Abstract
Objective
To compare the cyclic fatigue resistance of conventional and heat-treated "replica-like" reciprocating instruments with their original counterparts under single and double curvatures and assess tip size discrepancies against manufacturer-reported values.
Methods
Eighteen instruments were used per group for the study. Two measurements at the instrument's tip were made with a digital calliper. Cyclic fatigue resistance was evaluated under single (60°, 5 mm radius) and double curvatures (60°, 5 mm radius each) in a 37°C water bath. Time to fracture (seconds) was recorded and analysed with appropriate statistical tests (p=0.05).
Results
The tip sizes of all instruments were smaller than the value purported by the manufacturers (0.25 mm) and outside the range of values obtained, with significant differences for all groups (p<0.001). Time to fracture were as follows: Reciproc R25: single curvature: 171.5±38.9, double curvature: 133.4±47.4; Reverso Silver: single curvature: 169.0±104.8, double curvature: 57.8±20.0; Reciproc Blue R25 single curvature: 355.4±86.4, double curvature: 140.5±67.7; Reverso Blue: single curvature: 359.5±102.8, double curvature: 142.9±69.0. Reverso Silver presented with a significantly lower time to fracture overall when compared with the Blue instruments (p=0.002) and with all instruments in double curvatures (p<0.05). In single curvatures, blue files had longer times to fracture (p<0.05). When comparing single versus double curvatures, only Reciproc R25 had no significant differences regarding time to fracture (p=0.54).
Conclusion
The tip sizes of the instrument tested were smaller than what is reported by the manufacturers. The cyclic fatigue resistance of the conventional “replica-like” instrument (Reverso Silver) was significantly lower than the blue "heat-treated" comparators. Double curvature hastened fracture.
Keywords: Endodontics, instrument fracture, laboratory testing, root canal instrumentation
HIGHLIGHTS
In the presence of “S-shaped" curvatures a “replica-like" (Reverso) and original (Reciproc) instruments had a reduced cyclic fatigue resistance.
The mechanical performance of Reverso heat-treated instruments was superior to the non-heat-treated “replica-like" comparator, but not the original file.
The apical size of all files tested was less than what reported by the manufacturers.
“Replica-like” instruments require robust independent testing before their clinical use.
INTRODUCTION
Using any tested type of engine-driven Nickel-Titanium (NiTi) endodontic instrument is recommended for root canal preparation, based on the best available evidence (1). This is common practice (2), and the outcomes with rotary and reciprocating kinetics are considered comparable (3, 4).
NiTi endodontic instruments can originate from different sources and manufacturers, including “replica-like” systems which are becoming increasingly available in the market with limited cost, thus becoming attractive for end-users. These have been defined as “having the same number (i.e. size), colour coding and similar (or equivalent) nomenclature to original brand instruments” produced by the original manufacturer (5). “Replica-like” instruments are quality tested by the manufacturers and their legal information is commonly available online. “Replica-like” instruments can be produced as patents are mostly valid in the countries they are applied to, for 20 years starting from the date of the application filing, though it is difficult to give the exact duration during which a design is protected. Importantly, “replica-like” should be differentiated from counterfeit instruments, that are purported as produced by the original manufacturer (5), with the latter not being the object of the present study and will not be discussed further. To the author’s knowledge, it is impossible to suggest an accurate estimate of the number of “replica-like” instruments commercially available globally.
There is a paucity of literature testing “replica-like” instruments before commercialisation. This is crucial as highlighted by the European Society of Endodontic S3 level clinical practice guidelines (1), where the term “tested” is mentioned explicitly. Some studies have assessed various features of some “replica-like” instruments recently, however, only two studies have assessed instruments with reciprocating motion (6, 7). A popular “replica-like” instrument producer is Access (Paris, France –manufactured by Shenzen SuperLine technology, Shenzen, China), which offers instruments comparable to Reciproc R25 and Reciproc Blue R25 (VDW Dental, Munich, Germany), including post-manufacture heat-treatment for the latter. Importantly, heat treatment is likely to improve fatigue resistance to fatigue when compared to conventional instruments (2). Post-manufactured heat-treated files are often described as “blue”, a term that will be used in the present manuscript.
Cyclic fatigue resistance test is commonly used to test the mechanical performance of engine-driven instruments as their fracture due to this form of stress has been associated with flaws during manufacturing, the presence of contaminants and issues related to instrument design (e.g. core diameter, tip size, and taper) (8–10). Breakage of an instrument in a curved canal is related to alternating tension and compression cycles when flexed, in the area of maximum curvature. Laboratory studies should include single and double (S-shaped) curvatures as the latter are common (11) and are associated with adverse effects on cyclic fatigue resistance of instruments, as highlighted by a seminal study (12). To the best of knowledge, no previous study has assessed "replica-like" files using "S-shaped" canals and compared “blue” with conventional instruments. Also, inconsistencies in instrument tip sizes are relatively unexplored.
