Morphological Analysis Of The Masseter Muscle In CBCT Scans And Its Application In Orthodontics

As an important part of the orofacial system,the masseter muscle plays an important role in both morphology and function.Previous studies have found that after orthodontic treatment,the thickness of the cheek soft tissues of will decrease,but there haven't been research linking it to the thickness of the internal masseter muscle or other tissues.On the other hand,the masseter muscle outputs one of the most powerful bite forces in the masticatory system and research suggests that the physiological occlusal curve of the posterior teeth is gradually formed under the action of the bite force.Therefore,the bite force and its orientation provided by the masseter muscle should have a certain effect on the physiological occlusal curve of the posterior teeth.However,in the past,CT and MRI were usually used for three-dimensional imaging studies of the masseter muscle,and often with a small sample size and no consensus reached.Therefore,this study will further explore the morphology and function of the masseter muscle.The study is divided into three parts.The first part was a methodological study to establish the auto-segmentation of masseter muscle on CBCT scans and to evaluate the validity and reliability of CBCT masseter muscle segmentation by comparing with the MRI masseter muscle segmentation of the same patients.In the masseter muscle auto-segmentation task,our study applied CT scans and correspondent masseter muscle labels for the training of the segmentation network and unsupervised domain adaptation framework for automatic labeling of CBCT scans.The CycleGAN-based framework completed the unsupervised transference of CT and CBCT scans,in which case,the segmentation network can be used for CBCT masseter muscle segmentation with only labeling of CT scans.Compared with the state-of-the-art,the proposed approach showed higher similarity with manual segmentation with a DSC of 93.7%.On the other hand,seventeen volunteers were included in this study for comparison of masseter muscle segmentation between CBCT and MRI scans.CBCT and MRI scans of the volunteers were taken respectively within one month.The masseter muscles in the CBCT scans were segmented by the CycleGAN-based framework combined with manual check.The masseter muscles in the MRI scans were segmented manually.The segmentations were repeated by the first examiner and a second examiner.For cross-sectional area(CSA),paired t-test,intraclass correlation coefficient(ICC)and standard error of measurement(SEM)were calculated to evaluate the validity and reliability of the segmentations.The validity and reliability were also calculated by Dice similarity coefficient(DSC)and average Hausdorff distance(aHD)between different segmentations.Paired t-test showed that there was no significant difference in CSA between CBCT and MRI masseter segmentations.The ICCs were all larger than 0.95 and the SEM was less than 4.85 mm2 for CSA.The DSC were all larger than 0.94 showing over 94%of similarity between CBCT and MRI masseter segmentations.The aHD were all smaller than 0.1 mm showing great consistency of the contour of CBCT and MRI segmentations.Masseter muscle segmentation from CBCT scans was not significantly different from the segmentation from MRI scans.CBCT masseter muscle segmentation showed great validity compared with MRI scans,and great reliability in retests.The second part of the study aimed to evaluate orthodontic treatment-related thickness changes of the masseter muscles and surrounding soft tissues and the potential factors that would influence these changes during orthodontic treatment in female adults.Forty-two female adult patients were included in this retrospective study and were divided into extraction(n=22)and nonextraction(n=20)groups.Pretreatment and posttreatment cone-beam computed tomography(CBCT)images were superimposed and reconstructed.The thickness changes of the masseter area of facial soft tissue(MAS),masseter muscles(MM)and surrounding fat tissue(FT)were measured and compared between extraction and nonextraction groups.Spearman's correlation coefficient was calculated between the above-mentioned soft tissue changes and pretreatment age,treatment duration,sagittal relationship(ANB),and vertical relationship(Frankfort-mandibular plane angle,FMA).Regression analysis was conducted to confirm the causal relations of the variables.The thickness of MAS and MM significantly decreased in both groups,with larger decreases(>1 mm)in the extraction group.There were strong correlations(r>0.7)between the thickness decrease in MAS and MM and moderate correlations(r>0.4)between MAS and FT changes.A significantly greater decrease of MAS and MM were found to be moderately correlated with a smaller FMA.Scatter plots and regression analysis confirmed these correlations.Masseter muscles and the surrounding soft tissue exhibited a significant decrease in thickness during orthodontic treatment in female adults.Low-angle patients experienced a greater decrease in soft tissue thickness in the masseter area.But the thickness changes were clinically very small in most patients.The last part of the study further explored the relationship between size and orientation of the masseter muscle and the functional occlusal plane(FOP),using the same sample as the second part including pre-and posttreatment CBCTs of 42 patients.Firstly,the sample was divided into two groups,22 in the extraction group and 20 in the nonextraction group.The pre-and posttreatment CBCTs were superimposed and reconstructed.The horizontal plane was established by the bilateral orbital points and porion points before the treatment.The plane through the anterior and posterior nasal spinal(ANS and PNS)points and perpendicular to the horizontal plane was the sagittal plane,and the plane vertical to the horizontal plane and the sagittal plane was the coronal plane.All the buccal cusps of the maxillary second molar,the first molar and the adjacent bicuspid teeth were used to establish the pre-and posttreatment functional occlusal plane(FOP).In addition,we measured the depth of the Spee curve,the volume of the masseter muscle,and the long axis of the masseter muscle by principal component analysis(PCA)which was projected onto the sagittal plane as its sagittal orientation.Combined with cephalometric indicators,we analyzed the results.After treatment,the masseter muscle volume of both the extraction group and the non-extraction group was significantly reduced,and the Spee curve became flat,but the orientation of the masseter muscle did not change significantly.Analysis of variance was performed on the angle between the sagittal orientation of masseter muscle and functional occlusal plane(MM/FOP)in different vertical skeletal facial types,and it was found that there was no significant statistical difference between the three groups,and the mean values of all groups were very close to 90°.Finally,the pretreatment occlusal plane(FOP/FHpre)was used as the dependent variable,and the independent variables selected by the correlation analysis were subjected to regression analysis.The regression equation included the pretreatment orientation of the masseter muscle(MM/FHpre),the torque of upper and lower central incisor(Ul/NA,U1/PP,L1/MP),the inclination of the maxillary first molar(U6/PP),the protrusion of point A(NA/PA)and the mandibular plane angle(FMA).The R2 of the regression equation was 0.362.This study concluded that after orthodontic treatment,the masseter muscle was significantly reduced in volume,but there was almost no significant change in orientation.The angle between the masseter muscle and the functional occlusal plane was basically stable and close to vertical,and does not change with the mandibular plane angle.The sagittal orientation of the masseter muscle,the angle of the upper and lower central incisors,the protrusion of point A and the angle of the maxillary first molar will affect the physiological inclination of the functional occlusal plane of the posterior teeth.