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Dentinal Microcracks after Root Canal Instrumentation Using Instruments Manufactured with Different NiTi Alloys and the SAF System: A Systematic Review

: Aim: The aim of this systematic analysis was to assess the prevalence of dentinal microcracks at various levels (3, 6, and 9mm from the apex) after using instruments made with conventional, R-Phase, and M-Wire NiTi alloys and the SAF system. Materials and Methods: Electronic searches were conducted in the databases Embase, Cochrane Library, Scopus, PubMed, and Web of Science. To arrange search methods, “MeSH” terms and/or keywords typically associated with the subject were paired with the Boolean operators “AND” and “OR.” Additional searches were conducted on the websites of four separate endodontic journals. After reading the titles and excluding duplicates, 1000 of the 1343 documents originally found were eliminated. Upon reviewing the abstracts, 310 of the remaining 343 experiments were also eliminated. Based on qualifying requirements, only 13 of the remaining 33 articles were included in the qualitative review. Results: All systems trig-gered dentinal microcracks; however, when chemo-mechanical preparation was performed using Self-Adjusting File (SAF) and systems manufactured with R-phase technology—K3XF and Twisted File Adaptive (TFA)—less of these defects were found when compared to those manufactured with traditional NiTi—ProTaper Universal and Mtwo—and with M-Wire—ProTaper Next, Reciproc, and WaveOne. Conclusions: A lower prevalence of dentinal microcracks was observed after using SAF and endodontic systems manufactured with R-phase.


Introduction
Endodontic treatment is primarily performed to maintain or reestablish the health of peri-radicular tissues in the vicinity of the involved teeth [1]. In vital teeth, the pulp is removed, and the root canal is cleaned, shaped, and filled with a biocompatible material. Since the periapical tissues are not involved, the procedure is based on a "prophylactic target" to prevent the formation of a peri-radicular lesion [2]. The pulp cells in necrotic teeth are destroyed and permanently compromised, enabling microbial colonization of the root canal system (RCS) [3]. While a periapical lesion is not necessarily visible on irrigation of the root canal during the process, with the irrigant being activated further by the vibrating motion of the file, which induces turbulence in the root canal. Irrigation is delivered by a special rinsing unit that delivers the irrigant at a flow rate of 5 mL/min (VATEA, ReDent-Nova) [23,24].
The advancement of NiTi instruments has made endodontic care even more convenient for both patients and practitioners. However, clinicians and experts continue to be concerned with root canal anomalies [25,26], radicular perforations [25,26], instrument fracture [25,26], and dentinal microcracks.
Dentinal microcracks are complete or incomplete thickness cracks that occur on the root canal walls as a result of endodontic file operation. These defects result in vertical root fracture [27] and tooth extraction [28][29][30]. Regardless of the NiTi file system used during chemo-mechanical preparation, dentinal microcracks have been observed.
The goal of the study is to analyze the effect of different file systems with respect to their different composition of NiTi alloys and cross-sectional shapes in dentinal crack formation during root canal instrumentation.

Protocol and Registration
The current study adhered to the Preferred Reporting Items for Systematic Review and Meta-Analysis-PRISMA (http://www.prisma-statement.org) guidelines. It was accessed on 9 August 2020. However, since no clinical records were included, it was not registered in PROSPERO (International Prospective Registry of Systematic Reviews).

Search Strategies
Refer to Table 1. Table 1. Sources of information and search strategies.

Hand search
Australian Endodontic Journal, Iranian Endodontic Journal, International Endodontic Journal, and Journal of Endodontics 23

Data Sources
Two independent researchers (K.S. and A.P.) conducted an electronic literature search up to 10 August 2019, using MeSH terms and keywords together with the Boolean operators "OR" and "AND" to collect the relevant literature using suitable filters. The keywords "NiTi file systems", "rotary instruments", "reciprocating file systems", "dentin microcracks", "dentinal defects", "dentinal craze lines" and "endodontics" were combined with the Boolean operators "OR" and "AND" to gather the required literature using appropriate filters. The complete search strategy with the number of articles resulted from them is presented in Table 1.

