Root canal preparation using micro-computed tomography analysis: a literature review

This literature review has critically analyzed the published research related to the biomechanical preparation of root canals with three-dimensional analysis using micro-computed tomography (micro-CT). In December 2017, six databases (PubMed, Cochrane, Web of Science, Embase, Scopus, and Science Direct) were accessed using keywords to find articles including the use of the micro-CT analysis in biomechanical root canal preparation. There were 60 full articles that were selected, which were screened and read by two authors. The research that was reviewed and analyzed included root canal anatomy and sample selection, changes in canal shape and untouched canal areas, canal transportation and centering ability, and kinematics (motion). Of the studies selected, 49.18% discussed anatomical characteristics, with 54.1% of these studies describing mesial roots of mandibular molars with moderate curvature. Only 35% used a stratified distribution based on root canal system morphology and quantitative data obtained by micro-CT. The analysis of canal transportation and centering ability showed that transport values in the apical third exceeded the critical limit of 0.3 mm in mesial roots of mandibular molars with moderate curvature, especially in the groups in which a reciprocating system was used. In relation to kinematics, 91.70% of the reviewed studies evaluated continuous rotating instruments, followed by reciprocating rotation (38.33%), vibratory (15%), and the adaptive kinematics, which was in only 8.33%. The reciprocating kinematics was associated with higher canal decentralization and transportation indexes, as well as a greater capacity for dentin removal and debris accumulation. This literature review showed that the anatomy, the type of design and kinematics of instruments, and the experimental design are factors that directly influence the quality of biomechanical preparation of root canals analyzed in a qualitative and quantitative manner by micro-CT.


Introduction
Biomechanical root canal preparation is an important endodontic treatment step.The goal is the complete removal of remaining pulp tissue, microorganisms, and infected dentin; as well as shaping of the root canal system (RCS) through the mechanical action of endodontic instruments and the chemical action of auxiliary solutions providing adequate conditions for the sealing of the pulp cavity and repair of the periapical tissues. 1,2,3,4,5ince 2012, new protocols for biomechanical root canal preparation have been developed using nickeltitanium (NiTi) instruments, whose flexibility and resistance to torsion allow their use in a continuous rotating movement; these in turn reduce the working time, operator fatigue, and the risk of operative accidents. 6,7,8,9,10,11,12,13,14,15NiTi instruments have been developed with various geometric conformations, 8 with the most significant design difference being the taper with conicity ranging from 2% to 12%, compared to the standard 2% taper of stainless steel instruments established by ANSI/ADA in 1976 and updated in 1982. 8iTi rotary instruments were first commercialized in the 1990s, and since then about 700 studies have been published in journals indexed in PubMed before June 2010.They have evaluated the performance of these instruments, as well as the newly developed instrument designs.Studies have shown that despite the flexibility, torsional strength, and elastic memory of NiTi instruments, they still leave a significant percentage of the canal surface untouched.This is mainly due to the anatomical characteristics of root canals, such as flattening, curvatures, isthmuses, recesses, and ramifications, which hinder the performance of the instrument and may leave tissue and bacterial remnants 16,17,18,19,20,21,22,23 (Figure 1).
Another concern noted when NiTi rotary instruments first appeared was a screw effect, which added to the difficulty of preparing challenging anatomical areas.This led to the development of more than 150 mechanized systems with differing designs, including variable tapers as well as different alloy treatments and movement types. 2,8,24,25,26,27,28,29,30n this review, various mechanical systems have been reviewed and discussed according to the innovations in the design, type of alloy treatment, movements, and concept of recommended preparation, without commercial bias (Table 1).It is noted that some systems may incorporate more than one of the mentioned characteristics.
