Light Transmission Through a Zirconia-reinforced Lithium-silicate Material

Objectives: Light transmission efficiency through new zirconia-reinforced Celtra®Duo (ZLS, Dentsply; shade A3.5), was studied by measuring the light power-density (LPD) passing through specimens of different thickness and, indirectly, by determining the degree of conversion (DC%) of two dual-cure resin-based cements.
Methods: ZLS-A3.5 blocks were sectioned at 1, 1.5, 2mm using a microtome (Micromet-Remet). LPD (mW/cm²) through ZLS-A3.5 specimens (N=10 per thickness) was determined using a Managing Accurate Resin Curing spectrometer (MARC, BlueLightAnalytics). DC% of resin cements (Calibra®Ceram-CC and Calibra®Universal-CU, Dentsply) was evaluated using an FTIR-ATR equipment (Nicolet 6700-Thermo Scientific) working between 4000cm⁻¹ and 500cm⁻¹ (resolution 8cm⁻¹). Cements (N=5), were placed on the FTIR diamond stage (0.12x3mm mold) and photopolymerized for 40s through the ceramic specimens with the LED curing-light (VALO, Ultradent) in standard power mode. Polymerization kinetic (1spectrum/s) was recorded up to 300s. DC% was calculated at 40s and 300s, considering the ratio between the area of the reactive peak (1635cm^-1) and the internal reference peak (1715cm^-1). Data were statistically analyzed (ANOVA).
Results: LPD was dependent on the ceramic thickness: the highest value was measured through 1mm specimen, exponentially decreasing by increasing thickness. CC and CU DC%, resulted linearly affected by ZLS thickness, reaching the highest values using 1mm ZLS specimens, both at 40 and 300s of polymerization.
Conclusions: The decrease of LPD, was exponentially correlated to ZLS-A3.5 thickness increase following the Lambert-Beer law (f(x)=a*exp(-b*x), R²>0.99). Additionally, LPD decrease was correlated to the DC% decrease of both cements (CC and CU) at both polymerization intervals (40 and 300s), suggesting that, thicker ZLS specimens might not allow sufficient light passing through the material, ultimately causing insufficient photopolymerization of the tested cements.