Mechanical and elemental comparison of EndoStar and One Curve nickel–titanium endodontic files: An in vitro study

Abstract

Objective:

To compare the cyclic fatigue resistance, bending behavior, and elemental composition of two heat-treated nickel–titanium (NiTi) rotary file systems, Endostar E3 Azure and One Curve, using standardized in vitro mechanical and surface characterization protocols.

Methodology:

The study randomly allocated 90 new size 25/0.06 NiTi rotary files (n = 45 Endostar; n = 45 One Curve) into three subgroups (n = 15 each) for dynamic cyclic fatigue testing, bending resistance assessment, and scanning electron microscopy/EDS analysis. Fatigue testing was performed in a dynamic stainless-steel canal at 37°C ± 1°C using a pecking motion. Bending resistance was measured according to ISO 3630-1 using a universal testing machine. Elemental composition was analyzed with energy-dispersive X-ray spectroscopy. Statistical comparisons were made using independent-samples t-tests (P < 0.05).

Results:

Endostar exhibited significantly greater cyclic fatigue resistance compared with One Curve, showing higher number of cycles to fracture (2814.3 ± 488.9 vs. 1454.7 ± 452.5; P < 0.001) and longer time to fracture (567.1 ± 102.6 s vs. 288.5 ± 92.4 s; P < 0.001). Bending resistance was higher in Endostar (852.8 ± 36.6 gf) compared with One Curve (538.8 ± 57.1 gf; P < 0.001). EDS analysis revealed increased oxygen levels in Endostar and higher nickel and carbon levels in One Curve.

Conclusion:

Endostar E3 Azure demonstrated superior cyclic fatigue resistance, higher bending stiffness, and distinct surface chemistry compared with One Curve. Surface elemental variations and heat-treated differences may be associated with the observed mechanical variations; however, mechanistic interpretation requires further phase-transformation analysis.

INTRODUCTION

Nickel–titanium (NiTi) rotary instruments are fundamental to modern root canal preparation due to their flexibility, shape memory, and ability to maintain canal curvature compared with stainless-steel files.[1] Three key qualities are important to their clinical application. First, cyclic fatigue resistance represents an instrument’s ability to withstand repeated tensile and compressive stresses in curved canals.[2] Second, bending stiffness reflects the file’s ability to negotiate complex canal anatomies. Importantly, research emphasizes that flexibility does not necessarily predict fatigue resistance; bending stiffness and cyclic fatigue behavior are influenced by different metallurgical and structural factors. Elemental composition, assessed through Energy-dispersive spectroscopy (EDS), provides semi-quantitative surface information related to alloy characteristics and mechanical behavior. However, EDS does not determine bulk phase composition or transformation temperatures.[3]

Recent studies have shown that metallurgical characteristics, geometric design, and clinical loading conditions influence instrument failure.[4,5] Dynamic cyclic fatigue testing more accurately simulates the clinical pecking motion of rotary instruments and produces results that better reflect real intracanal behavior. Several studies have demonstrated that dynamic tests distribute stress more evenly along the file, resulting in longer fatigue life than static tests.[6,7] Meanwhile, EDS analyses revealed that NiTi and surface oxides differ between systems, and manufacturing protocols and heat treatment play a major role in elemental distribution and, hence, mechanical performance.[8,9,10] Dynamic cyclic fatigue testing incorporates axial movement, allowing the point of maximum stress to shift along the file.[11,12,13]

Although some studies have evaluated heat-treated systems, limited literature assesses cyclic fatigue resistance, bending response, and EDS characterization in combination.[14,15] Furthermore, modern systems with proprietary thermomechanical processes, such as Endostar E3 Azure (Azure HT technology, Poldent, Poland) and One Curve (C.Wire controlled-memory alloy), have not been examined together regarding both mechanical and chemical properties.[16]

Endostar E3 Azure files undergo Azure HT heat treatment, which produces a characteristic titanium-oxide “blue” surface layer and stabilizes the martensitic phase at body temperature. This enhances flexibility and delays crack propagation under cyclic loading. In contrast, One Curve files are produced using C-Wire controlled-memory alloy, resulting in high flexibility but different nickel distribution and surface characteristics. These metallurgical variations may contribute to differences in cyclic fatigue resistance and bending response; however, definitive correlations between elemental composition and mechanical behavior require further phase-transformation analysis.

