Tehran University of Medical Sciences Frontiers in Dentistry 2676-296X 21 0 2024 06 02 1 8 Nioosha Sarabi Department of Operative Dentistry, School of Dentistry, Tehran University of Medical Science, Tehran, Iran Mahshid Mohammadi-Bassir Department of Operative Dentistry, School of Dentistry, Shahed University, Tehran, Iran Forough Fadavi Private Practitioner, Tehran, Iran Mohammad Bagher Rezvani Department of Operative Dentistry, School of Dentistry, Shahed University, Tehran, Iran Fahimeh Dehestani-Ardakani Department of Operative Dentistry, School of Dentistry, Shahed University, Tehran, Iran Hossein Labbaf Department of Endodontics, School of Dentistry, Shahed University, Tehran, Iran 2021 11 29 2022 11 14 Objectives: Evaluation of the effect of grinding on flexural strength of zirconia after low temperature degradation (LTD) and pH-cycling. Materials and Methods: Sixty-four bar-shaped specimens of yttria-stabilized tetragonal polycrystalline zirconia were milled, sintered, wet-polished, and divided into 8 groups (N=8). The four control groups were not aged while artificial aging was performed in the 4 experimental groups in three steps including  LTD in steam for 40h, pH-cycling, and tooth brushing for artificial aging. All groups underwent surface preparations as follows: standard polishing without surface treatment (Sp), grinding with a blue-yellow band diamond instrument (Gr); grinding with a diamond rotary instrument (DRI) and then over-glazing (Gl); grinding with a DRI followed by two-step intraoral polishing (Po); standard polishing and aging (Sp-Ag); grinding and aging (Gr-Ag), grinding, over-glazing and aging (Gl-Ag); and grinding, polishing and aging (Po-Ag). Monoclinic content was assessed in one specimen of each group by X-ray diffraction (XRD). The 3-point flexural strength test was performed in a universal testing machine. The results were analyzed with two-way ANOVA and Tukey’s test (α=0.05). Results: Mean flexural strength (Mpa) was significantly higher in groups Gr and Po compared to group Sp (both, P<0.0001) and group Gl (both, P<0.0001). In XRD analyses, the highest monoclinic phase before aging was observed in group Gr (12.6%), and after aging in group Gr-Ag (51.2%). Conclusion: Grinding and polishing increased the flexural strength, while glazing did not exhibit any significant effect on this parameter.  Furthermore, aging did not adversely affect flexural strength. https://fid.tums.ac.ir/index.php/fid/article/view/4299 https://fid.tums.ac.ir/index.php/fid/article/download/4299/2047