The aim of this research was to evaluate the degree of monomer conversion of different resin cement shades when photocured under different feldspathic ceramic shades. The photocuring time was also evaluated as well as the translucency of each ceramic shade.
Three VITA VM7 ceramic shades (Base Dentin 0M1, Base Dentin 2M2 and Base Dentin 5M3) were used to determine the translucency percentage. A spectrophotometer MiniScan was used to measure the opacity percentage of each specimen (2-mm-thick) and then the translucency was calculated. To measure the degree of conversion (DC), the resin cement (Variolink II; A3 Yellow and transparent) specimens (thickness: 100 μm) were photocured under a ceramic block (2-mm-thick) for 20 or 40 s. Specimens photocured without the ceramic block were used as control. Sixteen groups ( n = 3) were evaluated. Micro-ATR/FTIR spectrometry was used to evaluate the extent of polymerization of all specimens after 24 h. The %DC was calculated of experimentally polymerized versus maximally polymerized composite.
The translucency percentages of 0M1, 2M2 and 5M3 ceramics were 12.41 (1.02)%, 5.75 (1.91)% and 1.07 (0.03)%, respectively. The %DC of both resin cement shades cured under ceramic 5M3 was significantly lower than the other groups ( p < 0.05). The %DC of 0M1 groups exhibited no significant difference from 2M2 groups ( p > 0.05), with the exception of the transparent cement photocured for 40 s.
Photocuring under 2 mm ceramic showed that the increase in chroma saturation significantly decreased Variolink II resin cement %DC (100-μm-thick).
Degree of conversion (DC) of the monomers in the composite polymerization reaction depends on the energy received. DC is the product of the light intensity and the exposure time . Studies have showed that DC depends also on the type and thickness of the restorative material . Additionally, the microhardness of the resin cement using different ceramic shades can be different and this test shows a relation with DC. Studies observed significant difference in the microhardness values according to the ceramic shade tested .
Watts and Cash evaluated the light transmission through different restorative materials and Peixoto et al. studied the light transmission of different ceramic shades. Other studies evaluated the effect of polymerization time and material ceramic thickness on the degree of polymerization of the resin cement, measured indirectly by the microhardness . For higher thickness, the cements showed lower microhardness.
Lower microhardness can be synonymous with the incomplete polymerization of the composite resin. As a result, a decrease in the cement mechanical properties can occur, increasing the water sorption and microleakage . Additionally, the molecules of the non-polymerized monomer can be debonded from the material, causing tissue inflammation . For this reason, it is important to optimize the polymerizing procedure of the resin cements in order to obtain the best physical properties and great clinical performance.
Nowadays, there is a large variety of resin cement shades that are used in esthetic restorations to adjust the restoration color . Depending on the cement shade, light absorption and transmission can vary, affecting the DC of these materials . Fan et al. verified that periods of light exposure recommended by the manufacturers of some resins are not enough to promote complete polymerization. A direct method used to evaluate the DC is Fourier transformation infrared spectroscopy (FTIR) .
Enough resin cement polymerization is a crucial factor to promote adequate bond strength in the interface of ceramic–resin cement and resin cement–dentin. Therefore, it becomes important to use criteria in choosing the materials and technique in order to optimize their physical proprieties . This improves the indirect restoration’s clinical performance and longevity. There are no reports evaluating the %DC of different dual-cure resin cement shades as well as no reports evaluating ceramic and cement shades with relevant clinical resin cement thickness. Therefore, the aim of this study was to evaluate the effect of different ceramic and resin cement shades as well as the photocuring time on %DC of dual-cure resin cement. The hypotheses are: (1) the darker the ceramic shade, the lower %DC of the resin cement; (2) the darker the resin cement shade, the lower %DC of the resin material; and (3) the polymerization time would affect the %DC of the dual-cure resin cement.
Materials and methods
The material types, brand names, batch numbers and manufacturers of the products that were used in the current study are presented in Table 1 .
|Material||Brand name||Batch numbers||Manufacturer|
|Ceramic||Veneering materials VITA VM7||0M1: #7505
|Vita Zanhfabrik, Bad Säckingen, Alemanha|
|Hydrofluoric acid 10%||Ceramic etching gel||#L806765||Dentsply, Petrópolis, RJ, Brazil|
|Resin cement||Variolink II||Transparent: base: #L01441, catalyst: #K27635; A3 Yellow: base: #K43442, catalyst: #K56289||Ivoclar Vivadent, Schaan, Leichtenstein|
Measure of the ceramic translucency
Ceramic disc preparation
A machined acrylic resin pattern (diameter: 25 mm, thickness: 2.5 mm) was duplicated with addition silicon impression (Elite H-D Putty Soft Normal Setting, Zhermach, Badia Polesine, Italy). This silicon mold was used to manufacture the same pattern of ceramic discs for all shades. The feldspathic ceramic (VITA VM7, Vita Zanhfabrik) was sintered (Vacumat, Vita Zanhfabrik), according to the manufacturer’s instructions. At the end of this process, three ceramic discs (20 mm × 2 mm) were produced: Base Dentin 0M1, Base Dentin 2M2, and Base Dentin 5M3.