Eligibility Criteria
A search of the literature was conducted to find in vitro studies that examined the occurrence of dentinal defects after using instruments made with conventional, R-Phase, and M-Wire NiTi alloys and SAF systems, and were published between 1 January 2012, and 10 August 2019. Based on the PICOS approach, two reviewers reviewed the full texts of the remaining papers and defined inclusion and exclusion requirements. Disputes were resolved by the decision of a third reviewer. Table 2 shows the inclusion and removal conditions. Table 2. Inclusion and exclusion criteria adopted.

Inclusion Criteria
In vitro experiments conducted on extracted human teeth; Root canals prepared with a file with conventional NiTi metallurgy (ProTaper Universal and Mtwo systems), hollow-file (Self-Adjusting File system), M-wire metallurgy (WaveOne, ProTaper Next and Reciproc systems), and R-phase metallurgy (Twisted File Adaptive and K3XF systems) using a standard technique according to the product manual; Inclusion of at least two of the four comparison groups: files with conventional NiTi metallurgy (ProTaper Universal and Mtwo systems), hollow-file (Self-Adjusting File system), M-wire metallurgy (WaveOne, ProTaper Next and Reciproc systems), and R-phase metallurgy (TFA and K3XF systems); Prepared teeth sectioned horizontally at 3, 6, and 9 mm from the apex, with the slices examined and numbered under a stereomicroscope or a scanning electron microscope (SEM); Dentinal cracks are measured in slices instead of the number of teeth; Studies published in English.

Exclusion Criteria
In vivo studies or studies not of human teeth; Root canals prepared with instruments other than files with conventional NiTi metallurgy (ProTaper Universal and Mtwo systems), hollow-file (Self-Adjusting File system) and M-wire metallurgy (WaveOne, ProTaper Next and Reciproc systems); Evaluation of the crack initiation at levels other than 3, 6, and 9 mm from the apex; Studies without a control group; Evaluation of dentinal cracks after the root canal filling procedure; Retreatment; Unavailable data. Note: If any lines, microcracks, or fractures were detected in the dentinal slice, it was defined as a "slice with cracks". A dentinal slice devoid of any craze lines, microcracks, or fractures on the external surface of the root or the internal surface of the root canal wall was defined as a "slice without cracks".

Study Selection
Two reviewers (K.S. and A.P.) were in charge of selecting/excluding publications based on a linear review of names, abstracts, and full texts (in cases of doubt). Based on the qualifying criterion, the remaining articles were read in their entirety before making the final decision (Table 2). This whole method was carried out separately, with the assistance of a third researcher (R.M.) in the cases of questions or disputes.

Data Extraction
After reading the full text, two researchers (K.S and A.P.) conducted independent sampling and data extraction to collect data from qualifying studies. Significant information was extracted and reported from the included research, including the first author, publication year, region, probability of bias, reference group, root curvature, number of tooth slices per group, identification of microcracks prior to the sample, file system used, tool used to classify dentinal microcracks, control group, and irrigant method. Owing to the methodological heterogeneity of the findings, a meta-analysis was not conducted.

Quality Assessment
Two researchers (K.S. and A.P.) independently assessed the possibility of prejudice in the included experiments using the Joanna Briggs' consistency evaluation of in vitro studies [31]. A third researcher (R.M.) was consulted if there were any contradictions.
The assessment tool includes the following domains: 1.
Congruity between the stated philosophical perspective and the research methodology.

2.
Congruity between the research methodology and research question or objectives.

3.
Congruity between the research methodology and the method used to collect data.

4.
Congruity between the research methodology and representation, and analysis of data.

5.
Congruity between the research methodology and interpretation of results. 6.
Statement locating the researcher culturally or theoretically. 7.
Influence of researcher on the research and vice versa addressed. 8.
Representation of participants and their voices. 9.
Assessment of research ethicality according to the current criteria for recent studies and if there is evidence of ethical approval by an appropriate body. 10. Assess whether the conclusion drawn in the project report flow from the analysis or interpretation of data.
Each area was assigned a Yes, No, Uncertain, or Not Available rating. According to Joanna Briggs' quality evaluation guidelines, these tests were recorded for each chosen sample. Each analysis was then classified as either an inclusion, exclusion, or obtain further detail for the systematic review.

