Effect of TEGDMA/BisGMA ratio on stress development and viscoelastic properties of experimental two-paste composites.

In this study, we explored the reduction of shrinkage stresses in restored teeth by stimulating viscous flow of adhesive restoratives during curing, by increasing the TEGDMA/BisGMA ratio in the resin of composite restoratives. We studied a series of experimental two-paste composites with different amounts of TEGDMA (30, 50, 70 wt%, respectively) in the resin by mechanical testing, infrared spectroscopy, and dilatometry, to determine how comonomer composition affects the mechanical and chemical properties of the composite during curing. It was found that the polymerization rate of BisGMA-TEGDMA composites is indicative of the viscoelastic behavior during curing: The higher the reactivity, the higher the stiffness and viscosity development. Composites with 50 wt% TEGDMA in the resin displayed the highest maximum polymerization rate. High amounts of TEGDMA in the resin only modestly increased the pre-gel viscous flow (= lowered viscosity) property of composites. Of these composites, high post-gel shrinkage is the decisive factor in high shrinkage stress development.

Modeling of viscoelastic behavior of dental chemically activated resin composites during curing.

Shrinkage stresses generated in dental resin composites during curing are among the major problems in adhesive dentistry, because they interfere with the integrity of the restored tooth. The aim of this study was to find a mechanical model to describe the viscoelastic behavior of a two-paste resin composite during curing, to aid our understanding of the process of shrinkage stress development. In this study, stress-strain data on Clearfil F2 during curing were obtained by a dynamic test method and analyzed using three mechanical models (Maxwell, Kelvin, and the Standard Linear Solid model). With a modeling procedure, the model's stress response was compared with the experimental stress data, and the material parameters were calculated. On the basis of the modeling and evaluation results, a model for describing the viscoelastic behavior of the shrinking resin composite was selected. The validation results showed that the modeling procedure is free of error, and that it was capable of finding material parameters associated with a two-parametric model with a high degree of accuracy. The viscoelastic behavior of the shrinking resin composite, as excited by the conditions of the test method, cannot be described by a single mechanical model. In the early stage of curing, the most accurate prediction was achieved by the Maxwell model, while during the remainder of the curing process the Kelvin model can be used to describe the viscoelastic behavior of the two-paste resin composite.

Visco-elastic parameters of dental restorative materials during setting.

Contraction stresses generated in restoratives during setting are among the major problems in adhesive dentistry, since they often result in loss of adhesion from the cavity walls or in post-operative pain. The rate of stress development and the ultimate magnitude of the stress, which determine the seriousness of these problems, depend on the relatively unknown visco-elastic behavior of the restoratives during setting. The aim of this study was to determine the visco-elastic parameters during setting, to aid our understanding of the process of contraction stress development. A dynamic mechanical method was used in which the materials were subjected to periodic strain cycles in a universal testing machine during the first 60 min of setting. The visco-elastic parameters (viscosity eta and Young's modulus E) were calculated by analysis of the experimental stress-strain data with a simple mechanical model according to Maxwell. Two restorative materials from different classes were investigated: a two-paste resin composite and a conventional glass-ionomer cement. A comparison of the results showed significant differences in the development of viscosity and stiffness in the early stage of setting. The resultant relaxation time (eta/E) of the glass ionomer remained at a low level during the first 15 min, whereas that of the resin composite increased markedly. This is of clinical importance, since it implies that, during the early setting stage, glass ionomers are better capable of reducing the contraction stresses than resin composites, thus increasing the likelihood that the bond with the cavity walls will form and survive during setting.

Developments in shrinkage control of adhesive restoratives.

PURPOSE: This article reviews material properties and application techniques important in minimizing effects of polymerization shrinkage during the curing reaction of resin composite restorative materials used in adhesive dentistry. MATERIALS AND METHODS: Relevant scientific publications were critically reviewed. RESULTS: Since it was recognized that shrinkage, which takes place during the curing reaction of resin composite restorative materials, may cause severe problems in adhesive dentistry, considerable effort has been put into reducing the negative effects. The most important problem is the debonding of the restoration-tooth interface, resulting in increased microleakage and, ultimately, in secondary caries. Despite all efforts, there is still no material or general application method that guarantees a leak-proof and durable restoration. CLINICAL SIGNIFICANCE: It is of the utmost importance that dental practitioners know how to deal with the problems related to resin composite shrinkage, so that they can choose the material and procedure most likely to produce a leak-proof and durable restoration, maximizing the potential for clinical success.