In vitro evaluation of the biaxial flexural strength of monolithic zirconia fabricated with CAD/CAM systems after different grinding procedures

Monolithic zirconia is frequently used in crown and bridge restorations by eliminating the risks of chipping of the veneered porcelain However, although these materials are produced by CAD/CAM systems, clinical adjustment still necessary. Grinding procedures are frequently apply by clinicians using high-speed and low-speed rotary instruments with different types of burs with or without water cooling. The purpose of this study is to evaluate the changing mechanical properties of monolithic zirconia materials after grinding procedures. Disk-shaped specimens (15 mm in diameter and 1.20± 0.2 mm in height) with a smaller cylinder at the center of the each disk (3 mm in diameter and 1 mm in height) were prepared by using monolitihic zirconia materials (Amann Girbach-Ceramill Zolid White, Zirkonzahn-Prettau, GC-Inıtıal Zr Disk) for the experimental groups. For the control groups, there were no special cylinder design at the center. Grinding procedures were applied by using 3 different type of burs (Meisinger 881Z4 (medium) , Meisinger Z732 (medium), Meisinger 640 (medium)) with and without water cooling until the cylinder at the center lost it's thickness during this process and the temperature values were recorded. After grinding, the surface roughness values were measured and the material was subjected to biaxial flexural test and vickers microhardness test. One-way variance analysis is used in comparisons of variables, Newman Keuls multiple comparison test in sub-group comparisons, one-way variance analysis in cross-group comparisons, Tukey multiple comparison test in sub-group comparisons, independent t-test in comparison of binary groups were used. The results were evaluated at p<0.05 level. When the results of our study were evaluated, the roughness averages of the untreated control groups were statistically lower than the roughness averages of all the experimental groups. The mean roughness values of the groups in which the aerotor and 881Z4 bur used in all conditions (with and without water cooling) were found to be significantly higher than the other experimental and control groups. When the biaxial flexural strengths were evaluated, the biaxial flexural strength of the experimental groups in which the grinding process was applied without water cooling had lower mean values than the experimental groups that the grinding process was applied under water cooling. In all monolithic zirconia materials, there was no statistically significant difference between the biaxial flexural strengths of experimental groups in all conditions (with and without water cooling). In our study, the highest temperature values without water cooling was measured 104,24 ± 13,27 oC and with water cooling was measured 27,91 ± 1,84 oC. In all monolithic zirconia materials, the vickers microhardness values of experimental groups which were grinded with aerotor and 881Z4 burs in all condition (with and without water cooling) were found to be significantly higher than the other experimental groups.