Influence of Ceramic Shades on Surface Hardness of Different Resin Cements

Resin cements react to different cure methods in various ways, depending on their composition and the ceramic with which they interact. Some studies have found that high filler-loaded cements polymerize primarily with immediate photo-activation while others have consistent polymerization regardless of the method of cure.¹⁰ Similarly, the chemical composition of the ceramic restorations has been found to influence the resulting surface hardness of the resin cement as it is light-cured. Ceramics reinforced with lithium disilicate cause a reduction in the surface hardness of luting cements at thicknesses greater than 1 mm. However, ceramics reinforced with leucite do not exhibit this effect even at a thickness of 2 mm.¹¹ The thickness and shade of the ceramic can also have an effect on the degree of conversion of the cement. In general, darker and thicker sections of ceramic require greater light intensity in order to cure the cement.^7,8

In an effort to reduce light exposure time, curing units have been developed that function at a higher intensity than conventional light-cure systems. However, in order to achieve the maximum degree of conversion of the resin cement, a cure time of at least 15 seconds is required regardless of the light-cure system employed.¹¹ During light-curing, the intensity of the light and the type of curing unit used can influence the degree of conversion of the cement. This influence has the potential to affect the surface hardness of the cement and the rate at which conversion occurs.⁹ Conventional quartz tungsten halogen (QTH) and high-intensity QTH lamps have traditionally operated at light output intensities of a much higher degree than the first generation of typical light-emitting diode (LED) curing units, thus allowing them to cure more effectively through ceramic materials of darker shades and thicker samples of resin cements compared with older LED light-curing units.¹

Modern LED units have overcome this limitation operating at higher power intensities compared with QTH lamps.¹² These curing units have been shown to use less power, work more efficiently, and produce less heat (dispersion) than conventional QTH curing systems. Traditionally, their narrow spectral range peaks at approximately 470 nm, which coincides with the ideal activation wavelength of camphorquinone, the photoinitiator found in many resin cements.¹ However, cements with initiators that absorb light at lower wavelengths, such as Lucirin® TPO (BASF Corp., www.basf.com), experience a lower depth of cure when exposed to conventional LED units in comparison with traditional tungsten systems,³ and consequently, the polymerized cements may exhibit reduced mechanical properties. In response to this discrepancy, newly developed LED systems are designed with a wider wavelength spectrum that emits light capable of activating these low wavelength photoinitiators.¹³ Some studies have used the Knoop hardness (KH) test to assess these properties among different light-curing units, composites,^13,14 and dual-cure cements, notably to compare the effect of the veneering material on the final hardness of cement.¹⁵

In this study, a comparison was made between two brands of dual-cure resin cements and two shades of ceramic. One light-curing unit was used to polymerize the resin cements, and the subsequent effects on the surface hardness of the cements were assessed using the Knoop hardness test. The null hypothesis is that the different ceramic shades have no effect on the surface hardness of the dual-cured resin cements.

Materials and Methods

Six study groups (n = 15), comprised of a control, 2M2 ceramic spacer, and 5M3 ceramic spacer, were prepared using two dual-cure resin cements (RelyX™ ARC [RLX], 3M ESPE, www.3MESPE.com; and Variolink® II [VLK], Ivoclar Vivadent, www.ivoclarvivadent.com) exposed to LED light-curing (Bluephase® G1, Ivoclar Vivadent) for 20 seconds. Two extra groups (RLX-SC, VLK-SC) were prepared using the self-cure mode for each resin cement. The materials used to perform this study and their respective compositions and characteristics are listed in Table 1.

A metallic matrix consisting of a metal ring with a 25-mm diameter, 2-mm thickness, and a central hole 5-mm in diameter was used with a glass slab to fabricate the specimens. An adhesive tape (Scotch® Tape, 3M) was placed on the underside of the ring to seal and allow proper placement of the resin cement. Resin cements were mixed in accordance with the manufacturer’s instructions and inserted into the center of the matrix. A clear matrix band (Polyester Matrix Tape, 3M ESPE) and a glass slide were positioned on top of the metallic ring, and the cement was light-cured for 20 seconds using an intensity of 1200 mW/cm². In the control groups (RLX and VLK), light was applied directly to the resin cements without the presence of a ceramic spacer, while the 2M2 and 5M3 groups had ceramic spacers.

The ceramic spacers (5-mm x 5-mm x 2-mm thickness) were adapted to the tip of the light-cure unit using a polyvinylsiloxane (PVS) impression material (Express™ STD base/catalyst, 3M ESPE), preventing the escape of light through the sides of the ceramic. After curing the samples, the clear matrix was removed and the top surface of the sample was polished with sandpaper of varying grits, beginning with 600 and increasing by 200 grits, up to 2000. The samples were cleaned with a 70% alcohol solution and air-dried. The self-cure resin cement group and the 5M3 ceramic group did not achieve sufficient hardness during the first 10 minutes of the experiment. These samples were stored in a light-free environment with a 1000 g weight placed upon them for 1 hour prior to being polished and cleaned.

To collect the KH values, a microhardness tester was used (Buehler model Micromet® 5114, www.buehler.com). The specimens were placed on the tester and subjected to five 50 g/15 s indentations (three samples in each group with five indentations each) at predetermined times. Each indentation was measured from left to right, and the result was converted automatically to Knoop hardness by the Micromet. The surface hardness was measured at intervals of 0, 1, 2, 3, 4, 6, 24, 168, and 336 hours in order to determine the point at which the cements would reach the maximum KH value for each sample. A statistical analysis of the data was conducted using two-way ANOVA and a Levene’s test at a significance level of 0.05.

