Effect of reciprocation usage of nickel‐titanium rotary files on the cyclic fatigue resistance

The use of reciprocating motion for a nickel‐titanium file has been claimed to increase its resistance to fatigue in comparison to continuous rotation. The purpose of this study was to investigate the effect using a reciprocating motion instead of continuous rotation for nickel‐titanium files on their cyclic fatigue resistance. Cyclic fatigue tests simulating clinical use with ProTaper F2 and ProFile #25/.06 instruments were carried out in an artificial stainless‐steel root canal with a 1.5 mm inner diameter, 5 mm radius and 60° angle of curvature. The instruments were driven using either continuous rotation or reciprocation until fracture. For the reciprocation, the rotation angles were set to 140° clockwise and 45° counterclockwise. The number of cycles to fracture was determined by measuring the time to fracture. Statistical analysis was performed using two‐way anova . There were no significant differences in the fracture cycles due to the use of reciprocation or continuous rotation for either instrument. The results found using reciprocation were comparable to those found using continuous rotation with regard to the cyclic fatigue resistance of nickel‐titanium rotary files.     

Mechanical behaviour of nickel‐titanium rotary endodontic instruments in simulated clinical conditions: a computational study

Aim To develop an accurate finite element (FE) model for studying rotary endodontic instruments and to demonstrate the usefulness of the FE method in improving the knowledge of the mechanical behaviour of these instruments during root canal preparation. Methodology An accurate geometrical model of a Ni‐Ti ProTaper F1 instrument was created. The interaction between the rotating instrument and differently shaped root canals during the insertion and removal procedure was studied using FE analyses. The complex thermo‐mechanical behaviour of the Ni‐Ti alloy was reproduced using an ad hoc computational subroutine. With the aim of demonstrating the enhanced performance of the shape memory alloy employment, the same analysis was performed on a ‘virtual’ ProTaper F1 made of stainless steel. Results The Ni‐Ti instrument operated in its pseudo‐elastic range and was able to recover its original shape and to follow the canal curvature without deviation. The radius and the position of the canal curvature are the most critical parameters that determined the stress in the instrument with higher stress levels being produced by decreasing the radius and moving from the apical to the mid root position. Conclusions The most demanding working conditions were observed in canals with sharp curves, especially in areas where the instruments had larger diameters. To prevent possible damage to instruments and fracture, it is advised that the instruments should be discarded following their use in such canals.     

Evaluation of cyclic flexural fatigue resistance of 25/0.04 and 25/0.06 twisted file rotary nickel‐titanium endodontic instruments

Twisted files (TFs) are rotary nickel‐titanium (NiTi) instruments that are produced with a newly developed manufacturing process that is supposed to improve their properties, especially flexibility and resistance to cyclic flexural fatigue failure. The aim of this study was to study the cyclic flexural fatigue failure resistance of tip size International Standards Organization 25 TFs with two tapers 0.04 and 0.06 and to compare them with the Profile (PF) rotary NiTi files of similar tip size and taper. Four groups of fifteen files were used in this study. TF (25/0.04), TF (25/0.06), PF (25/0.04) and PF (25/0.06) were tested using the cyclic flexural fatigue testing device. The time to failure during cyclic flexural fatigue testing was recorded. The mean time required for the instrument to fail under cyclic flexural fatigue testing was 235.5 ± 68 s for 25/0.04 TF, 188.5 ± 75 s for 25/0.06 TF, 180.3 ± 102 s for 25/0.04 PF and 156.3 ± 17 s for 25/0.06 PF. The difference between the time to failure of 25/0.04 TF and 25/0.06 TF was not statistically significant. The difference between the time to failure of TF and PF of similar tip size and taper was not statistically significant. The findings of this study indicate that size 25/0.04 and 25/0.06 TFs had similar resistance to cyclic flexural fatigue failure. In addition, TFs were not superior, in terms of resistance to cyclic flexural fatigue failure, to PF of similar tip size and taper.     

A comparison of cyclic fatigue resistance in reciprocating and rotary nickel-titanium instruments

The cyclic fatigue resistance of four nickel-titanium endodontic instruments was compared. K3, ProFile and GT Series X rotary instruments were examined along with SafeSiders reciprocating instruments. 30/0.04 instruments at 25 mm length were compared. Cyclic fatigue testing was conducted by operating instruments in artificially constructed stainless steel canals with 30° and 45° angles of curvature and 5 mm and 7.5 mm radii of curvature. The time and cycles to failure were recorded for 192 samples. Statistical analysis was performed with three-way anova and the Student-Neuman-Keuls multiple comparisons testing. With a 5 min maximum running time, no SafeSiders samples were observed to separate. ProFile and GT Series X instruments were found to be significantly more resistant than K3 instruments (P < 0.001) for all experiment groups. There was no statistical difference between ProFile and GT Series X files (P < 0.582).     

