The modified conservative access cavities have been the scope of interest for all bio-minimalistic researchers since 2018 (1). Minimally invasive endodontic treatment aims to preserve as much of the natural tooth structure as possible while ensuring effective debridement and disinfection of the root canal system. The contracted access cavity is a key component of minimally invasive endodontics, characterized by a conservative access design that minimizes removal of pericervical dentin and maintains the structural integrity of the tooth. The problem with contracted cavities is that they place the file under high stress due to the absence of a straight path for the enlarging tool into the canal (2).

Recent heat-treated files having higher cyclic fatigue resistance have been developed (3). Among these instruments, the E3 Azure files (Endostar, Zielonka, Poland), characterized by their unique design and material properties, have garnered attention for their performance and durability (4). The Endostar E3 Azure is an advanced endodontic system designed using Azure HT Technology, a specialized heat-treatment manufacturing process. This innovative technology enhances the flexibility and fracture resistance of the files, making them highly reliable even in complex clinical cases. The Azure HT Technology process alters the crystal structure of nickel-titanium files, enabling the transformation from martensite to austenite to occur near body temperature. The E3 Azure features a modified S-shaped cross-section, which reduces the file's core size to enhance debris removal and improve flexibility. It also incorporates a variable pitch, neutral rake angle, and a safe cutting tip for optimal performance (5).

In addition, recent kinematics were proposed as an alternative for continuous rotation and allowed increased time to failure for the instrument (6, 7). The reciprocating motion refers to the use of alternating clockwise and counterclockwise movements during root canal instrumentation. This motion reduces torsional stress on the file and the risk of instrument separation, especially in curved or narrow canals. The reciprocation motion has been tested through a lot of different factors and compared to rotational cutting motions in multiple aspects (6, 8–10).

The manufacturer's recommendation for the E3 Azure file is the use in a full rotation motion, a reciprocation motion, or a complex motion, which is comparable to the adaptive motion proposed earlier by Kerr for the TF adaptive file system. The file design, specifically, the neutral rake angle, allows for the use of this file in variable motions (6, 8–10).

The angle of file access into the root canal plays a crucial role in determining the cyclic fatigue resistance of endodontic instruments (11). When a file enters the canal at a sharp angle, it is subjected to increased flexural stress, particularly at the point of maximum curvature. This repeated stress accumulation accelerates crack initiation and propagation within the metal structure, ultimately reducing the file's lifespan and increasing the risk of separation. Therefore, maintaining an optimal file access angle through proper access cavity design and glide path preparation is essential in prolonging instrument longevity and ensuring efficient root canal shaping (5, 6, 12).

The 0° and 30° angles of file access are representative of conventional and contracted endodontic access cavities, respectively, because they simulate the straight-line path of an instrument entering the canal through differently designed access outlines. A 0° angle reflects the file being inserted along the long axis of the tooth, which is typically feasible in conventional access cavities due to total removal of the roof of the pulp chamber and a straight-line access to the apical foramen or the initial canal curvature. In contrast, a 30° angle mimics the angulated approach required in contracted and conservative access cavities, which necessitates an indirect or more oblique entry point due to preservation of the coronal dentine and lack of straight-line access to the canal (5, 12).

However, the combined effect of access cavity angulation and kinematics on the dynamic cyclic fatigue resistance of recently introduced heat-treated NiTi files, such as E3 Azure, has not yet been investigated. Furthermore, most available studies do not simulate clinically relevant conditions, such as dynamic movement (9) and body temperature (9, 12), limiting their applicability and clinical relevance.

Therefore, this study aims to assess the influence of angled canal access on file separation, along with the impact of different cutting motions. The null hypothesis states that there is no significant difference in the time to file separation between full rotation and reciprocation motions when the access angle is either 0° or 30°.

The two-way ANOVA results presented in Table 1 showed that there was a significant interaction between both tested variables (p < 0.001). The simple effects comparisons presented in Table 2 showed that regardless of access angle, samples instrumented using reciprocation motion had significantly higher time to fracture than those with continuous rotation (p < 0.001). Additionally, regardless of rotation motion, samples with zero-degree access angle had significantly higher time to fracture than those with 30° (p < 0.001). Mean and standard deviation values for the fracture-segment length were statistically not significant, as presented in Table 3.

Several factors contribute to file fracture; however, cyclic fatigue has been identified as a major cause, particularly when instruments are used in curved root canals (16, 17). Dynamic cyclic fatigue testing, despite its limitations in replicating the full complexity of clinical conditions, remains one of the most reliable and standardized methods for evaluating the fatigue resistance of endodontic files (6, 18, 19). Unlike static models, dynamic testing incorporates axial movements that closely mimic the clinical pecking motion, resulting in more realistic stress distribution along the instrument (18). This approach allows for controlled and reproducible comparisons between instruments, motions, and access designs, while isolating variables such as file design, kinematics, and access angle. In the present study, a custom-made stainless steel artificial canal with a 60° curvature and 3 mm radius was used to simulate clinical conditions (5, 11). The dynamic model better replicates intracanal stresses by distributing them along the file shaft, rather than concentrating stress at a single point, as observed in static models (20).

