Microhardness of resin cements after light activation through various translucencies of monolithic zirconia

Abstract

PURPOSE This study aimed to investigate the Vickers Hardness Number (VHN) of light- and dual cured resin cements cured through monolithic zirconia specimens (VITA YZ) of various translucencies: translucent (T); high translucent (HT); super translucent (ST); and extra translucent (XT) at 0, 24, and 48 h after curing. MATERIALS AND METHODS Four zirconia specimens from each translucency were prepared. Two light-cured resin cements (Variolink N LC; VL and RelyX Veneer; RL) and two dual-cured resin cements (Variolink N DC; VD and RelyX U200; RD) were used. The cement was mixed and loaded in a mold and cured for 20 s through the zirconia specimen. The upper surface of cements was tested for VHN using a microhardness tester at 0, 24, and 48 h after curing. The VHN were analyzed using two-way repeated, Brown-Forsythe ANOVA with Games Howell post-hoc analysis and independent t-tests ( P < .05). RESULTS All cements showed significantly higher VHN from 0 h to 24 h ( P < .001). At 48 h, the VHN of light-cured cements were significantly lower when cured under the T groups than under XT groups ( P = .001 in VL, P = .014 in RL). At each post curing time of each translucency, VD showed higher VHN than VL ( P < .05), and RD also showed higher VHN than RL ( P < .05). CONCLUSION The translucency of zirconia has an effect on the VHN for light-cured resin cements, but has no effect on dual-cured resin cements. Dual-cured resin cement exhibited higher VHN than the light-cured resin cement from the same manufacturer. All resin cements showed significantly higher VHN from 0 h to 24 h.

Influence of medium-translucency monolithic zirconia thicknesses and light-curing time on the polymerization of dual-cure resin cements

“…This result could be clarified by the fact that the RelyX U200 contains more inorganic filler (72 by weight) than TheraCem (60-65 by weight) 41 . The results of the current study are in agreement with other authors who stated that the surface hardness of resin cement depends on the proportion, type, and distribution of the inorganic filler content and that increasing the proportion of inorganic fillers enhances the surface hardness of the cement 14,15 .…”

“…A possible explanation for this result is that increasing the thickness of zirconia causes the light to be attenuated by the zirconia material. The combination of absorption, scattering, and reflection explains the attenuation of the light passing through the restoration, which negatively affects the polymerization reaction of the cement 10,15,[22][23][24][25] .…”

“…The "dark polymerization" of unpolymerized monomers may continue after the cessation of photoactivation 32 . "Dark polymerization" can adequately compensate for the deficiency in initial polymerization and reach the ultimate polymerization to obtain the best clinical performance 15 .…”

“…The direct method (Fourier -transform infrared spectroscopy) is commonly used for DC analysis and provides reliable results 10,11 and the indirect method (Vickers microhardness test) 12 . Several factors may influence polymerization, including ceramic thickness, translucency, type of luting agent, and light-curing time [13][14][15] .…”