Prevalence of defects and fractures in nickel-titanium instruments after single use in patients

doi:10.21203/rs.3.rs-3949280/v1

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Prevalence of defects and fractures in nickel-titanium instruments after single use in patients

https://doi.org/10.21203/rs.3.rs-3949280/v1

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We investigated defects and fractures in nickel-titanium, engine-driven, endodontic instruments/files that had been single-used in patients` mandibular and maxillary molars. A total of 169 instruments [n = 113 ProTaper Next® (PTN) (Dentsply Maillefer, Ballaigues, Switzerland), and n = 56 Reciproc® (R) (VDW, Munich, Germany)] were analyzed using scanning electron microscopy. A single operator had used the instruments under a standardized protocol in one patient`s molar. Two trained and calibrated evaluators analysed three locations for each instrument. For non-fractured instruments, each location/third (apical/medium/coronal) received one of the classifications: i) intact (no plastic deformation/no defect), or ii) crack and/or deformation/unwinding. For fractured instruments, the area of fracture was classified in: i) cyclic fatigue mode, or ii) shear mode (torsional fatigue). Chi-square test calculated frequency of defects. PTN and R presented the same low frequency of defects after one clinical use in patients` molars by the same dentist. Defects appear equally in apical, medium, and coronal – except for X3 PTN that had more defects in the coronal part. Only one clinical fracture occurred, with an X3 PTN instrument: a torsional fatigue fracture originated in a crack. It is appropriate to use either PTN or R, in a single use, to treat root canals of patients` molars.

Biological sciences/Biotechnology

Health sciences/Health care/Dentistry

Defects

Files separation

Fracture

Nickel-titanium

ProTaper Next®

Reciproc

ProTaper Next^® and Reciproc_®instruments presented the same low frequency of defects (4.33%) under SEM, after single-use in patients` molars by the same dentist.

One X3 ProTaper Next^® instrument had a cyclic fatigue fracture, originated in a crack.

Although an instrument separation/fracture does not always reduce the root canal treatment prognosis (even when the instrument remains inside the canal); it might affect the outcome of endodontic treatment. Studies showed that instrument fractures negatively affect the outcome of endodontic treatments in cases where microbial control is compromised, or periapical lesions preexisttive ^1–3.

In the early era of engine-driven nickel-titanium (NiTi) instruments a high risk of instrument fracture existed. In spite of the improvements of current NiTi instruments in relation to pioneers NiTi, the risk of instrument separation is still a concern for the practitioner ⁴, mainly because this event might occur without previous sign of instrument distortion ⁵.

Instrument separation in endodontics occurs mainly as a consequence of the metal fatigue (cyclic fatigue and torsional fatigue), manufacturing defects (micro-cracks or pits), corrosion of the instrument in the presence of NaOCl ^6,7, or a combination of these factors. Other reasons could be the lack of operator experience ^8–11, different operators included in a study (undergraduate students, specialists etc.), multiple uses of instruments ^12,13, and lack of coronal preflaring and glide path creation ^14–16.

The fabrication of NiTi instruments with different alloys, different designs, and different kinematics, have been attempts to reduce the potential fractures. Engine-driven instruments fabricated with M-Wire NiTi alloy has increased the resistance to cyclic fatigue in approximately 390% in relation to those instruments fabricated with the regular NiTi alloy ¹⁷. The design of ProTaper Next® system, for example, makes it rotates in an eccentric manner, away from the instrument's center of mass, which allows only two of the cutting edges contact the canal wall, when the other two rotates freely. This design aims to dislocate dentin debris in an apicocoronally direction and to reduce spiral twisting, consequently reducing fractures ^18,19. The reciprocating kinematics also intend to reduce the percentage of fractures, because it reduces torsional and cyclic fatigue. Reciproc® instruments initially rotates in counterclockwise direction, where the instrument penetrates and cuts the dentin, to further rotates in the opposite direction, where the instrument is released ^20,21.

