Effect of LED Light-Curing Spectral Emission Profile on Light-Cured Resin Cement Degree of Conversion

INTRODUCTION

The demand for ceramic esthetic restorations has increased over the past years.^1–4 The continuous development of ceramic materials improves their indications of use and esthetics.^3,5–7 Among ceramic options, lithium disilicate glass ceramics are often the restorative material of choice since they combine fracture resistance and esthetics.^3,8,9 An innovative manufacturing technology was introduced in 2016, designed to further enhance the properties of lithium disilicate glass ceramics.^8,10 Described as high-density micronization, this process results in smaller crystals that are better distributed and in higher density in the glassy matrix, leading to a ceramic (GC Initial LiSi Press, GC Co, Tokyo, Japan) with better physical properties, superior esthetics, lower wear potential to antagonist teeth, and higher polishability compared to any other lithium disilicate glass ceramic.^8,10

However, ceramic materials with excellent mechanical properties do not necessarily imply better clinical performance.^11,12 The success of ceramic restorations relies on the adhesion of the luting agent to tooth structure.^13,14 Resin-based cements are used to lute glass ceramic restorations.¹¹ The polymerization process of resin cements can be initiated by the application of a light source (light-cured resin cements), by a chemical redox reaction (chemical-cured resin cements), or by the combination of these two processes (dual-cured resin cements).^13,15,16 Light-cured resin cements have two major advantages: working time is controlled by clinicians, and better color stability is attained compared to both dual- and chemical-cured resin cements.^7,17 However, ceramic type^12,17–21 and its thickness,^{4,12,13,17,22,23} translucency,^2,7,12,24 and color²³ may attenuate the light emitted by a light-curing unit (LCU). Thus, the light source must be powerful enough to be transmitted through the ceramic restoration and reach the resin cement with sufficient irradiance to carry out the polymerization process appropriately.^{7,17,19,21,25,26}

One way of quantifying the polymerization of resin cements is to measure their degree of conversion (DC), which represents the percentage of monomers converted into polymers.^11,15 A high DC of resin cements has usually been linked to better physical and mechanical properties,^7,19,21,27 clinical performance,^7,23,27 biocompatibility,^16,28 color stability,¹³ and adhesion to tooth structure²⁵ in addition to lower solubility in the oral environment.^13,24 Light activation can be achieved by many light sources,^1,29,30 but LED LCUs are currently the “gold standard” to perform this process.²⁹ Camphorquinone is the most common photoinitiator present in resin cements, with a light absorption spectrum ranging from about 425 to 495 nm and thus compatible with the spectral emission profile of LED LCUs.^26,29,30 However, materials formulated with camphorquinone tend to be slightly yellowish.^26,30,31 The search for resin composite materials with lighter colors has led manufactures to develop alternative photoinitiators to circumvent this limitation,²⁶ the most common being Lucirin TPO, PPD, and Ivocerin.^26,30 The counterpoint to using alternative photoinitiators is the occasional need for a shorter light wavelength than that emitted by a single-peak LED LCU (420 to 480 nm), also classified as a first- or second-generation LED LCU.^26,30 Recently, third-generation LED LCUs (also classified as multipeak LED LCUs) have been developed that have a broader spectral emission profile (390 to 490 nm) compared to single-peak LED devices.^26,29–31

Ivocerin is a patented photoinitiator considered more reactive than camphorquinone with a light absorption spectrum ranging from about 390 to 445 nm (absorption peak at 408 nm).^30,32 It can be used alone, that is, without any additional coinitiator; this ensures better color stability of the materials that use it.^32,33 An example of a light-cured resin cement that contains this photoinitiator is Variolink Esthetic LC (Ivoclar Vivadent, Schaan, Liechtenstein). For reasons of industrial protection, its manufacturer does not describe the entire formulation of this light-cured resin cement, omitting the types and concentrations of the photo-initiators. Therefore, the characteristics that the LED LCU should have to light activate this material are unclear.^30,34 To date, literature is scarce regarding the use of different LED LCUs in Ivocerin-containing light-cured resin cements as well as the interaction of the light produced by these LED LCUs with lithium disilicate glass ceramics. Thus, the objective of the present study was to evaluate the DC of an Ivocerin-containing light-cured resin cement light activated with progressive exposure times (20, 40, or 60 seconds) through different thicknesses of a lithium disilicate glass ceramic using two LED LCUs and to evaluate the influence of the interposition of glass ceramic on irradiance and on spectral emission profile of the light emitted by the LED LCUs. The null hypotheses were that DC would not be influenced by the interposition of the glass ceramic, exposure time, or the LED LCU.

