Evaluation of the porcelain bonding of metal substructures fabricated by 3 different manufacturing techniques, in-vitro comparison of the effect of porcelain firing cycles on internal and marginal fit of crowns

The method of producing metal substructure by milling soft metal with CAD / CAM is presented as an alternative method to metal substructure production methods like conventional casting and laser sintering. This new technology has advantages such as reducing production time and cost, and milling form a block using the existing CAD / CAM machine without necessitating the purchase of an additional machine such as laser sintering. The aims of this study were to examine 1) the internal and marginal fit 2) the effect of the porcelain firing cycles on internal and marginal fit of restorations fabricated using casting, laser sintering and soft metal milling methods 3) evaluate metal-porcelain bond strength. Single unit metal substructures were produced by using soft metal milling (n=15), laser sintering (n=15) and conventional casting (n=15) methods on a right first maxillary molar metal die. The internal fit was measured using the silicon replica method and the marginal adaptations were measured by computerized light microscopy and software from 5 standard points on each surface. Then porcelain was applied on metal substructures with the help of a specially prepared index in order to standardize the porcelain thicknesses. The measurements were repeated following the porcelain application and glazing. Statistical analyzes were performed using Three-way ANOVA and Tukey-HSD (α = 0,05) tests. According to statistical analysis of marginal gap measurement values, statistically significant difference was found between all three production methods (p=0,000). The mean marginal gap values of the crowns were 31,4±13,8 μm for soft metal milling, 20,8±14,4 μm for conventional casting and 7,3±6 μm for laser sintering. There was a statistically significant difference (p = 0,000) between the metal and porcelain, metal and glaze measurement steps and no significant difference between the porcelain and glaze measurement steps (p = 0.072). In all three groups there was some increase in the marginal gap after porcelain application. A statistically significant difference was found between the three production methods for internal gap measurement values (p = 0,000). The mean internal gap values were 37±5,7 mg for conventional casting, 28±6,1 mg for laser sintering and 21,6±7,3 mg for soft metal milling. There was no statistically significant difference between the porcelain firing cycles (p>0,05), but the internal gap increased slightly for all compared groups. According to the statistical analysis of the fracture strength values, no statistically significant difference was found between all three production methods (p=0,266). The mean values of the fracture force values of the crowns (± standard deviation) were 360,9 ± 155,0 N for conventional casting, 340,7 ± 177,8 N for laser sintering and 441,1 ± 192,9 N for soft metal scraping, respectively . In conclusion, the soft metal milling method in our study had the best internal fit but the worst marginal fit. The results were well below the clinically acceptable values. All 3 metal substructure production techniques had clinically acceptable marginal gap and internal gap values. The porcelain firing cycle did not cause a statistically significant difference in the internal gap. However, a statistically significant increase in the marginal gap was observed. However, all values were within clinically acceptable values. When the fracture strengths of the specimens were evaluated, there was no statistical difference between the three methods. The test method used for testing metal ceramic bond strenght has to be improves.