Dynamic cyclic and torsional fatigue resistance of TruNatomy compared with different nickel–titanium rotary instruments

This study evaluated the dynamic cyclic and torsional fatigue resistance of recently introduced TruNatomy instruments (TRN) and compare with HyFlex CM (HFC), Vortex Blue (VB) and FlexMaster (FM) instruments. Size 20, 0.04 taper of TRN, HFC, VB and FM instruments was tested for dynamic cyclic and torsional fatigue resistance. Dynamic cyclic fatigue resistance was evaluated using an artificial canal with a radius of 5 mm and a 90° angle of curvature. The number of cycles to failure (NCF) was calculated. The dynamic torsional fatigue resistance was evaluated by holding a 5 mm of the tip of each instrument in a metal block with composite resin. Torsional fatigue resistance was recorded by counting the number of load applications before fracture for each instrument. The HFC instruments had greater fatigue resistance than VB, TRN and FM. FM had a higher resistance to torsional stress than TRN, HFC and VB instruments.     

Surface roughness and cyclic fatigue resistance of reciprocating and novel rotary instruments after use in curved root canals

This study evaluated surface wear and resistance to fracture of reciprocating and novel rotary instruments after use in curved canals. Twenty new instruments (n = 5/each group): Reciproc Blue, WaveOne Gold, XP-endo Shaper (XPS) and TruNatomy were used twice for the instrumentation of simulated curved canals in plastic blocks. The instrumentation was performed according to the manufacturer's instructions at body temperature. Surface roughness was evaluated, using a profilometer, before instrumentation and twice after the instrumentation. Instruments were, then, subjected to a cyclic fatigue test to evaluate the time to failure. Differences were assessed using ANOVA and post hoc tests (α = 0.05). The XPS instruments showed significantly higher values of roughness compared with TruNatomy at every stage, especially after the second use (p < 0.008). Reciprocating instruments showed superior cyclic fatigue resistance compared with novel rotary instruments (p < 0.05). TruNatomy showed minimal surface roughness, and XPS exhibited significantly higher surface roughness.     

Extrusion of debris after use of rotary nickel‐titanium files with different pitch: A pilot study

The aim of this investigation was to quantitatively compare the amount of debris extruded apically from curved canals when using rotary nickel‐titanium files with different pitches and sequences of use under the same preparation technique. Thirty‐six extracted human maxillary and mandibular molars with curved root canals were used. They were divided into two groups of 18 teeth each prepared by one of two different sequences with NRT files (MANI Inc., Tochigi, Japan). Each group was further divided into three subgroups according to file pitch length (short, medium and long). The debris extruded apically was collected in glass vials. The liquid inside was dried and the debris in each group was weighed and compared. Two‐way analysis of variance and Fisher's PLSD test were used to evaluate differences between the extruded debris weights among the experimental groups. No statistically significant difference was observed between two sequences. There were significant differences in sequence 1 between the short pitch group and the others. Also, there were significant differences between the long pitch and others in sequence 2, except between the short and medium pitches. In conclusion, short pitch design resulted in significantly less amount of debris extrusion compared with the other pitch lengths.     

Movement kinematics and cyclic fatigue of NiTi rotary instruments: a systematic review

The aim of this review was to provide a detailed analysis of the literature concerning the correlation between different movement kinematics and the cyclic fatigue resistance of NiTi rotary endodontic instruments. From June 2014 to August 2015, four independent reviewers comprehensively and systematically searched the Medline (PubMed), EMBASE, Web of Science, Scopus and Google Scholar databases for works published since January 2005, using the following search terms: endodontics; nickel–titanium rotary files; continuous rotation; reciprocating motion; cyclic fatigue. In addition to the electronic searches, manual searches were performed to include articles listed in the reference sections of high‐impact published articles that were not indexed in the databases. Laboratory studies in English language were considered for this review. The electronic and manual searches resulted in identification of 75 articles. Based on the inclusion criteria, 32 articles were selected for analysis of full‐text copies. Specific analysis was then made of 20 articles that described the effects of reciprocating and continuous movements on cyclic fatigue of the instruments. A wide range of testing conditions and methodologies have been used to compare the cyclic fatigue resistance of rotary endodontic instruments. Most studies report that reciprocating motion improves the fatigue resistance of endodontic instruments, compared to continuous rotation, independent of other variables such as the speed of rotation, the angle or radius of curvature of simulated canals, geometry and taper, or the surface characteristics of the NiTi instruments.     

镍钛机用器械循环疲劳的研究进展

镍钛机用器械与不锈钢器械相比较具有更佳的柔韧性,应用于弯曲根管预备能取得更好的成形和清洁效果,缩短预备时间。但器械折断一直是有待解决的重要问题之一。器械折断的发生会增加根管治疗的难度,甚至导致治疗失败,而器械使用中的循环疲劳是器械折断的主要机制。影响镍钛机用器械循环疲劳的因素很多,包括器械自身因素和外部因素。本文对近年来镍钛机用器械循环疲劳的研究进展进行综述,重点讨论器械循环疲劳的影响因素。