| Objective:To optimize the design of all-ceramic inlay reparation of MOD cavity, by using Ansys finite element method and changing the value of the MOD cavity parameter of the maxillary first premolar.Methods: This study is based on four experiments:1. Micro-CT was used to scan the extracted, intact maxillary first premolar .Collecting the point data images of enamel, dentin, and pulp by Mimics 10.0 software, which were imported into Imageware12.0 software to deal with point cloud data and inverse algorithm of curve. After surface fitting, the tooth solid model was accomplished by Ansys 11.0 software. Then, enamel, dentin, and pulp were integrated through Boolean operation in Ansys 11.0. At the same time, the solid model of periodontium and alveolar bone were constructed, and the 3D meshes were created respectively.2. Segmenting the established finite element model of maxillary first premolar in Ansys11.0 software, which simulated the MOD cavity of maxillary first premolar, and the 3D models were remeshed. By establishing shell element to simulate resin cement, the finite element models were established to restore the MOD cavity of maxillary first premolar.3. To set the distance from pulpal wall to gingival wall as H, the width of gingival wall as G, and grouped by the two parameters. The models were modified depended on defined parameters (H and G) in Ansys 11.0 software. Under decentralized vertical loading 200N, the figure and data of stress distribution were got by using finite element analysis method.4. The model with depth of pulpal wall(H)=1.0mm, wide of gingival wall(G)=2.0mm was chosen. To group the form of axiopulpal preparation: angle of 90 as A, rounded as B, axial wall to lean toward pulp wall 10 degrees as C, bevel as D, the figure and data of stress distribution were got by using finite element analysis methodResults:1. This study established a fine three-dimensional finite element model of maxillary first premolar including pulp, periodontium and alveolar bone. After 3D meshing, the enamel had 26,685 tetrahedral elements, the dentin had 114,082 tetrahedral elements, the pulp had 11,843 tetrahedral elements, the periodontium had 22,004 tetrahedral elements,the alveolar bone had 66,767 tetrahedral elements. In total, the model had 241,381 10-node-tetrahedral elements. The accuracy of the model was very high. It is real reflected the figure of the tooth and the curvilinear shape of enamelo-cemental junction .2. This study established a fine three-dimensional finite element model to restore maxillary first premolar by all-ceramic inlay, which included pulp, periodontium, alveolar bone and resin cement. The resin cement had 2,786 elements, and 5,711 nodes. The all-ceramic inlay had 38,027 elements and 55,854 nodes.3. The preparation of the MOD cavity influenced the stress distribution of all-ceramic inlay restored. Under the condition of constant width of gingival wall, the Maximum Von Mises of enamel and dentin decreased with the increase of the depth of pulpal wall. The enamel changed evidently,it was decreased 4.991% as G=2.mm, and 6.077% as G=2.5mm respectively. The Maximum major principal stress of enamel and dentin were minimum as H=1.0.With the increase of depth of gingival wall, both the Maximum Von Mises stress and the Maximum major principal stress of dentin and cement would increase, and the Maximum Von Mises stress increased by 14.519% and 24.75% respectively , and the Maximum major principal stress increased by 18.723% and 46.423% respectively to the largest extent than the former value.The stress distribution of cervical enamel surrounding the proximal edge of the inlay was more centralized.4. The different shape of axiopulpal was modified in Ansys11.0 software. For cement layer and dentin the Maximum Von Mises stress in group B,C,D was larger than that value in group A. From stress distribution chart the stress-centralized area was smaller in group B,C,D than that in group A; There is no apparent regular pattern in enamel, For all-ceramic inlay, the Maximum Von Mises stress and maximum major principal stress in group B, C, D were smaller than that in group A.Conclusions:By using and combining the Micro-CT technology and reverse-engineering software, this study established a three-dimensional finite element model of maxillary first premolar. Through shell element to simulate resin cement in Ansys software, and prepared MOD cavity to established a finite element model which restored with all-ceramic inlays. Tooth preparation can influence the stress distribution of restored all-ceramic inlay. The depth of pulpal wall is an important parameter of enamel, the value of stress increased with the decrease of the depth of pulpal wall; The width of gingival wall is an important parameter of dentin and cement layer, the value of stress increase with the increase of the width of gingival wall. The cervical enamel surrounding the proxiaml edge of the inlay is prone to fracture. Preparing the vertical axiopulpal to be rounded, axial wall to lean toward pulp wall 10 degrees and beveling can lower the stress inside of the all-ceramic inlay, and improve the stress distribution of cement, dentin, and enamel. Especially, by beveling axiopulpal line angle, we can lower the value of maximum major principal stress of all-ceramic inlay to prevent the inlay fracture, and provide some theoretical foundation for clinical operation.
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