.90 | 0.06 | 1.80 | 7.56 |
| Nondeviated | 4.14 ± 0.89 | 0.06 | 1.72 | 7.63 |
| CSA A5 (cm 2 ) | ||||
| Deviated | 4.26 ± 0.94 | 0.07 | 2.01 | 7.76 |
| Nondeviated | 4.19 ± 0.91 | 0.06 | 1.88 | 8.12 |
| CSA A10 (cm 2 ) | ||||
| Deviated | 4.29 ± 0.94 | 0.07 | 2.11 | 7.81 |
| Nondeviated | 4.26 ± 0.92 | 0.07 | 1.95 | 8.26 |
| Variables | Total (n = 197) | Male (n = 67) | Female (n = 130) | Asymmetry (n = 104) | Symmetry (n = 93) | Low (n = 41) | Mean (n = 110) | Nondeviated (n = 46) | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| D | ND | D | ND | D | ND | D | ND | D | ND | D | ND | D | ND | D | ND | |
| R B20 | 0.68 ± 0.16 | 0.67 ± 0.16 | 0.72 ± 0.16 | 0.70 ± 0.17 | 0.66 ± 0.15 | 0.66 ± 0.15 | 0.68 ± 0.16 | 0.67 ± 0.17 | 0.65 ± 0.16 | 0.66 ± 0.17 | 0.76 ± 0.12 | 0.76 ± 0.12 | 0.67 ± 0.16 | 0.66 ± 0.16 | 0.62 ± 0.17 | 0.63 ± 0.16 |
| R B15 | 0.81 ± 0.12 | 0.80 ± 0.12 | 0.82 ± 0.12 | 0.81 ± 0.13 | 0.80 ± 0.11 | 0.79 ± 0.12 | 0.81 ± 0.11 | 0.79 ± 0.13 | 0.83 ± 0.11 | 0.78 ± 0.13 | 0.87 ± 0.08 | 0.86 ± 0.09 | 0.80 ± 0.11 | 0.78 ± 0.12 | 0.77 ± 0.13 | 0.77 ± 0.13 |
| R B10 | 0.88 ± 0.09 | 0.87 ± 0.10 | 0.88 ± 0.09 | 0.88 ± 0.10 | 0.88 ± 0.08 | 0.87 ± 0.09 | 0.88 ± 0.09 | 0.87 ± 0.10 | 0.98 ± 0.09 | 0.79 ± 0.10 | 0.92 ± 0.08 | 0.91 ± 0.08 | 0.87 ± 0.08 | 0.86 ± 0.09 | 0.87 ± 0.10 | 0.86 ± 0.11 |
| R B5 | 0.92 ± 0.07 | 0.91 ± 0.08 | 0.91 ± 0.07 | 0.91 ± 0.08 | 0.92 ± 0.07 | 0.91 ± 0.08 | 0.92 ± 0.07 | 0.90 ± 0.08 | 0.92 ± 0.07 | 0.91 ± 0.08 | 0.94 ± 0.06 | 0.93 ± 0.07 | 0.92 ± 0.06 | 0.90 ± 0.07 | 0.91 ± 0.08 | 0.91 ± 0.09 |
| R F | 0.94 ± 0.05 | 0.92 ± 0.06 | 0.94 ± 0.05 | 0.92 ± 0.06 | 0.94 ± 0.05 | 0.93 ± 0.06 | 0.94 ± 0.05 | 0.92 ± 0.06 | 0.94 ± 0.05 | 0.92 ± 0.06 | 0.95 ± 0.04 | 0.93 ± 0.05 | 0.94 ± 0.05 | 0.92 ± 0.06 | 0.93 ± 0.07 | 0.93 ± 0.06 |
| R A5 | 0.95 ± 0.04 | 0.94 ± 0.04 | 0.95 ± 0.04 | 0.94 ± 0.04 | 0.95 ± 0.04 | 0.94 ± 0.05 | 0.95 ± 0.04 | 0.93 ± 0.05 | 0.94 ± 0.04 | 0.94 ± 0.05 | 0.96 ± 0.03 | 0.94 ± 0.04 | 0.95 ± 0.04 | 0.93 ± 0.05 | 0.93 ± 0.06 | 0.94 ± 0.04 |