Clinical and laboratorial studies have great variation in the rates of instrument separation/fractures, due to the differences in methodology and the specific outcomes evaluated. Authors found only two fractures in 1136 Vortex Blue NiTi instruments (sizes 30 and 45) that were used multiple times ²². On the other hand, a higher number of fractures (n = 28) were found from a total of 122 used ProTaper S1 instruments discarded at an endodontic university clinic over a period of 17 months ²³. Another study showed that a clinical evaluation of 3,780 root canal preparations done in primary and retreatment cases with the Reciproc® instruments led to the following resuls: 0.4% of separation per tooth and 0.2% per canal ²⁴.

Up to date, the clinical approach of discarding NiTi instruments after single use is the choice to prevent fractures. However, contemporary evidence has stated that separation/fractures of instruments has dropped recently and seems to be a minor problem in current endodontics ^11,23–26. This statement could be a reason for using NiTi instruments not as a single use, but perhaps, two or three uses without significantly increase the risk of instrument fracture.

Considering that the clinical use in patients` teeth is an important variable to investigate defects and fractures, this project collected rotary (ProTaper Next®) and reciprocating (Reciproc®) instruments that had been single used in a standardized manner in patients` molars. The sample of instruments originated from a clinical trial, where there was only one operator. Therefore this in vitro study analyzed defects and fractures in rotary and reciprocating NiTi instruments after a single clinical use by a single dentist under scanning electron microscopy.

The first null hypothesis was that the frequency of defects and fracture would be equal between location/thirds (apical, medium, coronal) within the same instrument type. The second null hypothesis was that no significant difference would exist in the frequency of defects and fracture between different types of instruments (ProTaper Next® and Reciproc®).

Study Design

This was a cross-sectional in vitro study that used scanning electron microscopy (SEM) to investigate defects and fractures in nickel-titanium (NiTi), engine-driven, endodontic instruments that had been single-used in patients` molars. The study was conducted in accordance with relevant guidelines and regulations of the Consolidated Standards of Reporting Trials Statement (CONSORT). The ethical approval for this current study was integrated with an umbrella clinical study, previously approved by the Research Ethics Committee of Ceuma University under the registration: CAAE 48302115.9.0000.5034 and registered in an online platform for clinical trials of the Brazilian Registry of Clinical Trials under the protocol REBEC U1111-1182-2800.

All methods were carried out in accordance with relevant guidelines and regulations and performed in accordance with the Declaration of Helsinki. Informed consent was obtained from all participants.

A total of n = 169 endodontic rotary and reciprocating NiTi instruments were analysed, as follows: 1) rotary n = 113 (crown-down technique) from the ProTaper Next® system (PTN) (Dentsply Maillefer, Ballaigues, Switzerland), including n = 31 X1, n = 31 X2, n = 31 X3, and n = 20 X4; 2) reciprocating n = 56 (single-file technique) from the Reciproc® system (R) (VDW, Munich, Germany), including n = 28 R25, and n = 28 R40.

Use of endodontic NiTi instruments

A single operator used the endodontic instruments/systems at random to clean and shape root canals of patients` molars. Patients were 18 to 66 years old, with indication to treat asymptomatic maxillary or mandibular molars with root curvatures less than 25^{o 27}, sufficient crown tooth structure that allowed a straightforward rubber dam isolation, and without advanced pulp chamber and/or root canals mineralization.

The X1 (17/04), X2(25/06), and X3 (30/07) PTN instruments were sequentially used (crown-down technique) in mesial canals (MB and ML) of mandibular molars and buccal canals of maxillary molars (MB1, MB2, and DB), having X3 as the master apical file. For the distal (D) canal of mandibular molars and palatal (P) canal of maxillary molars the same sequence was used (X1, X2, X3), added the X4 (40/06), as the master apical file. Each instrument/sequence had a single use i.e., they were used in one molar.

The R25 instrument (single file technique) was used in mesial canals (MB and ML) of mandibular molars and buccal canals of maxillary molars (MB1, MB2, and DB), and R40 (single file technique) was used in distal (D) canal of mandibular molars and palatal (P) canal of maxillary molars. Identical to the PTN system, each R instrument had a single use i.e., they were used in one molar.

