This study investigated viscosity and thermal kinetics of 10 selected preheated restorative resin composites and the effect of ultrasound energy on film thickness.
A range of different resin composites was tested: Charisma Diamond, IPS Empress Direct, Enamel Plus HRi, Essentia, Estelite Omega, Filtek Z100, Filtek Z350 XT, Gradia, TPH Spectrum and VisCalor. A flowable resin composite (Opallis Flow) and two resin cements (RelyX Veneer, Variolink Esthetic LC) also were tested. Viscosity (Pa s) was measured at 37 °C and 69 °C (preheating temperature) using a rheometer. Film thickness (μm) was measured before and after application of ultrasound energy. Temperature loss within resin composite following preheating (°C/s) was monitored. Data were statistically analyzed ( α = 0.05).
Viscosity at 69 °C was lower than at 37 °C for all materials except the flowable resin composite. Preheating reduced viscosity between 47% and 92% for the restorative resin composites, which were generally more viscous than the flowable materials. Film thickness varied largely among materials. All preheated resin composites had films thicker than 50 μm without ultrasound energy. Application of ultrasound reduced film thickness between 21% and 49%. Linear and nonlinear regressions did not identify any relationship between filler loading, viscosity, and/or film thickness. All materials showed quick temperature reduction following preheating, showing maximum temperature loss rates after approximately 10 s.
Distinct restorative resin composites react differently to preheating, affecting viscosity and film thickness. The overall performance of the preheating technique depends on proper material selection and use of ultrasound energy for reducing film thickness.
Use of preheated restorative resin composite as luting agent for veneers and other thin indirect restorations is increasingly popular. The topic has been investigated in clinical and laboratory studies [ ]. When compared to photopolymerizable resin cements and flowable resin composites, potential advantages of preheated restorative resin composites may include increased shade availability, lower cost, less polymerization shrinkage and marginal degradation, and improved mechanical performance due to their higher filler content [ ].
Preheating intends to reduce viscosity and increase flowability of restorative resin composite pastes [ ], but thicker films compared to resin cements are commonly observed [ , , ]. It has been reported that a poor marginal fit of indirect restorations could lead to resin cement dissolution and marginal discoloration [ ]. There is still no consensus, however, for limits of clinically acceptable film thickness. As a laboratory screening method, the ISO 4049 standard considers 50 μm as a limit for resin-based luting agents [ ]. Most authors suggest that films should be thinner than 120 μm in the clinics [ ], whereas clinical studies indicate that average marginal discrepancies in indirect restorations may vary between 100 and 315 μm [ ]. The film thickness yielded by different preheated restorative resin composites should be evaluated in order to aid the proper selection of an adequate material for the technique.
A new resin composite claiming a ‘thermoviscous technology’ (VisCalor, Voco, Cuxhaven, Germany) was recently introduced. VisCalor is primarily a bulk-fill restorative, but perhaps it could generate a thin film if used as luting agent. Recent reports observed that preheating reduced up to 66% the force required to extrude VisCalor from its compule, whereas the degree of C = C was not affected [ ] and no adverse effect of premature polymerization was observed [ ]. Another alternative to reduce film thickness, raised in previous work [ ], is the use of ultrasound energy, which could increase flowability of the restorative resin composite if applied over the ceramic restoration [ ].
Several restorative resin composite options are available in the market. Since most materials are not primarily intended to be preheated, chances are that dentists will choose anyone at hand. However, a recent study [ ] reported that different formulations of resin composites may react differently to preheating, affecting viscosity and film thickness, and ultimately influencing the mechanical performance of luted ceramic structures. Thermal loss after preheating is ceased will likely play a role on those aspects. Since not all clinical preheating techniques may provide adequate working time, the cooling patterns of different resin composites should be further studied. The best-case scenario would be understanding how a range of restorative resin composites react to preheating and the resulting flowability and film thickness, guiding proper material selection and the clinical procedures.
