Light-curing of restorative composite through milled and 3D-printed full-contour zirconia for adhesive luting

To evaluate the effect of different zirconia compositions and manufacturing processes on the light irradiance (LI), to measure the degree of conversion (DC) of solely light-curing restorative composite underneath these zirconia grades and to evaluate the respective zirconia microstructures.

Six dental zirconia grades (GC HT, GC UHT [GC]; Katana HT, Katana UTML [Kuraray Noritake]; Lava Esthetic, Lava Plus [3 M Oral Care]) were cut and sintered per manufacturer instructions. One 3D-printed zirconia grade (XJet [XJET]) was prepared according to previous research. Zirconia plates were ground to four thicknesses (0.5, 1.0, 1.5, 3.0 mm). The LI through these zirconias was measured using light spectrometry using two light-curing units (Demi Plus [Kerr], Bluephase G4 [Ivoclar]). Restorative composite (Clearfil AP-X [Kuraray Noritake]) was light-cured through the zirconia plates and the DC was determined by micro-Raman spectrometry 5 min, 24 h and 1 w after light-curing. Statistical analysis of LI and DC data involved linear mixed-effects modelling and multi-way ANOVA. Microstructural analysis of zirconia was performed by scanning electron microscopy.

Zirconia type and thickness, and LCU had a significant effect on LI (p < .0001). DC significantly increased over time (p < .0001) and was not influenced by curing-light attenuation if LI reached at least 40 mW/cm². Increased yttria content resulted in an increased zirconia grain size.

Despite significant light attenuation, DC of composite light-cured through zirconia at almost all thicknesses, approached DC measured without zirconia interposition for five out of seven zirconia grades. Additionally, the manufacturing process did not seem to influence LI or DC.