| The treatment and restoration of defected teeth are common cases in the dental clinical practice. When it is damaged under the margin of its gingiva or alveolar ridge because of various sources, the restoration of a tooth would become difficult, or its survival rate would be unpredictable. In those cases, crown surgical lengthening and forced orthodontic eruption are applied to restructure the biological with of the defected teeth. Permanent restoration can be taken after a certain period. However, longer treatment cycle, more complex procedure and multidisciplinary cooperation are required in both methods. Given those troubles, a novel post, called subgingival post, is designed and its flexibility is evaluated through finite element analysis in this study.The design of subginigval posts was depended on the successful application of implant, which used the remaining tooth structures to provide necessary retention and support for the maintenance of the essential initial stability for bone information. The hypothesis was that healthy soft tissue and osseointegration could generated around subgingival posts, which could successfully restored the morphology and function of defected teeth without being extracted or troubles of application other complex treatment.The purpose of the study is to structure four 3D models of the maxillary central incisor with different defected under the alveolar ridge and restored with the designed subgingival post and core crown. The von-Mises stress on the root, the periodontal ligament and the bone-post interface and the sliding distance on the bone-post interface were analyzed. The results were used to evaluated the flexibility of the designation of the subgingival post and provide the essential biomechanical support for further research.All the study can be divided into three parts.1. A solid model of the maxillary central incisor was generated in the computer-aided design software of Unigraphics NX,coupled with Mimics and Geomagic and based on the serial of computer tomography images from a intact standard isolated maxillary central incisor. Then the surrounding models of the tooth, including the cortical bone, the cancellous bone and the periodontal ligament were constructed, and the subgingival post and core crown was designed. The solid model was then imported into ANSYS Workbench to generate the FE model by using 10-node tetrahedral elements. The material properties of all parts in the model were assumed to be isotropic and linearly elastic. The 120N of labial oblique force was applied with 45o to the long axis of the maxillary central incisor on the one third incisal part of the lingual surface. The bottom of the cortical and cancellous bone was contrained in x, y and z directions (displacements=0) as the boundary condition. The interface between the bone and the post was assumed as being frictionless, with other contact surfaces being fixed.2. Four different models of the subgingival post and core restoration with 1mm, 2mm, 3mm and 4mm defect under the alveolar ridge were structured based on the model established in the first part of this experiment. In those models, the heights of post-bone interface were obtained up to 1mm, 2mm, 3mm and 4 mm respectively. The material properties, the boundary conditions, the application of forces and the assumption of contact conditions were set the same as the former part of this experiment. The distribution of the equivalent stress on the root, the periodontal ligament and on the bone-post interface, and the sliding distance on the bone-post interface were analyzed and compared.3. In order to make further research of it in the vivo studies, the prefabricated subgingival post was designed in the CAD software and optimized through finite element analysis. The subgingival post consists of three parts, which are the post part inserted in the prepared canal, the abutment connecting with bone and soft tissue and the core which provides retention and support for the crown. The post part was designed with screw on its surface, in order to get enough retention and support for the remaining root. The shape of abutment was designed as cylinder, which can be selected in restoration of teeth with various root shapes. The part of core was designed with different notches which provide retention for the core materials. The 3D parametric model of the prefabricated subgingival post and core restoration for the central incisor defected under the alveolar ridge, was established in the UG software through its automated modeling. Then it was inputted in the Ansys workbench and the finite element model was structured which be meshed and simulated the same as in the first part in the experiment. In the Design Exploration, the diameter of the abutment of the subgingival post and the outsider diameter of the screw part were defined as input parameters, with the former ranging from 6.0mm to 7.0mm and the later from 1.0mm to 2.0mm. Meanwhile, the maximum sliding distance on the bone-post interface and the maximum equivalent stress on the bone-post interface, on the root, on the periodontal ligament and on the screw part were defined as response parameters. The results were outputted in 3D images.The results of this study showed as following.1. By using the software of Mimics, Geomagic Studio, UG and Ansys, the FE model of the maxillary incisor restored with the subgingival post was successfully structured, which had precise anatomical features.2. The maximum sliding distances on the bone-post interfaces on all four models with various defects under the alveolar ridge were less than 50μm,which was increased with the increment of the defect of tooth structure within the bone and the height of the bone-post interface.The maximum equivalent stress was observed on the cervical part of the bone-post interface, which rose with the increasing height of the bone-post interface.3. Stresses were still existed on the root and the periodontal ligament. The maximum equivalent stresses decreased with the increasing height of the bone-post interface.4. All the response parameters were significantly influenced and decreased by the increment of the diameter of the abutment of the subgingival post. When it located in the range from 6.5mm～7.0mm which was larger than the largest diameter of the selected incisor on the cervix, the maximum sliding distance on the bone-post interface was relatively smaller.5. The increasing outsider diameter of the screw part has slight effects on the maximum sliding distance and the maximum EQV on the bone-post interface as well as the maximum EQV on the periodontal ligament. The maximum EQV on the root showed a increasing trend with the increment of outsider diameter of the screw part. When it ranged from 1.2mm～1.5mm,which is about one third to one half of the minimum diameter of the part of root in which the post was inserted, the maximum EQV on the screw part is relatively smaller.Conclusion:1.The micro-movement on the bone-post interface was bellow the reported threshold that allowed the possible formation of osseointegration around implant, which may suggest that osseointegration may be generated on the bone-post interface. The design of the subgingival post used to restore those root with serious defect into the alveolar ridge may be feasible. The remaining root and periodontal ligament play important roles to the initial stability on the bone-post interface after restoration.2.The diameter of the abutment of the subgingival post was suggested larger the maximum diameter of the restored tooth on the cervical section. The outside diameter of its screw part should be one third to one half of the minimum diameter of the part of root in which the post was inserted. |
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