Do high translucency zirconia shades contribute to the degree of conversion of dual-cure resin cements?

Objective: The opacity of the zirconia ceramic restoration may influence the degree of conversion (%DC) of dual-cured resin cements. The aim of this study was to evaluate the degree of conversion of resin luting agents photocured under different high translucency zirconia shades. The opacity of each ceramic shade was evaluated.
Method and materials: The opacity percentage of Lava Frame and three Lava Plus High Translucency Zirconia ceramic shades (W1, A2, and B4) was determined. A spectrophotometer MiniScan was used to measure the opacity percentage of each specimen (0.7 mm thick) and then the opacity was calculated. Specimens from three different resin cements (Panavia F2.0, RelyX Unicem 2 Automix Self-Adhesive, and PermaFlo DC) were prepared with 100-µm thickness. The specimens were photocured according to the manufacturers’ instructions under a ceramic block (0.7 mm thick). Specimens photocured without the ceramic block were used as control. Fifteen groups (n = 3) were evaluated. Micro-ATR/FTIR (micro-attenuated total reflectance/Fourier-transformed infrared spectroscopy) spectrometry was used to evaluate the extent of polymerization of all specimens after 24 hours. The %DC was determined using experimentally polymerized versus maximally polymerized composite.
Results: The opacity percentages (mean ± SD) of W1, Lava Frame, A2, and B4 ceramics were 72.41% ± 0.04%, 74.24% ± 0.09%, 77.63% ± 0.11%, and 78.17% ± 0.12%, respectively. There was a statistically significant difference in %DC among the different cements investigated (P = .003) and a statistical difference in %DC was also found among the different ceramic shades studied (P = .030). For the Lava Frame and A2 ceramic blocks, the resin cements tested presented the lowest %DC except for the RelyX Unicem resin cement.
Conclusions: The %DC of RelyX Unicem 2 cement under low zirconia opacities (W1 and Lava Frame) was greater compared to high zirconia opacities (A2 and B4). For the Panavia F2.0 cement, no effect on the %DC under different high translucency shades was observed. The highest %DC of PermaFlo cement was exhibited when the ceramic shade W1 was used. Clinical implications: Dual-cured resin cements exhibited different degrees of conversion depending on the opacity of the zirconia restoration. However, different brands demonstrated higher or lower dependency of the light transmittance on their degree of conversion.