An attempt to overcome the limitations imposed by the anatomical complexity of the RCS resulted in the development of the Self-Adjusting File (SAF) instrument (Redent-Nova Inc., Ra'anana, Israel) with an innovative manufacturing process.It is an instrument with a distinctive design that features a hollow, compressible, thin-walled body, composed of a delicate NiTi trellis covered by an abrasive layer.The use of this single instrument allows its adaptation inside the root canal, and by its vibratory movement (3000 to 5000 vibrations per minute) of low amplitude (0.4 mm), it promotes uniform dentine wear, which results in a canal with a cross section similar to the original but with slightly larger dimensions. 2,29,30,31,32,33,34,35everal reciprocating systems have been released 26 such as Waveone (Dentsply Maillefer, Baillagues, Switzerland) and Reciproc (VDW GmbH, Munich, Germany), which are based on the concept of root canal preparation with a single instrument.These instruments are manufactured with a M-Wire alloy from a thermal treating process of the NiTi alloy, which provides greater flexibility and resistance to cyclic fatigue than the conventional NiTi alloy. 4,9,11,13,15,33,34,35,36,37,38,39,40,41,42,43,44,45 Te movement dynamics of these instruments, known as reciprocating, is the rotation in the counterclockwise direction (cutting direction) followed by a less extensive clockwise rotation (instrument releasing direction), facilitating continuous and progressive movement toward apical. 15,26According to some authors, the reciprocating movement reduces the risk of torsion fracture because the instrument is not subjected to the stress levels caused by a continuous rotary motion. 15,26,46,47,48nother system for the preparation of the root canal based on the concept of a single instrument, but with continuous rotation, is the OneShape system (MicroMega, Besançon, France). 11,49,50This system is produced from conventional NiTi alloy using a tip diameter of #25 and continuous 0.06 taper, with three different cross-sections along the active part, variable pitch, and idle spirals, with the goal of reducing the BT Race (FKG Dentaire) Reciproc Blue (VDW) WaveOne (Dentsply) EZ-Fill Safesider system (EDS)

Vibratory
Self-Adjusting File (ReDent-Nova) -instrumentation.This system was designed to permit switching from a continuous clockwise motion (when the instrument is not subjected to stress within the canal) to an interrupted reciprocation motion (when undue tensions are generated by dentin) during instrumentation. 37,44,45,47,53,54,55,56,57,58here are also instruments designed with center of mass and/or center of rotation in offset that, in rotation, produce a mechanical wave that runs through the active part of the instrument.This results in improvement of the flexibility along the active part of the instrument and minimizes the instrument locking in the dentin, in addition to reducing the formation of debris. 8,30,34,41,59These features were incorporated into additional systems including Protaper Next (Dentsply Maillefer, Ballaigues, Switzerland), TRUShape (Dentsply Tulsa Dental Specialties, Tulsa, USA), Revo-S (Micro-Mega, Besançon, France), and One Shape (Micro-Mega, Besançon, France).
Recently, the concept of mechanical finalization of the biomechanical preparation was proposed.Following the mechanical preparation of the root canal, the instrument whips against the walls of the root canal allowing its action in untouched areas such as isthmuses, flattenings, and recesses.This action is possible with the XP-endo Finisher instrument (FKG Dentaire S.A., La Chaux-de-Fonds, Switzerland), which is produced with highly flexible NiTi MaxWire alloy (25/.00)(Martensite-Austenite Electropolish-FleX) that changes shape at different temperatures.The instrument is straight in the martensite phase (M-phase) of the alloy, which is reached when it is cooled, and when it is exposed to higher temperatures (such as body temperature) its shape changes due to molecular memory of the alloy to the austenite phase (A-phase).This makes the instrument assume a semi-circular conformation that, in rotation, allows it to reach an area of 6 mm in diameter that is 100 times greater than that of other instruments. 60,61In 2017, Leoni et al. 61 evaluated the possibility of using the XP-endo Finisher instrument as well as the SAF in the mechanical finishing of root canal preparation with isthmuses, demonstrating the reduction of accumulated debris after biomechanical preparation.
