Reciprocating instruments made from M-wire alloy have been proposed to reduce the risk of fracture. No information is available on the surface alteration after single and multiple uses in root canals. Two reciprocating NiTi instruments were used on extracted teeth up to three times. ESEM/EDS analysis was conducted to determine defects, alterations, and wear features of the apical third of instruments and metallographic analysis was performed on the cross-section of new and used instruments to compare alloy properties. Topography of apical portion was evaluated by AFM before and after uses. Extracted single-rooted teeth were divided into two groups and instrumented according to the manufacturer's recommendations with: (A) WaveOne Primary and (B) Reciproc R25. Each group was divided into three subgroups according to the number of canals instrumented: 1, 2, and 3, respectively. Chi square test was performed to verify homogeneity of defects distribution and GLM to evaluate the differences of RMS at baseline and after use for both groups (α level 0.05). No instrument fractured and no spiral distortions were observed under optical microscope even when the number of uses increased. Not significant differences were found for WaveOne and Reciproc. Blades presented a wrapped portion in WaveOne group and a more symmetrical feature in Reciproc group. Metallographic analysis revealed in both groups the presence of twinned martensitic grains with isolated flat austenitic areas. Both instruments demonstrated limited alteration, such as tip deformation and wear. This study confirmed the safe clinical use of both instruments for shaping multi-rooted teeth. SCANNING 36:517–525, 2014. © 2014 Wiley Periodicals, Inc.
The use of reciprocating motion as an alternative to the conventional continuous rotation has been recently suggested to be convenient in the instrumentation of curved canals with the employment of a single NiTi file (Kim et al., 2012). The balanced force technique was introduced by Roane et al. in 1985 for the instrumentation of even severely curved canals and nowadays applied in alternating rotation. The alternated movement of greater counterclockwise (cutting direction) and reverse clockwise motion (release of the instrument) makes continuously progress the instrument toward the apex of the root canal. This is documented to reduce the risk of cyclic fatigue caused by tension and compression (De-Deus et al., 2010; Varela-Patiño et al., 2010; Kim et al., 2012; Plotino et al., 2012). According to the manufacturers, the employment of a single NiTi instrument used with reciprocating motion could reduce the risk of torsional failure, leaving cyclic fatigue as only cause of instrument separation.
In 2011, two different reciprocating single file systems were launched to simplify root canal preparation. Only one single shaping file is required to provide the canal with an adequate size and taper. WaveOne files are designed with three radial lands in the apical 5 mm and then changes to a bladed design similar to ProTaper for the remaining 9 mm, while Reciproc have an S-shaped cross section with two cutting blades similar to MTwo NiTi rotary system along its entire length (Prichard, 2012). Both brands of instruments are produced with a new nickel–titanium alloy (M-wire) that presents improved mechanical properties (Plotino et al., 2012). Since WaveOne and Reciproc files tested in reciprocation had the longest fatigue life, the synergy of reciprocation and M-Wire alloy significantly improves the fatigue resistance of NiTi files and may reduce the risk for instrument fracture in root canal (Pirani et al., 2013).
According to the manufacturer's instruction, the single use instrument may be used up to three to four root canals of the same molar or of the same patient. Root canal preparation may damage NiTi instruments resulting in wear and deformation (Yamazaki-Arasaki et al., 2012) with a resultant decrease in cutting efficiency (Sattapan et al., 2000).
The aim of the present pilot study was to evaluate the surface alterations and topography of two different M-wire reciprocating instruments before and after shaping root canals. Moreover, the microstructure was metallographically analyzed and a comparison between different groups of files was made.
Materials and Methods
Collection of Instruments
Twelve brand new files were mounted on a stub in a standardized position and examined with environmental scanning electron microscopy (ESEM EVO50 EP, Carl Zeiss NTS GmbH, Oberkochen, Germany). Photomicrographs were taken at magnifications ranging from 70× to 5,000× at the tip and at 4 mm from the tip. Energy dispersive X-ray spectrophotometry (EDS Oxford Inca Energy 350, Oxford Instruments, Abingdon, Oxfordshire, UK) examination was performed to analyze the composition of NiTi alloy.
Single-rooted extracted human teeth with radiographically narrow and straight canals were selected and cut to obtain segments of the same length of 15 mm. Once that canal patency was verified with a K-file #10, glide path was achieved with K-files #15 and #20 at the full working length previously determined by radiographs. Irrigation was performed at any instrument change, with a total amount of 3 ml of 5% NaOCl (Niclor 5, Ogna, Muggiò, Italy) and 3 ml of 10% EDTA (Tubuliclean, Ogna, Muggiò, Italy) solutions, equally shared according to the number of instruments employed. EDTA was always placed and kept in canals before using any reciprocating system.
