A.J. Feilzer

Setting Stress in Composite Resin in Relation to Configuration of the Restoration

The setting stress in composite resins was studied as a function of restoration shape. The shape is described by the configuration factor, C, the ratio of the restorations bonded to unbonded (free) surfaces. In an experimental set-up, the shape of the restoration was simulated by cylindrical forms of various dimensions. The shrinkage stress was measured continuously. It was shown that in most of the clinically relevant cavity configurations, the stress-relieving flow is not sufficient to preserve adhesion to dentin by dentin-bonding agents.

The Competition between the Composite-Dentin Bond Strength and the Polymerization Contraction Stress:

The influence of contraction stresses, developed during the polymerization of composites, on adhesion to dentin treated with a dentin adhesive was studied for a chemically- and a light-activated microfilled composite, in both linear and 3-D models. The linear model consisted of an arrangement set up in a tensilometer in which the composites could be applied to a flat dentin surface fixed to the stationary cross-head at one end, and mechanically clamped to the cross-head connected to the load cell at the other end. The increase of the bond strength was measured at different time intervals from the start of mixing and was compared with the developing contraction stress. Throughout the complete polymerization process, the adhesion survived the contraction stress, which is explained by flow relaxation, which can occur sufficiently in this configuration. In the three-dimensional model, the composites are attached to more than two dentin walls. In this situation, flow is severely limited, and contraction stress values can exceed the bond strength, leading to separation. This was demonstrated in Class V cavities. The shape of the cavity is considered to be of great importance in conservation of the composite-dentin bond.

Polymerization shrinkage and polymerization shrinkage stress in polymer-based restoratives

OBJECTIVES This paper is intended to contribute to the recognition and understanding of problems related to polymerization shrinkage. DATA SOURCES Scientific publications of relevance with regard to this subject were critically reviewed. STUDY SELECTION The dimensional changes which develop during the curing of resin composites and glass polyalkenoate cements are studied, with special reference to methods of determining shrinkage, shrinkage stress and stress relief. CONCLUSIONS As no method for handling the adhesive restorative materials has yet been described which guarantees a leakproof restoration, the practitioner has to accept the problem of polymerization shrinkage and destructive shrinkage stress. Only a proper understanding of the mechanisms that cause these problems and the techniques that may reduce their effects will enable the practitioner to derive maximum benefit from the application of resin composites and glass polyalkenoate cements in restorative dentistry.

Quantitative determination of stress reduction by flow in composite restorations

In this study, the reduction of the polymerization shrinkage stress by flow of four chemically-initiated composites was investigated in relation to the cavity configuration. In an experimental set-up simulating restorations bonded to cavity walls, the developing shrinkage stress accompanied by flow was recorded as a function of time for several configurations. For each configuration, theoretical shrinkage stress curves were also drawn, excluding stress reduction by flow. These data were obtained from Youngs modulus determinations at the early setting stage and the corresponding polymerization shrinkage. By comparison of the theoretical stress with the experimentally determined stress, a measure for the ability to flow in the bonded situation could be obtained. It was found that the flow strongly depended on the type of composite and on the configuration of the cavity.

Increased Wall-to-Wall Curing Contraction in Thin Bonded Resin Layers

Wall-to-wall (WTW) polymerization contraction of filled and unfilled chemically and photo-initiated resins was studied in relation to the WTW distance. In an experimental set-up, the resins were bonded to two opposing disks, and the axial (WTW) displacement resulting from the polymerization shrinkage was measured continuously. It was found that the WTW contraction increased with decreasing WTW distance and ultimately reached a value of almost three times the linear polymerization shrinkage.

Selective infiltration-etching technique for a strong and durable bond of resin cements to zirconia-based materials

