Study Of Biomechanics In The Temporomandibular Joint During Mouth Opening

The temporomandibular joint (TMJ) is considered one of the load bearing organs inthe human body. Functional overloading is considered to play a major role in the etiologyof TMD. Excessive compressive and shear stresses are probably the most common sourcesof condylar resorption and disc perforation. Progress in disc displacement also seems to berelated to the nature of the stress distribution and biomechanical equilibrium in the TMJ.Therefore, the biomechanical environment in the TMJ is a key to understand the originand progression of TMD. Analysis of joint load distribution cannot be performedexperimentally in human. The finite-element (FE) method has been proven to be a suitabletool for approximating the distribution of loads in the structures of the TMJ. It can be usedto analyze the stress distribution patterns in the TMJ tissues after application of force ordeformation. Most of finite element (FE) analyses focused on the clenching movement, because the maximum of TMJ loading was recognized at maximum clenching oraggressive behavior. However, TMJ receives various loading not only during clenchingbut also during jaw opening. Furthermore, the condylar movement during mouth openingproduces remarkable disc motion and deformation. Up to now, few studies have beenreported to dynamically simulate the disc, considering disc displacement and deformationduring mouth opening. In this study, a two-dimensional dynamic finite element model ofthe human disc during mouth opening was developed based on the MR images in differentdegrees of mouth opening. Disc reaction forces of healthy and TMD subjects duringmouth opening were analyzed. We also discussed the feasibility of TMJ strain analysisduring mouth opening by means of Digital Image Correlation Technique.Part one: Dynamic magnetic resonance imaging of temporomandibular joint duringmouth openingObjective: This study aimed to evaluate the TMJ movement by taking magneticresonance images in different degrees of mouth opening. Methods: Sagittal MRI sliceswere obtained from a healthy subject and subject with TMD in every degree of mouthopening (6-25mm). The images of the same slice in every position were selected to make afilm. The position, shape, range of the motion and deformation of mandibular condyle anddisc were assessed. Results: The contours of glenoid fossa, condyle and articular disccould be clearly displayed on MRI. There are no artifact and “Jumping” sign duringcondyle and articular disc movement. Lateral pterygoid muscle was contracting duringmouth opening, dragging condyle and articular disc moving toward anterior. In the TMDsubject, articular disc abnormal displacement and deformation was observed during mouthopening. Conclusion: Magnetic resonance images in different degrees of mouth openingcan provide valuable information for assessing the dynamic TMJ movement, also can beused to establish FE model of TMJ during the movement of mouth opening. Part two: Finite element analysis of normal human articular discduring mouth openingObjective: The aim of this study was to establish finite element model of the normalarticular disc during mouth opening movement, and to analysis the stress distribution inthe articular disc. Methods: The contours of glenoid fossa, condyle and articular disc inevery degree of mouth opening were traced using a commercial software Auto-CAD byone trained dentist. Two-dimensional finite element models of articular disc in everydegree of mouth opening were developed with the ANSYS program. The discdeformation was used as the loading condition for stress analysis. The stress distributionin the healthy articular disc was evaluated during mouth opening. Results: Thereconstructed model of articular disc during mouth opening had good morphology, andhad good similarity to the real anatomic structure. This model could dynamicallysimulate the articular disc with the aim of studying stress distribution during the openingmovement. For the normal subject, the posterior zone suffered tensile stress, and thestress level increased with the progress of the opening movement. The highest tensilestresses were located in the anterior band, where is the attachment of lateral pterygoidmuscle. The compressive stress level in the intermediate zone is much higher, and keptincreasing through mouth opening. Lower tangential stresses were found in the healthyarticular disc during mouth opening. Conclusion: This methodology improved thebiomechanical similarity, and offered possibilities for articular disc functional analysisduring mouth opening. The stress increased with the degree of jaw opening, but remainedat a reasonable level. Thus, the jaw opening process will do no harm to the healthyarticular disc.Part three: Comparison of stress distribution of articular disc between health subjectand subject with TMD during mouth openingObjective: The aim of the work was to compare the stress distribution in the articular disc between a healthy subject and a subject with TMD during mouth opening. Methods:Two-dimensional finite element models of articular disc were developed according tomagnetic resonance images which were taken from a subject with TMD in differentdegrees of mouth opening. Differences of stress distribution in the articular disc duringmouthing opening between the subjects with and without TMD were evaluated. Results:The mean stress level of the disc was higher in the TMD than in the normal subject. Forthe TMD subject, tensile stresses in the posterior band were twice as high as in the healthyjoint, increasing remarkably with the progress of the opening movement. For the TMDsubject, anterior band suffered higher tensile stresses, rising gradually with the progress ofthe opening movement. So we think hyperactivity of the lateral pterygoid muscle plays animportant role in the progress of TMD. Tangential stresses in the disc of the TMD subjectwere higher than in the healthy one. Conclusion: It is suggested that TMD induces thechange of stress distribution in the disc, resulting in the secondary tissue damage, if notreatment is applied.Part four: Preliminary study of TMJ strain analysis during mouth opening based onDigital Image Correlation TechniqueObjective: To investigate the possibility of TMJ strain analysis during mouth opening inuse of Digital Image Correlation Technique (DIC). Methods: DIC is subsequently used toobtain displacement field by the image of the TMJ during mouth opening using MATLABsoftware. Results: Strain data calculated by Digital Image Correlation Technique wassimilar to the real displacement, Disparity was also found in many areas. Conclusion:DIC is suitable for accurate measurement of the TMJ strain field, with the development ofpretreatment for MR image and algorithms for DIC.