Effect of manufacturing techniques on the marginal and internal fit of cobalt-chromium implant-supported multiunit frameworks

Statement of problem. Subtractive and additive computer-aided design and computer-aided manufacturing (CAD-CAM) systems have been used in the fabrication of cobalt-chromium (Co-Cr) long-span restorations. However, the accuracy of fit of multiunit frameworks is unclear. Purpose. The purpose of this in vitro study was to compare the marginal and internal fit of implant-supported, cement-retained 3-unit, 4-unit, and 5-unit Co-Cr metal frameworks fabricated by the lost wax (LW), CAD-CAM milling, and selective laser melting (SLM) techniques. Materials and methods. A total of 90 Co-Cr metal frameworks were fabricated for 3-unit, 4-unit, or 5-unit implant-supported cement-retained restorations on stock abutments with 3 different manufacturing technique subgroups (LW, CAD-CAM milling, and SLM). The silicone replica technique was used to evaluate the marginal and internal discrepancy values. By using a light microscope at x45 magnification and a digital measurement program, the thickness of the silicone layer was measured at 16 reference points on each abutment for a total of 3360 measurements. The effect of manufacturing techniques and number of units (groups) on discrepancy values was evaluated using a full factorial ANOVA model. Group and manufacturing technique effects were analyzed separately by 1-way ANOVA in case of significant interactions. Pairwise comparisons were evaluated using the Tukey post hoc test (alpha=.05). Results. The mean marginal discrepancy of 3-unit frameworks showed no statistically significant differences in the LW (35 mu m) and SLM (25 mu m) techniques; however, the frameworks manufactured by CAD-CAM milling (68 mu m) had the highest marginal discrepancy values (P<.001). The mean marginal discrepancy values were 40 tm (LW), 33 mu m (CAD-CAM milling), and 25 mu m (SLM) for 4-unit frameworks, and no significant differences were found among the manufacturing techniques. For 5-unit frameworks, CAD-CAM milling techniques had the widest mean marginal discrepancy values (85 mu m), and copings manufactured by the LW technique had the lowest mean marginal discrepancy values (36 mu m). For all manufacturing techniques, axial discrepancy values were not affected with respect to the unit number (P=.526). The highest internal discrepancy values were measured at the occlusal area in all groups. Conclusions. CAD-CAM milling had the poorest marginal fit values for 5-unit frameworks, whereas the LW technique demonstrated the best results. Unit number had no significant influence on the marginal and internal fit of the LW manufactured frameworks.