Two trained operators performed all the endodontic instrumentations. Straight root canals were shaped with WaveOne Primary (Dentsply Maillefer, Baillagues, Switzerland) reciprocating files (Group A) and Reciproc R25 (VDW GmbH, Munich, Germany) (Group B) with a slow in-and-out pecking motion until the silicone stop positioned at WL was in contact with the reference point. The reciprocating motor (X-Smart Plus Dentsply Maillefer) was setup at the “WaveOne All” and “Reciproc All” configuration. All the instruments were used with a 6:1 reduction handpiece (Dentsply Maillefer) and divided according to the number of instrumented canals into three subgroups: (i) files single-used (n = 6), (ii) files used in two canals (n = 6), and (iii) files used in three canals (n = 6). After coded instrumentations, the instruments were washed in an ultrasonic bath containing detergent for 15 min.
The collected instruments were then mounted on a stub in the standardized position and re-analyzed under ESEM at the same magnification and positions as they had been before usage. The pre- and postoperative photomicrographs of the instrument's heads and cutting edges were compared and evaluated for the following criteria: microfractures, metal flash, metal strips, blunt cutting edge, disruption of cutting edge, debris, and tip flattening. Chi/squared test have been performed on both groups in order to verify homogeneity of defects distribution. Homogeneity was related to deviation of the observed defects with respect to the estimated defects in the null hypothesis. Significance level was set as 0.05.
Qualitative EDS examination (EDS Oxford Inca Energy 350, Oxford Instruments) was performed on used instruments to identify the composition of some particles adherent to the file surface.
Two observers assigned a score in a blind manner to the condition of each instrument from each group on each use and a consensus was reached between the two examiners regarding any discrepancies.
After ESEM-EDS analysis, specimens were cut with water-cooled low-speed diamond saw (Micromet M, Remet, Bologna, Italy), embedded in epoxy resin, wet ground with 600–2,000 grit size SiC papers and polished with a 3 and 1 µm diamond paste. Specimens were etched following the previously described procedures (Pirani et al., 2011) to disclose the microstructure of the NiTi matrix. Subsequently, these samples were analyzed under light microscope (Zeiss AXIO, Carl Zeiss NTS GmbH) to evaluate their metallographic features and with ESEM equipped with EDS (EDS Oxford Inca Energy 350; Oxford Instruments). Brand new instruments were used as control for metallographic inspection.
All the micrographs obtained were analyzed by using digital imaging software and the ESEM pre-instrumentation and post-instrumentation images were superimposed to assess modifications of the file outline with Adobe Photoshop (Adobe Photoshop CS4, Adobe Systems, Inc., San Jose, CA); ImageJ for Microscopy (McMaster Biophotonics Facility, Hamilton, Canada) for those obtained by light microscope.
Eight files (n = 4) were attached to metal holder on the atomic force microscopy (AFM) and analyzed on four points at 4 mm from the tip of the file. The same analysis was performed on instruments before and after three uses. Measurements were obtained with the atomic force microscope Burleigh Vista (Burleigh Instruments, Inc., Burleigh Park Fishers, NY) at room temperature and atmospheric pressure with 0.5 Hz speed scan. Scanned areas were squares of 1 µm × 1 µm. AFM images were processed with ImageStudio 3.3 software (Burleigh Instruments, Inc.) to obtain the values of the root mean square (RMS).
Deviation of distribution of the identified defects with respect to the estimated defects under null hypothesis has been evaluated by using chi/square as homogeneity test. GLM model for repeated measures was performed to compare the differences of RMS at baseline and after uses for Reciproc and WaveOne. α level was a priori set as 0.05.
During the root canal preparation no instruments fractured nor there were any significant differences in the incidence of the defects examined with respect to the different numbers of root canals prepared.
No significant deviations of distribution of the identified defects with respect to the estimated defects under null hypothesis were found both for WaveOne and Reciproc groups. Consequently defects formation in both groups of instruments cannot be attributed to systematic behavior.