Thus, the aims of the present laboratory study were: to compare the cyclic fatigue resistance using single and double curvatures of reciprocating “replica-like” conventional and heat-treated (Reverso Silver and Reverso Blue) versus the comparable original instruments (Reciproc R25 and Reciproc Blue R25) and to contrast their tip size against the values reported by the manufacturers. The null hypotheses tested were: that there are no differences in cyclic fatigue resistance between the above conventional and replica instruments when tested in reciprocating motion in single and/or double curvatures and that there are no differences in apical tip size between the files in question and the reference values.
MATERIALS AND METHODS
Seventy-two files (Reciproc R25, Reverso Silver, Reciproc Blue R25, and Reverso Blue; n=18 each) were evaluated. According to the manufacturer, the length, tip size, and taper of all files were 25 mm, 25, and 0.08, respectively.
Measurement of the Tip Size
Two measurements at the tip of the instrument were made with a 150 mm digital calliper (Vernier, Software & Technology, Beaverton, OR, USA) at 3.2× magnification in all the instruments, and a mark was made at 0.5 mm as the reference point for measurements. The mean value of three measurements was recorded for analysis.
Cyclic Fatigue Test
For each instrument type, the samples were randomly selected from different packs and distributed into two subgroups (n=9) according to the curvature type (single or double). Random selection was achieved using a computer algorithm program (http://www.random.org). This test was conducted using a custom-designed stainless-steel block that had slots with a lateral opening, a length of 18 mm, and an internal diameter of 1.5 mm. A curved segment (8 mm), either a single curvature with a 60° angle and a 5 mm radius of the curvature or a double curvature each with a radius of 5 mm and an angle of 60°, separated the initial (7 mm) and final segments (3 mm) of the slots (Fig. 1). To prevent the files from escaping the device, the slots were covered with transparent glass to allow observations.
Figure 1.
Artificial canal used to test cyclic fatigue. (a) Single curvature; (b) double curvature
To simulate clinical conditions, the device was immersed in a water bath heated at 37°C, and the temperature of the water in the plastic laboratory tray was confirmed using a laboratory thermometer (Ibdciencia, Alcorcón, Spain) (Fig. 2). Subsequently, each of the files was inserted into the device and connected to an E-Connect S Endo Motor (Eighteenth, Changzhou, China), which was activated immediately after positioning the file in the slot with Reciproc ALL mode and clockwise and counter clockwise rotations of 30° and 150°, respectively. Once the apical part of the instrument reached the most apical part of the slot, it was held at the same position in a vice. The time (s) until the fracture occurred (noted audibly and visually) was recorded using a stopwatch (Redmi 13, Xiaomi, Beijing, China) and the length (mm) of the broken fragment of the file was measured using the digital calliper.
Figure 2.
Experimental set-up used for static cyclic fatigue testing (37°C water bath)
Statistical Analysis
Measurement of the size at the tip of the instrument
The measurements obtained were compared to the manufacturer's reference (0.25 mm) using the one-sample t-test. For the Kruskal-Wallis test, with a confidence level of 95% and a mean effect size (f) of 0.4, 80% power was required to detect statistically significant differences. The level of significance was set at 5%.
Cyclic fatigue test
The Shapiro–Wilk test was used to determine whether the data (resistance to cyclic fatigue and fracture location) were normally distributed. A non-parametric analysis test was chosen based on the sample size per group-curvature combination. The Kruskal–Wallis's test was used to contrast the homogeneity of time and distance values of the four file groups. The Mann–Whitney and Bonferroni correction were used to evaluate the effect of the curvature and for comparisons between groups. The level of significance was set at 5%.
RESULTS
Measurement of the size in millimetres at the tip of the instruments using the digital calliper were (mean±SD and confidence interval [CI]): Reciproc R25 (0.224±0.11 [0.218–0.230]); Reverso Silver (0.226±0.15 [0.219–0.234]); Reciproc Blue R25 (0.222±0.11 [0.216–0.227]; Reverso Blue (0.235±0.11 [0.230–0.241]). The reference value purported by the manufacturers (0.25) is larger and outside the range of values measured, with significant differences for all groups (p<0.001).
Time to fracture in seconds (mean±SD) was influenced by the file and the study set-up, as presented in Table 1.
TABLE 1.
