Theoretical Analysis And Numerical Simulation During The Dental Restoration Process

Composites with particulate reinforced resin matrix are now increasingly applied for replacing traditional restorative materials, such as silver amalgam, because of their advantage mechanical properties and good aesthetic. However, polymerization shrinkage of the resin-based materials is of concern because the resulting shrinkage stresses can lead to failure of the interfacial bonds, which results in marginal leakage, premature failure of the restoration, and in some cases micro-cracking of the tooth. In addition, the composite resin appears to be capable of uptaking water in aqueous environment of oral, which exerts an obviously detrimental effect on the physical/mechanical properties, and even results in irreversible damage to the dental composite. To determine the properties of composite and restoration-tooth structure under polymerization shrinkage and water diffusion, the previous studies of others were based on trial-and error methods.As a numerical method, the finite element technology can be used to solve complex engineering problems. There are currently no options in commercial finite element programs specifically designed to address the issue which take into consideration both polymerization shrinkage and water sorption due to its difficulty.To simulate the behavior of the restoration-tooth under polymerization shrinkage and water diffusion, the following investigations have been done based on the three-dimensional unit cell model and the finite element analysis:1 Moisture distribution in the restoration-tooth structure which expose to oral environment can be evaluated by assuming that moisture diffusion is described by a Fickian process. The strain analysis and moisture diffusive finite element formulations were obtained, respectively. The finite element formulation applicable for hygro-mechanical problem was derived by coupling solution.2 Impact factors about the performance of restorative material were analyzed considering varying material properties. The performance of the particulate reinforced composites (PRC) was studied deeply using the cubic unit cell model. It was found: (1) The stiffness of the PRC increase as the PVF increase. (2)The strength of the PRC depended on the strength of the particulate. (3)The interphase property has the strong effect on the property of the PRC.3 Damage effects of water sorption on mechanical properties of the PRC have been predicted using 3D finite cell models. The results were found as following: (1) The damage value increases more significantly with the increase in PVF before 30% PVF, beyond which the increasing trend becomes gradually gentle. (2)Keeping the interphase strength and PVF unchanged, the damage increase with increasing moisture concentration in a nearly linear relation. (3)The results generated from the FCC model with consideration of the interphase debonding are in good agreement with the experimental data, meanwhile, the FCC model is also capable of predicting the critical load for the damaged and the undamaged dental composite subject too the 3-point flexural test.4 A 3D-fmite element analysis has been successfully exploited to examine the mechanical behavior of the restoration-tooth structure under the polymerization shrinkage and water sorption. It was found: (1) The low stiffness of the composite has the effect to limit the displacement occurring in the remaining tooth tissue. For lower interphase, the cuspal flexures are litter higher. The high interphase modulus acts as a 'shielding effect' for stress transfer. (2)The mildest composite results in the lowest tooth stress and displacement. The stiffest restoration-tooth interphase leads to the lowest displacement and highest stress in composite. The stresses generated in the models increased with the stiffness of the composite. The stress and displacement are reduced due to the compensation action of water sorption. (3)The hyper-water sorption results in maximum stress and displacement within the models, while the equi-water sorption lead to minimum value.5 The effect of properties of the interphase on the mechanical behavior of the restoration-tooth structure under the polymerization shrinkage was discussed. The behavior of polymerization shrinkage was described using the model of viscoplasticity. Some results were got using the FEM method: (1) The low stiffness and thick interphase can release the high residual stresses which are located at the occlusal surface along the tooth-composite joint, but the thick interphase can result in the stress concentration in the angle surrounding the pulp wall. (2)Sensitivity analysis indicated that the modulus and thickness of the adhesive have the strong effect on the residual stresses of the restoration-tooth, a thin interphase of a more flexible adhesive exhibits the same mechanical performance as a thick interphase of a less flexible adhesive for the restoration-tooth within limits.