Comparison of porcelain surface and fractural strength obtained by microwave and conventional oven glazing
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Purpose. The present study was designed to investigate the surface roughness and fractural strength achieved by glazing porcelain samples in a conventional and microwave oven. Aims. Specific aims were to study and compare surface roughness (measured by means of a profilometer), fractural strength (measured by Instron Universal testing machine) and cracks and porosities (studied with the help of SEM sections). Hypothesis. The research hypothesis was that there will be a difference in the surface character and fractural strength of hand polished, microwave glazed and oven glazed samples. Specifically, it is hypothesized that microwave glazed samples will show smoother surfaces and higher fractural strength than hand polished and conventional oven glazed samples. Materials and methods. Two commercial dental porcelains were used, Omega 900 (Vita) and d.Sign (Ivoclar-Vivadent. Inc.). A total of 60 samples were made, 30 for each type of porcelain. The samples were sintered in the conventional oven and finished with medium grit diamond to remove any irregularities. Ten samples from each type of porcelain were further divided into three groups: hand polished, microwave glazed and conventional oven glazed. Each specimen was evaluated for surface roughness and fractural strength. The surfaces and fracture sites of specimens were subjectively evaluated for cracks and porosities using scanning electron microscope. Results were analyzed statistically using two way analyses of variance (ANOVA). Results. A significant difference in surface roughness was found among the surface treatments of porcelains (F 2,54 = 3.98, p = 0.02). There was no significant main effect for porcelain (F 1,54 = 2.46, p = 0.12) and no significant interaction between surface treatment and porcelain type (F 2,54 = 1.11, p = 0.34). Follow up tests showed a significant difference in surface roughness between oven glazed and microwave glazed treatments (p < 0.05) but no significant difference between oven glazed and hand polished samples or between microwave glazed and hand polished samples. There was also a significant difference in fractural strength between the two porcelains (F 1,54 = 25.47, p < 0.001) but no significant difference in fractural strength by surface treatment (F 2,54 = 0.74, p = 0.48) and no significant interaction between porcelain type and surface treatment (F 2,54 = 2.27, p = 0.11). The surface of microwave glazed porcelain samples showed remarkably fewer voids and appeared to be smoother when compared to conventional oven glazed samples. Omega 900 samples appeared to have fewer voids and surface imperfections when compared to d.Sign samples. Conclusions. The surface character of microwave glazed dental porcelain was superior to oven glazed dental porcelain. Polishing alone without glazing was comparable to glazing with microwave or conventional oven. Irrespective of the manner in which the samples were treated, Omega 900 porcelain samples had an overall higher fractural strength than the d.Sign porcelain samples. When viewed under the scanning electron microscope, the external surface voids and imperfections seen in the samples of d.Sign porcelain exceeded those seen in the samples of Omega 900 porcelain. The presence of voids is suggested as one reason for lower values of fractural strength for d.Sign samples.