| Inlay is a dental prosthetic restoration or intracoronal retainer made of different materials which inset the inside of tooth to restore the configuration and function of tooth. For more severe dental caries, it can restore the configuration and syntopie accurately.The purpose of using inlay to restore a large area of defective tooth is to reserve tooth, restore masticatory function and keep patients'health. However, the cuneo-response introduced by inlay increases the stress and may cause the fracture of the remaining tooth tissue. In order to study the influence of the inlay on the stress of the remaining tooth tissue, three-dimensional finite element is used to analysis stress distributions of the tooth endured different MOD inlay restore at different materials and different load conditions. The results can provide evidence for the clinical restore using inlay.Object and methods:1. Establishing a 3D solid and finite element model of mandiblar molar restored with MOD inlay. The first permanent mandibular molar was scanned by CT and the images were processed by Mimics, Geomagic Studio 7. A three-dimensional solid model was established using UG software.Then software ABAQUS and MSC.MARC were used to establish 3D finite element models. Three types of model were constructed: the MOD cavity restored by inlay with/without Root Canal Therapy, and the MOD cavity restored by onlay after endodontic treatment.2. Analyzing the stress distribution of the models at different load conditions, the influences of different inlay materials were also investigated (composite resin, ceramic, gold alloy). A simulated chewing force of 45kg perpendicular and 45°deviation with respect to the tooth axis were applied to center of the teeth with. The stress distribution and stress curve in different inlays were investigated to evaluate the influence of different materials and restorations on the stress distribution of the remaining tooth.3. Based on the 3D finite element models, the stress distributions of the tooth tissue at different load conditions is studied. The load of perpendicular and 45°deviation were applied to center of the tooth with three materials. The stress distribution and stress curve in different inlays were used to study the trend of the stress of remaining tooth. The tendency of stress on inlay and onlay restoration is investigated.Results:1. A three-dimensional finite element model of the first permanent mandibular molar which includes dentin, adamantine layer, camera pulpi and alveolar bone was established. The model has favorable geometric similarity.2. Different materials do have effect on the stress distribution of the restored tooth. The biggest stress concentration was observed in the gold alloy group, while much lower stress concentration was observed inceramic and composite resin group.3. After root canal therapy, tooth restored with onlay have the best stress distribution regardless the materials. Compared with the pulpless tooth restored by inlay, the tooth preserving the pulp restored by the same restoration has a better stress distribution.Conclusions:1. CT scanning technology, digital image processing technology and finite element method can be combined to establish a 3D solid finite element model of first mandibular molar. Furthermore, the method in the paper is effective when applied to calculate stress distribution within complex structures.2. Among the three materials (composite resin, ceramic, gold alloy) investigated in this study, composite resin inlay could reduce stress concentration of the remaining tooth tissues, compared with other two materials.3. Within the limitations of this study, it can be concluded that onlay is the ideal prothetic, which caused the least damage to the remaining tooth tissues, when restoring a highly decayed tooth. So we suggested that, inlay should be carefully evaluated, especially when the tooth was endodontically treated, so as to avoid fracture of the tooth.
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