Indiscernible defects were more evident in Reciproc than in WaveOne group: the frequency of detected metal defects is represented in Table I.
|New||Three uses||New||Three uses|
|Blunt cutting edge||0||0||0||2|
|Disruption of cutting edge||0||0||1||2|
In WaveOne groups, blunt cutting edge and disruption of cutting edge were absent in brand new instruments and increased after three uses. Indiscernible metal strips were found in unused and decreased in used instruments. Tip flattening was never observed irrespective to the different numbers of root canals prepared (Fig. 1(a,b)). In every re-used instruments, debris were observed by ESEM adhering on the surface between the cutting edges or on the tip. EDS analysis indicated the presence of carbon, oxygen, calcium, and phosphorous in this contaminating material.
In Reciproc groups, the incidence of metal defects and tip flattening was higher in three times used files than in brand new ones (Fig. 2(a,b)). In both brands of instruments, microfractures were absent in new files but were detected in two of the observed instruments. A single microcrack formation perpendicular to the axis of the instrument was observed in two 3 times used files: one Reciproc and one WaveOne (Fig. 3(a,b)).
Debris were not found in used Reciproc instruments.
Surface analysis conducted by ESEM on new WaveOne and Reciproc files confirmed the presence of a regular surface texture with milling marks all along the surface, mainly perpendicular to the long axis of the files (Fig. 4(a,b)). These machining marks did not present alterations and were more shallow in WaveOne group if compared to those observed in Reciproc files. The two brands of new NiTi instruments presented remarkable differences all along the blade surface. In WaveOne groups the blades resulted rounded with a wrapped portion similar to a “wave-curl” that during canal instrumentations modified and flattened with an indiscernible loss of metal (Fig. 5(a–d)). In brand new Reciproc files, the blade surface was more regular, symmetrical on both sides and during clinical use, it flattened becoming more sharp-edged.
The tip of the brand new instruments was tapering and with a terminal rounded portion. After several times of being used, in WaveOne groups the outline was not altered (Fig. 6(a,b)). The superimposition of this part in WaveOne groups demonstrated lower surface wear over the three uses. The final part of the tip of every single-used R25 resulted worn-out and, in one case, it was observed a missing portion of several micrometer (Fig. 7).
No distortion of the spirals was observed in both groups.
Observations under optical microscope of specimens after metallographic etching revealed little differences between the alloy morphology of the two instruments. In both Reciproc and WaveOne metallographic observations revealed the presence of austenitic flat, non-symmetrical isolated areas embedded in a martensitic matrix. A highly predominance of symmetrical twinned martensitic phase is present in both groups, with a decreasing grain dimension at the tip.
Only in correspondence of the handle, no austenitic isles were found. In both groups, optical micrographs did not reveal different structures on the outer and the inner surface. The M-Wire NiTi alloy presented typical martensitic matrix with its twinning elements: “primary” formed martensites (larger ones) and “secondary” martensite clusters (smaller ones) (Otsuka and Ren, 2005). Martensitic grains were randomly oriented and only in the 1-mm tip portion of three uses groups, they were rearranged and directed in a unique direction, preferential in parallel to the long axis of the instrument (Fig. 8(a,b)).
The EDS analysis conducted on inclusions detected within the grains excluded a metallic composition whereas confirmed a composition of roughly 55% nickel and 45% titanium for the NiTi matrix.
According to the AFM images, the mean difference of topographic surfaces of new files and used files were obtained (Table II). Difference between means in RMS values at baseline and after uses was statistically significant and higher for WaveOne than Reciproc (P < 0.001).
|Mean difference (SD)||181.08 (10.952)||248.51 (57.96)|
The present pilot study aimed to compare the morphology of metal surfaces of two different brands of new reciprocating files after several times of being used.
SEM analysis was selected for the assessment of instrument wear because is a well-documented method to accurately evaluate morphological characteristics (Eggert et al., 1999; Kim et al., 2010; Pirani et al., 2013) and deformation of NiTi files (Troian et al., 2006). In the present study, visible inspection was not considered a reliable methodology to inspect the reused M-Wire NiTi instruments because the time of instrumentation is not enough to create plastic and macroscopically visible deformations.
The use of single file technique results in significantly shorter preparation time compared with other full sequence systems (Paqué et al., 2011; Bürklein et al., 2012). However, comparing the time of active instrumentation into the canal for each file, this is considerably higher for Reciproc and WaveOne.
During the present laboratory test, no instrument fractured and no macroscopic signs of plastic deformation or spiral distortion were observed even when the number of uses increased up to the third one. This confirmed that the number of canals treated was not too high and suggested a safe use in multi-rooted teeth.
The criteria to evaluate signs of wear were established for this study according to Eggert et al. (1999), excluding phenomena (pitting, fretting, corrosion) that were judged not suitable for this case.
