| PFM dental restorations have been attractive to both dentist and patients because of their advantages since 1950s, such as good wear resistance, color stability, improved esthetics, higher mechanical strength, more simplified fabrication procedures and so on. PFM is still one of the most widespread methods for fixed restorations although it needs to be improve.Color measurement is one of the most important quality criteria for PFM. One of the key to the success of PFM is making the color simulate effectively, so that we cannot even differ restorations from the adjacent natural teeth at a glance. Generally speaking, there are two main factors contributing to the failures of PFM correlating with color. One is that the color of metal coping is not successfully masked. The other is that the thickness of dentin isn't enough to mask out the color of opaque resulting in incorrect shade match. Opaque porcelain situates between metal coping and body porcelain, whose objective is to mask out the metal, provide the basic shade of the restoration, and aid in shade match together with body porcelain and vitreous enamel. The optimal thickness of opaque porcelain and body porcelain will be changed according to the color and ceramic used.Adaptation is an important quality criterion for fixed restorations, It reflects the precision and the seating conditions of restorations. Adaptation is a broad concept, includes casting precision, usually refers to the marginal gap between the tissue surface of restoration and abutment, or the thickness of cement after cementation. Unreliable marginal adaptation will result in accumulation of dental plaques, furthermore lead to secondary caries and marginal gingivitis. It will influence negatively not only the health of teeth and periodontal tissues, but also esthetic effects of gingival margins of restorations. Mclean and Van Fraunhofer have found that the maximum clinically acceptable marginal gap is 120 m, which has been used widely as a standard of marginal adaptation.There is no objective evaluation about the type of metal ceramic alloys on color and marginal adaptation, Among the published studies on the color of PFM, there is few about galvanoceramic restorations. This study was aimed to evaluate the effects that different metal copings and different porcelain thickness have on the colors of restorations, and to compare the marginal adaptation of restorations using different metal copings. We hope it can bring some clinical significance and be help in clinical practice.The colors of different PFM restorations was measured according to the CIE1976- L*a*b* color scale by a Minolta chromatics meter CR-321. The color difference (AE) was calculated and then analyzed. The optimal thickness of opaque porcelains on different metal copings was found and investigated the influence that different metal copings and different thickness of body porcelain have on the colors of restorations. Filled in the gap betweenrestorations and dies with silicone rubber. By measuring the thickness of silicone rubber, we compared the marginal adaptation of PFM before and after porcelain application. The marginal differences of PFM after cementation were measured by SEM and data were analyzed by multivariate ANOVA. The main findings of this study were as follows:1. The thickness of opaque porcelain is not linear with its masking ability, that is, the masking ability can be improved by the increase of thickness. But after the thickness reach a certain value, the color difference will no longer increase.2. As in the PFM whose coping made of Ni-Cr alloy or precious alloy, when the thickness of opaque porcelain arrives at 0.3mm, the color of metal coping can be successfully masked and it has no significant difference with those whose opaque porcelain arrive at o.2mm, It is suggested that when the thickness of opaque arrives at 0.2mm, the color difference cannot be distinguished by naked eyes. As to those made by galvanized forming, the minimum thickness of opaque porcelain is 0.1mm and AE i...
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