Results for cyclic fatigue test
| File | Single curvature (n=9 per group) | Double curvature (n=9 per group) | ||
|---|---|---|---|---|
| Time to fracture | Fragment length | Time to fracture | Fragment length | |
| Reciproc R25 | 171.5±38.9b,e | 4.33±0.91a | 133.4±47.4d,f | 11.87±0.57c |
| Reverso Silver | 169.0±104.8b,e | 6.06±0.62b | 57.8±20.0b,c,f | 12.93±1.04c |
| Reciproc Blue R25 | 355.4±86.4a,e | 6.32±1.01b | 140.5±67.7a,d | 11.82±1.27c |
| Reverso Blue | 359.5±102.8a | 6.04±0.71b | 142.9±69.0a,d,f | 13.17±1.02b,c |
Groups identified by different superscript letters indicate significant statistical difference for the outcome measure using Kruskal-Wallis's test/ Mann-Whitney and Bonferroni correction (level of significance 5%).
When comparing the files, Reciproc Blue R25 and Reverso Blue showed a significantly longer time to fracture compared with Reverso Silver (p=0.002), regardless of the number of curvatures. Regarding time to fracture for single curvatures, Reciproc Blue R25 and Reverso Blue showed a significantly longer time to fracture compared to Reciproc R25 and Reverso Silver (p<0.05). In the presence of double curvature, Reverso Silver presented with a significantly shorter time to fracture compared with all other files (p<0.05). Regarding one versus two curvatures comparison globally, the files presented a shorter time to fracture when tested against double curvatures; however, when the type of file was considered, significant differences were not found for Reciproc R25 (p=0.54).
Length of the Fractured File
The length at which the fracture occurred was entirely dependent on the curvature. Results are presented in Table 1. Significantly longer fragments were associated with double curvatures (p<0.001). Reciproc R25 was associated with shorter fragments compared with all other files in single curvatures (p<0.05) and with Reverso Blue in double curvatures (p=0.047). No further significant differences were found in the different statistical analyses.
DISCUSSION
Overall, and within the limitation of the present laboratory set-up, higher time to fracture during the cyclic fatigue test was associated with post-manufacture heat treatment, whilst “S-shaped” curvatures hastened the process. These are in agreement with previous literature (4, 12). Notably, the number of curvatures was not associated with time to fracture for Reciproc R25. This counterintuitive finding can be justified by the various features of the instrument. In addition, the size of the tip for all experimental groups was smaller than the reference standards reported. Therefore, the first null hypothesis was partially rejected, whilst the second one was rejected.
The findings of this laboratory study support the results of a recent global survey of endodontic practice, where the heat-treated NiTi instruments were used by a smaller majority of respondents (2). The risk of instrument fracture in “S-shaped” canals should be reiterated, also considering the high prevalence of double curvatures in planes that are not visible with two-dimensional imaging (11).
Mechanical resistance testing, including cyclic fatigue, has been associated with clinical performance and the risk of instrument fracture (10). There is no set threshold in regard to cyclic fatigue resistance for endodontic instruments and higher values may not offer further benefits clinically, thus the results of this mechanical test should be interpreted with caution. In addition, the results of this test can be influenced by instrument design features and manufacturing process (e.g. treatment of alloys, core diameter, tip size, taper) (9, 10, 13, 14); thus, differences in mechanical resistance between “original” and “replica-files” may suggest inconsistencies in the above features.
The literature reports inconsistent findings when comparing fatigue resistance of “replica-like” and original files. Some examples of “replica-like" files having higher resistance include U-File (Dentmark, Ludhiana, India), SuperFiles and Super Files Blue (Shenzhen Flydent medical, Shenzen China), versus ProTaper Universal and ProTaper Gold (Dentsply Maillefer, Ballaigues, Switzerland) (5); V TAPER Gold (Fanta Dental, Shanghai, China) versus ProTaper Gold (13); U-File (Dentmark), Edge Taper (Johnson City, TN, USA) and Super-File (Flydent, Shenzhen, China) versus ProTaper Universal (10). No differences were reported when comparing MG3 Gold (Perfect Medical Instruments, Shenzhen, China) versus ProTaper gold (13); RC Blue (Dental Perfect, Shenzen Perfect Medical Instruments, Shenzen, China), Only One File Blue (Denco, Shenzen Denco Medical, Shenzen, China), Recip One Blue (Rogin Dental (Shenzen Rogin Medical, Shenzen, China) versus Reciproc Blue (after 3 rounds of processing and instrumentation of canals ex-vivo) (7) as well as Go-Taper Universal (Access, Shenzhen, China) versus ProTaper Universal (10). No differences between Protaper Next and X-Files (Dentmark) were reported (15). One type of instrument (Micro Blue, Microdont, Shenzen, China) had significantly lower cyclic fatigue resistance compared with Reciproc Blue (7). This reiterates the high variability in mechanical performance between “replica-like” files, and justifies the need for robust independent testing before their use in clinics.
