Evaluation and Comparison of the Vertical Marginal Discrepancy of Ni-Cr and Co-Cr Alloys Before and After Porcelain Firing: An in Vitro Study

Introduction: Marginal seal is the critical factor for the success of any restoration. Accuracy of marginal seal of copings depends on alloy systems & manufacturing techniques. In every technique like laser sintered, conventional casting etcetera, significant marginal discrepancy takes place after porcelain firing. The aim of this study is to evaluate and compare the vertical marginal discrepancy of Ni-Cr and Co-Cr alloys before and after porcelain firing. Methods: Custom made stainless steel die model was prepared to simulate the dimensions of the mandibular molar with a round shoulder margin. Cast copings of two base metal alloys Ni-Cr alloy & Co-Cr alloy were prepared. Total ۴۰ copings were made and divided into two groups, G۱ (Ni-Cr alloy) & G۲ (Co-Cr alloy). According to alloy type, ۲۰ specimens were used for each group of the study. The vertical discrepancy was measured with an optical LASER microscope (Nikon, Japan) before and after application of porcelain on the copings. Wilcoxon signed ranked test and Mann Whitney U test were used for Statistical analysis. Results: The median vertical marginal discrepancy before and after porcelain firing for Ni-Cr alloy was ۴۸.۱۳ and ۵۲.۸۷ um respectively and for Co-Cr alloy was ۹۵.۳۳ and ۱۰۰.۰۴ um respectively. There is a highly significant difference (P < ۰.۰۰۱) in the median vertical marginal discrepancy between Co-Cr alloy copings & Ni-Cr alloy copings before & after porcelain firing. Conclusions: Co-Cr alloy PFM restorations have double vertical marginal discrepancy compared to Ni-Cr alloy restorations but both the values can be considered within clinical acceptability range of the marginal discrepancy.

Keywords:

Base metal alloy copings , firing cycles , Marginal seal