Study Selection Results
The PRISMA flowchart in Figure 1 shows the results of the literature scan. Initial electronic and manual searches yielded 1343 results. After reading the titles and excluding duplicates, 1000 of the 1343 documents originally found were eliminated. Since reviewing the abstracts, 310 of the remaining 343 experiments were also eliminated. Based on the qualifying criterion (Table 2), just 13 of the remaining 33 articles were included in the qualitative study.  Table 3 summarizes the characteristics of the 13 experiments chosen for q analysis. All of the experiments used comparative research classes. In total, 152 of mandibular teeth (incisors, premolars, and molars) were analyzed. The root ranged from 0 to 40 degrees. Prior to instrumentation, all of the teeth were sca a SEM or a stereomicroscope to detect prior dentinal microcracks. After the ins tion, the same was done. A placebo group of unprepared teeth was included in In both experiments, irrigant solutions were used to prepare the root canals. In  Table 3 summarizes the characteristics of the 13 experiments chosen for quantitative analysis. All of the experiments used comparative research classes. In total, 1529 samples of mandibular teeth (incisors, premolars, and molars) were analyzed. The root curvatures ranged from 0 to 40 degrees. Prior to instrumentation, all of the teeth were scanned with a SEM or a stereomicroscope to detect prior dentinal microcracks. After the instrumentation, the same was done. A placebo group of unprepared teeth was included in both trials. In both experiments, irrigant solutions were used to prepare the root canals. In all studies, the occurrence of dentinal microcracks was assessed in slices taken from the apex at 3, 6, and 9 mm. Out of the 13 studies [32][33][34][35][36][37][38][39][40][41][42][43][44]

Quality Assessment
All papers related to the current study were shortlisted and screened based on names, abstracts, and finally, the entire articles were read one by one and reviewed for quality evaluation using the Joanna Briggs' quality assessment of in vitro studies' guidelines. In all, 343 papers were searched, with 330 articles being omitted for reasons such as lack of clarification of the process, research nature and results, consistency problems, and data completeness. Articles that lacked congruence between the research approach and the research query or purpose, the procedure used to gather data, data representation and analysis, and outcome interpretation were omitted (Table 4). Since the total number of experiments used in each study was limited, no statistical assessment of publishing bias (Trimand-fill, Egger's test) was carried out. Similarly, due to the scarcity of available evidence, no susceptibility or subgroup studies were carried out.

