| Temporomandibular join is one of the important components of oral system,it takes a role in all the functions of oral. Because of its refined structure and complicated functions , it becomes the emphases and difficulty of the study of biological stresses in the oral system.The relationship of orthodontic treatment and temporomandibular joint defunction is always a dispute.In our clinical treatment ,we ofen use inter-arch elastics for harmonizing the cclude relationship. The truth is we have changed the stress distribution in TMJ, then cause its reconstruction,and change the position of the mandible and the lower arch. so there is a require for the study of biological stresses.Finite element analysis is a method which segments a consecutive elastomer into finite cells, , and by analyzing each cell's mechanical character gain the whole elastomer's displacement and stress distribution. This method can analyze complicated strcture conformation load and material mechanics, it has became the leading measure and method of the TMJ stress analysis.MBT is a modern straight wire technique. It has inherited all the straight wire technique's excellences ,and has a outstanding characteristic at dental's precise location, reasonable use of treatment stress and simple treatment program,because of good treatmen outcomes, it starts to be the main technique of orthodontic treatment. There have been some study which constructed 3-D finite element model of upper arch including MBT brackets,but none study of the biological behavior of it.Objective: 1.To construct a three-dimensional element model including: TMJ maxillary,mandibular and dentition with MBT straight wire appliance for mechanical analysis.2.Analyze the influence of four types(classⅡlong elastic,classⅡshort elastic,classⅢlong elastic,classⅢshort elastic)of inter-arch elastics on the stress distribution of TMJ, then presume which type make for a healthy TMJ,and which may bring discomfort or even pathological changes of the TMJ. This will be a guidance for clinical apply of elastic.Method: :Made scanning images of maxillary mandibular and detitions through multi-slice CT scanner of a healthy female volunteer, using Mimics,Ansys software to reconstrut a three-dimensional element model of TMJ,maxillary,mandibular,dentition with MBT straight wire appliance(0.016〞ss). Simulate the clinical conditions put four types elastic on the model. Condition①ClassⅡlong elastic, the load points are upper canine bracket's distal gingival corner and lower first molar mesial gingival corner, the value is 100g. Condition②ClassⅡshort elastic, the load points are upper canine bracket's distal gingival corner and lower first premolar mesial gingival corner, the value is 100g. Condition③ClassⅢlong elastic, the load points are lower canine bracket's distal gingival corner and upper first molar mesial gingival corner, the value is 100g. Condition④ClassⅢshort elastic, the load points are lower canine bracket's distal gingival corner and upper first premolar mesial gingival corner, the value is 100g. Analyze the articular disc and the condylar process's stress distribution and displacement trend.Result :1. A three-dimensional element model of the left TMJ,maxilla,mandible and dentition with MBT straight wire appliance was construted.Components can be added or subtracted for different studies.2.The influence of four different types inter-arch elastics on the stress distribution of TMJ have been on this three dimensional finite element model , the condyle and articular disc's stress distribution and the displacement change have been obtained under these conditions.①Stress distribution: condition1 and condition2: Pull and pressure coexisted on the condyle surface, the front part and middle presents the wide range the compressed stress area, the stress value outside is slightly higher than the inside, the joint rear slope presents the obvious tensile stress distribution area. Pull compressed stress coexist on the articular disc surface similarly, anterior band and intermediate zone are compressed stress area, the stress value outside in slightly higher than the inside, the posterior band is tensile stress area. The stress values of condition2 are lower than condition1. Condition3 and condition4: Pull and pressure coexist on the condyle surface, the condyle forward slope is the tensile stress area, and the stress value is the highest. Stress value outside is slightly higher than the inside. Articular disc: Pull and pressure coexist on the surface of the articular disc, posterior band is compressed stress area, the stress value outside is slightly higher than the inside .Moreover the articular disc internal surface stress's distribution is similar to the outside surface, but the stress value is higher than the articular disc outside surface slightly. But the stress values of condition4 are lower than condition3.②Displacement: condition1 and condition2: in y axis direction condyle migrated to the front, in z axis direction condyle migrated downward, but the lower edge of the condyle rear slope moved a little upward, prompted the condyle possibly have turned clockwise. In y axis direction the articular disc migrated to the front, but the amount of the displacement is smaller than the condyle, which prompted the condyle glided in the joint. in z axis direction the articular disc migrated downward, the posterior band's displacement is most obvious. Condition2's displacement quantities are smaller than condition1. condition3 and condition4: in y axis direction condyle migrated rearward, in z axis direction condyle migrated upward, but the lower edge of the condyle rear slope moved a little downward, prompted condyle possibly have turned anti-clockwise. In y axis direction the articular disc migrated rearward, but the amount of the displacement is smaller than the condyle,which prompted the condyle glided in the joint . In z axis direction the articular disc migrated upward, the posterior band's displacement is most obvious. Condition3's displacement quantities are smaller than condition4.Conclusion:①The model is comparable with the vivol issue on geometrical and mechanical,it can be used to research the mechanical behavior of MBT straight wire appliance.②The loading and restraint conditions of this model have met the request of 3-dimentional element analysis of TMJ's biomechanical character.③At the same stress value,short inter-arch elastics offer more gentle stress than long inter-arch elastics.④Joint's stress distribution which ClassⅡinter-arch elastics produced conforms to the normal physiological condition.
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