RESUMO
This study aimed to compare cyclic fatigue and torsional resistances of glide path creating instruments with different tapers and tip sizes. Two sizes (G1 and G2) from G-File system and three sizes (PathFile #1, #2, and #3) from PathFile system were used for torsional resistance and cyclic fatigue resistance tests (n = 10). The torsional resistance was evaluated at 2-, 3-, 4-, 5-, and 6-mm from the file tip by plotting the torsional load changes until fracture by rotational loading of 2 rpm. The cyclic fatigue resistance was compared by measuring the number of cycles to failure. Data were analyzed statistically using one-way ANOVA and Duncan's post-hoc comparison. The length of the fractured file fragment was also measured. All fractured fragments were observed under a scanning electron microscope (SEM). Although G-2 file showed a lower torsional strength than PathFile #3 at 2- and 3-mm levels (p < 0.05), they had similar ultimate strengths at 4-, 5-, and 6-mm levels (p > 0.05). The smaller files of each brand had a significantly higher cyclic fatigue resistance than the bigger ones (p < 0.05). PathFile #1 and #2 had higher fatigue resistances than G-files (p < 0.05). While G-1 had a similar fatigue resistance as PathFile #3, G-2 showed the lowest and PathFile #1 showed the highest resistances among the tested groups (p < 0.05). The SEM examination showed typical appearances of cyclic fatigue and torsional fractures, regardless of the tested levels. Clinicians may consider the instruments' sizes for each clinical case in order to get efficient glide path with minimal risk of fracture.
RESUMO
INTRODUCTION: Although nickel-titanium (NiTi) rotary instruments may produce a well- tapered root canal with a low tendency of aberrations, these are generally perceived to have a high fracture risk during use and may produce significant forces on root dentin during instrumentation, which may induce a dentinal defect or crack in the apical part of the root. This study compared mathematically the stress generated by the Self-Adjusting File (ReDent-Nova, Ra'anana, Israel) with conventional rotary instruments during the movement of 3 NiTi endodontic file designs in a curved root canal. METHODS: Stresses were calculated using finite element analysis. Three file designs with tip size ISO #20 were used in this study. Finite element models of ProFile #20/.06 (Dentsply Maillefer, Ballaigues, Switzerland) (a constant tapered shaft), ProTaper Universal F1 (Dentsply Maillefer) (a progressively changing taper shaft), and SAF 1.5 mm (a mesh shaft) were activated within a curved root canal model. The stress generations resulting from the simulated shaping movement were evaluated in the apical root dentin area. RESULTS: The SAF induced the lowest von Mises stress concentration and the lowest tensile principal stress component in root dentin. The calculated stress values from ProTaper Universal F1 and ProFile #20/.06 were approximately 8 to 10 times bigger than that of the SAF. CONCLUSIONS: Stress levels during shaping and the susceptibility to apical root cracks after shaping vary with instrument design. The design of the SAF may produce minimal stress concentrations in the apical root dentin during shaping of the curved canal, which may increase the chance of preservation of root dentin integrity with a reduced risk of dentinal defects and apical root cracking.
