Influence of the composition and shades of ceramics on light transmission and degree of conversion of dual-cured resin cements.

Abstract Objective Since the transmittance of ceramics can influence the degree of conversion (DC) of resin cements, ceramics composition and shade should be considered in the selection of resin cement. This in vitro study aimed to evaluate the effect of the transmittance of different composition, opacities and shades of ceramics on the degree of conversion of two dual-cured resin cements. Methodology Sixty discs were prepared from low translucency (LT) and medium opacity (MO) lithium disilicate ceramic, and zirconia ceramic (Z). Each group was subdivided into 5 subgroups (n=4) in shades A2, A3.5, B2, C2 and D3. The transmittance measurement was performed in a spectrophotometer. The Variolink II and Rely X U200 resin cements were photoactivated by LED (1400 mW/cm2) for 40 s through the ceramic discs and without the discs (control group). The DC was measured with infrared FTIR spectroscopy, immediately after light activation. Data were analyzed with Kruskall-Wallis and one-way ANOVA, following post-hoc comparisons by Tukey test and Pearson’s correlation test (P<0.05). Results LT ceramic exhibited higher transmittance values compared to MO and Z ceramics. LTA2 and LTB2 showed statistically higher transmittance values compared to MOA2, MOA3.5 and ZA3.5. For Variolink II, the ceramic interposition did not influence the DC, since there were no statistical differences between groups with ceramic interposition and the control group. For Rely X U200 cement, the interposition of some ceramics types/shades (LTA3.5, MOA2, MOA3.5 and ZA3.5) significantly decreased the DC values compared to control group. A positive correlation was found between the ceramic transmittance and DC values of both tested cements. Conclusions. The transmittance and DC values of the cements were influenced by composition and shades of the ceramics. The higher the transmittance of ceramics, the higher the DC values for both cements.


