Introduction
The process of manufacturing nickel-titanium (NiTi) rotary endodontic instruments was first introduced in 1988. 1 NiTi instruments offer great flexibility, superelasticity, superior cutting ability, and more centered root canal preparation. Therefore, they reduce the risk of canal aberrations and the time required for root canal preparation. NiTi instruments maintain the original canal shape better than a stainless steel file. 2, 3
To increase the functionality and fracture resistance of tools used in root canal preparation, nickel-titanium (NiTi) rotary files' metallurgy and designs have seen considerable advancements recently. 4 Lately, a variety of new thermomechanical treatments for NiTi have been created to get around some of the issues with the conventional NiTi alloy, such as instrument separation and distortion. 5, 6 When compared to typical superelastic Ni-Ti instruments, the producers assert that these new thermal treatments provide instruments that are more resistant to cyclic fatigue. 7, 8 The two distinct crystal phases of the NiTi alloy, martensite and austenite, are temperature-dependent. 9
These two stages' characteristics differ greatly from one another. Heat causes martensitic NiTi to turn into austenite, and once this process is complete, the alloy has acquired shape memory and manifests superelastic properties. 10 The austenite finish temperature is the temperature at which this phenomena is complete. 10
Since they hinder the disinfection of the entire root canal system, shattered fragments from NiTi endodontic rotary files within the root canal system pose a challenge for the operator. 11
However, researchers have focused their attention on the NiTi alloys, new surface treatments, and design improvements as crucial variables in the fracture resistance of NiTi endodontic rotary files. Numerous factors, such as root canal shape, instrument geometry, rotational speed, torque, disinfection cycles, the number of clinical uses, and the angle and radius of the root canal system, can end up causing NiTi endodontic rotary files to separate.12, 13, 14, 15 Figure 1a.
The physical characteristics of NiTi endodontic rotary files are therefore improved by surface treatments such electropolishing, ion implantation, cryogenic treatment, and heat treatments, improving their cyclic fatigue resistance. 16
Instruments made with ProTaper Gold (PTG; Dentsply Maillefer, Baillagues, Switzerland) superior metallurgy have recently been created using patented heat treatment technology. ProTaper Next (PTN; Dentsply Maillefer) files have the same geometry as ProTaper Next instruments. High Af temperatures and a 2-stage unique transformation process are characteristics of PTG files. 17, 18
Newer generation NiTi file systems still utilised in continuous rotational motion are called HyFlex EDM (HEDM; Coltene/Whaledent, Altstätten, Switzerland). The HyFlex CM (HCM; Coltene/Whaledent), the first file system created using the controlled memory (CM) technology, is represented by HEDM as a development of that system. 19 (Figure 1b)
The kinematic properties of files remained unchanged under the WOG system. To improve the flexibility of the files, the cross section, size, and geometry of the WOG system were altered. 20 Moreover, gold was heated to strengthen the WOG's resilience to cycle fatigue21. (Figure 1c)
Aim
The aim of this systematic review is to evaluate and compare the cyclic fatigue resistance of different nickel titanium rotary instruments.
Materials and Methods
This review was based on the PRISMA guidelines. The protocol was registered in PROSPERO International Prospective Register of Systematic Reviews CRD42021239408 Eligibility criteria:
The controlled vocabulary (MeSH terms) and free terms were used to define the search strategy based on the elements of PICOS question as follows as:
Population (P): Artificial root canals.
Intervention (I): Niti rotary instruments.
Comparison (C): Root canals prepared with any of rotary NiTi instruments.
Outcome (O): Cyclic fatigue resistance of rotary NiTi instruments.
Study designs (S): In vitro Experimental Trails.
Search strategy
The results for this review have been searched into databases – PubMed/MEDLINE, Google Scholar, and Cochrane Databases. The databases were searched up to 31 January 2020. The searches were restricted to English language only. Hand searching was done through various journals for relevant articles related to the topic.
To be considered for inclusion, published articles were required to contain combination of the following keywords NiTi instruments; cyclic fatigue; temperature; reciprocation movement. No limits were applied to the initial search.
Inclusion criteria
Nickel Titanium Rotary Instruments, NiTi Rotary files included are: ProTaper GOLD ProTaper Next Wave One Gold Hyflex EDM, Studies which included cyclic fatigue model as Artificial root canals/Blocks. Studies have included which perform Cyclic fatigue tests. Studies where included between 2001 to 2021 In-Vitro studies where included.
Exclusion criteria
Studies which does not contain NiTi rotary instruments/files where excluded, Reuse and damage files where excluded. Studies at least contain one of these files ProTaper GOLD ProTaper Next Wave One Gold Hyflex EDM and if not then excluded. Invivo or animal studies where excluded.
