| Many destructive factors can lead to teeth missing and even dentural unintegrated such as: dental caries, periodontal disease;trauma;tumor and congenital malformation, etc. when the denture is exercising the masticating function, the teeth beside the space of missing tooth will tilt towards mesial or distal direction very easily. The occlusal force on the tilted teeth can not be transmited to periodontal tissue along the long axis of abutment tooth. The force was transmited to the dental cervix periodontium through tooth and concentrated on these areas. The alveolar bone resorption occurred after a long time, for exemple subcrestal pocket was formed, and the periodontal tissue was damaged at last. Besides, dentist always reduce the stress on periodontal tissue by decreasing buccolingual diameter. Many researches showed that the fixed partial denture (FPD)is the better restorative procedure besides the implant denture to restore the missing teeth of which the abutments are tilted. At parent, there are not internal or external detail reports about the stress analysis on the periodontal membrane of abutment teeth in the two situations: different degree tilted abutment teeth and different buccolingual diameter of pontic.Through successive scanning by three-dementional laser scanner on the surface of the model, we obtained the point cloud data. And processed these point cloud data by relative softwares, we got the three-dementional finite element model (3-D FEM)finally. And get some messages of the two situations in which ①mandibular first molar is missing, rigid fixed bridge was taken, second molar was tilted towards the second premolar seriously.②mandibular first molar is missing, rigid fixed bridge was taken, the area of pontic was decreased to different extent by reduce the buccolingual diameter. Inthe first situation, we focused on the stress condition on the tilted abutment. In the second case, we observed the stress contribution on the two abutments. The finial goal of these researches is to analyze the influence of tilted degrees of abutment on the periodontium, and to evaluate whether reducing the buccolingual diameter of pontic is beneficial to the periodontium after the FPD restoration. And provide biomechanical knowledge to the clinicians when selecting the indications.Part one: Three-dimensional Finite Elemental Model of Mandibular Fixed Partial Denture constructed by laser scannerThe purpose of this research is to construct three-dimensional geometry models and finite element models of fixed partial denture. Based on these results, we can exercise the biomechanical study of FPD smoothly. Select a mandibular three-unit fixed partial denture model with abutment roots to undergo three-dimensional laser scanning and get point cloud data. And put all of the point cloud data into the computer, these data are processed by relative Geomagic Studio6.0 reverse engineering software. Based on these point cloud data, the software generated triangular grid entity automatically. And separate the crown and roots, and get the outline characteristics of periodontal membrane by offsetting 0.2mm towards normal direction of the roots. Then put this grid model into the norientation software (I-DEAS 10 NX Series), combined with anatomic knowledge, designed the entity model of alveolar bone and crown directly, and put these models as geometry data pattern (stl) into the Geomagic Studio6.0 again. Through the Boolean calculation among the different entity models, we constructed the three-dimensional entity models of periodontal membrane, alveolar bone, preparations and fixed bridge. And import these models into MSC.Marc to divide grids. At last, we got 153856 quadr-nodes-tetrahedron elements and 68762 nodes points. The emulational model is accuracy. It can meet the need of dynamic analysis in stomatognathic system. So we got the conclusion: Laser scanning technic can generate three-dimensional finite elemental model quickly and exactly. It can provide helps to the clinicians.Part two: Stress analysis of the dental surrounding tissues of different degree tilted tooth as a fixed bridge abutment: a three - dimensional finite element analysisThe objective of this study is to analysis the stress distribution on the peridental membrane of different degree tilted abutment after the restoration of three - unit fixed partial denture when mandibular first molar is missing and the second molar is selected as the abutments. And provide the theoretical foundation to the clinical exercises. Five finite element models of three - unit fixed partial denture (Ml, M2, M3, M4, M5) were constructed in which the second molar was tilted 0°> 12<\ 18°> 24<\ 30° toward the second premolar by the combination of the three-demensional laser scanning technology and Marc software. A load of 230N was applied to the occlusal surface of the pontic from different direction. Observe the stress distribution on the periodental membrane of the second molar. And we got the result: In the model of which second molar was tilted 24°,the stress peak value on the peridental membrane of mesio-dental cervix was twice the normal second molar. And this value has exceeded the periodental reservation of abutments. So the conclusion is that when tiled degree of the abutment tooth was bigger than 24°, this tooth could not be selected as an abutment in the restoration of fixed partial denture. In such cases clinicians can select the cantilever bridge, implant denture or attachments.Part three: The stress effects of decreasing buccolingual diameter of the pontic to the dental surrounding tissues after the restoration of FPD: a three - dimensional finite element analysisTo reduce the stress on the dental surrounding tissues, clinicians always took the measures of decreasing buccolingual diameter of the pontic in the situation that mandibular first molar is missing, rigid fixed bridge was taken. The objective of this study is to obverse the stress contribution of periodental membrane and certify whehter decreasing buccolingual diameter of the pontic can reduce the stress on surrounding tissues of abutments when decrease the buccolingual diameter to different extent.Divide the pontic of FPD obtained in the experiment part one again proportionally in Geomagic Studio6.0 reverse engineering software by the combination of the three-demensional laser scanning technology and Marc software, and keep the area of connector and the mesial-distal diameter invariable meanwhile. Design the buccolingual diameter is 100%. 90% ^ 66.7% (2/3) and 50% of the normal separately, and divide grid in the software of Marc. And get four 3-D finite element models: M1(100%)n M2(90%) > M3(66.7%)^ M4(50%), exert the stress on the models, calculate and analysis the stress contribution on the abutment periodental membrane. When exert the vertical even load on the pontic, the stress on the periodontal tissue decreased regularly with reducing of the occlusal area, but there are not obvious change of the stress when exerting the vertical concentrate load and oblique load. So we got the conclusion that when keep the occlusal force unchanged, purely decreasing buccolingual diameter of the pontic can not decrease the stress on the periodental membrane of abutments. To achieve the destination, besides this measure, clinicians must cut down the functional cusps, increase the number of spill way and lower the occlusal surface.
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