C.L. Davidson

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.

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.

Unreacted Methacrylate Groups on the Surfaces of Composite Resins

The unreacted methacrylate groups on the surface of conventional composites and microfilled composites were studied by means of multiple internal reflection infrared spectroscopy. The influences of polymerization time, temperature, and the effect of polishing were studied. We also attempted to relate the results of some mechanical tests to the degree of chemical unsaturation measured in the different resins. All surface treatments and manipulative variables caused a decrease in double bond content and interfacial bond strength.

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.

Marginal Sealing of Curing Contraction Gaps in Class V Composite Resin Restorations

When one uses composite resins, the curing contraction, the stiffness of the material, and the strength of the vulnerable dentinal bond are important factors in determining the marginal adaptation of the restoration. Calculations based on these intrinsic material properties have indicated that both bulk placement and incremental placement of the restorative material in the cervical cavity inevitably lead to marginal gap formation. Sealing of this gap with an unfilled low-viscosity resin, directly after the composite resin is cured, may lead to perfectly closed restoration margins, provided that composites with low linear curing contraction and low Youngs modulus are used. These conditions were ezperimentally shown to be valid for laboratory and clinical situations.

Quantitative evaluation of the wear resistance of posterior dental restorations: a new three-dimensional measuring technique

Abstract The wear resistance of Adaptic, Estic Microfill, Miradapt and Dispersalloy was quantitatively measured in a long-term clinical study. A new, reliable and accurate three-dimensional measuring technique for quantitative evaluation of the wear resistance of posterior dental restorations was developed. The abrasion and attrition resistance of the conventional composite was inadequate for use in Class I and II restorations. The attrition resistance of the microfilled composite was unsatisfactory. For the first time, promising wear values were obtained for the hybrid radioopaque composite resin, Miradapt.

Effect of Abrasion Medium on Wear of Stress-bearing Composites and Amalgam in vitro

Measurements of in vitro wear for several composite materials and an amalgam were conducted using a new device in which contact stress, moving speed, mutual slip, and third-body composition could be set. This preliminary study reports the results obtained with a fixed choice of contact stress, speed, and slip, but with different third-body media. With a Millet-seedIPMMA-beads mixture as a medium, the various types of restoratives showed a wear pattern and relative ranking comparable with that reported in clinical studies.

The influence of surface roughness on porcelain strength

OBJECTIVES In order to adjust occlusion, the functional surfaces of porcelain restorations are often ground and mechanical machining is even an essential part of the CAD-CAM process for these restorations. The aim of this study was to investigate the influence of the finishing procedures on the biaxial flexure strength of four commercial porcelains. METHODS Four commercial porcelains of which two are used for metal-ceramic restorations (Flexo Ceram Dentine and Vita VM K68) and two for veneers and inlays (Duceram LFC Dentine and Cerinate BODY) are used in this study. For each porcelain, sixty discs (Ø = 22 mm, h = +/- 2.0 mm) were produced using twelve different finishing procedures. Twenty discs were left untreated, twenty discs were milled, using a high-speed diamond disc, and twenty discs were machined in a high-speed grinding/polishing device. Half of the samples were glazed. In each of these six groups, half of the samples were stored for 16 h at 80 degrees C in a 4% acetic acid solution. The biaxial flexure strength was determined using the ball-on-ring method. In each group the roughness of the surface was determined and examined via SEM. RESULTS With the exception of Flexo Ceram Dentine, a significant correlation was found between the roughness of the surface and the biaxial strength: the smoother the surface, the stronger the sample. The differences in biaxial strength may be attributed to the stress concentration of an applied load due to the roughness of the surface caused by mechanical finishing or chemical action. The fact that the strength of Flexo Ceram Dentine was not affected by the different surface treatments is probably due to the size of the leucite particles, which apparently induce more stress concentration than the surface flaws and the roughness of the surface. SIGNIFICANCE It was concluded that surface roughness determines the strength of a porcelain material, except where the inner structure of the material causes greater stress concentration than that caused by the combination of surface roughness and surface flaws.

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.