RESUMO
The integrity of teeth would be affected by the thickness of remaining radicular dentin; therefore, the test hypothesis of this study was to evaluate the necessity of weakened endodontically retreated maxillary anterior teeth to be reinforced with fibre post. The radicular dentin thickness of sound teeth at coronal, middle and apical levels was measured [step 1] and compared with the remaining dentin thickness after endodontic treatment [step2], endodontic retreatment [step3] and after post hole preparation [step4] These meas:emeAV5 NNXC, ecsx-related with the fracture resistance of teeth submitted to corresponding steps. Finite Element Analysis [FEA] as a valid and reliable technique was used to study the stress distribution within the tooth structure in each of the previously mentioned situations. The results showed that significant reduction of radicular dentin thickness occurred after endodontic treatment in the coronal and the apical two thirds and this reduction continued insignificantly in the consequent steps On the other hand, middle one-third revealed significant reduction in endodontic retreatment at P o.o5 The fracture resistance of sound teeth [group I] was insignificantly different either with those of endodontically treated [group II] or teeth restored with post [group IV]. However, endodontically retreated teeth [group III] showed significant reduction in the fracture resistance when compared with other groups which pointed out the necessity to fibre post reinforcement. FEA showed the patterns of stress distribution within the tooth structure for each step and the similar results obtained by this technique clarify the same conclusion
RESUMO
Objectives: the purpose of this study was threefold; firstly; to determine and compare the fracture resistance of sound, class II prepared and restored human maxillary premolars with five different bonded resin- composite, ceromer and ceramic inlays, using mechanical testing and finite element analysis [FEA]. Secondly; Check the validity of the numerical model as a successful non destructive technique for teeth analysis by comparing the obtained numerical data to the experimental data. Thirdly; to assess the-stress distribution and maximum tensile stresses, using a three- dimensional FEA, in class II prepared and restored human maxillary premolars with the aforementioned five inlays
Materials and Methods: Forty two sound human maxillary premolars were selected, cleansed, scrubed and embedded in self-curing acrylic resin blocks. Teeth were randomly assigned into seven groups six teeth each. Group I comprised unprepared and unrestored teeth [controls]. Group II comprised teeth that were class II prepared but not restored. In groups III-VII, standardized class II cavity preparations were performed and restored with five different bonded inlays; resin-composite [Filtek Z/250]. Ceromer [Targis], fired porcelain [Vita Dur Alpha], CAD/CAM [Cerec 1] and copy-milled [Celay] respectively. All inlays were cemented with dual-component composite-resin based luting material [Panavia 21, Lot 31113; Kurary Co. Ltd]. All specimens were subjected to a compressive axial load on a universal testing machine [Lloyd testing machine, England], with a 5 KN compressive load cell and a constant rate of loading 0.5 mm/ mm, by means of a steel bar [8 mm in diameter]. The bar was centralized to the occlusal surface, parallel to the long axis of the tooth and contacted the buccal and palatal cuspal slopes beyond the margins of the preparations/restorations. The compression load was applied until the specimen fractures. The fracture load values were recorded in [KN] at the moment of fracture. Data was tabulated and statistically analyzed. For the FEA, a maxillary first premolar was three dimensionally modeled. The model symbolized the tooth with its different components [enamel, dentin, etc], the restoration, the resin cement, the acrylic resin block and the applied load. The properties of the restoration were varied to cover all used inlays. The cement thickness was also varied. A 170 N static load simulating the working load on the tooth was applied in a way similar to that applied during the mechanical testing
Results: Class II cavity preparation in maxillary premolars significantly decreased the tooth resistance to fracture. The fracture resistance of teeth restored with Filtek Z/250, Targis and Cerec inlays was not statistically significantly different from that of sound tested teeth, yet, the sound tooth fracture resistance was not re-established. Teeth restored with laboratory fired ceramic inlays showed significantly the lowest mean fracture load compared with machinable ceramics and other tested inlays. The fracture load numerical data was consistent with the experimental data in this study. Results of FEA stress distribution and maximum tensile stresses assessment within the tooth and the different inlays, implicate that the modulus of elasticity and the tensile strength of the inlay material influence the mode of fracture of restored teeth. The finite element analysis can be considered as a reliable non destructive method for assessment of the tensile stress distribution within the restored teeth and also as a predictor of the fracture load
Recommendations: Further studies are needed to determine the role of the resin cement properties under machinable ceramic inlays in improving the fracture resistance of teeth