Influence of porcelain-firing procedures on the marginal distortion of electroformed metal-ceramic restorations

Objective: To evaluate the influence of firing procedures on the marginal distortion of electroformed metal-ceramic crown restorations. Method and Materials: Twenty-four standardized specimens were fabricated using a gold-electroforming system and were characterized by 3 finish line forms: shoulder (S-type), rounded shoulder (RS-type), and deep chamfer (DC-type) preparations. Marginal discrepancies were measured at 6 stages: before the firing procedures, after the gold-bonder application, after the opaque porcelain firing, after the dentin porcelain firing, after the enamel porcelain firing, and after the glazing firing. Marginal discrepancy values were recorded at 60 randomly chosen points along the circumferential margin using a laser microscope. Marginal distortion values were defined as the differences in the marginal discrepancies between the baseline (prefiring) levels and those after each firing procedure. All statistical analyses were performed using the Friedman and Kruskal-Wallis tests (a = .05). Results: For all of the finish line forms, the marginal distortion values were altered by the subsequent firing procedures. The largest marginal distortion values were observed after the first firing procedure. The total marginal distortion values were 34.2, 6.9, and 5.2 µm for the S-, RS-, and DC-type preparations, respectively. Conclusion: The porcelain-firing procedures influenced the marginal distortion of electroformed metal-ceramic crown restorations. The marginal distortion values of the S-type preparations were the highest among the 3 groups tested. Keywords: crown, dental porcelain, electroforming, firing procedure, marginal adaptation, metal-ceramic restoration