A risk of bias which was estimated (low, medium, or high) was assigned to each of the included studies by the investigators. The disagreements were resolved by discussion.
Statistical and meta analysis
Meta-analysis is a quantitative, formal, epidemiological study design used to systematically assess previous research studies to derive conclusions about that body of research. It is a statistical analysis that combines the results of multiple scientific studies.
Statistical analysis
The meta-analyses, using random-effects model, were applied with RevMan 5.4 (RevMan 5.4, The Nordic Cochrane Centre, Copenhagen). Heterogeneity was assessed by Q test and quantified with I2 statistics. Data on mean and standard deviation were obtained from selected studies. Mean no. of cycles required for file to fracture (cyclic fatigue) and time taken to fracture were considered as the main outcome. Three separate comparisons were performed: Comparisons of cyclic fatigue between Twisted File & Protaper Next File, Comparisons of cyclic fatigue between Protaper Universal & Protaper Gold File, and Comparison of time taken to fracture between Waveone Gold File & 2shape, using mean difference (MD) for Cyclic fatigue and time taken to fracture. For analyses, if the test showed substantial heterogeneity (I2>50%), a random effects model was applied, or else (I2 ≤50%), a fixed effects model would be used.
Comparison of cyclic fatigue between twisted file & protaper next file
The meta-analyses comparing the cycle fatigue between Twisted File & Protaper Next File included two papers with four comparisons. Due to the inability to examine the reported data, the other research were disregarded. The overall comparative results have been represented as a forest plot. Because heterogeneity was greater than 50% (I2 = 95%) in the meta-analysis of the chosen papers, the random effect model was used. Twisted File needed more cycles to fracture the file than Protaper Next File, with a mean difference of 174.08 (95% CI = 100.57 to 247.59; Z value = 4.64) per cycle. There was a statistically significant difference in the number of cycles needed to split the file (p=0.001). Figure 2
Comparison of cyclic fatigue between protaper universal and protaper gold file
Meta-analyses evaluating cycle fatigue between Protaper Universal and Protaper Gold File includes two papers with five comparisons each. Because the reported data could not be processed, the other investigations were disregarded. A forest plot has been used to represent the results of the overall comparison. Because heterogeneity was greater than 50% (I2 = 96%) in the meta-analysis of the chosen papers, the random effect model was used. Protaper Gold required more cycles to fracture the file than Protaper Universal File did, with a mean difference of -332.65 (95% CI = -464.04 to -202.27; Z value = 5.00). This variation in the quantity of cycles needed to split the file was statistically significant (p=0.001). Figure 3
Comparison of time taken to fracture between wave one gold and 2 shape file
The meta-analyses comparing the cyclical fatigue between Waveone Gold and 2shape File contained two papers with three comparisons. Due to the reported data's inability to be examined, the other research were disregarded. The aggregate comparative findings are shown as a forest plot. The random effect model was used since, in the meta-analysis done for the chosen papers, heterogeneity exceeded 50% (I2 = 100%). With a mean difference of 33.00 (95% CI = -106.81 to 172.82; Z value = 0.46), the number of cycles needed to fracture the file were higher in Waveone Gold than in 2shape File. It was not statistically significant (p=0.64) how many cycles were needed to fracture the file. Figure 4
Results
Outcome of Search - In sum, the electronic screening of PubMed and Cochrane identified 1020 articles.
From the initial 1020 studies identified, after the removal of duplicates (565articles), 455 articles were assessed for relevance to the objective of the research.
After the exclusion of 427 articles, 28 articles were assessed for eligibility.
After excluding, invitro studies (10), and studies not in English (09), a total of 09 full-text articles were further assessed and included in the systematic review (Figure 1 ).
Of the 20 studies7 were categorized as having a moderate risk of bias, and 2 was considered to have a high risk of bias.
There were no statistically significant variations between the fracture length of separated instruments, according to two reviews that conducted studies at controlled temperatures, namely Gianluca Plotino et al and K olcay et al.22
Figure 1
Static cyclic fatigue device with embedded stainless steel artificial root canal systems;(b).Scanning electron microscopic appearances of HyFlex EDM; (c).Scanning electron microscopic appearances of WaveOne Gold.
Table 1
Selected studies for inclusion in systematic review:
Table 2
Selected studies for inclusion in systematic review
Table 3
Risk of bias assessment for non-RCT
Discussion
The primary purpose of this systematic review and meta -analysis was to summarize the evidence from multiple studies that have evaluation of cyclic fatigue resistance of nickel titanium rotary instruments. The 9 studies included have tested cyclic fatigue resistance of nickel titanium rotary instruments in different temperature, different angle of curvature of canals, its metallurgy, thermal treatment, cross section and its continuous & reciprocating motion.