Surface finish has an effect on the crack initiation process (Cheung and Darvell, 2007) and our findings showed a smooth superficial texture of new NiTi rotary instruments, revealing the absence of manufacturing defects left by the grinding process. Metal defects resulting from industrial process are considered disturbances of continuity or homogeneity, impairing the physic-mechanical properties of such materials. Structural irregularities of the instrument surface may compromise its integrity during clinical use making the file more susceptible to fracture (Kim et al., 2010). However, not every metal defects are essential to the functioning of the endodontic instrument.
In both groups, all the reused instruments presented limited surface defects, thus confirming that the operation number did not alter the cutting edges. The re-used WaveOne remained intact and presented little sign of deformation up to the completion of the third instrumentation.
Only two 3 times used instruments presented a microcrack, running along the machining grooves. These fine crack lines did not propagate and fracture. All of the instruments showed well-preserved milling grooves both in used and unused status.
EDS analysis highlighted the presence of dentin debris lodged along machining grooves and this could encourage the single use of endodontic instruments to reduce the possibility of cross contamination.
The absence of plastic deformation could be explained considering the reduced time of instrumentation for the single file, not sufficient to create a severe damage of the instrument. In some cases, WaveOne used two times exhibited a more evident wear than other three times used instruments. This could be related to the diameter of the endodontic canal and the variable hardness of the dentin. Our findings are in accordance with Eggert et al. (1999), showing that surface wear is not strictly correlated with the number of uses. In fact, the presence of defects on NiTi reciprocating instruments is not directly associated with the number of root canals instrumented. This result could be associated to the possibility that, each time the instrument touched the canal surface, its texture is abraded.
The micrographs obtained at the tip of single-used instruments revealed how the friction forces generated by the interaction of the file with the root canal walls were able to damage the terminal portion of the instruments. The most frequent defect observed was associated with a wear of the tip, thus resulting the part of the instrument that most suffers solicitations. The clinical effects of the damage of this part on the canal centring ability is difficult to be evaluated.
In Reciproc group, the more visible wear of the tip could be related to the high apical force applied during instrumentation: the missing portion observed in one file, suggested a strong friction of this part during the scouting phase of the root canal space. The creation of a glide path is not strictly recommended by manufacturer of Reciproc, differently from WaveOne where the apical portion of the tip remained rounded and did not present surface alterations.
It is noteworthy that canal enlargement before using NiTi instruments would reduce the torsional stress (Shen et al., 2009) and the creation of a glide path also for the Reciproc instrument could be recommended to lower the possibility of file wear.
A distinction is essential between metal defects of the fine structure and coarser defects that form during each stage of the manufacture and processing. Metal defects such as pores, cavities, cracks, flakes, forging defects, and laps may be local distributed in limited zones or distributed throughout the entire volume or surface of a product (ASM International, 1987).
AFM quantitative analysis was conducted to provide further information about the superficial topography of apical portion in endodontic files. Although this technique is a well-documented tool for material investigation (Valois et al., 2005; Inan et al., 2007), it was difficult to maintain the probe on the apical portion of the file surface to perform the scan. Three-dimensional images of AFM confirmed an increased surface wear of the apical portion of Reciproc and WaveOne files after having been used.
When observed with the optical microscope, etched NiTi files of every groups revealed similar and uniform microstructures on the outer and the inner surface, therefore confirming that the external surface was not interested by plastic deformation though being the most stressed part. Although manufacturing process of M-Wire NiTi rotary instruments is covert, it is well known that Nitinol is cold worked first in an austenitic and than in a martensitic state at the finished diameter.
Within austenite–martensite microstructure, non-metallic inclusions were assumed to be residuals of polishing diamond paste.
It was noteworthy the preferential orientation of martensitic grains in the tip portion of multi-used WaveOne files, parallel to the long axis of the instrument. This preferential direction is influenced by anisotropic stresses the instrument is subjected to. The characteristic lattice orientation within the grains have been mostly correlated to the external applied stress (Ganghoffer et al., 1991) that create high internal stresses in the surrounding of the new phase.
As a conclusion of this pilot study, defects formation in both groups of instruments cannot be attributed to systematic behavior. Changes in the superficial morphology of M-wire tested instruments revealed little amount of surface wear in the apical region of the active part of all the tested files that worked several times in straight canals. Major changes were observed in the tip of the Reciproc instruments. Those results confirmed the possibility to clinically use the reciprocating instruments for shaping multi-rooted teeth.
The authors would like to thank Dr. Fabrizio Tarterini for the AFM measurements.