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.     

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.     

Influence of canal curvature on the amount of apically extruded debris determined by using three‐dimensional determination method

The aim of this study was to introduce a new three‐dimensional curvature classification method to evaluate the canal curvature and analyse its effect on the amount of debris extrusion during reciprocating preparation. Freshly extracted mandibular molar teeth were collected. After performing access cavities, periapical radiographs were taken on both mesio‐distal and bucco‐lingual planes of each tooth using a digital sensor. The radii of the curvature were calculated and a new three‐dimensional classification method was used to classify canal curvature, based on radius as follows: Three‐dimensionally slight, moderate and severe curve root canal. Fifteen teeth for each curvature sort were randomly chosen for canal instrumentation. The extruded debris was collected in pre‐weighed Eppendorf tubes and calculated. A significant difference was noted between root canals with slight and severe three‐dimensional curvatures (P < 0.001). It can be concluded that root canal curvature plays a significant role in the amount of extruded debris.     

Comparison of curved canals preparations using profile, GT and Hero 642 rotary files

This study compared preparations of curved canals, using Profile, GT file and Hero 642 rotary files. A total of 30 mesiobuccal canals of extracted first and second mandibular molars were used in this study. The teeth were embedded in resin and a pre-operative radiograph was taken. The specimens were then randomly divided into three instrument groups. Pre- and post-instrumentation radiographic images were scanned and superimposed using the VISI CAD/CAM program. At nine horizontal sections at 1 mm increments from the apex, it calculated pre- and post-instrumentation cross sectional areas (mm(2)) and enlargements to the inner aspect and outer aspects (mm). A Kruskal-Wallis test found no statistical difference among the groups in terms of total areas removed by each instrument (P > 0.05). It also found no significant difference in any group between the total of the inner and outer enlargements (P > 0.05). However the GT file removed significantly more material from the outer side of the canal than the inside at sections 3-5, and the Hero removed significantly more material from the outer side at sections 7-9 (P < 0.05). All instruments produced preparations that maintained original canal curvatures.     

Comparative evaluation of different instrument kinematics on contact percentage,dentin removal and apical debris extrusion in oval canals

The aim of this study was to compare four different instrument kinematics in oval canals. Distal root of sixty extracted first mandibular molars with single oval canal were allocated randomly to four groups (n = 15) namely protaper universal [PTU] (Rotary), self-adjusting file [SAF] (Adaptive), TruShape [TS] (Rotary) and WaveOne [WO] (Reciprocating). The percentage of untouched and touched surfaces of the root canal was evaluated using micro-computed tomography (MCT). Pre- and post-instrumentation weight was assessed using digital microbalance for determining the amount of dentin removed and apical extrusion of debris. The results were tabulated, and their mean difference was statistically analysed using one-way ANOVA and Tukey's HSD test. PTU and SAF exhibited significantly less mean percentage of untouched surface compared to TS. TS files removed less dentin and extruded less debris in long oval canals.     

Comparison of cyclic fatigue resistance of novel nickel‐titanium rotary instruments

New files (ProTaper Next/HyFlex/OneShape) are made from novel nickel‐titanium (NiTi) alloys/treatments. The purpose of this study was to compare the cyclic fatigue resistance of these new instruments with that of Revo‐S instruments. Four groups of 20 NiTi endodontic instruments were tested in steel canals with a 3 mm radius and a 60° angle of curvature. The cyclic fatigue of the following NiTi instruments with a tip size 25 and 0.06 taper that were manufactured with different alloys was tested: ProTaper Next X2 (M‐Wire), OneShape (conventional NiTi), Revo‐S Shaping Universal (conventional NiTi) and HyFlex 25/0.6 (controlled memory NiTi wire). A one‐way anova and post‐hoc Tukey's test (α = 0.05) revealed that the HyFlex files had the highest fatigue resistance and the Revo‐S had the least fatigue resistance among the groups (P < 0.001).