The fatigue resistance and superelasticity of NiTi instruments have recently been improved through the use of various thermomechanical treatments. The temperature of the phase change has a significant impact on the mechanical characteristics because the superelastic property is the result of stress-induced martensite transformation, which occurs in conjunction with a reversible phase transformation between austenite and martensite. 4 According to certain authors, martensitic instruments are less likely to fracture than austenitic ones. However, the majority of these experiments were conducted at room temperature, and there is some indication that a rise in temperature may have an impact on the characteristics of NiTi alloys. 23
ProTaper Universal(S1F2) and ProTaper Gold(S1F2) nickel-titanium rotary files will be evaluated for cyclic fatigue resistance by Gianluca Plotino et al. (2016) at room and intra canal temperatures of 35 oc and with 300 RPM. And the findings indicate that both at ambient temperature and intracanal temperature, PTG instruments shown a significantly higher resilience to cycle fatigue than PTU devices.
Real teeth may mimic clinical conditions better, but because root canal architecture varies, it is impossible to choose real canals with the same curvature in terms of angle and diameter. 24 The kinematic features of the files play a significant impact in the cyclic fatigue resistance of the various instruments, in addition to the alloy used in file production and the heat treatments used. 25
The study by Taha Ozyurek et al. (2018) compared the cyclic fatigue resistances of four distinct NiTi systems with various metallurgical properties: HyFlex EDM (HEDM), WaveOne Gold (WOG), Reciproc Blue (RB), and 2Shape (TS). HEDM:500rpm & 2.5n/cm T, Wave one Gold, ReciprocBlue, and 2Shape:300 rpm & 2.5n/cm T are files that use rpm% torque. Recriproc Blue outperforms the other evaluated files in terms of cyclic fatigue resistance, and there is no discernible difference between the 2shape, WaveOne Gold, and HyFlex EDM groups. In canal curvatures, NCF values obtained by HEDM revealed a substantially higher cyclic fatigue resistance than TS.
Despite their higher flexibility, NiTi endodontic rotary files occasionally fracture inside the root canal system. 26 These fractures are generated by tension from cyclic fatigue, torsional fatigue, or a combination of both. 2, 27 The prognosis of the root canal system is impacted by the incidence of fracture of NiTi endodontic rotary files, which ranges from 0.09% to 5%. 28 This is why numerous studies on the cyclic fatigue resistance of NiTi endodontic rotary files have been conducted. 29 In order to examine the cyclic and torsional fatigue resistance of stainless steel endodontic hand files, the American National Standard Institute and the American Dental Association created a standardised approach in 2002. The International Standards Organization (ISO) (ISO 3630/1) also provided a description of the fatigue resistance for 2% taper stainless steel endodontic hand files. Nevertheless, there are no specifications for NiTi endodontic rotary systems exceeding 2% taper's cycle fatigue resistance. 30
Celia Ruiz-Sanchez and associates (2020) When comparing the time to failure for the NiTi alloy study groups, all pairwise comparisons, with the exception of those between the PTN and PVB study groups, showed statistically significant differences. ProFileVortexBlue, ProTaperNext, and ProTaperUniversal endodontic rotary files' NiTi CM-Gold wire alloy are more resistant to cyclic fatigue.
Akanksha Bhatt et al(2018) aim of the present in-vitro study was to evaluate and compare cyclic flexural resistance of total 125 rotary Hero Shapers, Protaper Next, Profile Vortex, Hyflex CM and One Shape NiTi files manufactured from M wire Technology, CM wire technology and Conventional NiTi alloy with 400 Rpm & 2.5N/Cm torque. Result states that Time taken to fracture ranged from 7 to 58s in different groups .Cyclic flexural fracture resistance among the experimental groups the cyclic fatigue resistance was observed in descendingorder Group IV (Hyflex CM) ˃ Group V (One Shape) ˃ Group II (Protaper Next) ˃ Group III (Profile Vortex) ˃ Group I (Hero Shapers). Hyflex CM files had longest survival time while minimal survival time was shown by Hero Shapers files.