One of the disc surfaces was glazed according to the manufacturers’ instructions. The opposite surface was leveled and polished in a machine using silicon carbide papers in sequence (600, 800 and 1200 grit) under water cooling (3M ESPE, St. Paul, USA). Then, 10% hydrofluoric acid was applied for 1 min .
The opacity percentage of three different ceramic shades was measured using a spectrophotometer MiniScan EZ (Model 4500S, USA) which uses a xenon flash lamp to illuminate the specimen. The light reflected from the sample is separated into its component wavelengths through a dispersion grating. The spectrophotometer was positioned with direct contact on the disc and three measurements were obtained of each specimen. Two backgrounds (one black and one white) were used to determine the opacity percentage. The translucency percentage was calculated from the opacity percentage value subtracting the opacity percentage by 100.
The factor analyzed was ceramic shade (Base Dentin 0M1, Base Dentin 2M2 and Base Dentin 5M3). The data was entered into a computer program (Excel 4.0; Microsoft Corp, Redmond, WA) for calculation of descriptive statistics. Statistical analysis was performed using a 1-factor (ceramic shade). The data was analyzed by one-way analysis of variance (ANOVA) and Tukey HSD-Pairwise. p values less than 0.05 were considered to be statistically significant in all tests.
Fourier transformation infrared spectroscopy (FTIR)
Specimen of composites 100-μm thick were prepared to evaluate if the ceramic and resin cement shades affect their final polymerization under a ceramic block. A commercial dual-cure luting agent was used (Variolink II).
Three different shades of VITA VM7 ceramic blocks (Base Dentin 0M1, Base Dentin 2M2, and Base Dentin 5M3) from a machined acrylic resin pattern with 9 mm × 9 mm × 2.5 mm were produced as described above. After sintered, the ceramic blocks showed 7.2 mm × 7.2 mm × 2 mm, and then one block of each shade was produced in order to simulate a ceramic restoration.
The A3 Yellow base and catalyst, transparent base and catalyst composite luting material were then manipulated. Specimens were prepared by mixing equal parts (by weight) of base and catalyst. Forty-eight specimens (100 μm × 5 mm × 5 mm) were produced ( n = 3). A small piece of a Mylar strip (0.07 mm; Du Pont Company, Wilmington, DE) was placed under and over the resin and pressed flat. The ceramic block was placed over the composite specimens and different polymerizing protocols were performed ( Table 2 ). A silicon (Speedex; Colte‘ne/Whaledent Inc., Cuyahoga Falls, OH) mold was used as a supporting structure for the composite/ceramic complex in order to decrease the reflectivity of the underlying surface toward each specimen . Specimens directly irradiated were prepared following the same polymerizing protocols that were used to produce the previous specimens. A high power blue LED-curing device (Elipar FreeLight 2/3 M ESPE, 900 mW/cm 2 ) was used. Film thickness was checked using a digital caliper (Digimatic Caliper CD-6__ OS, Mitutoyo Corp., Tokyo, Japan) before testing. Then the specimens were stored for 24 h in light-proof boxes after the polymerization procedure in order to avoid further exposure to light.
|Ceramic shade||Resin cement shade||Photo-activation time (s)||Groups a|
|No block||Base: A3 Yellow
Catalyst: A3 Yellow
|Base Dentin 0M1||Base: A3 Yellow
Catalyst: A3 Yellow
|Base Dentin 2M2||Base: A3 Yellow
Catalyst: A3 Yellow
|Base Dentin 5M3||Base: A3 Yellow
Catalyst: A3 Yellow
The method used for the DC assessment was the micro-attenuated total reflectance/Fourier transform infrared spectrometry (micro-ATR/FTIR) . A FTIR spectrometer was used (Bruker IFS 55, Milton, ON, Canada) which is equipped with a IRScope II microscope with a Burker micro-ATR attachment with a Germanium crystal operated under the following conditions: 4000–600 cm −1 range, 4 cm −1 resolution, 100 scans coaddition, Ge lenses. This experimental setup allows analyzing an area approximately 80–100 μm in diameter to a depth of 1–2 μm.
Composite specimens’ spectra were recorded. A small amount of uncured resin cement from each material was also scanned and its spectrum was used as unpolymerized reference. The amount of double vinyl bonds remaining in the specimen exposed to irradiation is shown by the intensity of the peak at 1637 cm −1 referring to the C C stretching of the vinyl group and have been used to determine polymerization of acrylates and methacrylates. The %DC was calculated by the two frequency technique, using the absorption peak of C C groups at 1637 cm −1 (analytical frequency), and the absorption peak of the aromatic C⋯C groups at 1608 cm −1 (reference frequency). The %DC was calculated using this equation:
% DC = 100 1 − Aa ( C C ) Ab ( C ⋯ C ) Ab ( C C ) Aa ( C ⋯ C )