Mechanism underlying ultraviolet-irradiation-induced discoloration of dental zirconia ceramics stabilized with 3 and 5 mol% yttria

Abstract

The present study aimed to analyze the mechanism underlying the color alteration of dental zirconia ceramics stabilized with 3 and 5 mol% yttria (3YZ and 5YZ, respectively) caused by 254-nm ultraviolet (UV) irradiation. Fully sintered specimens of 3YZ (Zpex, Tosoh) and 5YZ (Zpex Smile, Tosoh) were irradiated with 254-nm UV at 4 mW/cm2 (up to 60 min). Color alteration (ΔE) was analyzed using the CIE L*a*b* colorimetric system. The UV-induced excitation of oxygen vacancies, which capture two electrons in zirconia (i.e., F center) was analyzed using photoluminescence (PL) and electron spin resonance (ESR) spectroscopy. Furthermore, the spontaneous change in ΔE after cessation of UV irradiation was evaluated. UV irradiation increased ΔE to 13.4 in 3YZ and 10.3 in 5YZ within the initial 10 min of irradiation, and then, ΔE reached a plateau. A PL peak was observed at 435 nm, which decreased with UV irradiation. In addition, UV irradiation generated an ESR signal attributed to the F+ centers formed via one-electron detachment from F centers. The signal intensities of both 3YZ and 5YZ increased with irradiation time; however, the signal intensity was higher in 3YZ than that in 5YZ. After cessation of UV irradiation, ΔE gradually decreased to approximately 2, which took a few weeks. The discoloration of 3YZ and 5YZ caused by UV irradiation was due to the excitation of F centers. Although the specimens spontaneously reverted to their original color over time, clinical procedures involving UV irradiation, such as photo-functionalization, should be performed with caution to avoid damaging the esthetics of dental zirconia restorations.