This study investigated the effects of preheating on viscosity, film thickness, and temperature loss of 10 contemporary restorative resin composites. The effect of ultrasound energy application on film thickness also was investigated. Two resin cements and a flowable resin composite were included for comparison. The hypotheses tested were: (i) film thickness, viscosity and thermal loss would be material dependent, (ii) use of ultrasound would reduce film thickness.
Materials and methods
Study design and materials tested
This in vitro study evaluated the effect of preheating different restorative resin composites on their viscosity and film thickness, which were the primary response-variables. Ten restorative resin composites ( Table 1 ) were selected considering their range in classifications, formulations, and manufacturers. Dentins shades A1, A2, or similar were tested. A flowable resin composite and two resin cements were tested for comparison, and are herein referred as flowable materials. A 69 °C temperature was used as clinical desired temperature for luting with preheated restorative resin composites. The effect of ultrasound energy application on film thickness was also tested. Thermal kinetics within resin composite increments following preheating was monitored, with temperature loss and cooling rates as response-variables.
|Restorative resin composites||Resin phase||Filler wt% (vol%)|
|Charisma diamond||Nanohybrid||Kulzer, Hanau, Germany||Bis-GMA, UDMA, TEGDMA, TCD-DI-HEA||77|
|IPS empress direct||Nanohybrid||Ivoclar Vivadent, Schaan, Liechtenstein||Bis-GMA, UDMA, TCDDMA||60 or 79.6 a|
|Enamel plus HRi||Nanohybrid||Micerium, Avegno, Italy||Bis-GMA, UDMA, BDDMA||80 (63)|
|Essentia||Microhybrid||GC, Tokyo, Japan||Bis-GMA, UDMA, TEGDMA, Bis-EMA, Bis-MEPP||81 (65)|
|Estelite omega||Supranano||Tokuyama, Tokyo, Japan||Bis-GMA, TEGDMA||82 (78)|
|Filtek Z100||Microhybrid||3M ESPE, St. Paul, MN, USA||Bis-GMA, TEGDMA||80 (66)|
|Filtek Z350 XT||Nanofill||3M ESPE||Bis-GMA, UDMA, Bis-EMA, PEGDMA, TEGDMA||72.5 (55.6)|
|TPH spectrum||Nanohybrid||Dentsply Sirona, York, PA, USA||Bis-GMA, Bis-EMA, TEGDMA||75 (57)|
|VisCalor||Nanohybrid||Voco, Cuxhaven, Germany||Bis-GMA, aliphatic dimethacrylate||83|
|RelyX veneer||Light-cured cement||3M ESPE||Bis-GMA, TEGDMA||66|
|Variolink esthetic LC||Light-cured cement||Ivoclar Vivadent||UDMA, DDMA||(38)|
|Flowable resin composite|
|Opallis flow||Microhybrid||FGM, Joinville, SC, Brazil||Bis-GMA, TEGDMA, Bis-EMA||72|
Viscosity (n = 5) was measured using a dynamic oscillation rheometer (R/S-CPS+; Brookfield, Middleboro, MA, USA). Two temperatures were tested: 69 °C, as the initial temperature obtained clinically after preheating on the specific heater device used here (HotSet; Technolife, Joinville, SC, Brazil), and 37 °C (body temperature) as final temperature, simulating the clinical condition after seating the restoration. It was not possible to test the materials at 25 °C because some resin composites were too viscous at room temperature and exceeded the rheometer measuring range. The resin composites were taken from their original packages ( i.e. syringe or compule) with a spatula and placed in a half-circle mold for standardizing a 0.5 mL volume. The test material was dispensed on the lower plate of the rheometer and positioned with a 0.05 mm gap between the plates. Heating was provided by the rheometer itself. Viscosity (Pa s) was measured until reaching the designated temperature and for additional 45 s, at a constant shear rate of 2 s −1 . The flowable resin composite and resin cements were also tested in both temperatures.
