| Background:Anterior disc displacement of the temporomandibular joint is one of the most common diseases in the oral and maxillofacial regions.It may be accompanied with TMJ pain,clicking and limited mouth opening as the symptoms.These disorders have a strong impact on the life qualities of patients.The disc of patients with anterior disc displacement would likely become more anteriorly displaced without treatment,resulting in serious organic changes.Many studies have noted anterior disc displacement was associated with morphology and biomechanics characteristics of the temporomandibular joint.The interrelationships between them are not completely clear.So far there has been little information about morphology and biomechanics characteristics of the temporomandibular joint based on the degree of anterior disc displacement.To explore the changes of morphology and biomechanics in the temporomandibular joint as anterior disc displacement progress is helpful to understand the clinical manifestations and progression mechanism of the disease,thereby to provide experimental basis for clinical diagnosis and treatment of patients with anterior disc displacement.Objective:The purpose of this study is to investigate the influence of the degree of anterior disc displacement on the bone morphology and stress distribution in temporomandibular joints.Methods:Experiment I: The subjects were selected from the department of orthodontics from 2020 to 2021.CBCT and MRI were taken for clinically suspected temporomandibular disorders before orthodontics.According to the inclusion and exclusion criteria,178 temporomandibular joints from 97 patients were evaluated in the present retrospective study.The subjects were divided into four groups based the disc position in the sagittal plane of MRI,namely control group(without disc displacement),slight group(disc anterior displaced slightly),moderate group(disc anterior displaced moderately)and severe group(disc anterior displaced severely).The CBCT data of each joint was exported into Mimics software for the measurements of temporomandibular joint bone morphology,including condyle linear dimensions(condyle length,width and height),condylar volume and surface aera,the depth and length of glenoid fossa,anterior joint space,superior joint space,and posterior joint space.The sagittal position of the condyle was also assessed.Using one-way analysis of variance,the Kruskal Wallis test,or Chi-square tests to detect the differences of all the bone morphologic measurement parameters among the 4 groups.The correlations between disc sagittal position and all bone the morphologic measurements were also analyzed.Experiment II: An adult female volunteer(age: 30 years old)from control group of experiment I was selected.DICOM files of its CBCT and MRI were processed using Mimics software,to reconstruct the 3D models of bony structures and the articular disk.The finite model of the control group was completed after treating in Geomagic Studio software and NX software successively.According to the grouping criteria of Experiment I,the three models of disc displaced groups(slight group,moderate group and severe group)were obtained by artificially moving the disc position of the control group forward.Finally,all the four models above were exported to the Ansys software to execute finite operation,respectively.As a simulation of clenching,the jaw-closing forces were applied to the mandible of each model for 60 s.The maximum von Mises stress on the articular cartilage,the articular disc and the retrodiscal tissues of each group were evaluated.Results:Experiment I:1.There were significant differences about measurements of condyle linear dimensions,condylar volume and surface aera,the glenoid fossa depth,joint spaces and distribution of the condyle sagittal positions among the 4 groups(p<0.0001).Whereas there was no statistical difference in terms of the glenoid fossa length among the 4 groups(p=0.1421).2.Correlation analysis revealed statistically significant positively correlations were determined between condyle linear dimensions,condylar volume and surface aera,glenoid fossa depth,superior joint space,and disc position(p<0.01).Posterior joint space was negatively correlated with disc position(p<0.01).However,there were no correlations between glenoid fossa length(r=0.03073),anterior joint space(r=0.02978)and disc position.Experiment II:1.The maximum von Mises stress on the surface of the articular cartilages were higher than those on the disc surfaces in each group.The stresses on the condyle cartilage surface were mainly concentrated on the condylar crest or anterolateral part of the condyle.The stresses on the glenoid fossa cartilage surface were mainly concentrated on the anterior and lateral part of the fossa.At the end of clenching,the maximum stresses on the surface of the articular cartilages were reduced by 30-40%in the control group.However,in the other three groups,the stresses were reduced slightly or even increased.As the disc moved forward,the stresses on the articular cartilages increased significantly,especially in the late stage of anterior disc displacement.2.The maximum von Mises stress on the inferior and superior disc surfaces were decreased compared with the onset(t = 0.01s)of clenching in all groups.As anterior disc displacement progress,the locations of the maximum stresses shifted from the lateral intermediate zone to the lateral posterior zone of the disc or the retrodiscal tissues,and the values tended to decrease gradually.3.There was about 5% decrease of the maximum von Mises stress in the retrodiscal tissues after clenching for 60 s in the control group,and the stresses were concentrated on interior part of the retrodiscal tissues.However,clenching significantly increased the stresses in the retrodiscal tissues in the other three groups,and the stresses were concentrated on lateral part of the retrodiscal tissues.Besides,the maximum von Mises stress in the retrodiscal tissues gradually grew as the disc moved forward.Conclusions:1.A statistically significant correlation was determined between temporomandibular joint bone morphology and sagittal disc position.The more anterior the disc displacement,the more severe the bone degenerative changes of temporomandibular joint.Patients with anterior disc displacement should be treated as soon as possible to avoid disease progression.2.The stress distribution in the TMJ is affected by the sagittal disc position.As the disc moved forward,the stresses on the surface of articular cartilages and retrodiscal tissues increased significantly,especially in the severe group. |