Given the critical role of cyclic fatigue resistance, bending rigidity, and alloy constitution in preventing instrument separation, these factors are essential in clinical practice. However, the absence of comparative data on the two systems represents a substantial gap in the endodontic literature. The current in vitro, controlled comparative analysis of Endostar E3 Azure and One Curve files integrated standardized dynamic cyclic fatigue testing, bending resistance assessment according to ISO 3630-1, and EDS elemental profiling to address this deficiency.

The objective of the present study was to compare the cyclic fatigue resistance, bending characteristics, and elemental structure of Endostar E3 Azure and One Curve NiTi rotary files to generate evidence-based information that would support the selection of appropriate clinical instruments.

METHODOLOGY

Study design

This comparative, experimental in vitro investigation was conducted to evaluate the mechanical properties and elemental profiles of two commercial NiTi rotary file systems: Endostar E3 Azure and One Curve. Three standardized laboratory assays were performed: cyclic fatigue resistance testing, bending resistance testing, and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDS).

Ethical approval

Ethical approval was granted by the Institutional Review Board. All experimental procedures were performed in accordance with institutional and international guidelines governing in vitro biomedical materials research.

Grouping and sample selection

A total of 90 new, unused NiTi rotary files were evaluated: 45 Endostar E3 Azure and 45 One Curve instruments. All files were size 25, taper 0.06, and 25 mm in length. Each file was inspected under a stereomicroscope (x20) to exclude instruments with manufacturing defects. For each system, instruments were randomly allocated into three groups (n = 15 per test): cyclic fatigue testing, bending resistance testing, and SEM/EDS analysis. Randomization was performed using a computer-generated allocation sequence to avoid selection bias.

Cyclic fatigue resistance test

A custom-made dynamic cyclic fatigue testing device consisting of a reciprocating linear DC gear motor (Mainland, Guangdong, China) mounted on a portable tripod (Promate, Shenzhen, China), allowing controlled axial pecking motion. The canal presented a 60° curvature with a 5-mm radius to reproduce stress patterns encountered in curved root canals. A digital thermostatic water bath (DXY, China) filled with normal saline and maintained at 37°C ± 1°C was used to reproduce intra-oral temperature, as heat-treated NiTi alloys exhibit temperature-dependent mechanical behavior.

A torque-controlled electric motor (X-Smart Plus, Dentsply Maillefer, Ballaigues, Switzerland) connected to a 6:1 reduction handpiece was used. Endostar E3 Azure instruments were rotated at 300 rpm with a torque limit of 3 Ncm, and One Curve instruments at 300 rpm with a torque limit of 2.5 Ncm. A continuous 3-mm axial pecking motion was applied to simulate clinical instrumentation. Each test was continued until file separation occurred, and fracture time was recorded using a digital chronometer. The number of cycles to fracture (NCF) was calculated using the formula:

Fractured fragment lengths were measured immediately after instrument separation using a calibrated electronic micrometre calliper (Adoric, Taiwan).

Bending resistance test

Bending resistance was assessed according to ISO 3630-1 using a universal testing machine (Model 5967, Instron, Canton, MI, USA). Each file was clamped 3 mm from the tip, and a controlled load was applied at a crosshead speed of 2 mm/min until a 45° deflection was achieved. The required force (gf) was recorded as the bending resistance. Lower force values indicated greater flexibility. All tests were performed under identical environmental and mechanical conditions.

