Marginal Accuracy and Internal Fit of 3-D Printing Laser-Sintered Co-Cr Alloy Copings

Abstract

Laser sintered technology has been introduced for clinical use and can be utilized more widely, accompanied by the digitalization of dentistry and the development of direct oral scanning devices. This study was performed with the aim of comparing the marginal accuracy and internal fit of Co-Cr alloy copings fabricated by casting, CAD/CAM (Computer-aided design/Computer-assisted manufacture) milled, and 3-D laser sintered techniques. A total of 36 Co-Cr alloy crown-copings were fabricated from an implant abutment. The marginal and internal fit were evaluated by measuring the weight of the silicone material, the vertical marginal discrepancy using a microscope, and the internal gap in the sectioned specimens. The data were statistically analyzed by One-way ANOVA (analysis of variance), a Scheffe's test, and Pearson's correlation at the significance level of p = 0.05, using statistics software. The silicone weight was significantly low in the casting group. The 3-D laser sintered group showed the highest vertical discrepancy, and marginal-, occlusal-, and average- internal gaps (p < 0.05). The CAD/CAM milled group revealed a significantly high axial internal gap. There are moderate correlations between the vertical marginal discrepancy and the internal gap variables (r = 0.654), except for the silicone weight. In this study, the 3-D laser sintered group achieved clinically acceptable marginal accuracy and internal fit.

Keywords: CAD/CAM milled; Co-Cr alloy; internal fit; laser sintering; marginal accuracy.

Figure 1

The internal gap in the marginal, axial, and occlusal area for the casting, CAD/CAM milled, and laser sintered copings. The values represent the means and standard deviations (120 points/area of each group). The average internal gap values were calculated by the mean values of the marginal, axial, and occlusal area of each specimen in the group (5 measurements × 2 locations × 3 areas × 12 specimens × 3 groups, total 1080 points). The 3-D laser sintered group showed the highest average internal gap value which is significantly different from those of the casting and the CAD/CAM milled copings (p < 0.05). There was no significant difference between the casting and the milled group (p > 0.05).

Figure 2

(a) The cross-sectioned image and sizes of the abutment used in this study; (b) Schematic diagram of the fabricated Co-Cr crown-coping. The thickness of the coping was designed to be 0.5 mm, and the cement gap was set at 30 µm.

Figure 3

Workflow of the specimen preparation according to the fabrication methods.

Figure 4

Schematic sectioned view of reference points for evaluation (a) and measurement images (b) the marginal fit in this study. Marginal internal gap (MIG); the perpendicular measurement from the internal surface of the coping to the axial wall of the abutment at the end of the margin, Absolute marginal discrepancy (AMD); the angular combination of the marginal gap and the extension error which is measured from the margin of the coping to the cavosurface angle of the abutment. In this study, we measured the AMD as the two-dimensional vertical marginal gap. (5 points × 4 areas × 12 specimens × 3 groups, total 720 points).

Figure 5

(a) Specimens for internal gap measurements in the casting, CAD/CAM milled, and laser sintered groups; (b) Schematic view of six standardized measurement areas for internal gap: two marginal point areas (1, 1’), two axial point area (2, 2’), and two occlusal point area (3, 3’). Measurement location of the marginal gaps (1, 1’) was the center of chamfer-area, and the measurement location of axial gaps (2, 2’) was the center of the axial wall, starting at the end-point of the margin and continuing until the transition point with the occlusal area. Measurement location of occlusal gaps (3, 3’) included the center of the occlusal surface of the coping, on both sides of the access hole (5 points × 2 locations × 3 areas × 12 specimens/group). A small circle on the left side shows the internal gap measurements as the perpendicular distance between an outer surface of the abutment and the inner surface of the coping.