Later, with the combination of this NiTi MaxWire alloy and Booster Tip technology, the XP-endo Shaper (FKG Dentaire S.A., La Chaux-de-Fonds, Switzerland) offered greater flexibility, fatigue resistance, and penetration of the canals with ease and speed, expanding or contracting according to canal morphology and preserving the three-dimensional structure of the root canal. 62,63Similar to the preparation finalization instrument mentioned above, this instrument can react to temperature variations and acquires a predetermined shape at body temperature, with the taper of the instrument starting at .01 and reaching a minimum .04taper when it expands into the root canal.The Booster Tip has unique geometry that allows the operator to start the preparation after an initial glide path of at least ISO diameter 15, increasing its working range gradually until reaching ISO diameter 30, following the original canal path. 29,62n addition to the aforementioned innovations, the thermomechanical treatment process of the alloys used in the fabrication of these instruments alters the molecular structure of the alloy, providing resistance to cyclic fatigue and flexibility while reducing shape memory, allowing pre-bending of the alloys. 28,62,64,65,66se of this enhanced treatment process resulted in the development of blue rotary files. 28The Vortex Blue system (Dentsply Tulsa Dental Specialties, Tulsa, USA) and, more recently, the Reciproc Blue system (VDW GmbH, Munich, Germany) are features in a reciprocating kinematics.
Parallel to the development of mechanized instruments and systems for clinical use, experimental models for the evaluation of the biomechanical preparation of the RCS in human teeth were perfected.In previously available experimental models, biomechanical preparation was assessed by radiographic images 67,68,69,70 and root cutting series using the muffle system or its variations; 71,72 however, these methods allowed only a two-dimensional quantitative evaluation after preparation, and the muffle system was destructive. 73,74,75 solution for the three-dimensional and nondestructive evaluation of the RCS appears to be the use of computed tomography and magnetic resonance imaging in experimental procedures. 27,76,77,784,45,47,56,57,58,59,61,62,63,68,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109 The aim of this review is to present a discussion of the studies that evaluated the preparation of root canals by three-dimensional analysis based on the use of micro-CT while considering the evolution of NiTi instruments and biomechanical preparation evaluation methods, as well as the limitations imposed by the anatomical aspects of the RCS and the resulting need for sample selection for biomechanical preparation evaluation studies.The topics that will be addressed include root canal anatomy and sample selection, changes in canal shape and untouched canal areas, canal transportation and centering ability, and kinematics (motion).

Methodology
This literature review followed the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines for literature review to ensure the understanding, transparency, and fidelity of the results. 110,111he search strategy was used in six databases (PubMed, Cochrane, Web of Science, Embase, Scopus, and Science Direct), with the same key word combinations and MeSH Terms (Table 2), including titles, abstracts, and full texts.The search was performed in December 2017, and there was no initial restriction regarding the year of publication, article language, or journal publication.
After the initial search, all of the titles and abstracts were screened by two authors to find eligible full articles to be included in the literature review.
The evaluation of the mechanized (rotary, reciprocating, oscillatory) biomechanical preparation of the RCS by micro-CT was the inclusion criteria used in the article selection.Furthermore, to reduce the risk of bias, it included only studies in the English language that evaluated mechanized endodontic systems, used human permanent teeth (any type of tooth groups and root canals), and analyzed twodimensional and three-dimensional parameters (canal transportation, centering ability, changes in the volume, surface area, perimeter and area, and others) of the root canals before and after the biomechanical preparation.
Studies written in a language other than English; those which used endodontic hand files, deciduous teeth, acrylic/resin blocks with simulated root canals; and other types of analyses that did not include biomechanical preparation assessment were all excluded from the literature review.All studies found in the search were individually evaluated by two evaluators, and only after full agreement were they included in our study.

Search results
Initially using only titles and abstracts, a total of 102 articles were selected in the database search, and were saved in the Mendeley Reference Management Software & Researcher Network to organize and subsequently facilitate the search and reading of articles.The articles did not meet inclusion criteria for various reasons such as articles that duplicated research or were not accessible in English or that used different methodologies, samples, or analysis are presented in the Table 3.