The temperature changes significantly influence the cyclic fatigue resistance of heat-treated endodontic NiTi files. An increase in temperature promotes phase transformation to a more austenitic form, which reduces cyclic fatigue resistance and accelerates microcrack propagation (21, 22). The test was conducted at body temperature to most accurately replicate clinical conditions.

Endodontic files entering the canals at steeper angles, the stress concentration at the point of maximum curvature significantly increases, leading to a higher incidence of instrument fracture. For instance, studies utilizing artificial canals and simulated clinical conditions have reported that files subjected to greater access angle experience a marked reduction in the number of cycles to failure compared to those with a more aligned entry path (6).

This study evaluated the effects of kinematics and access angle on the cyclic fatigue resistance of blue heat-treated NiTi files in simulated root canals. The methodology was validated by the nonsignificant difference in fractured segment lengths across all groups (f-ratio = 0.23414), with fractures consistently occurring at the canal curvature, confirming cyclic fatigue as the failure mode. As all files fractured in a predictable and uniform manner, and since only one file system (E3 Azure) was tested without variation in design or metallurgy, scanning electron microscopy was not performed, as it would not have provided additional insights (18, 19).

The results of this study demonstrated that both the angle of access and the type of motion significantly influence the dynamic cyclic fatigue resistance of blue heat-treated NiTi files; therefore, the null hypothesis tested was rejected.

Reciprocating motion demonstrated significantly longer time to fracture than continuous rotation, likely due to its alternating movement, which allows periodic stress release and reduces crack initiation and propagation. The unequal forward and reverse angles in reciprocation help minimize continuous tension and compression cycles, delaying fatigue failure. In contrast, continuous rotation imposes constant stress at the same points, accelerating crack development. This finding supports previous studies reporting enhanced cyclic fatigue resistance with reciprocation due to stress relief from rotational reversal (6, 23). However, some studies found no significant difference, possibly due to variations in file design (9), or modifications in the reciprocation motion (24).

In addition, the results of this study showed that the access angle affected cyclic fatigue resistance, as the files with a zero-access angulation showed significant resistance to cyclic fatigue in comparison with the 30° access angle. This could be attributed to the minimal magnitude of flexural stress imposed on the file. When the file enters straight into the canal without angulation, the distribution of cyclic stress is more uniform along its length, reducing the likelihood of localized stress concentration. In contrast, a 30-degree access angle introduces additional bending forces, increasing strain at specific points along the file, particularly at the curvature, which accelerates crack initiation and propagation. This increased mechanical stress leads to faster fatigue failure, reducing the instrument's lifespan (5, 11).

The findings of this study align closely with those of Assaf et al. (5), who also studied E3 Azure files in comparison to OneCurve files in full rotational motion. Their study demonstrated that a smaller curvature angle and a more coronal curvature position enhanced the fatigue resistance of the tested nickel-titanium rotary files.

Pedullà et al. (12) results align with our results evaluating the cyclic fatigue resistance of Reciproc and Reciproc Blue files at various angles of file access in a reciprocation motion. The smaller the angle of access, the higher the cyclic fatigue resistance of both tested files.

Despite their value, in vitro studies using simulated canals at body temperature have limitations. Artificial models cannot fully replicate the complexity of natural teeth, including anatomical variations, dentin properties, and clinical factors like debris, lubrication, and operator technique. While body temperature testing is more relevant than room temperature, it still fails to capture dynamic intraoral conditions. Therefore, caution is needed when applying these results clinically, and future research using in vivo studies and advanced simulations, such as finite element analysis, is recommended to better reflect real-world performance (18, 19).

There is a growing need for a more standardized and clinically relevant method of measuring the cyclic fatigue resistance of endodontic rotary instruments. Future research should focus on developing advanced testing protocols that incorporate realistic anatomical variations, simulated body temperature, and dynamic loads to better predict clinical performance. Additionally, integrating high-resolution imaging and finite element analysis could provide deeper insights into crack initiation and propagation, ultimately improving instrument design and clinical safety (19).

In conclusion, both the mode of motion and the angle of file access significantly influence the cyclic fatigue resistance of endodontic instruments. Reciprocation motion, which alternates clockwise and counterclockwise movements, reduces stress accumulation and delays crack propagation, thereby enhancing file longevity compared to continuous rotation. Additionally, a smaller access angle minimizes excessive flexion and stress concentration at the point of maximum curvature, further improving fatigue resistance.

These findings emphasize the importance of optimizing both motion kinematics and straight-line access to root canals to enhance the safety and efficiency of endodontic instrumentation.