Instruments were used according to the manufacturers` recommendation. Cleaning/shaping technique was standardized in relation to the confection of a glide path before shaping (using K-files #08, #10, and #15), foramen patency (K-file #10), number of movements applied to the NiTi instruments (in-and-out repetitions) and irrigation (15 mL, pH = 11, 2.5% sodium hypochlorite + final flush with 2mL 17% EDTA and 2mL2.5% sodium hypochlorite).

After using, the instruments were collected, decontaminated in an ultrasonic cleaner with 99% absolute alcohol for 90 s, and sterilized in autoclave.

Scanning Electron Microscopy (SEM) Analysis

All the 169 instruments were visualized using SEM (Tabletop Microscope TM3030; Hitachi, Tokyo, Japan) under x50, x100, and x300 magnifications, in 3 location/thirds (apical, medium, and coronal portions), and in 1 surface (front). Instrument “third” was defined as approximately 6 mm-interval. As control, one brand-new instrument of each type was identically visualized using SEM. To know the exact location to be visualized, we positioned an electronic caliper besides the instrument pointing the location to be photomicrographed.

Fractured instruments were measured under a stereoscopic magnifying glass to calculate the length of the tip fragment, using the rest of the fractured instrument length as a parameter. For SEM fractographic examination, the fractured instrument was mounted on a stub with the long axis perpendicular to the beam (20x to 600x). Two trained and calibrated evaluators analysed the photomicrograph of instruments. Both evaluators joined a one-hour training and discussion session before starting. One calibration session was conducted using 50 photomicrographs chosen at random.

For non-fractured instruments, each location/third (apical, medium, coronal) received one of the following classifications: i) intact (no plastic deformation/no defect), or ii) crack and/or deformation/unwinding²².

For fractured instruments, the area of fracture was analysed under magnifications from 20 to 600x and it was classified in: i) cyclic fatigue mode, or ii) shear mode (torsional fatigue) ²³. Figure 1 shows the study's methodological scheme.

Data Analysis

For non-fractured instruments, the classifications (intact, crack, deformation/unwinding) were recorded according to the instrument type (X1, X2, X3, X4, R25, R40) and the location/third (apical, medium, coronal). Chi-square tests and likelihood ratios were used to calculate frequency of defects between instrument thirds for the same type of instrument, and to calculate frequency of defects between type of instruments. Fractures (frequency and percentage) were descriptively reported. Analyses were conducted using SPSS 26.0 software (IBM, Armonk, NY, USA), with a significance level of 5%.

Table 1 shows the frequency of defects between instrument thirds for the same type of instrument. There was difference (P < 0.05) between thirds only for X3 instrument from PTN system, where the coronal third had more frequency of defects.

Table 1

Frequency (absolute and relative %) of defects between instrument thirds for the same type of NiTi endodontic instrument: ProTaper Next® system (X1, X2, X3, and X4), and Reciproc® (R25, and R40). Defects included: crack and/or deformation/unwinding.
Instrument	Location/Third	Defects		p-value
Instrument	Location/Third	No	Yes	p-value
X1 (n = 31)	Coronal	29 (93.5%)	2 (6.5%)	^#0.809
	Medium	29 (93.5%)	2 (6.5%)
	Apical	30 (96.8%)	1 (3.2%)
X2 (n = 31)	Coronal	30 (96.8%)	1 (3.2%)	^#0.429
	Medium	30 (96.8%)	1 (3.2%)
	Apical	28 (90.3%)	3 (9.7%)
X3 (n = 31)	Coronal	27 (87.1%)	4 (12.9%)	*0.043
	Medium	30 (96.8%)	1 (3.2%)
	Apical	31 (100%)	0 (0.0%)
X4 (n = 20)	Coronal	19 (95.0%)	1 (5.0%)	*0.237
	Medium	20 (100%)	0 (0.0%)
	Apical	18 (90.0%)	2 (10,0%)
R25 (n = 28)	Coronal	26 (92.9%)	2 (7.1%)	*0.241
	Medium	27 (96.4%)	1 (3.6%)
	Apical	28 (100%)	0 (0.0%)
R40 (n = 28)	Coronal	28 (100%)	0 (0.0%)	*0.363
	Medium	27 (96.4%)	1 (3.6%)
	Apical	28 (100%)	0 (0.0%)
#Chi-square, *Likehood ratio

Table 2 shows frequency of defects between different type of instruments. In total the average of defects was 4.33%. There was no difference (p > 0.05) in relation to the frequency of defects between instruments.