File fracture is the main concern throughout canal shaping procedures. 31 Several factors are responsible for file fracture in use; however, cyclic fatigue has been reported to be a significant cause as the rotary file might be used in curved root canals. 32 The geometric designs, structural characteristics and surface texture have a significant influence on the susceptibility of NiTi instruments to fracture mechanically. 33, 34
A.M Elnaghli (2014) aim of this study was to compare the cyclic fatigue resistance of the new ProTaperNext files with other files including Twisted Files, HeyFlex and ProTaper with Protaper:20.07F1TF: 25.06, PTN:25.06 X2, Hyflex: 25.06 & TF ,HF:500 rpm & PTN, PT:300rpm.Results are as Twisted Files had a significantly higher resistance to cyclic fatigue than the other instruments . No significant difference was found in NCF between PTN and HF , however, there was a significant difference of both these systems with PT, which exhibited the lowest mean NCF.
Examining ProTaper's resistance to cyclic fatigue is the goal of Pedulla E et al's (2017) study. Then, using EndoWave, Revo-S, Mtwo, Twisted Files, or both to reciprocate Optimal Torque Reverse motion or rotate continuously (OTR). PTN:25.0X2, Endowave:25.06, and Revo S SU:25.06 Mtw25.06o:TF:25.06 at 300 Rpm. According to the findings, Mtwo and TF had significantly greater TtF than all other instruments, both in continuous rotation and reciprocation of OTR motion, while PTN had stronger cycle fatigue resistance than Revo-S and EndoWave, both in continuous rotation and reciprocation of OTR motion.
In order to increase the cycle fatigue resistance of the NiTi files, the research suggests that reciprocating motion is substantially more effective than continuous rotation. 35, 36 It is possible that the transition from martensite to austenite in the NiTi files was brought on by a rise in temperature. While operating in the root canal, where the temperature is about 37°C, the properties of the NiTi files at room temperature may change. Because the temperature in the root canal is higher than the austenite starting point (As) value, the martensitic characteristics of the NiTi files can change from the martensitic phase to the austenitic phase. 37
The WOG, PTN, and TS NiTi instruments were the focus of a 2019 study by K olcay with the purpose of assessing their cycle fatigue resistances. Using an immersed transducer temperature sensor, these were tested in a specially constructed stainless steel metal block submerged in a water bath. The reciprocal movement showed the best performance in terms of cycle fatigue resistance, with WOG > PTN > TS according to the TTF data. Also, compared to the TS instruments, the PTN instruments displayed NCF values that were noticeably longer.
Mohsen AminSobhami et al (2015) aim to compare the cyclic fatigue resistance of Neoniti A1 #25, RaCe 25/.06, Mtwo 25/.06, Twisted file 25/.06 and ProTaper Next X2 file. With 300Rpm and 2N/cm Torque. Results states that Neoniti showed the highest and RaCe showed the lowest number of cycles to fracture (NCF) values (P<0.05), indicating the highest and lowest fatigue resistance.Regarding the radius of curvature, Twisted file had the lowest and Neoniti had the highest values.
Mechanized instrumentation saw significant advancements that allowed for the introduction of a number of systems with unique designs and differentiating metallurgy (40). ProDesign R and ProDesign Logic files have not received as much attention as WaveOne Gold's principal system, 21, 38, 39 according to studies. 40 ProDesign R and ProDesign Logic files are produced in the exact same way; the only difference is the cutting direction. When spinning in the opposite direction from ProDesign Logic, PDR is active.
To determine the fracture resistance of WaveOne Gold main, ProDesign R (25/0.06), and ProDesign Logic (25/0.06) files, with various RPM and Torque as WaveOne Gold, prodesigm, Silvio Emanuel Acioly Conrado de Menezes et al. (2017) set out to do. Prodesign logic:350 rpm & 1N/Cm T, R:350 rpm & 4N/Cm T. As a result, The ProDesign Logic file had significantly reduced performance when compared to the other files (P=0.019) but was still superior than WaveOne Gold in terms of cycle fatigue resistance. The WaveOne Gold group offered a lower NCF as well as a smaller sum of NCI+NCF (748.33), whereas ProDesign Logic (182.07 sec) and ProDesign R provided the longest timeframes to failure.
Conclusion
The findings of this systematic review and meta-analysis point to the possibility that the temperature at which the instruments are tested may have an impact on the results of cyclic fatigue resistance on NiTi files with varying behaviour depending on the alloy's heat treatment, and that an increase in the angle of curvature of artificial canals has a detrimental effect on cyclic fatigue resistance. The temperature inside the canal or at room temperature has no effect on the cyclic fatigue resistance of instruments made with the gold heat treatment.
For cycle fatigue resistance, reciprocal movement demonstrated the best performance. Due to the higher cycle fatigue resistance at body temperature, reciprocating motion can be used more safely than continuous rotation in curved canals. The instrument's alloy's thermal treatment, its cross section, and the glide path appear to have an impact on how resistant it is to cyclic fatigue.
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