Autoclave Sterilization Differentially Degrades Cyclic Fatigue Resistance in EdgeFile X7 and Fanta File Nickel-titanium Rotary Instruments

1. INTRODUCTION

Complete chemomechanical debridement of the root canal system is arguably the number one objective in endodontic treatment, and since the acceptance of nickel–titanium (NiTi) rotary files, this process has been revolutionized [1]. The unique superelasticity and shape-memory properties of NiTi root canal instruments enable more focused and efficient shaping of root canals, particularly in complex curved forms [2]. Cyclic fatigue and torsional loading are the two principal causes for instrument separation [3]. While that is still a major clinical problem, despite recent advances in instrument development, artifact remains an unresolved issue at this time. Cyclic fatigue is an instrument fracture that results from the fragmentation of the file while it is subjected to repetitive tensile and compressive stresses as it rotates within a passively curved canal, leaving its mark on the outcome [4].

A new series of NiTi instruments, which are manufactured by a different proprietary thermomechanical process [5], has been developed to minimize the risk of fracture and enhance clinical performance. Depending on the specific heat treatment employed, these thermomechanical treatments change the crystal structure of the alloy and shift the phase transition region (austenite, martensite, R-phase), which is activated to establish an ideal balance of flexibility, strength, and fatigue resistance against cyclic stressing [6, 7]. The FireWire™ treatment employed in the manufacture of Edge File X7 (EdgeEndo, Albuquerque, NM, USA) results in a very flexible file, as does the application of “AF-H Wire” technology, as introduced for Fanta File (Fanta Dental, Shanghai, China). Such ongoing improvements represent an evolution of endodontic metallurgy and are developed to provide the practitioner with improved, safer instruments [8].

Parallel to these materials' developments, financial limitations in clinical practice often require an increasing reuse of NiTi files. This constraint creates a firm requirement for sterilization by means such as steam autoclaving to prevent cross-contamination and ensure patient safety [9]. Nevertheless, there is still significant inconsistency and disputable supporting evidence in the literature regarding the impact of multiple autoclaving cycles on the mechanical and metallurgical properties of these advanced heat-treated alloys. Recent expert opinion and consensus recognise that dry heat and autoclave sterilization alter the mechanical behaviour of NiTi instruments, but the extent and type of these alterations vary depending on the file system and testing procedure [10].

A comprehensive 2024 review by Abdellatif et al. highlighted this controversy, noting that while some studies report a decrease in cyclic fatigue resistance after autoclaving, others suggest a potential increase, possibly due to a shape recovery effect or microstructural annealing [11]. This discrepancy is further complicated by variations in experimental protocols, such as testing at room temperature versus the more clinically relevant body temperature, which has been shown to decrease the fatigue strength of heat-treated instruments [11, 12]. Furthermore, recent studies have produced divergent results; Bardan et al. (2025) found that autoclaving did not affect the dynamic cyclic fatigue of several systems but did reduce torsional strength [13], whereas Patel et al. (2025) observed an increase in cyclic fatigue resistance for other file systems after sterilization [14].

This lack of consensus creates a significant knowledge gap for clinicians, who must make evidence-based decisions about the reuse of specific, contemporary file systems such as EdgeFile X7 and Fanta File. While some research has investigated the effects of sterilization on various NiTi instruments [15, 16], there is a paucity of data directly comparing these two popular, modern, heat-treated systems. Therefore, this study aimed to provide clarity by evaluating the impact of multiple autoclave sterilization cycles on the cyclic fatigue resistance of EdgeFile X7 and Fanta File. The null hypothesis was that autoclave sterilization would have no significant effect on the cyclic fatigue resistance of the tested instruments. The rationale for this study is rooted in the urgent clinical need to establish clear, evidence-based guidelines for the safe reuse of modern heat-treated NiTi files, thereby minimizing the risk of unexpected intracanal separation and ensuring optimal patient safety.

