Effect of repeated heating and cooling cycles on the degree of conversion and microhardness of four resin composites

Mechanical and Optical Characterization of Single-shade Resin Composites Used in Posterior Teeth

OBJECTIVES

This study aimed to evaluate the optical and mechanical performance of two single-shade resin-based composites (RBCs) compared to those of a conventional RBC for restoring posterior teeth.

METHODS

Two single-shade RBCs, Omnichroma (Tukoyama) and Vittra Unique APS (FGM), and a conventional RBC, Filtek Z350XT shade A2 (3M Oral Care), were evaluated in this study. The optical shade-matching performance was measured using a spectrophotometer. The light emitted by VALO Grand (Ultradent) and transmitted through the 2.0-mm RBC specimens was evaluated using beam profiling. Knoop hardness (KH, N/mm2), degree of conversion (DC, %) at the top and bottom, flexural strength (FS, MPa), elastic modulus (E, GPa), postgel shrinkage (Shr, %), and shrinkage stress (MPa) were evaluated. Scanning electron microscopy (SEM) was used to characterize the filler. Data for FS, E, and Shr were analyzed by one-way analysis of variance (ANOVA) and KH and DC by repeated one-way ANOVA measurement followed by Tukey test (α=0.05). The modified von Mises stress values, light transmission, and SEM images were analyzed qualitatively.

RESULTS

All single-shade RBCs exhibited higher chromatic adaptation than the Filtek Z350XT (p<0.001). Omnichroma exhibited less color difference than Vittra Unique APS, irrespective of the shade. The light transmitted through Omnichroma and Vittra Unique APS increased during polymerization. KH and DC values were significantly reduced from the top to the bottom of the specimens for all RBCs (p<0.001); however, the ratio values were always higher than 80%. In general, all RBCs demonstrated similar mechanical properties. All RBCs exhibited a similar FS (p=0.083) and Shr value (p=0.144). Filtek Z350XT exhibited significantly higher E (p<0.001) than both single-shade RBCs. All RBCs exhibited similar shrinkage stress during restoration and similar residual stress during occlusal loading.

CONCLUSIONS

Single-shade Omnichroma and Vittra Unique APS increased light transmission during light-activation, demonstrating better chromatic adaptation than conventional Filtek Z350XT. In general, Omnichroma and Vittra Unique APS exhibited similar mechanical properties and shrinkage stress distributions as Filtek Z350XT during light-activation and occlusal loading.

Does the use of preheated restorative resin composite as a luting agent influence the adaptation of fixed dental prostheses? A systematic review

STATEMENT OF PROBLEM

Although studies have explored the physical and mechanical properties of different composite resins with the preheating technique, evidence that the use of preheated restorative composite resin as a luting agent influences the adaptation of fixed dental prostheses is lacking.

PURPOSE

The purpose of this systematic review was to answer the question: Does the use of preheated restorative composite resin as a luting agent influence the adaptation of fixed dental prostheses?

MATERIAL AND METHODS

Seven databases and nonpeer-reviewed literature were searched, without language or year restrictions. Studies directly comparing the adaptation of fixed dental prostheses cemented with preheated restorative composite resin or resin cement were considered eligible for inclusion. Assessment of the risk of bias was based on a 9-item checklist.

RESULTS

Of 2109 retrieved articles, 4 in vitro studies met the inclusion criteria. Three of them concluded that preheated restorative composite resin produced a greater mismatch than resin cement, and the remaining study reported that composite resin improved the adaptation of fixed dental prostheses. All included studies had a medium risk of bias. The high level of heterogeneity among the studies precluded meta-analysis.

CONCLUSIONS

The use of preheated restorative composite resin as a luting agent seems to negatively influence the adaptation of fixed dental prostheses. It was not possible to evaluate whether variables such as the formulation of the resin material or its heating time and temperature influence the adaptation of fixed dental prostheses because of the high heterogeneity of the included studies.

Effect of repeated preheating on monomer elution from a bulk-fill composite resin

Background

Due to incomplete polymerization of composite resin restorations, residual monomers adversely affect their mechanical properties and biocompatibility. Preheating of composite resins is advised to increase the degree of conversion and reduce monomer elution. This study aimed to analyze the effect of preheating and repeated preheating on the amount of monomer released from a bulk-fill composite resin.

Methods

Forty samples were prepared using Filtek One Bulk Fill Restorative composite resin. Samples in one group were fabricated at room temperature, whereas the composite resins in the other groups were cured after 1, 10, or 20 repeated preheating cycles (55 °C), 10 in each group. Eluted urethane dimethacrylate (UDMA) and bisphenol-A-glycidylmethacrylate (BisGMA) monomers were measured with high-performance liquid chromatography (HPLC) 24 hours and 30 days after immersion. The data were evaluated using one-way ANOVA and post hoc Tukey tests. Paired-sample t tests were used to test the differences between time intervals.

Results

At both time intervals, the greatest amounts of released BisGMA, UDMA, and total monomers were obtained from the control group, whereas 10 preheating cycles resulted in the least monomer elution. The decrease in monomer elution was not statistically significant after 10 preheating cycles compared with that after one preheating cycle (P>0.05). The group with 20 preheating cycles showed a greater amount of monomer elution compared to that with 1 and 10 cycles, which was statistically significant (P < 0.05). The amount of released monomers on day 30 was significantly higher than on day 1 (P<0.01).

Conclusion

Preheating of the bulk-fill composite resin was shown to be effective in reducing monomer elution. However, monomer elution was adversely affected after repeated preheating cycles of 20.