The limitations of “replica-like” instruments regarding standardisation have been assessed previously, with contrasting results. One study reported that tip size and other design features were comparable between U-File, SuperFiles, Super Files Blue, ProTaper Universal and ProTaper Gold (5). Conversely, tip size was reported to be significantly different for Only One File R 25 (Shenzen, Denco Medical Shenzhen, China) (reciprocating file) when compared with the Reciproc R25, ProTaper Next X2 and X File X2 (NIC, Shenzhen Superline technology, Shenzen, China) (8). Within the limitations of the methodology, all instruments tested were smaller than the reported size in the present study.
The methodology of the present study needs further discussion. There is no standardization in cyclic fatigue testing, including whether the instrument is rotated in a fixed hand piece (i.e. static testing) or when in motion (e.g. pecking) is given to the hand piece to mimic the clinical reality (i.e. dynamic testing) (13); this motion also distributes the stress alongside a larger part of the instrument is associated with higher fatigue resistance in laboratory studies (5). In addition, static testing is considered more repeatable, though less representative of clinical practice. Nonetheless, the various features of hand piece movement would depend on the canal morphology and the operator, thus not replicable during cyclic fatigue tests (5), and a broader spread in results has been associated with dynamic testing (8). Laboratory models use canal curvatures of haphazard clinical translation. Inconsistencies of the methods for measuring root canal curvature clinically (16), and the potential role of various curvature features (level, height, radius, length and shape) have been previously reiterated (17). The experimental set-up of the present study included immersion in water at 37°C, which is crucial as the cyclic fatigue resistance of heat-treated NiTi instruments diminishes significantly at body temperature (3). The sample size of the present study for cyclic fatigue testing is similar to that of previous comparable studies (6, 7). However, in the absence of an a priori power calculation, the results should be interpreted with caution, because inadequate power cannot be ruled out even in the presence of multiple significant differences.
The length of the fractured instrument is a secondary outcome measure of the present study, normally used to confirm the correct placement of the instrument and validate the reproducibility of a static cyclic fatigue testing set-up (5, 10). The absence of multiple significant differences within a curvature type (i.e. single or double) for this particular outcome and the small standard deviation for all instruments tested in the present study confirms the consistency of placement of the instruments, thus allowing a reliable comparison (5, 12). The position of fracture in S-shaped models will depend on various curvature features, including the radius of the curvature (12). Significant differences in fractured fragments according to the type of instrument have been reported in a comparable study (8), and suggest that the level of the file more prone to fracture due to cyclic fatigue stress also depends on the instrument per se.
Despite that the lower prices of “replica-like” instruments make it more attractive than the original ones, it is crucial to reiterate that clinicians should consider only tested instruments. Instrument selection should consider the morphological complexities on a case by case basis, and blue files should be used in the presence of S-shaped curvatures. Legally, dental instruments are subject to regulations addressing safety and effectiveness, amongst others, and the fulfilment of these essential requirements lies with the manufacturer. Further multimethod laboratory studies are required to understand better the technical characteristics of Reverso and other “replica-like” files. Local and/or global schemes for reporting adverse effects or problems with medical devices (including endodontic instruments), are desirable as these allow the monitoring of their clinical safety. These should be accessible to operators as required.
CONCLUSION
The cyclic fatigue resistance (time to fracture) of the conventional “replica-like” instrument (Reverso Silver) was significantly lower than the heat-treated comparators (Reverso Blue and Reciproc Blue). Double curvatures were associated with a shorter time to fracture compared with single curvature for all instruments. The tip size of all instruments assessed was smaller than the values reported by the manufacturers. Conventional "replica-like" instruments demonstrated inferior cyclic fatigue resistance compared to blue instruments, particularly in double curvatures, highlighting their limitations for clinical use in complex canal anatomies.
Footnotes
Please cite this article as: Tarragó C, Valencia de Pablo O, Loroño G, Conde A, Perez-Alfayate R, Rossi-Fedele G, et al. Cyclic Fatigue Resistance of “Replica-like" and Original Reciprocating Instruments in Single and Double Curvatures. Eur Endod J
Disclosures
Authorship Contributions
Concept – R.E., G.L., C.T.; Design – R.E., A.C., G.L., C.T.; Supervision – R.E., G.L., O.V.D.P.; Funding – R.P.A., O.V.D.P.; Materials – R.P.A., O.V.D.P.; Data collection and/or processing – A.C., O.V.D.P.; Data analysis and/or interpretation – J.V., R.P.A., C.T., G.L.; Literature search – G.R.F., J.V.; Writing – G.R.F., J.V., G.L.; Critical review – G.R.F., A.C., G.L.
Conflict of Interest
All authors declared no conflict of interest.
Use of AI for Writing Assistance
The authors declared that no artificial intelligence (AI)– assisted technologies (such as Large Language Models [LLMs], chatbots, or image creators) was used in the production of submitted work.
Financial Disclosure
The authors declared that this study received no financial support.
Peer-review
Externally peer-reviewed.
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