2. MATERIALS AND METHODS

3. RESULTS

The results of the cyclic fatigue testing are summarized in the tables and figures below. Both file type and the number of sterilization cycles had a significant effect on the cyclic fatigue resistance of the instruments (p < 0.001).

3.1. Time to Fracture and Number of Cycles to Failure

EdgeFile demonstrated significantly higher initial fatigue resistance compared to Fanta File. The mean NCF for the control group of EdgeFile was 2649 ± 348 (95% CI: 2451-2847), while for Fanta File, it was 1353 ± 168 (95% CI: 1258-1448). After two sterilization cycles, the NCF of EdgeFile was reduced by 38.6% to 1626 ± 90 (95% CI: 1575-1677), while Fanta File showed a 19.5% reduction to 1089 ± 281 (95% CI: 932-1246). The distributions of fracture times and the progressive reduction in fatigue life are illustrated in Figs. (1 and 2), respectively. Detailed results for TtF and NCF are presented in Table 1 and Table 2.

Table 1.

File Type	Sterilization Cycles	Time to Fracture (seconds) Mean ± SD (95% CI)	Number of Cycles to Failure Mean ± SD (95% CI)
EdgeFile X7	0 (Control)	529.8 ± 69.6 (490.2-569.4)	2649 ± 348 (2451-2847)
EdgeFile X7	1	386.4 ± 54.2 (355.8-417.0)	1932 ± 271 (1779-2085)
EdgeFile X7	2	325.2 ± 18.0 (315.0-335.4)	1626 ± 90 (1575-1677)
Fanta File	0 (Control)	270.6 ± 33.6 (251.6-289.6)	1353 ± 168 (1258-1448)
Fanta File	1	243.0 ± 42.0 (219.0-267.0)	1215 ± 210 (1095-1335)
Fanta File	2	217.8 ± 56.2 (186.4-249.2)	1089 ± 281 (932-1246)

Abbreviations: SD, standard deviation; CI, confidence interval.
Note: All values represent means ± standard deviations with 95% confidence intervals in parentheses. n = 14 files per group.

Table 2.

File Type	Sterilization Cycles	NCF Mean ± SD (95% CI)	Percentage Reduction (%) from Control
EdgeFile X7	0 (Control)	2649 ± 348 (2451-2847)	0.0 (Baseline)
EdgeFile X7	1	1932 ± 271 (1779-2085)	27.1
EdgeFile X7	2	1626 ± 90 (1575-1677)	38.6
Fanta File	0 (Control)	1353 ± 168 (1258-1448)	0.0 (Baseline)
Fanta File	1	1215 ± 210 (1095-1335)	10.2
Fanta File	2	1089 ± 281 (932-1246)	19.5

Abbreviations: NCF, number of cycles to failure; SD, standard deviation; CI, confidence interval.
Note: Percentage reduction calculated as [(Control NCF - Sterilized NCF) / Control NCF] × 100. n = 14 files per group.

3.2. Interaction Effects

A significant interaction effect was observed between file type and the number of sterilization cycles (F = 12.45, p < 0.001, η²p = 0.42), as illustrated in the interaction plot (Table 3 and Fig. 3). This large effect size indicates that the two file systems responded differently to the sterilization process. EdgeFile, while initially more durable, showed a more pronounced reduction in fatigue resistance with each sterilization cycle compared to Fanta File.

Table 3.

Source of Variation	Sum of Squares	Degrees of Freedom	Mean Square	F-value (p-value)	Partial η2 (Effect Size)
File Type	8,456,234	1	8,456,234	58.685 (< 0.001)	0.58 (Large)
Sterilization Cycles	4,812,567	2	2,406,284	16.688 (< 0.001)	0.35 (Large)
File Type × Sterilization Cycles (Interaction)	1,795,432	2	897,716	12.450 (< 0.001)	0.42 (Large)
Error (Within Groups)	11,234,876	78	144,037	-	-
Total	26,299,109	83	-	-	-

Abbreviations: ANOVA, analysis of variance; η², eta-squared (effect size measure).
Note: Effect size interpretation: η² = 0.01 (small), 0.06 (medium), 0.14 (large). All main effects and interaction effects were statistically significant at p < 0.001. n = 84 total observations (6 groups × 14 files per group).