Table 2

Frequency (absolute and relative %) of defects between different type of NiTi endodontic instruments: ProTaper Next® system (X1, X2, X3, and X4), and Reciproc® (R25, and R40). Defects included: crack and/or deformation/unwinding.
Instrument	Defect		p-value
Instrument	No	Yes	p-value
X1 (n = 31)	88 (94.6%)	5 (5.4%)	*0.601
X2 (n = 31)	88 (94.6%)	5 (5.4%)
X3 (n = 31)	88 (94.6%)	5 (5.4%)
X4 (n = 20)	57 (95.0%)	3 (5.0%)
R25 (n = 28)	81 (96.4%)	3 (3.6%)
R40 (n = 28)	83 (98.8%)	1 (1.2%)
*Likehood ratio
*3 locations for each instrument (n = 31) were analyzed

Figure 2 illustrate three situations of defects in different parts for the same type of instrument: (A) crack; (B) deformation/unwinding; and (C) crack propagation.

Only one instrument had fracture (X3 instrument from PTN; Fig. 3), and the fracture mode was classified as torsional fatigue (shear mode). This fracture originated in a crack. The fractured instrumented measured 21 mm – corresponding, then, to an apical third fracture location, at 4 mm from the tip. This fracture corresponded to 0.59% from the total of instruments used in this study, 0.88% from the total of PTN/rotary instruments, and 3.22% from the total of X3 instruments.

This study aimed to investigate defects and fractures in rotary and reciprocating NiTi instruments following clinical use, and it was able to contribute to the body of knowledge showing low frequency of defects and fracture in contemporary NiTi instruments - after a single use in patients` molars. Our results found that both kinematics had a few defects, and we found no statistical difference between instruments (these findings are showed in Table 2). When parts of the instrument were analyzed, there was no difference in the frequency of defects comparing the apical, medium, and coronal thirds/location for most of the instruments. The only instrument that had more defects in the coronal third (compared to the medium and apical) was X3 from PTN. These results rejected our first null hypothesis because the frequency of defects and fracture was different between location/thirds (apical, medium, coronal) within the same instrument type. However, our second null hypothesis was accepted because no difference existed in the frequency of defects and fracture between PTN and R.

Similarity in the frequency defects between PTN and R means that similar number of defects were induced after the clinical use and/or manufacturing defects already existed. Although these instruments have different kinematics and designs, they are fabricated with the same alloy, the M-Wire alloy - which is manufactured following a thermomechanical processing procedure^24–26. The mechanical performance of NiTi alloys is extremely sensitive to the microstructure and the associated thermomechanical treatment history; and the thermal process have proved to improve fatigue resistance and microstructure of NiTi instruments ^27–31. Therefore, the quality and appropriability of the M-Wire alloy may be responsible for the low frequency of defects found for PTN and R.

Although we found no statistical difference between instruments in relation to the defects after using, when we look at the numerical frequency of defects, we may notice that small instruments had more defects compared to the bigger instruments. For example, in the 28 R25 evaluated instruments, there were 3 defects, while in the 28 R40 evaluated instruments there was only 1 defect. Similar pattern occurred for PTN instruments. This finding raised awareness about the possibility that small instruments may acquire more defects - which may be related to the lower alloy mass ^32–36 or for being used in a root canal that is still underprepared in relation to the bigger instruments.

In fact, either smaller or bigger instruments are prone to defects and fracture if the operator is illiterate on mechanical functionality. Recognition of the physical properties and stress limitations of files is critical. Rotation of a file with a large diameter in a curved canal increases cyclic fatigue compared with a file with a smaller diameter. To reduce cyclic fatigue of an instrument in a curved canal, a file with a smaller diameter should be used with reduced rotational speed. In the meantime, rotating a small diameter file in a tight canal makes it more susceptible to fracture because of torsional fatigue. To reduce torsional fatigue of an instrument, the rotational speed should be increased, and the file should be used with light apical pressure ¹.

