Perfection of unused NiTi endodontic files , myth or reality ? A scanning electron microscope ( SEM ) study

Endodontic rotary files have been developed to improve the quality of root canal preparation. However, such instruments have a risk of separation. In most cases, the files are not checked for quality before use. Instead, they are often removed from the packages, sterilized and used without being examined for the presence of imperfections or debris. Practitioners were always to be blamed of such mishaps, but recently there has been some investigations claiming that these files are not flawless and far from perfection. A sample of 50 files from 5 companies were analyzed under scanning electron microscope (SEM) with 100× magnification and given to 3 blinded endodontic observers to investigate the presence of manufacturing defects. The products of these companies were: Protaper, K3fx, Easy Race, Wave One and Twisted File. All files from each company presented all the 6 categories of defects. Each file presented at least two types of defects. Significant variation was found between companies and defects (p value = 0.03). This study showed that all the brands that had been examined had manufacturing defects in addition to imperfection in packing conditions; however, the electropolished instruments had significantly less imperfection, particularly in term of grooves and scratches.


INTRODUCTION
Using rotary instruments to prepare the root canal system has improved the quality of root canal cleaning and shaping.Traditionally, canal preparation used to start from the apical third progressing to the coronal third, and then this sequence has been converted in which the canal preparation now starts from the coronal part progressing apically.This approach facilitates apical file size determination and reduces the stress of the files within the canal with less incidence of canal ledge and instruments fracture (Alapati et al., 2013;Kuhn et al., 2001;Karn et al., 2014).
Despite these advantages of the rotary instruments, studies reported that they are "not flawless" and have surface defects (Bonetti-Filho et al., 1998;Chianello et al., 2008).The grinding process across the grains of a NiTi wire, limits the overall strength of the instrument owing to the formation of machining defects along the flute surface and residual stress within the internal structure (Gambarini, 2001;Tripi et al., 2006;Yao et al., 2006).Electropolishing procedure (EP) during the manufacturing process of the ground NiTi files reduces the number of surface defects and residual stress.This procedure removes the outer layer of a metal from the manufactured files, leaving the surface free of microcracks, contaminants, and work-induced residual stress.Electropolishing is a method of surface finishing used by manufacturers of rotary NiTi instruments to remove surface defects that may remain after machining.It is a controlled chemical process that involves submerging the instrument, acting as an anode, into an electrolytic solution that contains a cathode.When a low current is passed through the solution, a balance is achieved between the formations of a passive layer and dissolving of the surface into the electrolyte, leading to selective removal of protruding surface defects; therefore, a leveling and smoothing of a rough surface will occur at a rate of approximately 3.5 µm/min.Electropolished instruments appear shinier to the naked eye than those that have not been polished (Callister, 2000).Recently, a new rotary endodontic file has been introduced and manufactured differently which is twisted file (TF).This manufacturing process includes twisting of a file blank, Rphase heat treatment, and specific surface treatment, to decrease the formation of machining defects during the grinding process (Alapati et al., 2003).Machining defects such as scratches and microcavities on the instrument surface can act as sites for crack initiation increasing the possibility of instrument fracture (Kuhn et al., 2001;Karn et al., 2004).
Endodontic instrument fracture might jeopardize the success rate of the whole treatment (European Society of Endodontology, 2006).Two modes of endodontic instruments fracture are recognized: torsional fracture that occurs when the instrument (generally the tip) becomes interlocked in the canal while the file shank continues to rotate; and flexural fatigue which occurs when the instrument continuously rotates freely in a curved canal generating tension/compression cycles at the point of maximum flexure, which eventually results in fracture (Parashos et al., 2004).
Causes of endodontic instrument fracture while preparing canals are several including operator skill, instrumentation technique, dynamics of instrument use, canal geometry, tooth type, effect of cleaning and sterilization, number of uses, instrument design, in addition to manufacturing process (McGuigan et al., 2013).
Manufacturing process of endodontic files might create irregularities in the file surface (surface imperfections) that appear as grooves, micro-cavities, debris and pits (Alapati et al., 2003).Such flaws can never be seen with the bare eyes and need tools with extremely high magnification such as scanning electron microscopy (SEM), the device that was used a lot to examine several AbuMostafa et al. 17 types of endodontic files either hand or engine-driven.The aim of this study was to check whether the practitioner is always blamed in cases of file separation inside the root canal system or does the blame fall also on the file imperfections.
Each specimen was carefully removed from its package and examined without any surface treatment, attached in a metal slot for SEM (JSM, 6360LV, Tokyo, Japan) micrographs with 100× magnification.
The digital images acquired from the SEM were placed in a folder for each company, thereby 5 folders were created then named anonymously as A, B, C, D and E. These folders were copied on 3 CDs, then given along with evaluation forms to 3 blinded, skillful, calibrated Endodontists to evaluate the defects in those Ni-Ti files.The evaluation form criteria includes: debris, grooves, microcavities, scraping, cracks and cutting tip of the file.
(1) Debris: Loose fragments or remnants of something not attached to the file.
(2) Grooves: Scratches or line-marks on the file surface.
(3) Microcavities: Pits or holes in the file surface.(4) Scraping: Part of the file surface that has been peeled off but is still attached.
(5) Crack: A partial split or separation in the file surface; a fissure.Data were analyzed using t test.

