Statement of Problem: In order to eliminate the esthetic problem of metal collar in porcelain fused to metal crowns various techniques, such as shoulder porcelain, have been suggested.

Purpose: The aim of this study was to investigate the effects of repeated porcelain firing cycles on the marginal integrity of shoulder in porcelain fused to metal crowns, made of two kinds of alloys: noble and base-metal.

Materials and Methods: In this experimental study, four groups of ten brazen models (Die) were used. Metal crowns of groups A&B were made of a noble alloy and those of C&D of a base-metal alloy. Groups A&C had the shoulder width of 1mm and groups B&D had the shoulder width of 1.5mm. After degassing and opaqing, shoulder porcelain was completed in the area of shoulder. At this stage, the average vertical gap of margin was measured by a reflective microscope (400). Data were analyzed using ANOVA and Duncan tests.

Results: No simoltaneouse interaction between the type of alloy and the shoulder width was found (P=0.709). The type of alloy had a significant effect on marginal integrity (P=0.021) attributing the best marginal integrity of shoulder porcelain to base-metal alloys. Shoulder width played also a significant role on marginal integrity (P=0.00) indicating that the reduction of shoulder width would increase the marginal integrity.

Conclusion: According to these findings, the best marginal integrity of shoulder porcelain is achieved through shoulder width of 1mm along with base-metal alloys.