3.4. Fracture Location Analysis

The mean fracture location was 4.12 ± 0.52 mm from the file tip for EdgeFile and 3.89 ± 0.48 mm for Fanta File, with no significant difference between groups (p = 0.124), as shown in Fig. (4). All fractures occurred within the curved portion of the canal, confirming that failure was due to cyclic fatigue rather than torsional stress.

3.5. Correlation Analysis

A weak negative correlation was found between the time to fracture and the length of the fractured fragment (r = -0.174, p = 0.112), as shown in the correlation matrix (Table 4 and Fig. 5). This suggests that there is no strong relationship between the length of a file and the length of the piece that breaks off.

Table 4.

Parameter	Time to Fracture (TtF)	Number of Cycles to Failure (NCF)	Fracture Fragment Length
Time to Fracture (TtF)	1.000	0.998*** (p < 0.001)	-0.174 (p = 0.112)
Number of Cycles to Failure (NCF)	0.998*** (p < 0.001)	1.000	-0.168 (p = 0.128)
Fracture Fragment Length	-0.174 (p = 0.112)	-0.168 (p = 0.128)	1.000

Abbreviations: TtF, time to fracture; NCF, number of cycles to failure.
Note: Values represent Pearson correlation coefficients (r) with p-values in parentheses. *** indicates p < 0.001 (highly significant correlation). n = 84 total observations (6 groups × 14 files).

3.6. SEM Fractography Findings

SEM Tests in all specimens exhibited the distinctive appearance of cyclic fatigue fracture upon examination of their fractured surfaces in SEM tests. More specifically, fractographic results indicated clear zones of crack initiation at the edges of the files with less tortuous and more mirror-like features. These turned into fatigue striations indicative of constructive, slow crack propagation inward. Structures at the centre displayed dimpled overload zones, indicative of catastrophic ductile failure Figure 6. Differences in the morphology of fractured files (sterilized–none) were noted, but these differences did not appear to indicate a fundamental difference in component fatigue failure, as fatigue resistance was reduced overall regardless of sterilization.

4. DISCUSSION

The present in vitro study confirms that autoclave sterilization significantly degrades the cyclic fatigue resistance of both EdgeFile X7 and Fanta File NiTi rotary instruments, leading to the rejection of the null hypothesis. This finding aligns with a substantial body of literature demonstrating the detrimental effects of autoclaving on the mechanical properties of NiTi alloys [18, 19]. The clinical implications of these findings are profound, particularly in the context of file reuse. While a single-use policy remains the gold standard for preventing both cross-contamination and fracture [9], the economic pressures of daily practice often make reuse a necessity. Our data suggest that even a single sterilization cycle significantly compromises the fatigue life of these instruments. For EdgeFile, the NCF dropped by 36.5% after just one cycle. This substantial decrease in durability must be factored into any clinical decision-making framework for file reuse. Based on our findings, we recommend that if these files are to be reused, it should be for a very limited number of cases, perhaps only one additional use, especially when negotiating canals with severe curvatures.

The most striking finding of this study is the significant interaction effect between file type and sterilization, indicating that the two systems responded differently to the autoclaving process. EdgeFile X7, which exhibited nearly double the initial fatigue resistance of Fanta File, suffered a much greater percentage reduction in its number of cycles to failure (38.6% vs. 19.5% for Fanta File after two cycles). This suggests that the proprietary FireWire™ heat treatment, while imparting superior initial durability, may render the alloy more susceptible to the microstructural changes induced by autoclaving compared to the AF-H Wire technology of the Fanta File. This differential response underscores that findings from one NiTi system cannot be extrapolated to another, a point reinforced by the conflicting results prevalent in the literature [20, 21]. The core of this degradation lies in the metallurgical response of the alloy to the high temperatures and pressures of the autoclave cycle. These conditions can induce microstructural alterations, such as the precipitation of brittle intermetallic phases (e.g., Ti₂Ni, Ti₄Ni₃), which act as stress concentration points and facilitate crack initiation and propagation under cyclic loading [22, 23]. Recent studies, such as Karaś et al. (2025), have further corroborated that autoclave sterilization, especially after multiple cycles, increases surface roughness and causes microcracks in NiTi instruments, making them more prone to damage [24].

