Optical characteristics of experimental dental composite resin materials

OBJECTIVES

To derive the K-M optical coefficients of experimental composites and compare the inherent CIE L*, a* and b* color parameters, translucency parameters and both perceptibility and acceptability thresholds.

METHODS

Experimental composites were prepared with 4 base-monomers: Bis-GMA, UDMA, Bis-EMA and Fit852 with TEGDMA used as a co-monomer and 3 filler:resin fractions (50:50wt%, 60:40wt% and 70:30wt%). The optical absorption (K) and scattering (S) coefficients over the visible spectra were derived. Corrected reflectivity spectra were calculated using the corrected Kubelka-Munk reflectance model and were used to calculate CIE color parameters (X, Y, Z) values. Translucency parameter (TP) was calculated using the CIEDE2000 color difference. A three-way repeated measures ANOVA was used to analyze the CIE L*a* and b* color parameters at infinite thickness. A two-way ANOVA was used to analyze the translucency parameter at 2 mm thickness. Pairwise comparisons were assessed using Bonferroni-corrected Student's t-tests. For all statistical testing α = 0.05 was used. Color parameters (ΔE₀₀) were calculated for every experimental composite using the CIEDE2000 color differences. Perceptibility threshold (PT), acceptability threshold (AT) and translucency differences (ΔTP) were used to compare experimental composites in both filler fraction within every resin and every resin within each filler fraction.

RESULTS

The statistical analysis revealed a 3-way interaction (P < 0.0001) between base monomer, filler and direction factors. For the translucency parameter, when comparing filler fraction within base monomers, there were statistically significant differences between the filler fraction within all base monomers. The analysis of color differences (ΔE₀₀) of base monomers within filler fraction revealed that the comparison between experimental composites where beyond the acceptability threshold. The comparison of the differences in translucency parameter (ΔTP) of base monomers within filler fraction were beyond the perceptibility threshold, except between base monomers UT and FT.

SIGNIFICANCE

Different base monomer and filler fraction combination influences the optical characteristics of experimental composites such as: light transmission, translucency, and color appearance.

1/(2+Rc): A simple predictive formula for laboratory shrinkage-stress measurement

OBJECTIVE

This paper presents and verifies a simple predictive formula for laboratory shrinkage-stress measurement in dental composites that can account for the combined effect of material properties, sample geometry and instrument compliance.

METHODS

A mathematical model for laboratory shrinkage-stress measurement that includes the composite's elastic modulus, shrinkage strain, and their interaction with the sample's dimensions and the instrument's compliance has previously been developed. The model contains a dimensionless parameter, R_c, which represents the compliance of the instrument relative to that of the cured composite sample. A simplified formula, 1/(2+R_c), is proposed here for the normalized shrinkage stress to approximate the original model. The accuracy of the simplified formula is examined by comparing its shrinkage-stress predictions with those given by the exact formula for different cases. These include shrinkage stress measured using instruments with different compliances, samples with different thicknesses and composites with different filler fractions.

RESULTS

The simplified formula produces shrinkage-stress predictions that are very similar to those given by the full formula. In addition, it correctly predicts the decrease in shrinkage stress with an increasing configuration factor for compliant instruments. It also correctly predicts the value of the so-called flow factor of composites despite the fact that creep is not considered in the model.

SIGNIFICANCE

The new simple formula significantly simplifies the prediction of shrinkage stress for disc specimens used in laboratory experiments without much loss in precision. Its explicit analytical form shows clearly all the important parameters that control the level of shrinkage stress in such measurements. It also helps to resolve much of the confusion caused by the seemingly contradictory results reported in the literature. Further, the formula can be used as a guide for the design of dental composite materials or restorations to minimize their shrinkage stress.

Shrinkage stress development in dental composites--an analytical treatment

OBJECTIVES

The aim of this paper is to develop a comprehensive mathematical model for shrinkage stress development in dental composites that can account for the combined effect of material properties, specimen geometry and external constraints.

METHODS

A viscoelastic model that includes the composite's elastic, creep and shrinkage strains, and their interaction with the sample's dimensions and the external constraint is developed. The model contains two dimensionless parameters. The first one represents the compliance of the external constraint relative to that of the composite sample, and the second controls the rate of shrinkage stress decay through creep. The resulting differential equation is solved for two special cases: zero compliance and zero creep. Predictions for shrinkage stress measurements are then made using the analytical solutions for instruments with different compliances, samples with different thicknesses and composites with different filler fractions.

RESULTS

The model correctly predicts how shrinkage stress increases with time, its dependence on the interaction between the entire system's compliance and the material properties, and the effect of the filler fraction on its maximum value. Comparisons with reported shrinkage stress measurements have provided very good agreement between theory and experiments.

SIGNIFICANCE

The results provided by the model can help to resolve most, if not all, of the seemingly conflicting experimental observations reported in the literature. They can also provide some useful guidelines for optimizing the mechanical performance of dental composite restorations. The compliance ratio, a new parameter derived from the model, represents a fuller description of the constraints of the system.