A.J. de Gee

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.

Relaxation of Polymerization Contraction Stresses by Flow in Dental Composites

The hardening stresses in wall-to-wall bonded composites were measured and related to calculated values obtained from freely shrinking materials. Since the ultimate tensile strength of the materials contracting under restricted conditions was not lower than that of composites which were not hindered during the polymerization shrinkage, the conclusion was drawn that the contraction was compensated for by flow instead of internal disruptions.

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.

Polymerization contraction and conversion of light-curing BisGMA-based methacrylate resins

The aim of this study was to investigate the polymerization contraction and the conversion of light-curing methacrylate resins based on bisphenol-A bis(2-hydroxypropyl)methacrylate (BisGMA) diluted with triethylene glycol dimethyacrylate (TEGDMA), methyl methacrylate (MMA), hydroxypropyl methacrylate (HPMA) or (+/-)-2-ethylhexyl methacrylate (EHMA). The contraction measurements were carried out with a linometer, a simple device to determine true linear polymerization contraction of liquid monomers at ambient temperature. The contraction increased with the amount of diluting monomer. The estimated conversion of the BisGMA-TEGDMA, calculated using the contraction, is consistent with literature values. The BisGMA-HPMA mixtures showed high conversions at moderate contraction.

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.

Occlusal wear simulation with the ACTA wear machine

This paper describes how the clinical conditions in stress bearing areas are thought to be simulated in the ACTA wear machine. The wear types simulated are erosive wear and contact sliding wear in the presence of a third-body medium, consisting of natural food substances. Wear due to surface fatigue may also be studied with the wear machine. Wear rates for a wide range of resin composites, an amalgam and a glass polyalkenoate (ionomer) cement obtained with the ACTA wear machine correlated with an average correlation coefficient of 0.90 with data collected from clinical trials. The results justify the experimental set-up as being one that includes a realistic simulation of the complexity of clinical wear.

A modified dilatometer for continuous recording of volumetric polymerization shrinkage of composite restorative materials.

Abstract A method for continuous recording of volumetric polymerization shrinkage of chemically, as well as photoactivated composite materials is described. The percentage shrinkages of Concise-Cap-C-Rynge, Nuva-Fil and Fotofil determined in this way were 2·55, 2·67 and 2·03 respectively.

The Dependence of Shrinkage Stress Reduction on Porosity Concentration in Thin Resin Layers

The development of polymerization contraction stress was determined as a function of the surface area of porosity, so that the contribution of voids in resin composite to stress relief could be investigated. Experiments were carried out on 200-um-thick layers of resin bonded from wall to wall in a restrained condition. The resin samples were divided into three groups: Group A was without porosity, group B contained a small number of pores, and group C contained a large number of pores in comparison with group B. For each group, porosity area, maximal stress, and stress development rate were determined. The mean maximal stress and stress development rate were inversely proportional to the mean porosity surface. These characteristics differed significantly (p < 0.01) between group A and C. For determination of whether shrinkage stress reduction has to be ascribed to flow from the outer surfaces of the voids or to inhibition of the setting reaction by oxygen in the voids, resin containing only nitrogen bubbles was also tested. The results indicated that both aspects contributed substantially to shrinkage stress relief. Incorporation of pores by the stirring of a luting cement contributes to stress reduction and can therefore be considered as a contribution to the maintenance of marginal integrity.

Early and Long-term Wear of Conventional and Resin-modified Glass Ionomers

Various studies have shown that glass ionomers are susceptible to brittle fracture and acid conditions and that they undergo long-term changes in their mechanical properties. Little information is available on how brittleness, acid susceptibility, and long-term changes are reflected in the wear characteristics of glass ionomers. The purpose of this study was to evaluate long-term changes in conventional glass ionomers, metal-reinforced glass ionomers (including a cermet), and (light-curing) resin-modified glass ionomers by wear experiments simulating the wear process in occlusal contact-free areas. The wear tests were conducted periodically over a period of one year. In addition, wear was determined after one year at a pH of 5 or 6, for assessment of acid susceptibility, and at a condition as found in the occlusal contact areas. All materials showed high early-wear rates which decreased significantly during the one-year test period. This long-term process may be related to a slow progression of the acid-base reaction extending over several months. At each stage, the resin-modified glass ionomers wore significantly faster than the acid-base setting glass ionomers. Most of these materials were not affected at a pH of 6.0, while at a pH of 5.0 only the conventional and the metal-reinforced glass ionomers showed increased wear. Direct contacts with the antagonist led to a significant increase in wear in comparison with contact-free wear, probably as a result of sub-surface fatigue phenomena. In view of the unfavorable wear characteristics of the resin-modified glass ionomers and the high early wear of the conventional glass ionomers, including the metal-reinforced glass ionomers, it was concluded that none of these materials can yet be recommended for use in high-stress-bearing situations.