Marginal and internal adaptation of milled cobalt-chromium copings

The application of computer-aided design and computer-aided manufacturing (CAD/CAM) systems to produce complete coverage restorations with different materials continues to increase. To date, insufficient information is available regarding the adaptation of recently introduced milled cobalt-chromium (Co-Cr) copings for metal ceramic restorations.

The purpose of this in vitro study was to evaluate the marginal and internal fit of milled Co-Cr copings produced by CAD/CAM with 2 different marginal preparation designs.

Four master dies were developed from 2 ivorine central incisors and 2 ivorine maxillary molars, 1 of each prepared with a 0.8-mm chamfer and a 1.2-mm rounded shoulder. These 4 groups of teeth were replicated with polyvinyl siloxane and used as templates to fabricate epoxy dies (n=10) for each of the 4 groups; a total of 40 epoxy resin dies. Cobalt-chromium copings of standard thickness (0.4 mm) were fabricated for each die with CAD/CAM technology. Next, the working dies were scanned with a 5-axis laser scanner to produce a 3-dimensional model. A thin layer of low-viscosity polyvinyl siloxane material was placed inside each coping and seated on the die until the material set. Copings were removed from the dies, leaving the polyvinyl siloxane intact, and these silicone-coated dies were scanned. The software superimposed the 2 scans, and the marginal openings and internal fit were measured at multiple locations. The marginal opening was determined at 4 locations: mid-buccal (mB), mid-lingual (mL), mid-mesial (mM), and mid-distal (mD), and the mean of these 4 measurement locations was referred to as the group variable "edge." The internal occlusal adaptation was measured at the midpoint from buccal to lingual and mesial to distal locations and referred to as mid-occlusal (mO). Means and standard deviations for edge (marginal adaptation) and mO were calculated for each of the 4 groups. A 2-sample t test was performed to detect differences among groups. A regression analysis was done to evaluate the interaction between the variables mO and edge (α=.05).

Significantly smaller mean marginal openings (P=.017) were observed overall for the chamfer marginal design (anterior chamfer: 61 ±41 μm; posterior chamfer: 52 ±27 μm) compared with the shoulder design (anterior shoulder 103 ±49 μm, posterior shoulder 113 ±110 μm). The anterior chamfer had a statistically significant (P=.055) smaller mean marginal opening (61 ±41 μm) than the anterior shoulder (103 ±49 μm). No statistically significant differences (P=.119) were found between the posterior chamfer and posterior shoulder. The internal adaptation at the mO location was not significantly different among all 4 groups (P>.05). However, a regression analysis demonstrated a strong correlation (R=.842; P<.001) between the occlusal seat (mO) and marginal opening, with the smaller mean marginal opening of the chamfer design coinciding with the smaller occlusal seat values (61μm; mO: 182 μm) anterior chamfer; (52 μm; mO: 172 μm) posterior chamfer versus (103 μm; mO: 235 μm) anterior shoulder; (113 μm; mO: 242 μm) posterior shoulder.

The milled Co-Cr copings produced with a CAD/CAM system in this study demonstrated clinically acceptable marginal fit in the range of 52 to 113 μm before ceramic application.