METHODS AND MATERIALS

RESULTS

Degree of C=C Conversion

DC showed a statistically significant difference for LED LCU (p<0.001), glass ceramic thickness (p<0.001), and exposure time (p<0.001). The interaction between LED LCU and glass ceramic thickness (p<0.001) and the interactions between the three factors were statistically significant (p<0.001). Results for DC are shown in Table 1. DC increased significantly from 20 to 60 seconds of light activation in all ceramic thicknesses for both LED LCUs (p<0.05). Increasing light activation from 40 to 60 seconds was statistically significant only when the multipeak LED LCU was used on a 1.0-mm-thick ceramic specimen (p<0.05). For the multipeak LED LCU, DC for 20 seconds of light activation was statistically higher in the 0.3-mm-thickness group than the control and the 2.0-mm-thickness groups (p<0.05). Regarding 40 seconds of light activation, the control and the 0.3-mm-thickness groups presented the highest DC. As for 60 seconds of light activation, the lowest DC was for the 2.0-mm-thickness group. For the single-peak LED LCU, DC for 20 seconds light activation was statistically higher in the 1.0-mm-thickness group than the 0.3-and the 2.0-mm-thickness groups (p<0.05). Regarding 40 and 60 seconds of light activation, DC for the control and the 2.0-mm-thickness groups were statistically similar.

Table 1: Degree of C=C Conversion (%) of the Light-Cured Resin Cement According to Glass Ceramic Thickness and Exposure Time for Each LED Light-Curing Unit (LCU) (Mean ± SD) (n=5)^{a ,}

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Comparing the LED LCUs, multipeak LED LCU showed higher DC than single-peak LED LCU. The multipeak LED LCU showed higher DC for 20 seconds of light activation for both the 0.3- and the 2.0-mm-thickness groups (p<0.05). Regarding 40 seconds of light activation, the multipeak LED LCU showed higher DC for the control, the 0.3-mm-thickness, and the 2.0-mm-thickness groups (p<0.05). As for 60 seconds of light activation, the multipeak LED LCU showed higher DC only for the control group (p<0.05).

Irradiance, Spectral Emission Profile, and Light Transmittance

Irradiance showed a statistically significant difference between LED LCUs and thicknesses (p<0.001). The interaction between the factors was also statistically significant (p<0.001). Irradiance results are shown in Table 2. The irradiance of both LED LCUs decreased significantly as the glass ceramic specimen thickness increased (p<0.05). The multipeak LED LCU showed the highest irradiance values (p<0.05) for all thicknesses. The spectral emission profiles of the single-peak LED LCU and the multi-peak LED LCU are shown in Figure 1 and Figure 2, respectively. The multipeak LED LCU showed a spectral emission profile with two distinct peaks at 394 and 459 nm; the single-peak LED LCU had only one peak at 457 nm.

Table 2: LED Light-Curing Unit Irradiance According to Glass Ceramic Thickness (Mean ± SD; n=5) ^a

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No statistically significant difference in light transmittance was observed for the LED LCUs (p=0.894), only for the different glass ceramic specimen thicknesses (p<0.001). The light transmittance mean values are shown in Table 3. The light transmittance for both LED LCUs decreased significantly as the glass ceramic specimen thickness increased (p<0.05).

Table 3: Light Transmittance According to Glass Ceramic Thickness for Each LED Light-Curing Unit (Mean ± SD; n=5) ^a

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DISCUSSION

The null hypotheses were rejected. DC was influenced by the interposition of glass ceramic, exposure time, and LED LCU.

The glass ceramic ingot translucency selected in the present study was medium translucency due to its vast indications for use (thin veneers, veneers, inlays, onlays, crowns, and three-unit bridges).⁸ Following the preparation design guidelines for this ceramic material, the following minimum thicknesses are recommended: 0.3 mm for thin veneers; 1.0 mm for table tops, inlays, and onlays; and 1.5 mm for incisal/occlusal crown surfaces.⁸ This is the ultimate reason for evaluating the interposition of glass ceramic specimens of 0.3- and 1.0-mm thickness and its effect on the DC of the light-cured resin cement (Variolink Esthetic LC). This light-cured resin cement is indicated for luting glass ceramic restorations up to 2.0 mm in thickness.³³ This indication led glass ceramic specimens with 2.0-mm thickness to be included in the present study. The Light+ shade of light-cured resin cement was chosen due to its higher opacity among the five available shades.³³ Leloup and others⁴⁰ found that an increased opacity of a light-cured composite may reduce its DC. Thus, selecting the Light+ shade to lute a 2.0-mm glass ceramic thickness restoration would represent an extreme indication for the studied light-cured resin cement. A limitation of the present study was the evaluation of only one level of translucency of a single glass ceramic brand and only one shade of an Ivocerin-containing light-cured resin cement. Future investigations should consider comparing other glass ceramic shades and opacities as well as light-cured resin cements with other alternative photoinitiators.