Introduction
The use of resin cements has grown significantly in recent years due to the increase in demand for ceramic restorations. 1 The adhesive properties of these cements to the enamel, dentin, metal alloys and ceramics, their biocompatibility and low solubility justify their use, since they promote retention and reduction in the stress concentration at the tooth/ cement/restoration interface. 2,3 So that the physical, mechanical and biological properties of the resin cements are preserved, the light intensity of the lightcuring unit must generate sufficient energy to activate and promote an appropriate degree of conversion (DC), since the reduction of the radiation acts negatively in the curing process of these cements. [4][5][6] It has been observed that the DC of the monomers in the dual polymerization resin cements is influenced when they are activated under ceramic restorations, 4,6,7 since these can reduce the radiation reach from the light source to the cement, 8 and, consequently, alter the properties of the material, which may compromise the success of the ceramic restoration in the medium and long term.
The amount of light that passes through the ceramic depends on its translucency, 5 which is an optical property that exerts important influence on the aesthetics of the restoration, 9 and is defined as the property of a material of which most of the transmitted light undergoes scattering, reflecting and transmitting through it. The greater the amount of light through the object, the greater the translucency. 10 The factors affecting ceramic translucency are numerous, including ceramic composition, crystalline content, thickness, [11][12][13] porosity, 14 and fabrication technique, among others. 3 The methods for quantifying the transmittance are direct transmission, total transmission (including scattering) and spectral reflectance, and it can be performed in a spectrophotometer. 7 Among the methods used to evaluate the DC of resinous compounds, the spectrophotometer tests are the most suitable methods for this purpose because they allow detailed measurements of the non-reactive methacrylate groups. 15 Considering that the transmittance of ceramics can influence the degree of conversion (DC) of resin cements and consequently the longevity of the restoration, the composition and color of ceramics in the selection of resin cement should be considered.       between different studies is jeopardized by differences in methodologies used, as well as the different composition and shades of ceramics. 9,10,18,19,22,23 Among the several methodologies to determine the polymerization quality of composites, the spectrometry has been widely employed in various studies. 6,12,[24][25][26][27][28][29][30][31][32][33] This research showed that the two cements behaved differently when light-activated under the same conditions, although both were dual cure resin cements. The polymerization reaction of the cements is influenced by the composition, such as monomer type and inorganic particle content. 24,29 Since the DC is dependent on the composition of the and Z ceramic in shade D3. Nonetheless, despite some statistical differences within ceramic groups, the most important finding was that the interposition of ceramic disc did not influence the DC of this cement, since there were no significant differences between groups without (control group) and with interposition of ceramic discs in different shades, which is a reassuring result for the clinician. This fact may be attributed to the amount of photoinitiators and sensitivity of the cement when light-cured even under ceramic discs.
As explained by Ilie and Hickel 4 (2008), the velocity of the photo-initiation polymerization reaction is limited and an unrestricted increase in irradiation will not be able to accelerate this process. Furthermore, in a light activated polymerization system, the effectiveness of an initiator is limited by a deactivation mechanism. Not only can an active photoinitiator start a polymerization reaction but, by recombination with another active photoinitiator or reaction with an activated polymer chain, it can also stop it. Therefore, the amount of light that reached the cement layer underneath the ceramic was enough to achieve a degree of polymerization equivalent to the control group without ceramic interposition. Although some authors obtained similar results between specimens without and with the superposition of ceramics disc, 34 different results were found by other investigations. 6,19 The Rely X U200 cement presented significant differences between the groups without and with disc superposition for the LT ceramic in shade A3.5, for the MO in shades A2 and A3.5, and for the Z ceramic in shade A3.5. Other studies also found differences with the Rely U100 cement when light-activated without (56.7%) and under IPS Empress 2 (49.7%) with 2.0 mm thickness. 31 Flury, et al. 35 (2013) showed that discs of IPS Empress CAD and IPS e.max CAD with a thickness of 1.5 mm did not significantly influence the DC of the Rely X Unicem 2 cement (50.2% and 49.5%, respectively) when compared to the DC of the polymerized cement without the superposition of discs (51.1%). It is observed that the above-mentioned studies used cements with different trademarks, but basically with the same composition.
When correlating the transmittance and DC results, the Pearson's correlation test showed a significant weak positive correlation between these two aspects for both cements, showing that the higher translucency value, the higher was the DC of the cements. The results of this study are in agreement with previous scientific evidence, 4,19,20,25,26,36,37 which stated that differences in shade and opacity of restorative materials influence transmittance and, consequently, the polymerization of the cements.
It seems reasonable to emphasize that monomer conversion of resin cements is closely related to the amount of total light transmitted (direct and scattering light) through the indirect restoration. Nonetheless, in the current study the values of transmittance were obtained using the direct transmission method (direct beams, without quantifying scattering light, as previously mentioned). Accordingly, this fact could explain the lower correlation found in comparison to other authors. 4,20,25,36 In general, the DC values of the cements under the conditions tested in this work were similar as a function of the 2.0-mm-thickness of the discs. This aspect is corroborated by other studies 4,11,28,33,36-38 that did not show differences in hardness and DC values when the ceramic presented thickness up to 2.0 mm.
Despite the significant differences between the DC of the resin cements under different ceramic composition and shade, its important to highlight that the DC values of the two cements were between 50 and 70%, with a slight difference concerning the control group (approximately 5%). As demonstrated in previous investigations, 6,39 it seems reasonable to emphasize that such DC values are sufficient for resin cements to have an acceptable clinical performance. It also is important to highlight that the DC measurements were performed immediately after light activation. It is expected that the DC gradually expands as a function of time, with a substantial increase in the first 30 min and with a linear increase up to the first 24 h. 8,24,40 Therefore, according to the results, it can be stated that the cements used in this study may be used with the ceramics tested without causing relevant clinical impact in the DC of these cements.

Conclusions
Within the limitations of this in vitro study, the following conclusions were made: There was a significant difference in the values of transmittance between some composition and shades of ceramics. The LT ceramic presented the highest transmittance values, followed by the MO and Z ceramics. The superposition of the ceramic discs had no influence on the DC of Variolink II cement. For Rely X U200, the ceramic disc negatively influenced the DC.
There was a positive correlation between transmittance of ceramics and DC of Rely X U200 and Variolink II.