Porcelain crown restoration after a period of time some patients will have a gingival edge gray situation, the formation of this gray may be due to the following reasons: 1) the edge of the restoration is not close, exposing the color of the hard tissue of the teeth; 2) the edge of the porcelain crown porcelain layer thickness is insufficient to penetrate the metal color of the inner crown; 3) ordinary porcelain crowns due to the use of nickel-chromium alloy, it will be in the restoration of the metal elements after 3-5 years of free, so that the local soft and hard tissue staining; 4) Inadequate preparation of the shoulder table of the porcelain crown, resulting in the restoration compressing the gingiva, making the gingiva ischemic and dark; 5) Inadequate depth of preparation of the margin of the restoration, although the restoration itself is well prepared, the color of the tooth that has been discolored or the nucleus of the nickel-chromium alloy stump is exposed through the marginal gingiva. In the case of anterior teeth, due to the appearance of gray lines affecting aesthetics, it is recommended that the original restoration be removed and restored.