Effect of Two Polishing Systems on Surface Roughness, Topography, and Flexural Strength of a Monolithic Lithium Disilicate Ceramic

摘要

Purpose To evaluate the effect of overglazing and two polishing procedures on flexural strength and quality and quantity of surface roughness of a monolithic lithium disilicate ceramic computer-aided design (CAD) after grinding. Materials and Methods This in vitro study was conducted on 52 partially crystalized bar-shaped specimens (16 x 4 x 1.6 mm) of monolithic lithium disilicate ceramic. The specimens were wet polished with 600-, 800-, and 1200-grit silicon carbide papers for 15 seconds using a grinding/polishing machine at a speed of 300 rpm. Then, the specimens were crystalized and glaze-fired in one step simultaneously and randomly divided into four groups of 13: (I) Glazing group (control); (II) Grinding-glazing group, subjected to grinding with red band finishing diamond bur (46 mu m) followed by glazing; (III) Grinding-D+Z group, subjected to grinding and then polishing by coarse, medium, and fine diamond rubber points (D+Z); and (IV) Grinding-OptraFine group, subjected to grinding and then polishing with a two-step diamond rubber polishing system followed by a final polishing step with an OptraFine HP brush and diamond polishing paste. The surface roughness (Ra and Rz) values (mu m) were measured by a profilometer, and the mean values were compared using one-way ANOVA and Tamhane's test (post hoc comparison). One specimen of each group was evaluated under a scanning electron microscope (SEM) for surface topography. The three-point flexural strength values of the bars were measured using a universal testing machine at a 0.5 mm/min crosshead speed and recorded. The data were analyzed using one-way ANOVA and Tamhane's test (alpha = 0.05). Results Statistically significant differences were noted among the experimental groups for Ra, Rz (p 0.0001), and flexural strength values (p 0.009). The lowest Ra and Rz values were found in the grinding-OptraFine group (0.465 +/- 0.153), which were significantly lower than those in glazing (p 0.03) and grinding-glazing (p 0.001) groups. The Ra and Rz values were not significantly different between the two polishing systems (p = 0.23 and p = 0.25, respectively). The highest flexural strength was found in the glazing group (283.350 +/- 49.854 MPa) without significant differences compared to grinding-glazing (p = 0.98) and grinding-OptraFine groups (p = 0.86). The lowest flexural strength was found in grinding-D+Z group (225.070 +/- 17.299), which was significantly different from the value in glazing (p 0.03) and grinding-glazing (p 0.04) groups. SEM analysis of polished surfaces revealed regular morphology with some striations. Conclusions The OptraFine system created smoother and more uniform surfaces in terms of quantity (p 0.03 for Ra, p 0.01 for Rz) and quality of roughness compared to glazing. The flexural strength of lithium disilicate ceramic after polishing with the OptraFine system was similar to that after glazing (p = 0.86). Despite similar surface roughness after polishing with the two systems, the D+Z system reduced the flexural strength of ceramic (p 0.03).