Only 60 papers met all inclusion criteria and were included in this literature review for 2003 to 2017 (Figure 2; Table 4).

Antomical aspects that influence the sample selection and biomechanical preparation in studies in vitro
The study of the internal anatomy of human teeth only aroused the interest of researchers at the end of the 19th century.In 1901, Preiwerk 112 conducted the first studies with the injection of molten metal inside human dental canals.In 1913, Prinz 113 developed the traditional diaphanization method, which was then used by Okumura (1927), 114 a pioneer in the classification of root canals based on their anatomical characteristics.Through diaphanization, Vertucci  (1984)  115 also classified the RCS of permanent human teeth into eight morphological types according to the number of canals and the location of their divisions in the same root, and this classification system was most commonly cited in the studies of internal anatomy.
Subsequently, other studies added more than 30 morphological types to this classification 3,36,116,117,118,119,120,121 evidencing that a root with a conical canal and a single foramen was not commonplace. 1o increase the accuracy of the methods previously proposed for the evaluation of dental anatomy, the micro-CT allowed the non-destructive threedimensional analysis of additional canals, multiple foraminas, apical deltas, isthmuses, C-shaped roots and canals, and accessory canals 3,33,82,122,123 (Figure 1).
In addition, it obtained three-dimensional quantitative data of volume, surface area, and the structure model index (SMI); and two-dimensional parameters of area, perimeter, major and minor diameter, roundness and form factor of the root canal. 3,32,33,108,122,123,124,125btaining these quantitative micro-CT anatomy data may have contributed to in vitro studies producing more reliable results from a more selective sample, with the formation of homogeneous groups for the degree of curvature, diameter, and internal morphology, which will result in a better understanding of the action of each instrument according to the internal anatomy of the RCS.In this way, we will next discuss the anatomical aspects that were used in the 60 studies included in our literature review.
In the studies we reviewed, teeth with immature apices, resorptive defects, fractured roots, or root canal fillings or obstructions were excluded.For inclusion, the mandibular molar was generally the dental group of choice in 60% (n = 36) of the studies   reviewed, and in 83.33% of those (n = 30) in which the mesial root was selected for the root canal preparation evaluation.The curvature of the selected roots was evaluated in 71.66% (n = 43) of the studies with a higher prevalence of roots with moderate curvature and a bend angle between 20° and 40° in 58.13% (n = 25).
Regarding selection of the t ype of canal, Vertucci's classification for the evaluation of the root canal preparation showed the use of the Type I (single canal that extends from the pulp chamber to the apex) for incisors, 62 Type II (two canals that leave the pulp chamber and join near the apex to form a single canal), 43 and Type IV (two distinct and separate canals that extend from the pulp chamber to the apex) in mesial roots of mandibular molars. 12,14,40,44,45,47,97,101,104Also, in mesial roots of mandibular molars, isthmus classification Type I (narrow sheet and complete connection existing between two canals) 128 or Type III (incomplete isthmus existing above or below a complete isthmus) 57,58 was seen.
Another aspect of the internal anatomy used for the sample selection in the analyzed studies was the canal flattening, which classified the root canals as oval in 21.67% (n = 13) of the studies rev iewed, 11,15,29,30,33,45,62,102 long-oval 31,42,63 and flat-oval 85 in 53.84% (n = 7) of these studies, evaluating the degree of flattening by means of the ratio between the buccolingual and mesiodistal dimensions of the root canal on radiographs 15,29,30,33,42,85 and in CBCT images. 11The studies did not use quantitative data obtained by micro-CT (roundness, factor form, major and minor diameter, and SMI), which would have provided greater acuity of this type of anatomical variation.
