Background/purpose: The manufacturer claims that Twisted Files (TFs) are superior to file systems made by the traditional grinding method due to their cyclic fatigue resistance, flexibility, and better cutting efficiency. The aim of this study was to compare the shaping ability of TFs with instruments produced by a traditional NiTi grinding process (RevoS and ProTaper). Materials and methods: A total of 60 simulated resin blocks were divided into three experimental groups, each comprised of 20 resin blocks that were prepared with TFs, RevoS, and ProTaper using a crown-down technique. The preparation shape was assessed with a computer image analysis program on superimposed pre-and postoperative images. Material removal was measured at 10 points beginning 1 mm from the end-point of the canal. Mean total widths and outer and inner width measurements were determined on each central canal path, and the data were statistically analyzed using the Kruskal-Wallis and Mann-Whitney U-tests. Results: The Revo-S and ProTaper instruments removed more material from the inner side of the curvature compared with the TF instruments. Differences among the three rotary NiTi systems were statistically significant (P < 0.05) except for the first 3 mm and the last 2 mm. On the outer side of the canal, both the ProTaper and Revo-S instruments removed more material than the TF instruments in the apical third (P < 0.05). Conclusion: According to the results of this study, TFs respected the original canal curvature better than did ProTaper and RevoS rotary NiTi instruments. TF instruments also provided a more-centered apical preparation of the simulated canals at the apical third.

The aim of this study was to investigate the influence of denture cleansing solutions on micro hardness of a heat-cure acrylic resin.Materials and Methods: Fifty specimens (15 mm × 15 mm × 3 mm) were prepared with a heat-cure acrylic resin. Specimens were randomly distributed into groups and immersed in the following solutions: 0.5% sodium hypochlorite (NaOCl), 1% NaOCl, alkaline peroxide tablet solution and, 0.2% chlorhexidine gluconate. The control group was immersed in distilled water. An immersion was detected 20-minute for all solutions. The microhardness value of each specimen was obtained using the microhardness testing machine recording the Vickers hardness number before the beginning of the immersion cycles and after completion of 720 immersion cycles and compared. Data were imported to SPSS 15.0 software and were analyzed by paired t-test and One-Way ANOVA and Tukey HSD test were used for comparison among groups (p< 0.05). Results:The results show that alkaline peroxide tablet solution caused the statistically significant reduction of the microhardness of the acrylic resin. Other solutions tested in this study did not cause a significant effect on the microhardness of the acrylic resin.Conclusions: It was concluded that immersion in distilled water, chlorhexidine gluconate, 0.5% NaOCl and, 1% NaOCl solutions, simulating 720 days of 20 min daily soaking did not cause clinically significant adverse effects on the microhardness of the heat-polymerized acrylic resin, except for the alkaline peroxide tablet solutions.

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