Scanning electron microscopy and EDS analysis

Scanning electron microscopy

SEM was used exclusively for qualitative fractographic analysis of NiTi rotary instruments following cyclic fatigue testing. SEM examination was performed to evaluate fracture morphology, including crack initiation sites, fatigue striations, and final ductile rupture characteristics.

Fractured specimens were ultrasonically cleaned in ethanol, air-dried, mounted on aluminum stubs using conductive carbon adhesive, and sputter-coated with a thin layer of gold to enhance surface conductivity. SEM imaging was performed using a scanning electron microscope (JEOL Ltd., Tokyo, Japan) at magnifications ranging from ×220 to ×1000.

SEM was used to characterize fracture morphology and confirm cyclic fatigue as the failure mode. In contrast, EDS served to evaluate baseline surface chemistry rather than postfracture compositional changes. These analyses were designed to address distinct aspects of instrument behavior.

Energy dispersive X-ray spectroscopy (EDS)

EDS analysis was performed on unused, intact instruments to assess baseline surface elemental composition before mechanical testing. Fractured instruments were excluded to prevent potential alterations in surface chemistry caused by plastic deformation, localized heat generation, or oxidation during cyclic fatigue failure. This approach ensured standardized elemental comparison between systems independent of mechanical loading effects. EDS analysis was limited to semi-quantitative assessment of elemental weight percentages of nickel (Ni), titanium (Ti), oxygen (O), and carbon (C).

EDS analysis was conducted using an EDS detector (X-MaxN, Oxford Instruments, Abingdon, UK) integrated with a scanning electron microscope (JSM-6360 LV, JEOL Ltd., Tokyo, Japan). Elemental data acquisition and analysis were carried out using AZtec™ software (Oxford Instruments, Abingdon, UK). Fractured instruments and fracture surfaces were not included in the EDS analysis.

Fractographic examination

Fractographic evaluation was performed to classify failure patterns. SEM images were analyzed for characteristic features of cyclic fatigue, including crack origin, propagation pathways, and the final ductile fracture zone. Comparative assessment between the two systems was undertaken to identify metallurgical differences potentially contributing to variations in mechanical behavior.

Statistical analysis

Statistical analyses were performed using SPSS version 26.0 (IBM Corporation, USA). Data normality was assessed using the Shapiro–Wilk test. As variables followed a normal distribution, intergroup comparisons between Endostar E3 Azure and One Curve files were conducted using independent-samples t-tests. Parameters analyzed included NCF, bending resistance values, fractured fragment lengths, and EDS-derived elemental composition. Statistical significance was set at P < 0.05. Results were reported as mean ± standard deviation. Post hoc power analysis was performed based on the primary outcome variable, the number of NCF. Using the observed effect size and sample size (n = 15 per group), the calculated statistical power was approximately 0.80 at a significance level of 0.05. As this analysis was conducted retrospectively after data collection, its inferential value is limited and should be interpreted cautiously.

RESULTS

The mean ± standard deviation values for elemental composition (oxygen, carbon, nickel, and titanium), bending resistance (maximum load), and cyclic fatigue parameters (fractured fragment length, number of NCF, and time to fracture) for Endostar E3 Azure and One Curve instruments are summarized in Table 1. A total of 90 experimental units were analyzed (Endostar, n = 45; One Curve, n = 45), with each system equally subdivided for cyclic fatigue testing, bending resistance assessment, and energy-dispersive spectroscopy (EDS) analysis (n = 15 per subgroup). The primary outcome variable was the NCF. Power analysis performed using G* Power 3.1.9.7 (Franz Faul, University of Kiel, Germany) indicated adequate statistical power (0.799, approx. 80%).

Table 1.