Regarding the composition of the experimental groups, only 35% (n = 21) of the studies used a stratified distribution proposed by Versiani et al. 33 based on the similar internal anatomy and morphological dimensions of the root canal (statistically similar dimensions of the root canal) from the two-dimensional and three-dimensional data obtained by the micro-CT prior to the preparation. 27,29,33,34,45,57,63,97,98ecause studies have shown that the result of the biomechanical preparation depends more on the original anatomy of the root canal than on the instrument or technique used, 6,27,33,42,45,64,129,130,131,132 it becomes important to properly select samples with two-dimensional and three-dimensional values in order for the experimental groups to be balanced in terms of anatomical characteristics, which can improve the understanding of the results of each instrument against the different morphological characteristics of the root canals. 33,63,129

Changes in the canal geometrics and untouched areas after biomechanical preparation
The action of mechanized instruments inside the RCS promotes dentin wear that results in changes in the geometric configuration of the root canal, and these can be qualitatively and quantitatively observed through micro-CT 30,33,45,63 by images obtained at different steps in endodontic treatment by aligning the three-dimensional spatial coordinates of x, y, and z on specific software.
The quantitative parameters used for the evaluation of changes in the geometric configuration resulting from the biomechanical preparation are the three-dimensional parameters of volume, surface area, and (SMI); and two-dimensional parameters of area, perimeter, roundness, form factor, and major and minor diameter. 32,33,43,45,63The reviewed studies corroborate that after the biomechanical preparation, there is an increase in these parameters, regardless of the type of instrument or technique used. 4,27,36,41,44,47,102,104n addition, it is also possible to quantify the percentage of root canal walls touched and not touched by instruments.The touch or action of the instrument on the canal walls and the changes in volume and SMI are the parameters most used (68.85%) to evaluate the preparation by means of micro-CT. 4,11,15,42,47,57,62,83,96,102,103,105he change in canal volume is related to the effects of biomechanical preparation on dentin removal, 7,25,44,83,132,133 showing that the volume increase after preparation is proportionally higher in the cervical and middle third than in the apical and can be attributed to the cervical preparation 7,32,33,84 and to the greater taper of the instruments in the cervical region. 11,47Clinically, the increase in canal volume in the cervical third can mean the improvement of the reach of irrigating solutions in the apical third, or that the apical mechanical debridement was not as effective as cervical 7,12 (Figure 3).
An important aspect observed in this review was that 50% (n = 30) of studies evaluated the average percentage of canal walls untouched after biomechanical preparation and showed that no system or technique was able to touch all the walls of root can als, 4,27,29,30,42,43,44,45,57 showing a range of 2.6% to 80% of the walls being untouched. 27,57This variation may be related to changes in dental morphology, 25,27,45,79,84,101,104 the characteristics of the instruments used, 4,11,30,32,62,63,102 or the evaluation methodology used. 25,31,84n studies in which there was concern about sample selection, the percentage of untouched walls ranged from 8.17% to 58.8% for the whole canal in groups of teeth with flattened canals, 62,63 and from 3.13% to 51.03% for the apical third. 15,102nother aspect that may explain the range in the percentage of untouched walls is the design variability of instruments such as taper, diameter, and cross-section. 4,11,30,32,62,102The SAF, XP-endo Shaper, and TRUShape instruments showed highest percentages of touched walls when compared to other systems. 4,29,30,31,32,33,63,102egarding the final diameter, the reviewed studies showed that the final diameter of the instruments used for the biomechanical preparation ranged from diameter 25 to diameter 40 with a diameter 30 being most common in these canals.The mesial canals of mandibular molars were the most frequently used sample among the reviewed studies, and the diameter of these canals at 1 mm of the apical foramen varied between 0.28-0.40mm in the buccolingual direction and 0.21-0.28mm in the mesiodistal direction. 134.The surgical diameter was similar to the anatomical diameter, which may also explain the large percentage of untouched walls.Therefore, the standardization of the anatomic diameter should be a concern for sample selection during the experimental design as this data can be determined by the parameters of major and minor diameter obtained by micro-CT as well as being clinically described by Pécora et al. 135 In addition to the anatomical features of the dental group and instruments used for the preparation, another factor that can interfere in the results of touched walls is related to the evaluation methodology.The parameter of untouched walls is calculated by the difference between the number of static voxels and the total number of voxels on the surface of the root canal (voxels present at the same position on the canal surface before and after preparation). 25Both the overlap of the images and the resolution used for the acquisition of these images may be important factors in the interpretation of the obtained results 31,84 .The values of the resolutions used in the studies in this literature review ranged from 11.88 μm to 36 μm, 25,42,79,88 with the best resolutions in Yang et al., 88 with more values closer to 20 μm.Therefore, the observed differences in the percentage of untouched walls between studies may also be related to methodological differences.
