Abstract
The loss of interfacial integrity was identified as one of the major causes for replacement of resin composite restorations. Preheating procedure has been proven to enhance flowability and adaptation of resin composites and increase their degree of conversion. The purpose of this study was to investigate polymerization contraction stress produced in resin composites after preheating to 37℃ and 60℃, and measure microleakage of Class V restorations restored with preheated composites. Three resin composites (GC Kalore, Gradia Direct X, Filtek Supreme XT) at room temperature, 37℃, and 60℃ were investigated. Maximum contraction stress of the composites (n = 5) was evaluated in a modified low-compliance device. Samples were light-cured for 40 seconds and the maximum force was recorded during 15 minutes. Calculations were done to adjust for the system’s compliance and obtain linear shrinkage values of composites. Data were analyzed by Multivariated Analysis of Variance (MANOVA) and Tukey’s test for multiple comparisons (α = 0.05). Seventy-two Class V cavities were prepared on the buccal surfaces of extracted premolars and divided into 9 groups. The teeth were restored with composites at 3 temperatures and were thermo-cycled between 5℃ and 55℃ with a one-minute dwell-time for 1000 cycles. The teeth were sealed with wax and nail vanish before placed in 0.5% toluidine blue dye for 24 hours. The teeth were embedded in self-curing resin and sectioned bucco-lingually with a slow-speed diamond saw, providing 3 sections per restoration. Microleakage was rated by two evaluators using a 0 - 4 ordinal scale at the occlusal and cervical margins under light microscope. Microleakage data were analyzed with Kruskal-Wallis ANOVA and Mann-Whitney U test (α = 0.05). Results indicate that preheating composites to 37℃ and 60℃ significantly increased polymerization contraction stress of composites (p 0.05). A significantly greater amount of leakage was found at the cervical margins (p 0.05). For all tested materials, preheating composites to 60℃ resulted in significantly less microleakage at the cervical margin.
Highlights
Improvements in resin composites’ mechanical properties and their reduced polymerization shrinkage during the past decade encouraged clinicians to use resin composites more frequently for posterior restorations
The purpose of this study was to investigate polymerization contraction stress produced in resin composites after preheating to 37 ̊C and 60 ̊C, and measure microleakage of Class V restorations restored with preheated composites
Results indicate that preheating composites to 37 ̊C and 60 ̊C significantly increased polymerization contraction stress of composites (p < 0.05)
Summary
Improvements in resin composites’ mechanical properties and their reduced polymerization shrinkage during the past decade encouraged clinicians to use resin composites more frequently for posterior restorations. Increasing the filler load resulted in higher viscosity and led to concerns about handling, packing, and adaptation of the material. Many attempts were made to enhance composite adaptation and decrease microleakage between composites and dental cavity, either by using the flowable composite as a base material, chemical and laser treatments of dentin or by preheating the composites to lower their viscosity. The effect of lower viscosity in improvements of adaptation has been proven important. This is the primary basis for producing flowable resin composites in which the lower viscosity can be achieved by decreasing their filler contents and making changes in the matrix chemistry, which severely reduce their physical properties. Many polymers exhibit lower viscosity when they are heated because thermal vibrations force the composite monomer further apart and allow them to slide by each other more
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