Film thickness (n = 3) was measured based on the ISO 4049 standard [ ]. Only restorative resin composites were preheated in this analysis. Two optically flat, square glass plates with 200 mm 2 contact surface area were used. The combined thickness of the two glass plates stacked in contact was measured with a digital caliper (Mitutoyo, Tokyo, Japan) with 1 μm accuracy. Increments of restorative resin composites were preheated to 69 °C for 10 min in order to achieve and stabilize this temperature before testing [ ]. The increment was placed directly on the preheating device using a spatula. A standard 0.1 mL volume of the preheated material was dispensed on the center of a glass plate and the other plate was placed on top. A 150 N force was centrally and vertically applied via the upper plate using a loading device (Odeme Dental Research; Joaçaba, SC, Brazil). After 180 s, the loading system was released and the combined thickness of the two glass plates was measured again. Film thickness was calculated as the difference between the two readings. Three different specimens were tested for each material. The thickness of each specimen was read three times and the average value was recorded as the film thickness for that specimen. No light-polymerization was carried out because the same specimen was used next for testing the effect of ultrasound energy, in accordance with the clinical workflow of seating indirect restorations [ ]. The ultrasound energy was applied through the upper glass plate for 30 s using a polyacetal tip. The tip was positioned statically at the center of the glass plate with slight hand pressure, the ultrasound equipment operated at 40% power (DentSurg Pro; CVdentus, São José dos Campos, SP, Brazil). It should be highlighted that the resin composite between the glass plates was not warm anymore during the ultrasound application step, simulating what happens in the clinical scenario when luting indirect restorations with preheated resin composite. Film thickness after ultrasound application was measured anew.
Resin composite increments (2 mm in thickness, mass ∼130 mg) were placed over a polyester stripe and inside the preheating device. The preheating device has spaces that allowed the increments to be placed without overflowing during preheating. This was important to avoid reduction in increment thickness that could affect the temperature measurements. A type-K thermocouple was used (TM902C, Yarboly, China), the tip (diameter = 1 mm) was inserted within the increment to monitor temperature. When it reached 70 ± 1 °C, the polyester stripe with increment was removed from the preheating device and placed over the bench at room temperature (25 °C). Temperature (°C) within the increment was recorded every second for 2 min after placing the resin composite over the bench (n = 3). This time was enough for all resin composites to approximately reach room temperature. Plotted temperature vs. time data were adjusted by curve fitting (R 2 > 0.997) and temperature loss rates were calculated using these fitted plots.
Viscosity data were submitted to a Two-Way Analysis of Variance – ANOVA (material vs . temperature). Viscosity data were transformed to ranks before the analysis. Data for film thickness without use of ultrasound were analyzed using One-Way ANOVA. Film thickness data of restorative resin composites including the use of ultrasound were analyzed using Repeated Measures ANOVA (one factor repetition). All pairwise multiple comparison procedures were carried out using the Tukey method. Regression analysis were used to investigate the relationship between filler load (wt.% and vol%), viscosity, and/or film thickness. Significance level was set at α = 0.05 for all analyses. Thermal loss within resin composite and cooling rates were analyzed descriptively.
Results for viscosity at 37 °C and 69 °C are shown in Fig. 1 . Materials are listed in ascending order of viscosity at 69 °C (top to bottom). Average reductions in viscosity by preheating (%) are presented. Both factors and their interaction were statistically significant (p < 0.001). The viscosity at 69 °C was significantly lower than at 37 °C for all materials (p ≤ 0.027) except the flowable resin composite (p = 0.45). Significant differences in viscosity were observed in almost all comparisons between materials, including at 69 °C ( Table 2 ). In either temperature, all restorative resin composites were significantly more viscous than the flowable resin composite and Variolink Esthetic LC resin cement. When preheated, four resin composites had lower viscosity compared with RelyX Veneer resin cement (at room temperature): Essentia, Gradia, VisCalor, and Estelite Omega. Filtek Z350 XT showed remarkably higher viscosity than all other materials in both temperatures tested. At 69 °C, Filtek Z350 XT showed viscosity around 14 kPa s, whereas all other preheated materials were at least 3-fold less viscous. Preheating also reduced viscosity of the resin cements. VisCalor (92%), TPH Spectrum (82%), and Essentia (81%) showed the highest viscosity reductions by preheating.