The effect of different finish line and convergence angle on the marginal fit of zirconia all-ceramic restorations

Background and Objectives: The precise degree of marginal fit is one of the most significant criteria for the durable success of all-ceramic restorations. Since the finishing line designs and preparation angles can affect the marginal fit, the aim of the present study was to compare the effect of different finish line and convergence angle on the marginal fit of zirconia all-ceramic restorations. Materials and methods: In the present in vitro study a total of four brass dies were designed with Auto CAD software and manufactured with CNC machine (Siemens, SI Numerik 802D-SL; USA) as follow: Die A: Convergence angle of 12 degrees and radial shoulder margin, Die B: Convergence angle of 6 degrees and sloping shoulder of 135 degrees, Die C: Convergence angle of 6 degrees and radial shoulder margin, Die D: Convergence angle of 12 degrees and sloping shoulder of 135 degrees. A total of 40 plaster dies were divided in four groups of 10 (n=10), ten impressions were carried out by the double impression technique with a poly vinyl siloxane impression material for each master die. The impression was poured with type IV Plaster. A ceramic restoration was made for each plaster die then the marginal gaps were measured with SEM. Data were analysed using the two-way analysis of variance (ANOVA) and SPSS ver. 16. Results: The lowest mean (+/- standard deviation) marginal gap was observed in group A (31.9423), group D (36.03), group B (40.9537), group C (46.703), respectively. Different convergance angle had significant effect on marginal fittness (P <0.05), but marginal design had no significant effect on marginal fitness (p>0.05) and both of them together could affect on marginal fitness (p<0/05). Conclusion: The type of the finish line did not influence the marginal fit. The convergence angle influences the distance from the edge of the crown to the edge of the tooth finish line. It seems that use the convergance angle of 12 degree and radial shoulder margin can reduce the marginal gap in zirconia all-ceramic restorations.