ANALYTICAL ESTIMATION OF THE TOOTH STRENGTH RE-STORED BY DIRECT OR INDIRECT RESTORATIONS
Keywords:
contact interaction, stress-strain state, tooth, filling.Abstract
This work presents the results of analytical estimation of stress-strain state of reconstructed tooth hard tissues with consideration their contact interaction with a monolithic filing or a dental onlay. The tooth with a defect was simulated as a continuous isotropic cylindrical body with a non-through hole, and the filling or onlay is an elastic deformable cylinder. The ideal contact relations describe the contact between the tooth and the fillings (onlay). The final strength estimation was carried out according to the energy criterion. The mechanical and mathematical model of the system "tooth-filling (onlay)" was developed, which make possible to take into account the influence of elasticity and strength and formulating the boundary-contact problem in a form suitable for analytical solving on the object of study.
The stress-strain state parameters were calculated, in particular the axial, contact and circumferential stresses (analytical expressions (1 - 4), which take into account the elastic-mechanical properties of the hard tissues of the tooth and the material of the filling or onlay). Based on these data, plots of these indicators related the TLDI index was obtained. This index is convenient for practical application, because it is easy to obtain by direct measurements.
Analysis showed that increasing the stiffness of the filling (onlays) reduces the axial stresses in the tissues of the tooth. Therefore, the restoration material with the larger Young’s module should be used in case of the TLDI index decreasing. In addition, the contact stress between the filling (onlay) and the tissues of the tooth can be both compression and stretching, depending on the ratio of their Poisson’s coefficients. For this reason, it is impractical to use filling materials with for tooth restoration, because contact stresses will become stretched, which may lead to deterioration of adhesion or even to tear off the filling from the tooth. In addition, the contact stresses are, averagely, two orders less than the axial ones, so they can be neglected when evaluating the tooth strength.
The restored tooth strength assessment was made on the basis of the Guber-Mises energy criterion by the magnitude of the maximum equivalent stresses. The strength of the wall thickness should be no less then; if wall thickness is lees the filling is not effective and the ceramic onlay should be used to restore the tooth. In addition, the neglect of the Poisson effect can significantly affect the values of equivalent stresses.
Based on the analysis, practical recommendations for damaged tooth crown reconstructions were developed.
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