An evaluation of the marginal integrity of porcelain fused to base metal crowns after extension of porcelain to the border of the margin

In PFM restorations, marginal coverage of the metal framework with porcelain, without producing an overcontoured margin is necessary to enhance esthetics. The aim of this study was to evaluate the marginal integrity of a porcelain fused to base metal crown after extension of porcelain to the border of the margin. Thirty brass standard dies were prepared and divided into 3 groups of 10, based on different finishing lines: 90° shoulder, 1.2-mm-wide 135° slopping shoulder, 1.2-mm-wide and shoulder, 0.7-mm-wide with a 450 bevel, 0.5 mm-long. For each die a brass cap was fabricated according to the specific finishing line to accommodate a 0.5 mm thickness. Two metal copings were made on each die which were used as test and control samples. The marginal gap of each coping was measured at 4 points under a reflective microscope after casting and oxidation. Porcelain was applied and extended to the border of the margins in the test group and 0.5 mm short of the borders in the control group. Marginal gaps were measured once more at the same points after porcelain firing. Statistical analysis was performed using t-test. The marginal gaps of all copings increased after porcelain firing. Extension of porcelain to the border of the margins did not significantly increase the marginal gaps. These results indicated that porcelain can be extended to the border of the crown’s margin without producing a significant increase in the marginal gap.