Results

The effect of type of cure and aging time on the true hardness values for Variolink II and RelyX cements are displayed in Figure 1 and Figure 2. It can be seen from both figures that the plateau was reached after 24 h of aging. The mean KH values at 0 h and 336 h of aging for Variolink II and RelyX cements are presented in Table 2 and Table 3, respectively. Control groups exhibited significantly higher KH values than the other groups (P < 0.001) at 0 h following light-curing. The KH values for RelyX-2M2 was not significantly different than the control group (P > 0.05) after 336 hours. All groups tested with 5M3 spacers had KH values that were not significantly different from the groups with self-cure mode, P > 0.05. The KH value for RelyX self-cure cement was significantly higher than for Variolink II self-cure, P = 0.003. RelyX-2M2 had KH values that were not significantly different from that of Variolink-2M2, P > 0.05.

Discussion

Dual-cure resin cements were developed to combine the desirable properties of chemical- and light-cure polymerization materials, in order to achieve an appropriate degree of conversion, especially in deep areas of restorations.¹⁶ New technologies such as spectroscopy¹⁷ and the universal hardness test¹⁸ have improved the methods for testing physical and mechanical properties, as well as degree of conversion, of polymers.^19,20 Hardness tests are commonly used to indicate the degree of conversion for resin cements21; a lower surface hardness value means an incomplete polymerization of the resin cement.^22,23

The success or failure of a ceramic restoration depends mainly on the adhesion durability of the ceramic, cement, and enamel/dentin complex. This adhesion may be obtained by appropriate cement polymerization, ie, there should be a reasonable amount of light energy shining through the ceramic restoration so that the resin cement can achieve appropriate hardness.¹¹

Several factors may affect photopolymerization of the cement, such as composition, opacity, shade of the ceramic restoration, as well as type and concentration of photoinitiators in the resin cement.⁶ A study showed that there is no difference in surface hardness of photopolymerized cement cured with or without ceramic spacers using an LED light-curing unit.⁴ Additionally, the thickness of the ceramic had more influence on the surface hardness of the cement than color. The combination of thicker ceramic restoration and darker ceramic shade has a cumulative negative effect on the resin cement surface hardness.⁷

The present study shows that the control groups (RLX and VLK) achieved higher and similar KH values throughout the duration of testing for both cements (Figure 1 and Figure 2, Table 2 and Table 3). Groups using the 2M2 spacer exhibited lower KH values when compared with the control group for both cements (Table 2 and Table 3) with the exception of RLX-2M2, which reached KH values similar to those of the control group after 24 hours (Figure 2). The VLK-2M2 KH values were lower than RLX-2M2 at 0 h (Table 2) yet reached the maximum KH at 24 h similar to RLX-2M2 (Figure 1). These results are similar to previous studies that used ceramic spacers of different shades positioned between light-curing units and resin cements.^15,24

The KH values for groups RLX-5M3 and VLK-5M3 were not significantly different from the self-cure groups (RLX-SC, VLK-SC) at 0 h, P > 0.05, Table 2. However, at the maximum KH values (Table 3) RLX-SC exhibited significantly higher KH values (P < 0.001) compared with VLK-SC. It must be noted that at the maximum KH value the light-curing process had no influence on the KH of the 5M3 spacer groups when compared to all but one of the self-cured groups. The exception was VLK 5M3, which was significantly higher when compared with VLK-SC, which displayed the lowest KH values compared to the other groups (Table 2 and Table 3). Both 5M3 groups also showed similar behavior to self-cured groups that could be evaluated in the first 10 minutes after preparation.

All groups exhibited higher KH values 24 hours after light-curing (Table 2 and Table 3) except VLK-SC, which reached its peak of hardness 168 h following light-curing (Table 3 and Figure 1). This finding is consistent with a previous study.²⁵

This study revealed that the darker ceramic shade had a negative influence on the photopolymerization of dual-cure cements. These cements were not able to take advantage of the combined chemical- and light-cure systems to achieve sufficient degree of polymerization¹⁶ to attain optimum physical and mechanical properties and desired clinical performance.^4,26

Conclusions

Based on the outcomes of this study, the authors conclude the following:

• The control groups (no spacers) exhibited significantly higher initial KH values (P < 0.001) when compared with the other groups.

• The KH values for the 2M2 groups at 0 h aging were significantly higher than those of the 5M3 groups (P < 0.001), demonstrating that ceramic spacer shades have tremendous effect on the KH values of RelyX and Variolink II. Darker ceramic shades (5M3 spacers) interfere with the absorption of light by the cements tested. In addition, using only the self-cure mode in the dual-cure resin cements that were tested is insufficient to achieve optimal microhardness.

Acknowledgments

Part of this research was funded by a research grant from Ivoclar Vivadent, Inc. The authors would also like to thank Emerging Leaders of America Program (ELAP) and National Counsel of Technological and Scientific Development (CNPq – Brazil) for their support.

Disclosure

The authors do not have any financial interest with the manufacturers mentioned in the article.

About the Authors

Vinicius C. Bianco
PhD student, Department of Prosthodontics, Bauru School of Dentistry, University of São Paulo, São Paulo, Brazil

Maria Jacinta M.C. Santos, DDS, MSc, PhD
Assistant Professor, Schulich School of Medicine and Dentistry, University of Western Ontario, London, Ontario, Canada

José H. Rubo, DDS, MSc, PhD
Professor and Head, Department of Prosthodontics, Bauru School of Dentistry, University of São Paulo, São Paulo, Brazil

Amin S. Rizkalla, BSc, MEng, PhD
Associate Professor, Chair of the Division of Biomaterials Science, Schulich School of Medicine and Dentistry, University of Western Ontario, London, Ontario, Canada

Gildo C. Santos Jr., DDS, MSc, PhD
Associate Professor and Chair, Division of Restorative Dentistry, Schulich School of Medicine and Dentistry, University of Western Ontario, London, Ontario, Canada