3,68,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107 The untouched walls, especially in areas of anatomical challenges such as isthmus, recesses, and flattened root canals can retain bacteria and serve as a potential cause of persistent infection.137,138 This demonstrates the need for new strategies such as improving the performance of the irrigating solution, physical action of ultrasonic activation, and intracanal dressing to complement suboptimal mechanical action.30,61,79 Controversially, one study showed that there was no significant correlation between the elimination of active  bacteria and the average percentage increase in volume, surface area, and untouched walls when analyzed by micro-CT.138 In addition, a recent study of the correlation between data obtained after biomechanical preparation using micro-CT and histology 29 showed that even with the presence of untouched walls, SAF, TRUShape, and XP-endo Shaper can remove pulp tissue in oval canals.29 To analyze the changes in the dimensional shape of the root canal, the commonly used parameter is the SMI, which evaluates the convexity of the surface.Studies have shown that SMI values increased after preparation, indicating that the small and flatter irregularly tapered canal changed to a rounder and smoother tapered canal.27,31,33,36,45,68,86,88,102,103 Lower values for changes in SMI are related to the maintenance of the original shape of the root canal.27,45,102 Considering the limitations between the studies for analyzing the preparation related to changes in the geometry of the canals due to the methodological differences, the instruments showed that the most promising results regarding uniform dentin wear, smaller percentages of untouched walls, and the canal shape maintenance (evaluated by the SMI) especially on flattening canals, were the SAF, TRUShape, and XP-endo Shaper systems.4,29,30,31,33,63,85,102

Expression of canal transportation and centering ability during the biomechanical preparation
In 2017, the American Association of Endodontists 139 defined the canal transportation as a removal of canal wall structure on the outside curve in the apical half of the canal due to the tendency of files to restore themselves to their original linear shape during canal preparations, which may lead to ledge formation and possible perforations.Gambill et al. 140 proposed the term "centering ability" as a measurement of the ability of the instrument to stay centered in the canal.These measurements are still being used in various endodontic studies to assess the quality of the biomechanical preparation of root canals with instruments and techniques using different methodologies 141 .This was observed in this literature review in which 63.33% (n = 38) of the studies evaluated the canal transportation and the centering ability.
Several factors can influence the canal transportation and centering ability such as errors in treating endodontic cavities and in glide path, use of non-flexible instruments, instrument design (cross-section, taper, tip) and the absence of specific treatments and alloys in endodontic instrument manufacturing (stainless steel, nickel-titanium, thermic treatment).In addition, negligence during irrigation protocols and the operator's experience in determining the most appropriate preparation technique for each situation should be considered. 140,141,142The importance of anatomical knowledge of the RCS and its variations such as radius and curvature degree, canal configuration, and dentin thickness which can be observed in two-dimensional radiographic examinations is noteworthy. 3,140,141,142his literature review showed that approximately 52.63% (n = 20) of the studies reviewed evaluated the canal transportation and centering ability using the method proposed by Gambill et al. 140 for a CBCT analysis.This methodology consists of the measurement of the shortest distance, in mesial and distal directions, from the limit of the noninstrumented root canal to the limit of the tooth in comparison with the same measurements of the instrumented areas, represented by the formula (X¹ − X²) − (Y¹ − Y²).The X¹/X² and Y¹/Y² values represent the shortest distances from the outside and inside of the curved root to the periphery of the non-instrumented and instrumented areas of the root canals, respectively.Values may be seen from 0 (indicating no canal transportation) to 1 (indicating the perfect centering).