Last decade`s endodontic literature presents several evidence in relation to defects and fractures of NiTi instruments ^5,8,9,36; however current instruments have improved, and the strengthen of this study is the ability to analyse contemporary instruments, single-used in patients` molars by only one endodontist – which is very important because it reduces variability between operators. Importantly, we are aware that the canal complexities also play an important role in produce instruments stresses – and because of this all cases were selected based in similar curvatures, and a coronal enlargement and glide path was performed before instrumentation ^27,37.

Regarding the comparison of defects between different thirds/location of instruments, X3 from PTN was the only instrument that had a statistical difference: from the 31 X3 analyzed, 4 defects occurred in the coronal third, 1 defect in the medium third and 0 in the apical third. This circumstance may be related to the only clinical fracture occurred in this study – with a X3 instrument.

Analysing the ‘fracture outcome’ - the fracture occurred with the PTN rotary instrument corresponded to 0.88% of all PTN instruments used, while R had 0%. The photomicrograph analysis showed that the X3 fracture mode corresponded to torsional fatigue, and the fracture originated in a defect (crack), located at the apical third (at 4 mm from the tip) of the active part of the instrument. Torsional failure occurs when the tip of the instrument binds at the canal walls while the shank of the file (driven by the handpiece) continues to rotate. Consequently, shear fracture of the material occurs when the maximum strength of the material is exceeded. Literature have shown that cyclic fatigue seems to be more prevalent in curved root canals, whereas torsional failure might happen even in a straight canal. Authors recommended that instruments be used in a reciprocating motion when either torsional or cyclic fatigue is expected to occur (while shaping a severely curved canal) and that the instruments be discarded after use to prevent instrument fracture. In fact, in our study, reciprocating motion may have prevented a torsional fracture (since it occurred only with one rotary instrument) ^34,38. A recent systematic review stated that there is no difference in the clinical incidence of fracture of NiTi instruments between reciprocating and rotary motions. However, the systematic review found an overall incidence of files fracture of 2.27%, with a trend for higher incidence with rotary motion (2.43%) than with reciprocating (1.0%), though without significant differences ³⁹.

Rotary (PTN, ProTaper Next®) and reciprocating (R, Reciproc®) instruments presented the same low frequency of defects after one clinical use in patients` molars by the same dentist. Comparing the presence of defects between different part of the instrument, they appear equally in apical, medium, and coronal part – except for X3 from PTN system that had more defects in the coronal part. Only one clinical fracture occurred, with a X3 PTN instrument. Implication for the clinical practice may be the appropriability of using either PTN or R instruments to treat root canals of patients` molars; and single use is indicated, because - even with the presence of very few defects – one of them can repercuss in a fracture.

Author Contribution

P.S.O. : investigation, methodology, writing original draft. K.C. P. : investigation, methodology, writing original draft. L.G.D. : investigation, methodology. R. G.S.: writing, review & editing supervision. J. B.: project administration, data curation, conceptualization. E. M. C.: software, formal analysis. E. M. M.F.: software, formal analysis. M. C. F.: software, formal analysis. C. N. C.: project administration, data curation, conceptualization. All authors have read and approved the manuscript.

Data availability statement: additional research data are not shared

Funding statement: Foundation for Research and Scientific and Technological Development of Maranhão - FAPEMA, Brazil (PAEDT-01918/15 and INFRA-03015/18).

Conflict of interest disclosure: Authors deny any conflict of interest related to this study

Ethics approval statement: The ethical approval for this current study was integrated with an umbrella clinical study, previously approved (CAAE 48302115.9.0000.5034) and registered in an online platform for clinical trials (REBEC U1111-1182-2800).

Patient consent statement: N/A

Permission to reproduce material from other sources: N/A

Clinical trial registration: N/A

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No competing interests reported.

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Prevalence of defects and fractures in nickel-titanium instruments after single use in patients

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