STATEMENT OF PROBLEM Establishing a strong and a stable adhesive bond between yttrium, partially stabilized, tetragonal zirconia, polycrystal materials (Y-TZP) and resin luting agents has proven to be difficult using conventional surface roughening and coating methods. PURPOSE The purpose of this study was to evaluate the zirconia-resin bond strength and durability using a selective infiltration-etching technique. MATERIAL AND METHODS Seventy-two Y-TZP discs (19.5 x 3 mm) were airborne-particle abraded with 110-mum aluminum oxide particles and divided into 4 groups (n=18). One test group received the selective infiltration-etching surface treatment. Three commercial adhesive systems (Panavia F 2.0, RelyX ARC, and Bistite II DC) were used to bond the airborne-particle-abraded zirconia specimens to preaged restorative composite resin discs (Filtek Z250). Panavia was used to bond the selective infiltration-etched specimens. The bonded specimens were cut into microbars (6 x 1 x 1 mm), and a microtensile bond strength test (MTBS measured in MPa) was conducted immediately, after 1 week, 2 weeks, 3 weeks, and after 1 month of water storage (5 microbars/disc/time interval/group, n = 450 microbars/group). Scanning electron microscopy was used to examine the fractured microbars. The density (g/cm(3)) and the 4-point flexure strength (MPa) of the selective infiltration-etched and airborne-particle-abraded specimens were measured to evaluate the effect of selective infiltration etching on the structural integrity of the Y-TZP specimens. A repeated measures ANOVA with 1 within-subjects factor (time, 5 levels) and 1 between-subjects factor (technique, 4 levels) was used to analyze the data (alpha=.05). Pairwise comparisons were made using the Bonferroni post hoc test. RESULTS There were significant differences in the initial MTBS values (MPa) between the 4 bonding techniques (P<.001). Airborne-particle-abraded specimens bonded with either Panavia F 2.0, RelyX ARC, or Bistite II DC resulted in a mean (SD) bond strength of 23.3 (2.4), 33.4 (2.1), 31.3 (2.8) MPa, respectively, while the highest bond strength of 49.8 (2.7) MPa was achieved for the selective infiltration-etched specimens bonded with Panavia F 2.0. There was a significant interaction between water storage time and the bonding technique (P<.001) as reduction in MTBS values was observed with time, except for the specimens bonded with Panavia (selective infiltration-etched and airborne-particle-abraded specimens). Additionally, the observed failure mode was primarily cohesive for the selective infiltration-etched specimens, in contrast to the other groups, which showed primarily an interfacial failure. CONCLUSIONS For the materials used in this study and under the same testing conditions, selective infiltration etching is a reliable method for establishing a strong and durable bond with zirconia-based materials.

Setting stresses in composites for two different curing modes

As a continuation of a study into the development of the polymerization shrinkage stress of chemically initiated composites (CC), the development of the polymerization shrinkage stress of light-initiated composites (LC) in relation to the configuration-factor was determined. During setting, the LC composites generated a higher polymerization shrinkage stress, developed higher cohesive strength to resist this stress and showed less flow than their CC analogues. Trying to find a comprehensive explanation for the differences in behavior of stress development in LC and CC composites, the effect of mixing-in of porosity was also investigated on LC composites. Mixing-in of porosity slowed down and decreased the shrinkage stress development. This was attributed to either oxygen inhibition due to admixed air or to an increase of free surface from the presence of pores within the bulk of the 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 η and Youngs 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 (η/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.

Polymerization contraction stress in thin resin composite layers as a function of layer thickness

OBJECTIVES In the present study, the effect of layer thickness on the curing stress in thin resin composite layers was investigated. Since the value of the contraction stress is dependent on the compliance of the measuring equipment (especially for thin films), a method to determine the compliance of the test apparatus was tested. METHODS A chemically initiated resin composite (Clearfil F2, Kuraray) was inserted between two sandblasted and silane-coated stainless steel discs in a tensilometer. The curing contraction of the cylindrical samples was continuously counteracted by feedback displacement of the tensilometer crosshead, and the curing stress development was registered. After 20 min, the samples were loaded in tension until fracture. The curing stress was determined for layer thicknesses of 50, 100, 200, 300, 400, 500, 600, 700 microns, 1.4 mm and 2.7 mm. The compliance of the apparatus was calculated with the aid of a non-linear regression analysis, using an equation derived from Hookes Law as the model. RESULTS None of the samples fractured due to contraction stress prior to tensile loading. The contraction stress after 20 min decreased from 23.3 +/- 5.3 MPa for the 50 microns layer to 5.5 +/- 0.6 MPa for the 2.7 mm layer. The compliance on the apparatus was 0.029 mm/MPa. SIGNIFICANCE A measuring method was developed which was found to be suitable for the determination of axial polymerization contraction stress in this films of chemically initiated resin composites. The method makes it possible to estimate the stress levels that occur in resin composite films in the clinical situation.

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.