| The temporomandibular joints(TMJ)are load-bearing joints and rebuild all the life.The load on the TMJ has been proven to play an important role in its structure and function as well as the etiology and therapy of TMJ disorders. Occlual force is the most direct origin of the load. Occlusion,TMJ and masticatory muscles are the major components of the masticatory system of the interconnected biomechanical system. It is clear that the elements of the articulatory system are inescapably connected. They are obviously interdependent because a change in any part will clearly affects the other parts The change of Occlusion and the rebuilding ability of the TMJ is the impoint precondition of their coordination. If the coordination is broken down, temporomandibular disorder (TMD) may occur.The pathogeny of TMD is more complex. At present, most people hold the view that TMD is due to a combination of factors, in which the occlusion and spirit are more concerned by many scholars. The pathogenic factor of TMD recognized at the earliest is abnormal occlusion. Although the occlusal factor still exists in the large dispute, people can not deny that the occlusal factor is one of pathogenic factors of TMD and that occlusion treatment is effective for TMD.At present, scholars believe that the teeth put in order according to Spee curve can maintain multi-teeth close contact in the chewing process and the mandibular protrusion movement. And maxillary longitudinal occlusal curve assorts with mandibular Spee curve in harmony, which can improve the masticatory efficiency as function of the bio-mechanics. Spee curve is one of important morphological features of dentition, and a important indicator of the vertical position of teeth. Smooth occlusal curve can harmonize the mandibular movement in all directions. FEM (Finite Element Method) is a practical and effective method of theory stress analysis. This method was first applied to the design and optimization of dental restoration by Farah in 1973. With the rapid development of computer technology and a variety of full-featured software developed in recent years, the finite element technique can be applied to the mechanics research of a variety of complex problems. Finite element method has been widely used in the field of oral medicine. It has obvious advantages compared with other traditional methods of experimental stress analysis.From support of theories and research findings, three-dimensional FEM, this paper studies that the morphology change of vertical direction of occlusion has the effects on the stress distribution of condylar process and articular disc of temporomandibular joint or not by altering Spee curve with a view to have guidance for the clinical denture prosthetics and prevention of TMD.First, a healthy young female volunteer as a subject was selected and scanned from the top of condyle to the bottom of chin. CT data were obtained at 1.0 mm slice thickness, 0.65mm pitch value, 0.45mm interpolation. Data processing was performed in the graphics workstations of CT machine and directly exported to 230 DICOM format images.Second, the DICOM files were imported into the MIMICS software, according to the different gray-scale profiles, the mandible and teeth images were generated separately. Their three-dimensional model were generated by calculating all CT slices. Teeth thickened by 0.3mm minus the original teeth, then cut and got the periodontal ligament of uniform thickness. Alveolar spaces were obtained by subtraction of periodontal ligament from mandibular model. The mandible model thickened by 2mm minus the original mandible, then cut and got the similar articular disc. The establishment of similar glenoid fossa was a small cortical bone block minus similar articular discFinally, the finished three-dimensional models were imported into Abaqus software in the form of surface mesh, then converted the surface mesh to a volume mesh. All models were attributed elastic linear isotropic behavior. The mandible is considered as a non-uniform material and set 150 kinds of materials according to the gray value, other materials were assumed uniform. The outer surface of the glenoid fossa and the mandibular angle were fixed. We configured the contacts of teeth and periodontal ligament, as well as the periodontal ligament and alveolar, and articular disc and the glenoid fossa as constraint-tie, but the articular disc and condylar as non-frictional interaction. Imposed concentrated load near the incisal margin of incisor, the dental cusp of canine and the buccal cusp of premolar and molar. At last, we built the finite element model of intercuspal occlusion.We got the stress distribution of the condyle and the articular disc after caculating previous finite element model. Then change the shape of Spee curve via rebuilding the model of teeth, so that new finite element models were built. Recalculated and got the stress distribution under the different curve of Spee.The analysis of the results from different kinds of Spee curve conditions showed that the stress in all cases are concentrated in contact area of condylar and articular disc. That area was condylar anterior slope and intermediate zone of articular disc. When Spee curve's curvature is 0mm, the stress of condylar anterior slope and intermediate zone of articular disc had the addition with statistical significance compared with the original value; when Spee curve's curvature is 4.5mm, the stress of condylar anterior slope and intermediate zone of articular disc had the subtraction with statistical significance compared with the original value; when the second premolar and first molar was taken out, so that Spee curve was changed, the stress of condylar anterior slope and intermediate zone of articular disc increased with statistical significance compared with the original value. Hence, the change of Spee curve had an obvious effect on the stress value of temporomandibular joint with statistical significance (P<0.01) . In this study, thin-layer CT technology and finite element method were organically combined and some advanced hardware and software tools were used, such as Philip Brilliance 64 spiral CT, the medical modeling software Mimics10.01, finite element analysis software Abaqus6.7, which were applied to build three-dimensional finite element model of temporomandibular joint, mandible and teeth. This method was more rapid and precise than the previous ones. We Studied the relationship between the condyle and the articular disc through the method of contact finite element, which improved significantly compared with previous researches. For material assignment of mandible, according to the correlation of gray-scale, density and elastic modulus, as well as the empirical formula, the mandible was assigned to multiple materials, which reflected the characteristics of non-uniform materials of mandible, which was more accurate than the previous study in which mandible was considered as cortical bone and cancellous bone separately and closer to reality about the last results. Some results validated that the condylar anterior slope and intermediate zone of articular disc were the weight-bearing areas in the temporomandibular joint, and that the condylar anterior slope was destructed by some joint diseases and the intermediate zone of articular disc was easily broken and perforated. The final results showed that the stress value in the temporomandibular joint varied with the change of Spee curve.
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