Effect of Two Polishing Systems on Surface Roughness, Topography, and Flexural Strength of a Monolithic Lithium Disilicate Ceramic
Authors: Mohammadibassir Mahshid , Rezvani Mohammad Bagher , Golzari Hossein , Moravej Salehi Elham , Fahimi Mohammad Amin , Kharazi Fard Mohammad Javad 
Published: March 2017

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 × 4 × 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 μm) 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 (μm) 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 (α = 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).