The selection of the LED LCUs to be tested in the present study was performed by measuring the irradiance of LED LCUs by a handheld curing radiometer (Demetron L.E.D. Radiometers), a device used by clinicians and some researchers to monitor the output from their LCUs.⁴¹ Two LED LCUs with similar irradiance of approximately 1000 mW/cm² were selected. However, according to the results of the present study, a discrepancy was noticed between the irradiance measured by the handheld curing radiometer to those measured by the MARC-RC device; thus, the selection of the LED LCU was a major limitation of the present study, as ideally both LED LCUs should have the same irradiance measured by a MARC-RC device.

Several studies have shown that interposition of a restorative material may^{4,12,17,19,42,43} or may not^{15,17,24,27,42} be able to reduce the DC of light-cured resin cements. In the present study, there was no consensus for the influence of glass ceramic interposition on DC since the increase in the glass ceramic specimen thickness did not necessarily result in worse results. It is noteworthy to mention that there was a situation in which the interposition of a glass ceramic specimen with 0.3-mm thickness did not reduce but actually increased the DC compared to the control. This apparent abnormality may be explained by slower and smaller formation of free radicals due to light absorption by and diffusion into the structure of glass ceramic, reducing the polymerization rate, delaying the light-cured resin cement viscosity, and extending time for free radical diffusion, thus allowing greater conversion of monomers into polymers.^26,42 A similar result was found by Faria-e-Silva and Pfeifer,⁴² where a 0.5-mm-thick glass ceramic interposition tended to present higher DC than the control (without glass ceramic interposition).

A 40-second light activation (except in one group) was enough to obtain the highest DC for both LED LCUs. The increase in exposure time of a light-cured resin cement yielded similar results in other studies.^2,18 Alshaafi and others¹⁸ showed an increase in DC by increasing light activation not only from 20 to 40 seconds but also from 40 to 60 seconds. This can be partially explained by the greater thickness of the light-cured resin cement specimen (0.5 mm) and by the use of another light-cured resin cement (Variolink II base past, shade A1, Ivoclar Vivadent) in that study. Archegas and others² tested different exposure times (40, 80, and 120 seconds) and did not find any significant difference between 40 and 80 seconds, whereas differences between 40 and 120 seconds were significant. That study also used another light-cured resin cement (RelyX Veneer, shade A3, 3M Oral Care) specimens with 0.5-mm thickness.

When the light from the LED LCU reaches the surface of a glass ceramic, a considerable portion of its energy is lost (absorbed by and diffused into the structure of the material) and another portion transmitted.⁴⁴ It was observed that light transmittance of different glass ceramic thicknesses was statistically similar between LED LCUs, showing that there is no relation between light transmittance and emission profile of LED LCU. Thus, the tested multipeak LED LCU presented higher irradiance in all thicknesses since the value of its irradiance without glass ceramic interposition was higher than that of the single-peak LED LCU. The irradiance transmitted by the multipeak LED LCU through glass ceramic in the present study as well as that by Faria-e-Silva and Pfeifer⁴² was measured by the same light spectrometer (MARC-RC; however, the light transmittance that they found was slightly higher than that of the present study, probably because they evaluated a glass ceramic with higher translucency (IPS Empress Esthetic, shade ET1, Ivoclar Vivadent).

The silicone guides made for DC measurement ensured that only light transmitted through the glass ceramic specimen would reach the light-cured resin cement. Thus, an interesting finding was that DC was not always related to the irradiance received by the resin cement. There were situations in which the DC was not reduced compared to the control despite a significant reduction in light irradiance. This indicates that the irradiance received by the resin cement was sufficient to perform an acceptable monomeric conversion. Similar results have been found in other studies.^15,21,42

