NIR Upconversion to Enhance Resin Cement Polymerization Under Lithium-disilicate Ceramics

Objectives: To evaluate the efficacy of near-infrared (NIR) activated upconversion phosphor (UP) in polymerizing resin cements under lithium-disilicate ceramics.
Methods: Lithium-disilicate disks of different thickness (0.5, 1.0, 1.5, and 2.0 mm) were prepared with IPS e.max Press (Ivoclar Vivadent). Light transmission rates of a NIR diode laser (980 nm) through these disks were examined by a powermeter, and compared with those of blue light (BL, 475 nm) from a quartz-tungsten-halogen unit (Optilux 501, Kerr). The NIR-to-BL UP powder (NaYF₄:Yb³⁺/Tm³⁺) was milled to 0.5 μm and added into a light-curable resin cement (Variolink II base) at 5 wt%. The experimental cement was cured with irradiations of either BL or BL combining NIR through ceramic disks. The surface hardness of resin cement under ceramic disks was measured by a microindenter to evaluate the degree of polymerization.
Results: Light transmission of BL (900 mW/cm²) and high intensity NIR (4000 mW/cm²) decreased to 43% and 64% through 0.5-mm thick disk, as well as 21% and 53% through 2-mm thick disk, respectively. Both lights at lower intensity (624 mW/cm²) revealed reduced light transmission, but NIR still showed higher light transmission through all the disks. The microindentation test revealed decreased surface microhardness on the resin cements as the ceramic disk thickness increased. For the uncovered cement, light irradiation with BL 20s followed by NIR 40s (BL20s+NIR40s) showed the highest microhardness. However, BL40s+NIR20s showed the highest microhardness values on the irradiated cement under all disks, followed by BL20s+NIR40s, and BL60s.
Conclusions: NIR exhibits higher transmission rate through lithium-disilicate ceramic than BL does. The NIR adjunct polymerization enhanced the degree of conversion of UP-modified resin cement under the ceramic disks of different thicknesses. This technique will improve the polymerization of resin cements used in luting lithium-disilicate restorations.