Comparison between Endostar E3 azure and one curve in relation to cyclic fatigue resistance, bending resistance, and surface elemental composition (EDS weight %)

Type of test	Variables	Ni-Ti files		P §
		Endostar, mean±SD	One curve, mean±SD
EDS	Oxygen (weight %)	40.4±5.13	29.1±2.96	<0.001**
	Carbon (weight %)	24.6±3.41	29.0±2.14	<0.001**
	Nickel (weight %)	22.3±1.84	29.9±3.89	<0.001**
	Titanium (weight %)	12.9±2.84	11.9±1.76	0.230
Bending test	Maximum (gf)	852.8±36.6	538.8±57.1	<0.001**
Cyclic fatigue test	Length (mm)	4.18±0.94	3.37±0.29	0.004**
	NCF	2814.3±488.9	1454.7±452.5	<0.001**
	Number of seconds	567.1±102.6	288.5±92.4	<0.001**

**Significant at P<0.05 level, ^§P-value has been calculated using Independent Sample t-test. Elemental values represent surface weight percentages (weight %) obtained from semi-quantitative EDS analysis. SD: Standard deviation, EDS: Energy dispersive spectroscopy, NCF: Number of cycles to fracture, Ni-Ti: Nickel-titanium

Comparative analysis revealed significant differences between the two NiTi systems across most evaluated parameters [Table 1]. Endostar E3 Azure demonstrated significantly higher oxygen content, greater bending resistance, longer fractured fragment length, higher NCF, and longer time to fracture than One Curve (P < 0.05). In contrast, One Curve exhibited significantly higher surface carbon and nickel concentrations (P < 0.001), while titanium content did not differ significantly between the systems (P = 0.230). Overall, Endostar E3 Azure showed higher bending stiffness and superior cyclic fatigue resistance under dynamic testing conditions at body temperature.

Figure 1 illustrates the mean differences in surface elemental composition between Endostar E3 Azure and One Curve instruments as determined by energy-dispersive spectroscopy (EDS). Endostar E3 Azure exhibited a significantly higher mean oxygen content compared with One Curve (P < 0.001). In contrast, One Curve demonstrated significantly higher mean values of carbon and nickel than Endostar E3 Azure (P < 0.001 for both). No statistically significant difference was observed in titanium content between the two systems (P = 0.230).

Figure 1.

Mean surface elemental composition (weight%) of Endostar E3 Azure and One Curve nickel-titanium instruments as determined by energy-dispersive spectroscopy (EDS). Endostar demonstrated higher oxygen content (40.4 ± 5.13 wt%) compared with One Curve (29.1 ± 2.96 wt%; P < 0.001). One Curve showed higher carbon (29.0 ± 2.14 wt% vs. 24.6 ± 3.41 wt%; P < 0.001) and nickel content (29.9 ± 3.89 wt% vs. 22.3 ± 1.84 wt%; P < 0.001). Titanium content did not differ significantly between Endostar (12.9 ± 2.84 wt%) and One Curve (11.9 ± 1.76 wt%; P = 0.230). Values are presented as mean ± standard deviation

Finally, fractographic examination using SEM revealed similar failure patterns in both systems following cyclic fatigue testing. The fractured surfaces exhibited characteristic features of ductile fatigue failure, including crack initiation zones, fatigue striations, and dimpled rupture morphology [Figure 2]. These observations confirm that cyclic fatigue was the predominant mechanism of failure for both file systems under the applied dynamic conditions.

Figure 2.

Scanning electron microscopy images of fractured nickel-titanium rotary instruments following dynamic cyclic fatigue testing (×220–×1000). (a-c) Endostar E3 Azure files showing characteristic cyclic fatigue features, including crack initiation at the outer curvature, fatigue striations, and a central ductile fracture zone. (d-f) One Curve files demonstrating similar morphology with visible crack origin, fatigue striations, and final ductile rupture region. These fractographic patterns confirm cyclic fatigue as the predominant failure mode in both systems

DISCUSSION

In the current study, it was established that there are clinically significant and evident differences between Endostar E3 Azure and One Curve rotary files when considering cyclic fatigue resistance, bending behavior, and surface chemistry. Endostar files were shown to be substantially more resistant to cyclic fatigue, both in terms of their greater NCF and longer time to fracture, and also in requiring a greater maximum bending load.

