Effect of lithium disilicate ceramic thickness, shade and translucency on transmitted irradiance and knoop microhardness of a light cured luting resin cement

Abstract

This in vitro study evaluates the influence of pressed lithium disilicate thickness, shade and translucency on the transmitted irradiance and the Knoop microhardness (KHN) of a light-cured resin cement at two depths. One hundred and thirty-five ceramic discs of IPS e.max Press (Ivoclar Vivadent) were fabricated and divided into twenty-seven groups (n = 5) according to the association between translucency: HT (hight translucency), LT (low translucency), and MO (medium opacity); shade: BL2, A1 and A3.5; and thickness: 0.5 mm, 1.5 mm, and 2.0 mm. One side of each ceramic disc was finished, polished and glazed. The irradiance (mW/cm²) of a multiwave LED light curing unit (Valo, Ultradent) was evaluated with a potentiometer (Ophir 10ª-V2-SH, Ophir Optronics) without (control group) or with interposition of ceramic samples. The microhardness of Variolink Esthetic LC resin cement (Ivoclar Vivadent) was evaluated after 24 h at two depths (100 μm and 700 μm). Data were submitted to ANOVA followed by Tukey’s test (α = 0.05). Irradiance and KHN were significantly influenced by ceramic thickness (p < 0.0001), shade (p < 0.001), translucency (p < 0.0001) and depth (p < 0.0001). Conclusions: the interposition of increasing ceramic thicknesses significantly reduced the irradiance and microhardness of resin cement. Increased depth in the resin cement showed significantly reduced microhardness for all studied groups. Increased ceramic opacity reduced the KHN of the resin cement at both depths for all ceramic thicknesses and shades.

Effect of CAD/CAM aesthetic material thickness and translucency on the polymerisation of light- and dual-cured resin cements

“…A similar study recorded a drop in irradiance from 700 mW/cm 2 to 100 mW/cm 2 with an interposed 2-mm resin composite substrate [18]. Concurring with previous studies, linear inverse correlations were confirmed for substrate translucency [23,32,33,40] and light irradiance [30,34,45] versus thickness. The minimum irradiance range to achieve efficient resin polymerisation is 400-500 mW/cm 2 for 20-40 s with a 1 mm thickness substrate, depending on photoinitiator type and content [24,32].…”

“…At 0.5 thickness, the measured I t and T′ % increased in the following sequence, VE < VM = GB < CS. At 0.5 and 1 mm, all materials had I t above 500 mW/cm 2 which represent the minimum irradiance recommended for sufficient polymerisation of Ivocerin™-containing luting cements for 20 s [32]. However, this minimum irradiance was not met in substrates thicker than 1 mm.…”

“…Furthermore, these optical features regulate the light irradiance that penetrates to the underlying luting cement, required to initiate monomer conversion [1,23]. These light attenuating features depend upon the substrate material's microstructural composition, shade and thickness [18,[32][33][34].…”

“…However, adequate polymerisation could be estimated indirectly by monitoring the development of optical or mechanical properties associated with the polymerisation process [29]. Changes in refractive index or microhardness (Vickers and Knoop) have been used to indicate the progress of polymerisation [32,33,41]. Recently, Martens hardness (H M ) has been measured via a series of force-controlled indentations to analyse the ageing behaviour of six dual-cured luting cements after photoactivation through a 1-mm thick zirconia substrate over 7 d [25].…”

“…The effect of ceramic substrates including composition, shade and thickness on the polymerisation of different luting cements has been determined using degree of conversion and classical microhardness measures [1,7,32,38]. However, conflicting polymerisation results were noted for dual cured luting cements underneath CAD/CAM resin-based substrates compared to ceramic substrates [23,33,40].…”