The high resolution and acuity of micro-CT allowed the evolution of this analysis, as introduced by Peters et al., 131,143,144 comparing each slice voxel by voxel from all data sets obtained before and after root canal preparation to obtain consistent results for centers of gravity.In this case, each slice was defined by a series of coordinated data for the x-, y-and z-axes, and the centers of gravity were calculated for each slice, connected along the z-axis by a fitted line which can be analyzed to determine canal curvature as the second derivative.Thus, the comparison of the centers of gravity before and after preparation showed the results of canal transportation, 45,97,131,143,144 as shown in 47.36% (n = 18) of the studies in this review.
In general, besides the metallurgical properties and design of instruments, other factors including the procedural protocol and the anatomy of the RCSs can influence the canal transportation during instrumentation.This literature review showed that 86.84% (n = 33) of the studies used mesial roots of mandibular molars with moderate curvature.In these studies, the results obtained showed that canal transportation at the apical region tended to occur in the lateral and outer surface of the canal curvature and in the inner surface of the canal curvature at the cervical and middle thirds.68,79,81,84,86,87,88,90,93,94,95,96,98,101,102,103,105,106 Regarding kinematics of the instruments, it was possible to verify that the instruments that act in vibratory mode presented less variations of transport and centralization considering the entire root canal (0.03 mm-0.14 mm), 84 followed by the instruments in adaptive (0.01 mm-0.058mm), 47,58, , continuous (0.00 mm-0.62 mm), 14,27 and reciprocating action (0.04 mm-0.48 mm).37,45 According to Wu et al., 134 transportation in the apical third more than 0.3 mm could negatively affect the obturation sealing, 38,86,134 and the values found in this review have not exceeded this critical limit in this root third.
Although the majority of the results did not present a statistical difference between the experimental groups in the performed studies, the reciprocating instruments were more likely to present values greater than 0.3 mm.These higher values could be explained by the cross-sectional design of these instruments which, in the Reciproc instruments, has a sharp double-cutting edge and S-shaped geometry, while the WaveOne is characterized by a modified triangular cross section with radial lands at the tip and a convex triangular cross section in the middle and coronal portions of the instrument.These characteristics provide greater cutting power and consequently more canal transportation during root canal biomechanical preparation. 37,86Also, the reciprocating motion allows a balanced force for movement into the root canal. 86,94,145he necessity of carefully evaluating these parameters is evident because deviation from the original canal can lead to incomplete touch of the root canal walls, residual debris and necrotic tissues that can affect the root canal filling, or the formation of zips and perforations.Another problem is overpreparation, which can result in excessive dentin removal, root weakening, and fractures. 97,131,140,141,142,144e influence of the kinematic and/ or motion type on the root canal biomechanical preparation Parallel to the advent of different materials and instruments from the use of stainless steel hand files to NiTi alloy mechanical systems, 4,15,33,35,45,57,58,85,97,146 different kinematics were developed such as a continuous rotation, vibratory, reciprocating, and recently an adaptive motion, to prevent the screw effect. 6,15,33,43,62,85,86,105,146,147he use of systems in continuous kinematics has been widely studied, and 91.80% of the reviewed studies evaluated the effectiveness and the shaping ability of these instruments in the biomechanical preparation of the root canals, which may be related to the greater number of commercially available systems with the easy handling of continuous rotation.