Conversely, the One Curve files demonstrated lower bending load values, indicating greater structural flexibility, and the nickel and carbon surface signals on EDS analysis were higher. There was no significant difference between groups in titanium composition. These fundamental distinctions are consistent with contemporary findings suggesting that heat treatment, cross-sectional geometry, and surface integrity continue to be critical predictors of mechanical performance in NiTi endodontic instruments.[17,18,19]

Moreover, it is important to note that testing at 37°C was essential because NiTi alloys are temperature-dependent. Testing at body temperature ensures behavior closer to clinical performance. Furthermore, the current study explains that Endostar E3 Azure files undergo Azure HT heat treatment, producing a titanium-oxide “blue” surface layer and stabilizing the martensitic phase at body temperature. This process is reported to enhance flexibility and may contribute to delayed crack propagation, although the present study did not directly evaluate phase transformation behavior. In contrast, One Curve files are manufactured using C-Wire controlled-memory alloy, which increases flexibility but results in different nickel distribution and surface properties.

Eid et al. evaluated Endostar E3 Azure instruments under dynamic cyclic fatigue conditions at body temperature and emphasized that both access angle and kinematic motion significantly influence fatigue performance, reinforcing the methodological importance of simulating clinical motion patterns in fatigue testing.[20] Similarly, Assaf et al. directly compared One Curve and E3 Azure under dynamic cyclic fatigue and demonstrated that factors such as access angle and curvature location markedly affect fatigue resistance, with E3 Azure exhibiting superior performance in standardized dynamic models.[21] These studies align with the present findings, confirming that E3 Azure’s enhanced fatigue resilience is evident.

The use of a dynamic cyclic fatigue protocol at physiological temperature is a major methodological strength of this study. Compared with static fatigue assessment, dynamic axial-pecking motion more accurately represents clinical kinematics and has been shown to produce extended fatigue life by distributing mechanical stresses across a larger segment of the instrument.[22,23] Modern heat-treated alloys demonstrate pronounced thermosensitivity, where testing at 37°C is essential because the martensite/austenite ratio and fatigue characteristics are temperature dependent.[24,25,26] More recent thermomechanical investigations confirm that body-temperature testing induces fatigue patterns that better reflect true intracanal performance.[5] Hence, the adopted protocol enhances the clinical validity of the present findings.

The first assumption would be that the increase in the NCF with a greater bending load on Endostar would seem counterintuitive, as increased flexibility should be correlated with a longer fatigue life. However, according to the metallurgical studies, fatigue resistance is regulated not only by the bending hardiness but also by an amalgamation of the impact of heat-treatment, mass distribution, and crack propagation resistance.[27,28] Even comparable high (quasi-static) bending stiffness, the S-shaped cross-sectional shape of Endostar, which is presumably the source of its stable titanium-oxide surface, might augment crack blunting and retard crack propagation. The findings support the literature indicating that the optimization of the thermomechanical process can help to increase fatigue resistance without requiring the most versatile tool.[5,19]

Comparative SEM energy dispersive X-ray spectroscopy (EDX) studies of contemporary heat-treated NiTi single-file systems have consistently reported pronounced inter-brand variability in oxygen, carbon, nickel, and titanium signals under standardized testing conditions, reflecting a common metallurgical trend rather than identical elemental compositions, a pattern that is also evident in the present comparison between Endostar E3 Azure and One Curve.[29]

