Influence of repeated porcelain firing cycles on the microstructure, marginal and internal fit of implant-cemented Co-Cr frameworks

This study investigated the effect of porcelain firing cycles and fabrication techniques on the microstructure, marginal, and internal fit of implant-cemented cobalt-chromium alloys (Co-Cr) frameworks. Two groups of Co-Cr specimens were prepared: one using selective laser melting (SLM) and the other through the lost wax-casting technique (C). Each group was further divided into two subgroups: as-manufactured and porcelain-fired specimens. The microstructure was analyzed using x-ray diffraction (XRD) and scanning electron microscopy with energy-dispersive x-ray spectroscopy (SEM/EDX). Ten Co-Cr frameworks per manufacturing technique were fabricated, with marginal and internal discrepancies measured using micro-CT before and after the firing cycles. The results were analyzed by two-way ANOVA and Tukey multiple-comparison tests. The results showed a mixed microstructure of γ-fcc and ε-hcp cobalt-based phases, with porcelain firings increasing ε-Co content, especially in group C. The C specimens exhibited distribution of small pores, while no pores/defects were found in the SLM group. The SLM group, although not at a statistically significant level, showed reduced marginal and internal discrepancy values after firings. In the same behavior pattern, the C group exhibited statistically significantly lower discrepancy values. Ultimately, all subgroups demonstrated clinically acceptable marginal adaptation, but the SLM frameworks exhibited superior marginal and internal fit which remained unaffected by the firing cycles.