Many people who have had porcelain restorations find that there are unsatisfactory gray lines on the edges of the porcelain crowns, which directly affect the aesthetics. So why some porcelain teeth will appear gray line? 1, nickel-cadmium porcelain teeth are prone to the phenomenon of gray line teeth, because the chemical instability of the metal, its metal ions free out, deposited in and porcelain teeth near the gum or root tissue, so that and porcelain teeth contact parts of the gums gray, affecting the aesthetics. 2, porcelain teeth edge fit is not good, porcelain teeth and gums do not fit, after wearing on the real teeth and real teeth between the gap, then porcelain teeth will appear a series of problems, including gum gray line. This is the main cause of gum line. Generally is the doctor’s technical level problem. 3, the quality of porcelain teeth is not good, the thinness of the inner wall is not the same degree, revealing the metal color shows the gray line color. This is due to the porcelain crown edge porcelain layer thickness is insufficient, revealing the metal color of the inner crown caused. 4, porcelain crown shoulder preparation is insufficient, resulting in restorations pressed against the gums, so that the gums are ischemic, dark color. 5, the restoration edge preparation depth is insufficient, although the restoration itself is well prepared, but has been discolored teeth or nickel-chromium alloy pile core color through the edge of the gingiva exposed.