Marginal and internal discrepancies of zirconia copings: Effects of milling system and finish line design

Context: Discrepancies at the abutment/crown interface can affect the longevity of zirconia restorations. Aim: The aim was to evaluate the marginal and internal discrepancies (MD and ID) of zirconia copings manufactured by two milling systems with different finish lines. Materials and Methods: Three aluminum-master-dies (h = 5.5 mm; Ψ =7.5 mm; 6), with different finish lines (large chamfer [LC]; tilted chamfer [TC]; rounded shoulder [RS]) were fabricated. Twenty impressions were made from each master die and poured. Sixty zirconia copings were manufactured and divided according to the factors finish line and milling system (n = 10): CAD<inf>LC</inf> = Computer-aided design/computer-aided manufacturing (CAD/CAM) + LC; CAD<inf>TC</inf> = CAD/CAM + TC; CAD<inf>RS</inf> = CAD/CAM + RS; MAD<inf>LC</inf> = manually aided design/manually aided manufacturing (MAD/MAM) + LC; MAD<inf>TC</inf> = MAD/MAM + TC; and MAD<inf>RS</inf> = MAD/MAM + RS. For MD analysis, each coping was fixed, and the distance between the external edges of the coping and the edge of the cervical preparation was measured (50 points). Using the same copings, the ID of each coping was evaluated, by the replica technique, at 12 points equally distributed among the regions (n = 10): Ray (R), axial (A), and occlusal (Occl). The measurements were performed by optical microscopy (&KHgr;250). The data (μm) were subjected to parametric and non-parametric statistical analyses. Results: For the MAD/MAM system, the finish line (P = 0.0001) affected significantly the MD median values (μm): LC = 251.80a, RS = 68.40a and TC = 8.10b (Dunn′s test). For the CAD/CAM system, the median MD values (μm) were not affected by the factor finish line (P = 0.4037): LC = 0.82a, RS = 0.52a, and TC = 0.89a. For the ID, it was observed interaction between the finish line types and the region (P = 0.0001) and between region and milling system (P = 0.0031) (RM-ANOVA). Conclusions: The CAD/CAM system presented lower MD values, regardless the finish line. However, the MAD/MAM system showed ID values smaller than those of CAD/CAM.

Keywords

Computer-aided design/computer-aided manufacturing system, Internal discrepancy, Manually aided design/manually aided manufacturing system, Marginal discrepancy, Replica technique