RESULTS
Figures 1 to 4 SEM captures show some of the manufacturing defects detected in the endodontic instruments examined in this study.The percent distribution of the evaluation criteria for each endodontic instrument is presented as shown in Table 1.Significant variation was found between products and defects (p value = 0.03).It was found out that all instruments examined in this study have grooves and scratches, but with variable percentage.EasyRace significantly has fewer grooves and scratches (36.67%) when compared with K3, Protaper, TF and Waveone that got 100, 93.33, 100, and 100%, respectively.Microcavities were found in all examined K3 files, and in almost 70% of the examined  EasyRace, Protaper, TF and Wave one files.Scrapings and cracks were also found in examined files, however those defects were less found in Protaper as compared to the other products.

DISCUSSION
One of the most significant imperfections found in this study were debris.Debris on the files decreases its dentine cutting efficiency and canal shaping ability; additionally, it was able to demonstrate that debris carried to the apical area impairs healing (Eldik et al., 2004).
Grooves, scratches and microcavities may present a concentration of corrosion and possibly become sites susceptible to instrument breakage.The findings of this study were consistent with Chianello et al. (2008) who found 0% microcavities in EasyRace before use.This is attributed to the electropolishing surface treatment of the EasyRace instruments.Furthermore, studies of Lopes et al. (2010) and Larsen et al. (2009) proved that the polished instruments are more resistant to fracture and displayed a significantly higher number of cycles to fracture when compared with non-polished instruments.
Having scrapings on the file's surface increases its binding possibility, therefore increasing the incidence of file fracture.Cracks are the most dangerous defect a file can have; when binding to the canal walls it fractures immediately if the file did not reverse the rotation.For Kuhn et al. (2001) in the breakage process, the crack nucleation stage is facilitated by a high density of surface defects and the successive fatigue failure is largely   due to a crack propagation process.
Originally, the NiTi rotary endodontic files were all machined to create the desired taper, cutting edge and the flutes design.Recent technology has enabled twisting the NiTi alloys (Twisted file™ [TF] SybronEndo, Orange, CA, USA); it is claimed that this technology increased torsional resistance, flexibility and strength of TFs as compared to ground files (Anderson et al., 2007).However, it seems that this technology did not enhance the surface finishing of the TFs.This study showed debris, scratches and grooves in 100% of the examined TFs, in addition to microcavities and scraping in more than 70%.Another recent technology of NiTi alloy is introducing the M-Wire (Dentsply-Tulsa Dental Specialties, Tulsa, OK, USA), WaveOne™ (DentsplyMaillefer, Ballaigues, Switzerland) is an example of a file system availing of this technology.Some studies reported increased resistance of instruments with M-Wire to cyclic fatigue when compared with conventional machined instruments (Johnson et al., 2008;Al-Hadlaq et al., 2010;Gao et al., 2010).Again, it seems that this innovative technology did not improve the surface quality of the instruments.Grooves were found in all of the examined WaveOne instruments, whereas debris was found in 90% and microcavities in 73.33%.

Conclusion
Advancements in the dental field are racing towards perfection.This is true as regards to instruments, materials and machines, but the Ni-Ti endodontic files are still far from perfect.This study showed that all the brands that had been examined had manufacturing defects in addition to imperfection in packing conditions; however, the electropolished instruments had significantly fewer imperfections particularly in term of grooves and scratches.

Figure 1 .
Figure 1.SEM capture of the apical 3 mm of a scanned file showing grooves.

Figure 2 .
Figure 2. SEM capture of a scanned file showing debris.

Figure 3 .
Figure 3. SEM capture of the apical 3 mm of a scanned file showing microcavities.

Figure 4 .
Figure 4. SEM capture of the apical 3 mm of a scanned file showing scraping.

Table 1 .
The percent distribution of the evaluation criteria for each endodontic instrument.