Discussion
The main aim of endodontic therapy is to clean and fill the RCS in order to preserve or regain the integrity of the periapical tissues [45]. Despite continuous developments in instruments to improve RCS cleaning [46], dentin microcracks continue to be a source of concern for clinicians and researchers [47,48] as they often contribute to the instrumented tooth fracturing and extraction [49][50][51]. Although, there are multiple factors that contribute to dentinal crack formation like the design of the file, flexibility of file, and stress on the tooth surface used during instrumentation.
Files considered in the study are ProTaper Universal file and Mtwo which belong to the conventional NiTi file system group. The cross-sectional design of ProTaper Universal resembles that of a reamer, with three machined cutting edges and convex core [52]. The Mtwo files cross-sectional design resembles that of the S-file [53]. It has no radial lands, progressive blade pitch from tip to shaft, positive rake angles, and a non-cutting tip. Mtwo files have two cutting edges with minimal radial contact providing maximum space for dentin removal. It is advised by the manufacture to use all files in the instrumentation sequence to the full length of the root canal [54]. ProTaper Next is fabricated utilizing M-wire technology which helps to increase file flexibility. The off-centered rectangular cross section and the unique design of the file generate enlarged space for debris [55]. WaveOne made of M-wire metallurgy have a reverse helix and 2 distinct cross-sections along the length of their active portions. From D1-D8, the WaveOne files have a modified convex triangular cross-section, whereas from D9-D16, these files have a convex triangular cross-section [56]. Reciproc also made of M-wire technology consist of s-shaped cross section, the variable taper, the cutting angles and the thermally improved raw material provides high efficiency and cutting performance. The file's tip is non-cutting for a gentle treatment near the apex [57]. Twisted File Adaptive (TFA) is made up of R-phase heat treatment, twisting of the metal wire, and a special surface conditioning [58]. As a result of this it is consider superior to traditional NiTi instruments respect to their flexibility; cyclic fatigue resistance, cutting efficiency, and their ability maintain the original canal shape with minimal transportation [59,60]. K3XF consists of modified triple and developed by R-phase heat treatment [61]. Self-adjusting files system (SAF) file system is a hollow-file system that adapts to wall of the root canal according to anatomy of the canal [62]. Although there are different design and cross sections of the files developed there is still development on dentinal cracks during root canal instrumentation.
Another important factor that contributes to dentinal cracks is improper tooth preparation. It is often seen that rough dehydration leads to fracturing, cracking and volumetric shrinkage [63]. It recommended to prepare the tooth with lubricants to replace water before root canal instrumentation for desired results to avoids necessary factors affecting the desired results [64]. Although the step of tooth preparation is an important step in root canal instrumentation it is described in detail in the studies included in the systematic review.
The aim of this systematic analysis was to assess the prevalence of dentin microcracks after using instruments made with conventional, R-Phase, and M-Wire NiTi alloys and the SAF system. A systematic review of the literature is a coordinated review focused on a pre-established approach to classify, pick, and critically examine important studies regarding a topic or doubt. The aim of systematization is to reduce the significant amounts of prejudices that are typically present in a traditional or non-systematic analysis of the literature [65]. Critically, studies that involved the use of mandibular teeth with a maximum curvature of 40 degrees and no prior dentinal microcracks were included. The studies also processed a control group and comparative classes that were used in the review. As a result, the frequency of microcracks could be determined using a SEM and a stereomicroscope, which examined cross-sections at 3 mm, 6 mm, and 9 mm from the apex.
In this systematic analysis, 1529 teeth samples were studied, with mandibular premolars and central incisors being the most commonly used teeth. Three research [33,39,42] recorded teeth with root curvature, while others reported teeth with a straight root. The curvature specifics were not defined by Zhou et al [41]. Conventional NiTi file systems were the most commonly used, followed by M-wire file systems. All experiments that used traditional NiTi files as an experimental group used the ProTaper Universal method. WaveOne method was used in 7 studies for M-wire, with ProTaper Next and Reciproc following it. R-TFA phase's system was the most widely used.
When the occurrence of dentinal microcracks at various cross-sections was examined using SEM after using traditional NiTi file systems, a greater number of defects were found after using the ProTaper Universal method [32,34,[36][37][38]41,43,44]. WaveOne was associated with the highest number of dentinal defects among M-wire systems [33,34,36,42]. Four M-wire studies [32,38,40,42] that used ProTaper Next have registered a rise in the number of dentinal cracks scanned. There were less dentinal microcracks after using R-phase and SAF (hollow-file system). Moreover, it should be noted that R-phase and SAF instruments were the least often included in the studies included in this systematic analysis. Kansal et al., in 2014, Saha et al., in 2017, and Kumari and Vishwas, in 2016, did not disclose any data at 6 and 9 mm cross-sections, so their conclusions might be inadequate. Zhou et al., in 2015, used all three mechanisms and found the lowest number of dentinal cracks as compared to the other findings in the study. The highest number of dentinal cracks was observed at 3 mm from the root apex, although as the distance from the apex increases, the number of microcracks decreases, with the least recorded at the 9 mm cross-section. See Table 5 for more information. All of the experiments had a control group for reference and a well-described protocol that was reported and evaluated using the Joanna Briggs' quality evaluation of in vitro studies' criterion [31]. The papers were evaluated based on their congruence between the research methodology and the research topic or purpose, the process used to gather data, data representation and analysis, and interpretation of findings. Only papers that met these conditions were included (Table 6).  The development of emerging technology used in the manufacturing of endodontic instruments seems to reduce the occurrence of dentin microcracks. NiTi files generated using M-wire and R-phase technologies had a lower incidence of dentinal microcracks, especially in the apical third.

Limitations
The literature review was undertaken using major databases; however, articles that are published elsewhere may have been overlooked. As a result, the papers used in the present systematic review were written in English, and some important evidence may have been overlooked as several papers may have been published in other languages. Only a few studies yielded inconclusive findings. The lack of standardization of cross-section length from the root apex resulted in varying cross-section lengths. The existence of selective research performed using new generation file systems was the main drawback of this systematic analysis.

Conclusions
Despite the shortcomings of this systematic review, it is possible to infer that the use of endodontic NiTi files manufactured using M-wire and R-phase technology provides a better biomechanical preparation by preventing the development of dentinal microcracks. The conclusion of the systematic review is drawn based on the limited number of publications.

Data Availability Statement:
The data used to support the findings of this study are available from the corresponding authors upon request.

Conflicts of Interest:
The authors declare no conflict of interest.