In recent decades, rotary instruments have revolutionized endodontic science and are preferred by most clinicians and specialists because they provide higher quality and safer treatments. 33,45,62,87,97,147The constant evolution in the metallurgy and the design features of the NiTi alloy with its superelastic and shape memory properties improves the centering ability of the instruments through the root canal and decreases the canal transportation. 82,133,146,147,148egarding root canal shaping, the NiTi rotary files using a continuous or adaptive motion have enhanced the quality of root canal preparation with less canal transportation, better centering ability, and minimal procedural errors such as zips and perforations, 15,21,83,85,86,91,149,150,151 when compared with a reciprocating motion. 15,35,45,91Furthermore, reciprocating instruments allow more dentinal removal which may be caused by the different cross sections of these instruments. 4,7,13,57,86,152nother factor that may be related to the instrument kinematics is the amount of debris accumulation along the root canals and its extrusion, which may lead to postoperative pain and reinfection. 4,6,11,15,38,39,57,62,84,153,154everal studies have demonstrated that reciprocating motion is associated with more extrusion debris when compared with all of the other types of motion, since the material has no escape in the cervical direction, which favors the accumulation of debris in the apical and periapical regions. 38,147,155,156t is worth noting that despite the greater tendency to extrude debris, the reciprocating motion has been related to a greater reduction of bacteria, which may be associated with the modified convex triangular cross-section with radial lands at the tip and the convex triangular cross-section in the middle and coronal portion cross-sectional design, which allows greater cutting ability. 37,49,88,147,157,158,159

Conclusions
The analysis of the biomechanical preparation studies evaluated with micro-CT show that anatomy, the design, and kinematics of the instruments, as well as the experimental design are factors that directly affect the quality of the biomechanical preparation of root canals.Despite the fact that the micro-CT presents high acuity, both for the study of the internal anatomy and for the biomechanical preparation, the difficulty in establishing criteria for selection and standardization of the sample in consideration of anatomical challenges is observed in the reviewed studies, which makes it challenging to determine the most effective instrumentation system (taper, kinematics, cross section and single or serial use) for each root canal morphology.
Also, the studies reviewed in the present analysis showed through micro-CT that none of the evaluated systems was able to completely touch the walls of the root canals.The effectiveness of the biomechanical preparation, as well as the maintenance of the channel shape, kinematics, and design of the instruments was evaluated.On the other hand, a greater trend of decentralization and transport of the root canals in the apical third was observed, as well as accumulation of debris after the use of reciprocating instruments.
In view of the results obtained in laboratory experiments by means of micro-CT, the need to transpose these data into clinical practice is clear.It is expected that in the future, the improvement and creation of new software for use in CBCT may allow the development of languages and logarithmic calculations in the correction of possible distortions generated during the scan, making possible the alignment and registration of images and subsequent clinical validation of the information obtained only by means of micro-CT thus far.With the development of these software programs, it will be possible to create tomographic models which will contribute to the planning of each clinical case.This new technology will determine more accurately the length of the canal, foraminal output, degree of curvature, major, and minor diameter of the canal, and the presence of isthmuses and accessory channels.
In addition, from the two-dimensional and threedimensional data of the root canal obtained by means of micro-CT and clinically by means of CBCT, it will also be possible to develop applications capable of simulating the results of the isolated action of different instruments in the various alternatives in SCR, which may contribute to the planning of biomechanical preparation, as well as to the development of new instruments, increasing quality, and predictability during all stages of endodontic treatment.

Figure 1 .
Figure 1.Three-dimensional models of root canal system of human teeth obtained from microcomputed tomography scanning showing the importance of the diagnosis of anatomical challenges prior to the biomechanical preparation of the root canal system.(A) Presence of accessory and lateral canals, and apical deltas.(B) Isthmus.(C) Flattened canals.(D) Presence of moderate and severe curvatures.

Figure 2 .
Figure 2. Flow diagram with the database search and articles include (based on PRISMA).

Figure 3 .
Figure 3. Representative 2D and 3D reconstructions of the internal and external anatomy of the mandibular molar before and after root canal preparation with mechanized instruments.(A) Buccal view of superimposed 3D models before (green) and after (red) root canal preparation.(B) Lingual view of superimposed 3D models before (green) and after (red) root canal preparation.(D) Representative cross-sections of the superimposed root canals before (green) and after (red) preparation at the cervical (c), middle (m) and apical (a) thirds.

Table 2 .
The search strategy using during the literature review.

Table 4 .
Summary of studies included in this literature review according to author and year of publication, sample used, sample selection features, instrumentation system and final apical diameter evaluated.