الفهرس 
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Abstract
The resistance to deformation and/or fracture of four different nickel-titanium rotary systems was evaluated. The tested systems were: Group (1): ProTaper (Dentsply Maillefer, Ballaigues, Switzerland), Group (2): RaCe (FKG Dentaire, La-Chaux-de-Fonds, Switzerland), Group (3): K3 (Kerr Sybron Endo, Orange, CA, USA), Group (4): Liberator (Miltex Inc, York, PA). Extracted human permanent mandibular first and second molars with root canals curvature between 25 degrees and 35 degrees were selected. Before use, the teeth were decontaminated by immersion in 5.25% sodium hypochlorite for 30 min. Teeth were then cleaned and scaled to remove any surface deposits and/or calculus. All teeth were cleaned and shaped according to the assigned group following the to the manufacturer instructions. All root canals were prepared throughout the study by the same operator. Final apical file size of #30 file was chosen for all the tested rotary systems. Instruments were continuously inspected during usage for any signs of deformation using a magnifying lens 16X. Selected samples were examined under the scanning electron microscope at magnification between 30X-1500X.
The following data was collected:
iv. The total number of root canals prepared by each rotary file of each group until deformation and/or fracture.
v. The distribution of deformation and/or fracture of all the tested files after preparation of 5, 10, 15 and 20 root canals.
vi. The total deformation and/or fracture percent for all the tested systems.
All data was tabulated and subjected to statistical analysis using one way analysis of variance (ANOVA) at P < 0.05. Significant differences between groups were tested by Duncan’s test at P ≤ 0.05.
The results showed that K3 files were found to be the superior one regarding resistance to deformation and fracture while the Liberator files were the worst. The ProTaper and the RaCe systems ranked moderately. The highest mean number of root canals prepared until deformation and/or fracture was for the K3 group (13.8±0.31/box), followed by the RaCe group (8.6±1.2/box) then the ProTaper group (8.3±0.6/box) and lastly the Liberator group (6.1±0.56/box). Statistically, the mean number of canals prepared by files of K3 group was significantly higher than those of the other three groups. On the other hand, the mean number of canals prepared by files of Liberator group was significantly lower than those of the other three groups. There was no significant difference between the mean number of canals prepared by files of RaCe group and those of ProTaper group.
The results also showed that the deformation rate was higher than the fracture rate for the ProTaper, RaCe and K3 rotary systems. On the other hand, the Liberator system showed a higher fracture rate than the deformation rate.
Based on the results of the present study, the following conclusions could be drawn:
1. The geometrical designs of the tested nickel-titanium rotary systems have a direct influence on their rate of deformation and/or fracture.
2. The K3 nickel-titanium rotary system showed superior resistance to deterioration while the Liberator system was inferior.
3. Macroscopic examination of fluted nickel-titanium rotary systems can help discard deformed files before being fractured.