Clinical context and challenges of cyclic fatigue

In endodontics, the fracture of NiTi instruments during the shaping of curved canals constitutes a major complication impacting the prognosis of the treatment. This study evaluates the mechanical performance of two instruments benefiting from a "blue" heat treatment: the VDW Rotate (VR) and the Azur Blue (AzB). The central problem addressed is the balance between dynamic cyclic fatigue resistance and flexural stiffness, two determining factors for clinical safety when negotiating complex trajectories.

Objectives and experimental framework

The objective of this work is to accurately compare the fatigue resistance, stiffness, and phase transformation characteristics of these two systems at a simulated intracanal temperature of 34 ± 1°C. To achieve this, 58 new instruments were analyzed (n=29 per group). Dynamic cyclic fatigue tests were performed in an 18 mm long artificial canal with a 45° curvature and a 5 mm radius. The study tests the hypothesis that the disparities in mechanical performance between the VDW Rotate and the Azur Blue are more closely related to their metallurgical phase transformation behavior than to their external geometry alone.

Design and experimental protocol

This in vitro comparative study focused on 58 new endodontic instruments, equally divided between two systems benefiting from a "blue" heat treatment: VDW Rotate (VR) and Azur Blue (AzB) (n=29 per group). The experiment was conducted at a constant simulated intracanal temperature of 34 ± 1°C.

The protocol segmented the samples according to the following analyses:

• Dynamic cyclic fatigue resistance (n=12 per group): tested within an artificial canal 18 mm long and 1.4 mm in diameter, featuring a 45° curvature and a 5 mm radius. The time to fracture was recorded and then converted into the number of cycles to failure (NCF).
• Flexural rigidity (n=12 per group): evaluated with a 3 mm deflection.
• Metallurgical analyses: use of differential scanning calorimetry (DSC, n=1), X-ray diffraction (XRD, n=2) and energy dispersive X-ray fluorescence (EDXRF, n=2).

Fracture morphology and surface characteristics were examined by scanning electron microscopy (SEM). Finally, the length of the fractured segments was measured to precisely locate the point of failure under stress.

Results: A marked superiority of Azur Blue in cyclic fatigue

The evaluation of mechanical performance at a simulated intracanal temperature of 34 ± 1°C reveals significant differences between the two heat-treated nickel-titanium (NiTi) instrument systems.

Mechanical performance and fracture resistance

The Azur Blue (AzB) instrument demonstrated a significantly higher resistance to dynamic cyclic fatigue compared to the VDW Rotate (VR). Meanwhile, the VDW Rotate proved to be stiffer during bending tests. The quantitative data are summarized in the following table:

The length of the fractured fragments was significantly greater for Azur Blue (4.87 mm versus 3.27 mm for VR), suggesting a different distribution of stresses during dynamic rotation in the artificial canal with a 45° curvature.

Metallurgical analysis and phase behavior

Differential Scanning Calorimetry (DSC) analyses revealed distinct phase transformation behaviors, explaining the observed variations in flexibility:

• Austenite finish temperature (Af): It was measured at 35.8°C for VDW Rotate and at 39.4°C for Azur Blue.
• Crystallographic state: At the test temperature (34°C), the Azur Blue instrument is more in a martensitic/R-phase than the VDW Rotate, the latter being closer to its full austenitic phase.

Additional analyses by X-ray diffraction (XRD) and energy-dispersive X-ray fluorescence (EDXRF) confirm that these mechanical properties result more from the phase transformation behavior induced by the blue heat treatment than from the external geometry of the instruments alone. Scanning electron microscopy (SEM) imaging complemented these observations with a fractographic analysis of the fractured segments.

Analysis of mechanical and metallurgical performance

This study demonstrates that the superiority of Azur Blue (AzB) over VDW Rotate (VR) in terms of cyclic fatigue resistance (2039 cycles versus 1609) is intrinsically linked to its phase transformation behavior. With an austenite finish temperature (Af) of 39.4°C compared to 35.8°C for VR, the AzB instrument maintains more pronounced martensitic properties at the simulated intracanal temperature of 34°C. Clinically, this translates into significantly higher flexibility (stiffness of 0.85 N vs 1.07 N) and better stress tolerance in 45° curvatures.

The highlight of this analysis is the direct correlation between metallurgy and clinical safety: AzB offers a superior margin of maneuver before failure. Notably, the length of the fractured fragments differs (4.87 mm for AzB vs 3.27 mm for VR), highlighting the combined impact of geometry and heat treatment on stress distribution. However, the major limitation remains the in vitro nature of the test: the 1.4 mm diameter artificial canal does not replicate the complexity of dentinal debris or real anatomical variations, although the differential scanning calorimetry (DSC) and X-ray diffraction (XRD) data validate the robustness of the mechanical results.

Conclusion

L'Azur Blue features superior cyclic fatigue resistance and increased flexibility compared to VDW Rotate, characteristics dictated by its specific heat treatment and its martensitic phase stable at body temperature.

Summary of results

L'instrument Azur Blue surpasse le VDW Rotate en termes de résistance à la fatigue cyclique (2039 ± 250 cycles contre 1609 ± 222) et de flexibilité (0,85 N contre 1,07 N de rigidité). Ces performances sont directement liées à sa température de finition austénitique plus élevée (39,4°C), garantissant un comportement plus martensitique et ductile à la température intracanalaire de 34°C.

In concrete terms, for the practitioner:

• Secure complex curvatures: prioritize Azur Blue for canals with pronounced curvatures (45° and more) to minimize the risk of cyclic fatigue fracture.
• Optimize the passage of calcifications: use the VDW Rotate when you are looking for superior rigidity to progress through narrower or mineralized canals.
• Anticipate complication management: in the event of a fracture within a 5 mm radius curvature, expect a potentially longer fragment with Azur Blue (approx. 4.9 mm) than with VDW Rotate (approx. 3.3 mm).

Technical lexicon of the study

Dynamic cyclic fatigue: Ability of an endodontic instrument to resist fracture under repeated cycles of compression and tension during reciprocating motion in a curved root canal. In this study, it is expressed by the number of cycles to failure (NCF) at a simulated temperature of 34 ± 1°C.

Bending stiffness: Force required to induce elastic deformation of the instrument. It is measured here by a 3 mm deflection, reflecting the file's ability to follow the canal anatomy without exerting excessive forces on the walls.

Austenite finish temperature (Af): Critical thermal point (35.8°C for VDW Rotate and 39.4°C for Azur Blue) marking the end of the transition to the austenitic phase. An Af higher than the intra-canal temperature promotes the presence of more flexible phases (martensite or R-phase), improving fatigue resistance.

Differential Scanning Calorimetry (DSC): Thermal analysis method used to identify the phase transformation temperatures of NiTi alloys by measuring the heat flows absorbed or released during crystalline state changes.

X-ray Diffraction (XRD): Structural characterization technique used to analyze atomic organization and identify metallurgical phases present (austenite, martensite, R-phase) on the surface of instruments.

Energy Dispersive X-ray Fluorescence (EDXRF): Spectroscopic analysis used to determine the qualitative and quantitative elemental chemical composition of the tested nickel-titanium alloys.

Blue Heat-Treated Instruments: Endodontic files that have undergone a specific surface heat treatment creating a blue titanium oxide layer, aimed at modifying phase transformation characteristics to increase flexibility and cyclic resistance.

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