| R A6 | 0.95 ± 0.04 | 0.94 ± 0.04 | 0.95 ± 0.04 | 0.94 ± 0.04 | 0.95 ± 0.04 | 0.94 ± 0.04 | 0.95 ± 0.04 | 0.93 ± 0.05 | 0.95 ± 0.04 | 0.95 ± 0.05 | 0.96 ± 0.03 | 0.94 ± 0.04 | 0.96 ± 0.03 | 0.94 ± 0.04 | 0.93 ± 0.05 | 0.94 ± 0.04 |
| R A7 | 0.95 ± 0.04 | 0.94 ± 0.04 | 0.96 ± 0.04 | 0.95 ± 0.04 | 0.95 ± 0.04 | 0.94 ± 0.04 | 0.96 ± 0.04 | 0.94 ± 0.05 | 0.96 ± 0.04 | 0.95 ± 0.05 | 0.96 ± 0.03 | 0.95 ± 0.03 | 0.96 ± 0.03 | 0.94 ± 0.04 | 0.94 ± 0.05 | 0.94 ± 0.04 |
| R A8 | 0.96 ± 0.04 | 0.94 ± 0.04 | 0.96 ± 0.04 | 0.95 ± 0.04 | 0.95 ± 0.04 | 0.94 ± 0.05 | 0.96 ± 0.04 | 0.94 ± 0.05 | 0.95 ± 0.04 | 0.94 ± 0.05 | 0.97 ± 0.03 | 0.95 ± 0.03 | 0.96 ± 0.03 | 0.94 ± 0.04 | 0.94 ± 0.05 | 0.95 ± 0.05 |
| R A9 | 0.96 ± 0.04 | 0.95 ± 0.04 | 0.96 ± 0.03 | 0.95 ± 0.04 | 0.95 ± 0.04 | 0.94 ± 0.05 | 0.96 ± 0.04 | 0.94 ± 0.05 | 0.96 ± 0.04 | 0.94 ± 0.05 | 0.96 ± 0.03 | 0.96 ± 0.03 | 0.96 ± 0.03 | 0.94 ± 0.04 | 0.95 ± 0.05 | 0.95 ± 0.05 |
| R A10 | 0.96 ± 0.04 | 0.95 ± 0.05 | 0.96 ± 0.03 | 0.96 ± 0.04 | 0.95 ± 0.04 | 0.95 ± 0.05 | 0.95 ± 0.05 | 0.95 ± 0.05 | 0.95 ± 0.05 | 0.94 ± 0.05 | 0.96 ± 0.05 | 0.96 ± 0.04 | 0.96 ± 0.04 | 0.95 ± 0.04 | 0.95 ± 0.05 | 0.95 ± 0.06 |
| R A15 | 0.90 ± 0.08 | 0.92 ± 0.08 | 0.93 ± 0.06 | 0.93 ± 0.07 | 0.89 ± 0.09 | 0.92 ± 0.08 | 0.89 ± 0.09 | 0.91 ± 0.09 | 0.88 ± 0.09 | 0.91 ± 0.09 | 0.88 ± 0.09 | 0.91 ± 0.08 | 0.92 ± 0.07 | 0.93 ± 0.06 | 0.88 ± 0.09 | 0.90 ± 0.09 |
| R A20 | 0.78 ± 0.13 | 0.81 ± 0.12 | 0.83 ± 0.13 | 0.85 ± 0.12 | 0.75 ± 0.13 | 0.79 ± 0.11 | 0.75 ± 0.13 | 0.81 ± 0.13 | 0.75 ± 0.13 | 0.81 ± 0.13 | 0.74 ± 0.13 | 0.78 ± 0.13 | 0.80 ± 0.11 | 0.83 ± 0.10 | 0.76 ± 0.17 | 0.78 ± 0.13 |
| R A25 | 0.60 ± 0.19 | 0.65 ± 0.17 | 0.68 ± 0.19 | 0.73 ± 0.18 | 0.56 ± 0.18 | 0.61 ± 0.15 | 0.55 ± 0.20 | 0.64 ± 0.19 | 0.55 ± 0.20 | 0.64 ± 0.19 | 0.55 ± 0.20 | 0.61 ± 0.18 | 0.63 ± 0.17 | 0.68 ± 0.15 | 0.57 ± 0.23 | 0.63 ± 0.20 |
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