If you have an older, porcelain-fused-to-metal dental crown, you have likely been able to enjoy many years of smiling and eating with complete comfort. These durable, lifelike restorations were long the gold standard for protecting teeth that had extensive damage from cavities and decay. But unfortunately, over time, this type of crown can begin to exhibit a dark side: in some cases, patients may notice a black line forming around the bottom where the porcelain fuses to the metal. If this happens to you, here’s what you can do about it.