The visible light absorption spectrum of Ivocerin corresponds approximately to a wavelength ranging between 390 and 445 nm with an absorption peak at 408 nm.^30,33 Both LED LCUs emitted light with a spectrum within this range. Since the multipeak LED LCU has a spectral emission profile with two distinct peaks (at 394 and 459 nm), it was able to activate the photoinitiator better.^26,30 However, it did not necessarily result in higher DC. In some groups, the single-peak LED LCU—with only one emission peak at 457 nm and transmitting lower irradiance to the light-cured resin cement—presented results statistically similar to those of the multi-peak LED LCU. This can probably be explained by the higher reactivity of Ivocerin than camphorqui-none, thus optimizing the polymerization reaction.^30,33 Therefore, a lower irradiance—even though not at Ivocerin’s absorption peak—would be able to result in enough DC. Another possible explanation may be the presence of other photoinitiators with an absorption spectrum about 457 nm, such as camphorquinone or PPD.²⁶ However, the manufacturer classifies the resin cement used in the present study as amine free,^32,33 and the CQ-amine combination is the most commonly used photoactivation system in light-cured composites.^45,46

The glass ceramic used in the present study behaved as a filter for neutral density, meaning that the spectral emission profile of LED LCUs though the three glass ceramic thicknesses did not change. Alshaafi and others¹⁸ and Faria-e-Silva and Pfeifer⁴² found similar results. However, LED LCU spectral emission profiles did not prove to be a decisive factor to improve Ivocerin-containing light-cured resin cement DC. AlQahtani and others¹ also evaluated the influence of different LED LCUs on DC. These authors found similar results regarding the spectral emission profile effect on DC using another light-cured resin cement (Variolink II base paste, shades A1 and A4).

Light transmittance and the translucency parameter presented a near perfect positive nonlinear correlation for both LED LCUs; that is, increased translucency parameter of the tested glass ceramic was associated with increased light transmittance. However, since this correlation is nonlinear, the value of one of these parameters cannot be established when the other is known. This finding differs from that of the study by Oh and others,¹² in which the evaluated glass ceramics presented a near perfect linear positive correlation between these parameters. Thus, in that case, it would be possible to determine the value of one parameter knowing the value of the other. However, it is worth mentioning that the methodology used by Oh and others¹² to evaluate light transmittance and the translucency parameter differs from that used in the present study.

Although DC was evaluated at room temperature, a higher temperature, such as human body temperature (37°C), would probably not result in higher DC. Another study demonstrated that increasing the temperature from 23°C to 54°C did not result in statistically higher DC for a light-cured resin cement (RelyX Veneer).⁴⁷ In the present study, the DC was measured immediately after resin cement light activation. No new DC measurement was performed 24 hours after light activation because the specimens could not be repositioned on the diamond crystal of the ATR module as they were for the initial measurement. In spite of that, in another study,¹² it was reported that there was no significant increase in DC of a light-cured resin cement (Variolink N base paste, Ivoclar Vivadent) when measured immediately and 24 hours after light activation, probably because the resin cement specimens were about 50 μm thick (same thickness as in the present study), thus allowing enough light irradiation to reach maximal DC.

The success of ceramic restorations depends not only on the mechanical properties of the ceramic system but also on the resin cement’s mechanical properties and adequate polymerization reaction.^{7,13,16,19,21,23,27,28} Thus, since mean DC ranged from 71.1% to 80.1%, Ivocerin-containing light-cured resin cements can be selected for luting lithium disilicate glass ceramic restorations of medium translucency using either a single- or multipeak LED LCU. Several studies have evaluated light-and/or dual-cured resin cements DC through ceramic;^a however, methodological differences (such as the use of another resin cement, ceramic type, LED LCU, DC measurement method, and/or spectrometer to measure light transmittance and light irradiance) do not allow an adequate comparison with the present study.

A recently published systematic review and meta-analysis selected 13 clinical trials of laminate veneers by methodology quality with a median follow-up period of nine years. Laminate veneers resulted in a high estimated overall cumulative survival rate (89%).⁴⁸ Laminate veneers resulted in low complication rates; the following clinical outcomes of interest were debonding (2%), ceramic fracture/chipping (4%), secondary caries (1%), severe marginal discoloration (2%), and endodontic problems (2%).⁴⁸ The present article’s authors speculate that most of these complication outcomes might be partially attributed to ineffective light activation during the luting procedure that could have resulted in low DC. Light-activation luting procedure protocols still need to be investigated in further laminate veneer clinical trials.

Although the multipeak LED LCU proved to be superior to the single-peak LED LCU in the present study, it is difficult to state whether differences between mean DC using these two LED LCUs could have clinical implications. However, the use of a multipeak LED appears to reduce light-activation time of Ivocerin-containing resin cement, reducing chair time.

CONCLUSIONS

The results of this laboratory study seem to indicate that the interposition of medium translucency lithium disilicate glass ceramic has a minor effect on degree of conversion for the analyzed light-cured resin cement. In spite of that, using a multipeak LED light-curing unit seems to achieve a higher degree of conversion in a shorter light-activation time.