| BackgroundCoronal fractures of anterior teeth expecially maxillary central incisors were the most frequent form of acute dental injuries that mainly affects children in the developmental stage. In the past, such teeth were restored with composite resin, veneer and crown. The color of composite resin was vulnerable to change and fall off. The gingival margin of young permanent teeth was instable and the pulp horn was high. So the veneer and crown therapies were questioned for children. Reattachment was conform to the modern dentistry concepts, first it was a minimal invasive technique comparing to traditional restoration methods, and the technique could restore the outline and contact surface, color, function of the fracture teeth, what was more, the clinical operation was easy and economical affordable. Nowadays research methods on reattachment were confined to clinical studies and in vitro teeth mechanics experiments. Preparation technique was one of the most important factors affecting reattachment therapy, which had been reached a consensus. However, some critical problems needed to be further addressed to. Such as how did different preparation techniques affect the results of reattachment, how did different angles and length of bevel affect the outcome of reattachment. In order to provide more evidence for clinical applications, the following investigation were designed combining the finite element analysis that had been widely used in the oral medicine with reattachment technique.ObjectivesThe experiment combined Micro-Computed-Tomography scanning technology with Mimicsl0.0, Hypermesh10.0. Ansys10.0software to construct the3D solid model and the three-dimensional finite element model of whole and1/3crown across fracture of the maxillary central incisor, and tested the the model validation. With the same method, different preparation techniques and different bevels were constructed. The first principal stress, the third principal stress and the Von-mises stress distribution patterns within models were analyzed the differdnt preparation techniques and different bevels on reattachment with different preparation techniques or different bevels to provide reference evidences for clinical selection.Materials and MethodologiesPart I Establishment of3D FEM of whole and1/3crown across fracture of the maxillary central incisor1、collect the primitive dataThe whole maxillary central incisor was selected according to Chinese standards after fixing with epoxy resin, the tooth scanned from root to crown using a micro-CT scanner. The section was vertical to long axis of the tooth with layer distance of0.125mm. The primitive data and pictures was saved according to a certain order.2、establish and test model of the maxillary central incisorThe Micro-CT pictures were switched into Mimics10.0software for geometric reconstruction, including enamel, dentin and pulp. After correcting and dividing mesh with Hypermesh10.0softmare, the model was subsequently inputed into Ansys10.0software. The overall size of each finite element mesh was controlled at0.1-0.2mm. The3D FEM model include alveolodental ligament and alveolar bone was established. Thickness of alveolodental ligament was0.2mm.Various materials and tissues in model were hypothesized to be homogeneous and continuous. The tooth was fixed in alveolar bone with isotropic property. The stress was static loading at the junction between1/3middle and1/3incisal at lingual side of the tooth. The aspect of stress was at the angle of45°with the long axis. Observation of index was Von-mises stress for model validation.3、establish model of1/3crown across fracture of the toothWith the same method, the model of1/3crown across fracture of the maxillary central incisor was established.Part II Establishment and analysis of3D FEM of four preparation technique1、Establishment of3D FEM of four preparation techniqueWith the same method, the models of the maxillary central incisors were established, using four different preparation technique. For example, the simple reattachment protocols, the overcontour, the internal dentin groove technique and the dentin removal. Fragment, resin composite material and remnant were established. The contact surface between fragment, resin composite material and remnant was relative stable.2、Analysis of3D FEM of four preparation techniqueConditions of loading were the same as part Ⅰ. Observation of indexes:the maximum stress and the stress distribution patterns of S1, S3and Von-mises of fragment, resin composite material and remnant. How did different preparation technique affect reattachment. Part Ⅲ Establishment and analysis of3D FEM of four preparation bevels1、Establishment of3D FEM of four preparation bevelsWith the same method, the model of four preparation bevels of the maxillary central incisor were established at the angle30°length with lmm. angle30°with length3mm, angle60°with length1mm, angle60°with length3mm. The model of fragment, resin composite material and remnant were established respectively and the contact surface between fragment, resin composite material and remnant was relative stable.2、Analysis of3D FEM of four preparation bevelsConditions of loading were the same as partⅠ and observation of indexes were the same as part Ⅱ. How did different preparation bevels affect reattachment.Results1、The3D solid model and the3D FEM of the maxillary central incisor was established, including enamel, dentin, pulp, periodontal ligament and alveolar bone. The maximum stress distribution of the Von-mises stress was mainly con centrated on cervical and loading surface. The maximum stress was69.9MPa.2、The3D FEM of1/3crown across fracture of the maxillary central incisor was established. The3D FEM of four preparation techniqueand four preparation bevels were established. Fragment, resin composite material and remnant were established independently3、The maximum stress of remnant was larger than resin composite material and remnant but the maximum stress of fragment, resin composite material and remnant was lower than tooth and resin. For the maximum stress of the Von-mises stress, the complete teeth was lower than the remnant.4、With static surface load, the difference of the maximum stress of different preparation technique of fragment, resin composite material and remnant were not significant. The stress distribution of different preparation technologies were similar. The first principal stress, the third principal stress and the Von-mises stress of fragment were mainly concentrated on the distal marginal ridge of tongue. The first principal stress of resin composite material was mainly concentrated on distal surface, but the third principal stress and the Von-mises stress were mainly concentrated on the distal marginal ridge of tongue. The first principal stress, the third principal stress and the Von-mises stress of remnant were mainly concentrated on the lingual tooth neck.5、With static surface load, the maximum stress of different length bevel of fragment, resin composite material and remnant had no regularity, but the distribution were similar. The stress of bevel with60degrees and1mm was the largest. The first principal stress was6.338MPa, the third principal stress was-15.924MPa and the Von-mises stress was13.198MPa. With the same angle, the longer bevel lenth was, the smaller stress of fragment was, but the bigger stress of resin composite material and remnant was.Conclusions1、It is possible to establish3D FEM of the maxillary central incisor. The model features geometric similarities and perfect mechanical.2、Reattachment was useful for crown fracture of composite tooth.3、The difference of four preparation technologies are not significant in oral biomechanics. Size and location of the stress were no significant difference in different preparation techniques. The preparation technologies should be selected based on clinical.4、The angle is no effect on strsee of fragment, resin composite material and remnant in oral biomechanics. The longer length can decrease the force of fragment, but increase the force of resin composite material and remnant. The preparation technologies should be selected based on clinical. Length of bevel should be moderate, not as long as possible. |
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