2017 年 125 巻 3 号 p. 79-84
Leucite (KAlSi2O6) glass-ceramic dental prostheses are fabricated by firing glass-ceramic powder into the shape of a tooth crown on an abutment-tooth frame made from a precious metal alloy. It is known that, if this alloy contains silver, the prosthesis suffers from an unattractive yellow discoloration. Although the addition of antimony trioxide to the glass-ceramic mixture has been shown to reduce such yellowing, this compound is believed to be toxic to humans. In this study, we investigate yellowing with the addition of cerium oxide, which is a promising replacement for antimony trioxide due to its low toxicity to biological organisms. Using scanning electron microscopy, energy dispersive X-ray spectrometry, and ultraviolet and visible spectrophotometry, we analyze the fine-grained structure and composition of the fired glass-ceramic body that comes into contact with silver, as well as the nature and extent of the resulting yellowing. Our findings indicate that, as previously thought, the yellowing is indeed caused by the diffusion of silver ions into the fired glass-ceramic body to form colloids, but they also reveal a contribution from sodium at the interface between the fired body and the silver. To investigate methods suppressing the yellowing, we fabricate several model glasses with different sodium-to-cerium ratios, and study the nature and extent of the yellowing for each glass. Our results reveal that the extent of yellowing is correlated with increased sodium content and that the yellowing may be suppressed by adjusting the sodium-to-cerium ratio.