Our results contribute to a complex and often contradictory body of recent literature. Bardan et al. (2025), in a methodologically robust study using dynamic fatigue testing, found that autoclaving up to seven times did not affect the dynamic cyclic fatigue resistance of several modern NiTi systems. However, it did reduce torsional strength [13]. Conversely, Patel et al. (2025) reported an increase in cyclic fatigue resistance for other systems after autoclaving, postulating a beneficial shape-recovery or stress-relief effect [14]. This aligns with recent research by dos Reis et al. (2023), who found that replica-like instruments were reliable for up to two cases but showed significant SEM-visible distortions after the third use [19]. The discrepancy between our findings and those of Bardan et al. could be attributed to differences in the file systems tested (our study included EdgeFile and Fanta File, which were not in their analysis) and the testing methodology (static vs. dynamic fatigue testing). The static model used in our study, while standard, may represent a more aggressive test of pure cyclic fatigue by concentrating stress at a single point, potentially highlighting the effects of sterilization-induced micro-brittleness more readily than a dynamic test. This static method was deliberately preferred to dynamic testing, as it concentrates stress at a single, constant point of maximum curvature—allowing for reduced variability in the measurement and the assessment of pure cyclic fatigue resistance by highlighting effects from sterilization-induced micro-brittleness.

This study has several strengths, including a sample size determined by a priori power analysis, the use of a standardized artificial canal, and the inclusion of two contemporary, widely used file systems that have not been extensively compared in this context. Future research should aim to correlate mechanical testing with such metallurgical analyses to provide a more complete mechanistic understanding [25, 26].

In conclusion, this study provides clear evidence that even limited autoclave sterilization significantly reduces the cyclic fatigue resistance of EdgeFile X7 and Fanta File instruments. The magnitude of this reduction is system-dependent, with the initially more durable EdgeFile showing greater susceptibility to sterilization-induced degradation. These findings provide a crucial piece of evidence for clinicians, urging caution and suggesting a highly limited reuse protocol for these specific instruments to ensure patient safety and mitigate the risk of intraoperative instrument fracture during root canal therapy.

5. LIMITATIONS

The primary limitation of this method is the use of a static cyclic fatigue model at room temperature. Testing at body temperature (37°C) is critical, as the phase transformation behavior of many heat-treated NiTi alloys changes near this temperature, often leading to reduced fatigue resistance. Secondly, this investigation was limited to two sterilization cycles. Evaluating an extended series of cycles (e.g., 3, 5, 7) would offer a more complete picture of the degradation curve. Finally, our analysis did not include advanced metallurgical characterization such as Differential Scanning Calorimetry (DSC) or X-ray Diffraction (XRD), which could have provided direct evidence of the microstructural changes underlying the observed mechanical degradation. Future research should aim to correlate mechanical testing with such metallurgical analyses to provide a more complete mechanistic understanding.

CONCLUSION

Within the limitations of this in vitro study, it can be concluded that autoclave sterilization significantly reduces the cyclic fatigue resistance of both EdgeFile X7 and Fanta File NiTi rotary instruments. While EdgeFile demonstrated superior initial fatigue resistance (95.8% higher NCF than Fanta File), it was more negatively affected by repeated sterilization cycles, experiencing a 38.6% reduction in NCF after two cycles compared to 19.5% for Fanta File. The significant interaction effect between file type and sterilization (η²p = 0.42) confirms that the two systems respond differently to autoclaving. These findings underscore the importance of considering the effects of sterilization when making decisions about the reuse of heat-treated NiTi files in clinical practice, with recommendations for highly limited reuse protocols to ensure patient safety.

REFERENCES