Marginal fit of metal-ceramic crowns fabricated by using a casting and two selective laser melting processes before and after ceramic firing

Statement of problem: Few studies have investigated changes in the marginal fit of metal-ceramic restorations fabricated by selective laser melting (SLM) techniques after the application of veneering ceramic. Purpose: The purpose of this in vitro study was to evaluate the marginal fit (silicone replica technique) and internal porosity (cross-section analysis) of cobalt-chromium (Co-Cr) alloy metal crowns prepared by using 2 SLM processes together with a casting technique before and after ceramic veneering. Material and methods: Cast single Co-Cr crowns and SLM-processed crowns with large (SLML) or small (SLMS) porosity were prepared (n=20/group), and half were subjected to ceramic veneering. On a single Co-Cr master die, the marginal discrepancy (MD) and absolute marginal discrepancy (AMD) of the crowns were measured by using the silicone replica technique, in which each replica was cut into 4 sections before and after ceramic veneering (n=10 for each subgroup). After marginal fit measurements, each metal coping was cross-sectioned into 4 parts, and 5 rectangular optical microscope images were acquired on both outer corners of each quarter. The porosity was then calculated as the ratio of the black-to-white pixels on the binarized images. The data were analyzed by 2-way ANOVA and the post hoc test (Tukey or Student t test) (α=.05). Results: Before ceramic veneering, the 2 SLM groups showed significantly larger MDs than the casting group (56.4 ±10.4 μm) (P<.05). A significant increase in MD after ceramic veneering was detected only in the SLML group (P<.001). The AMD values showed a similar trend with MD values. The 2 SLM groups (in particular, SLML) showed a significantly higher amount of porosity than the casting group before ceramic veneering (P<.001). Only the SLML group showed a significant decrease in the amount of porosity after ceramic veneering (P<.001). Conclusions: Within the limitations of this in vitro study, large internal porosity within the SLM-fabricated Co-Cr metal copings affected the marginal fit of the metal-ceramic crowns. However, all the MD values of the 3 groups were lower than the acceptable range even after the application of veneering ceramic.