Since 2020, comparative clinical and laboratory investigations have provided a broad contextual background for interpreting the mechanical behavior of contemporary NiTi instruments. Although earlier studies reported favorable fatigue performance of One Curve relative to its preceding generations, recent dynamic-temperature research has emphasized that meaningful comparisons require careful consideration of differences in instrument architecture, heat treatment, and testing conditions.[5,23] In the present study, Endostar demonstrated superior fatigue resistance, supporting the view that mechanical performance is highly instrument-specific and strongly influenced by metallurgy and cross-sectional geometry. Moreover, recent evidence indicates that rotation kinematics influence fatigue life; however, most variations in performance appear to correlate more closely with intrinsic structural design than with motor settings.[23]

Systematic reviews further help contextualize these findings. Dos Reis-Prado et al. reported that irrigant composition and solution temperature may affect fatigue behavior, although heat-treated instruments tend to show lower sensitivity to such variables.[24] Because the current study used saline at 37°C without corrosive agents, the enhanced performance of Endostar cannot be attributed to chemical artifacts and is more plausibly related to inherent structural or surface characteristics.

SEM/EDS analysis also supports this mechanistic interpretation. Recent studies by Azizi et al.[27] and Ertuğrul and Arslan demonstrated that chemical and microstructural heterogeneity is evident among new-generation NiTi systems and is associated with instrument-specific differences in mechanical performance.[5] These surface-chemistry profiles align with the trends observed in this study and reinforce the emerging view that oxide-layer characteristics and surface nickel distribution may be associated with variations in fatigue resistance; however, causative interpretation requires complementary analyses such as differential scanning calorimetry (DSC) or X-ray diffraction (XRD).

The clinical implications are reflected in the performance trade-offs. The higher fatigue limit of Endostar suggests that it may be advantageous in preparing severely curved canals, where cyclic fatigue is the predominant failure mode. Conversely, the greater flexibility of One Curve may be useful in extremely curved or narrow canals, where reducing bending moments helps maintain canal anatomy and minimizes procedural errors. Recent shaping literature similarly emphasizes selecting instrumentation systems based on curvature severity, anatomical complexity, and operator preference.

This study has several strengths. It employed a standardized in vitro design with controlled testing conditions, minimizing operator-related and instrument-related variability. The use of dynamic cyclic fatigue testing, bending resistance assessment according to ISO standards, and SEM/EDS analysis within the same experimental framework allowed for a comprehensive evaluation of both mechanical behavior and surface elemental composition. Assessing two contemporary heat-treated systems under identical conditions provides meaningful comparative insight that is often lacking in previous literature.

Some limitations should still be acknowledged. Artificial canals, although standardized, cannot fully reproduce the complexity of natural root canal morphology or tactile feedback during instrumentation. It is important to emphasize that Energy Dispersive X-ray Spectroscopy (EDS) provides semi-quantitative surface elemental analysis and does not assess bulk alloy composition, phase transformation temperatures, or martensite–austenite distribution. Therefore, while surface oxygen and nickel variations were observed between systems, these findings should be interpreted as associative rather than mechanistic. Comprehensive metallurgical characterization would require complementary techniques such as DSC or XRD to evaluate transformation behavior and crystallographic phase composition.

Although phase transformation behavior plays a critical role in the mechanical performance of heat-treated NiTi instruments, DSC analysis was beyond the scope of the present investigation. Future studies incorporating DSC could further elucidate transformation temperatures and correlate them with fatigue resistance outcomes.

CONCLUSION

The present in vitro investigation demonstrated that Endostar E3 Azure exhibits superior cyclic fatigue resistance and greater structural endurance compared with One Curve, despite having higher bending stiffness. Elemental analysis further showed distinct surface chemistry profiles between the two systems, suggesting that heat-treated and oxide-layer characteristics are relevant to performance differences. These findings reinforce the importance of alloy treatment, cross-sectional design, and temperature-sensitive mechanical behavior in shaping instrument reliability. Overall, the study provides evidence-based insight to guide file selection in clinically challenging anatomies, emphasizing the need for system-specific evaluation rather than general assumptions about heat-treated NiTi instruments.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.