Changes in the Surface Area and Volume of Hyflex EDM Instruments after Multiple Uses

Abstract

Objective

The aim of the study was to evaluate the surface areas and volumes of HyFlex EDM (HEDM) nickel-titanium (NiTi) files before and after multiple uses in mandibular molar root canal preparation.

Materials and methods

Twelve HEDM files with sizes of 10/.05 and 25/.08 were evaluated by micro–CT before and after a single use or multiple uses in root canal preparation. The mesiobuccal and mesiolingual root canals of thirty-six human mandibular molar teeth were instrumented using twelve 10/.05 (300 rpm and 1.8 Ncm) and twelve 25/.085 Ncm) HEDM files according to the manufacturer's recommendation. HEDM files were evaluated using micro–CT at four different timelines: 1) Intact file, 2) After instrumentation of one canal, 3) After instrumentation of two canals, and 4) After instrumentation of three canals. Each group's volume and surface area were analyzed by applying Friedman's test. Each group comparison was assessed using the Wilcoxon signed-rank test with Bonferroni correction.

Results

Intact HEDM 10/.05 and HEDM 25/.08 files showed significantly larger volumes and surface areas compared to the used instruments (p < 0.05).

Conclusions

A significant decrease in the volume and surface area of the NiTi files was observed as the number of uses increased regardless of the file group. These findings emphasize the importance for clinicians to understand the structural changes in HEDM files during repeated use to ensure safe instrument reuse and prevent endodontic treatment failure.

Introduction

Nickel titanium (NiTi) rotary file systems are frequently used to prepare root canals due to their excellent cutting ability and flexibility. (1, 2) Despite these advantages, files are exposed to pressure and stress during root canal treatment. These stresses cause cyclic fatigue, thus leading to sudden breakage or distortions in the instruments (3) and negatively affecting the outcome of root canal treatment (4).

There are many factors that increase the susceptibility to fracture of root canal files. While fabricating files, cracks, grooves, and pits often form on the instrument surface. (5) These areas are stress-inducing and can result in instrument breakage during clinical use. (6) HyFlex EDM (HEDM; Coltene/Whaledent) is produced from a controlled memory (CM) alloy using electrical discharging machining (EDM) technology. EDM technology is based on a noncontact thermal erosion process that partially melts and evaporates a wire through high-frequency spark discharges (7). By creating a unique shape on a file surface, the file can exhibit high abrasion resistance and good mechanical properties (8). The files produced with this method are stronger and up to 700% more resistant to cyclic fatigue compared to those produced with other methods. In addition, these features reduce file usage during root canal preparation while preserving original anatomical forms.

NiTi files might be used more than once in dental practice for financial reasons; however, studies have reported that long-term clinical use of NiTi files significantly reduces cyclic fatigue resistance (9-11). Currently, it is not possible to establish a definitive guideline regarding the safe number of uses for rotary NiTi files, as this varies depending on factors such as the type of tooth, operator, and root canal anatomy. Nonetheless, there is a clear trend toward advocating for the single-use of rotary NiTi files during root canal treatments (12). HEDM files return to their original shape after sterilization. If the files do not return to their original shape after sterilization, their use is not recommended (8). The irregularities on the surface of intact HEDM files do not cause risk on file surface preservation after repeated use due to noncontact production of NiTi files with EDM technology (13). The spark-machined surface was shown to remain unaffected after multiple uses, confirming a high wear resistance in previous studies (8). However, there are no studies in the literature on the post-use changes in surface area and volume of HEDM rotary files. The aim of this study was to evaluate the surface areas and volumes of HEDM files before and after three instances of instrumental use with microcomputed tomography (micro–CT). The null hypothesis of the study was that there would be no significant changes in the volume and surface area of HEDM rotary files after repeated use compared to their pre-use measurements.

Materials and methods

Sample size calculation

The sample size for this study was calculated using software (G * Power 3.1.7 for Windows; Heinrich Heine, University of Dusseldorf, Dusseldorf, Germany), and the t test for 2 independent groups was selected with an alpha-type error of.05 and a beta power of 0.95 for all variables. A previous study was used to determine an effect size (f) of 1.71 (13). The minimum sample size required was seven specimens per group. The sample size was increased to twelve files per group after considering the potential risk of file fracture during root canal preparation and the possibility that the files would not return to their original shape after autoclaving.

Sample selection

After the study received ethical approval by the Ankara Yıldırım Beyazıt University IRB Committee (2019-336), 36 mandibular molar teeth featuring mesial roots with Vertucci’s type IV(Two unique and independent canals from the orifice to the root apex) canal configurations (14) and curvature angles between 20° and 40° (15) were selected from a pool of extracted human teeth. The homogeneous distribution of the teeth and their compliance with the inclusion criteria were confirmed using digital periapical radiography and micro–CT (16). The inclusion criteria were as follows: teeth that had not undergone root canal treatment before, mature roots, no diagnoses such as root caries or internal or external resorption, and no calcification, cracks or fractures. The crowns of the teeth were removed from the enamel–cement junction with a diamond bur (Diatech, Charleston, U.S. A) by water cooling; the root length was 12±0.5 mm.

Sample preparation

The sizes of the access cavities were uniformly prepared in all samples. After access cavity preparation, the working length (WL) was determined by measuring the length of the #10 K-file (Dentsply, Maillefer, Ballaigues, Switzerland) when its tip became visible at the apical foramen. HEDM files were used with a VDW Gold (VDW, Munich, Germany) endodontic motor. The mesiobuccal and mesiolingual root canals of thirty-six human mandibular molar teeth were filed using 10/.05 (300 rpm and 1.8 Ncm) and 25/.085 Ncm) HEDM files according to the manufacturer's recommendations. The shank parts of the files were marked with an ISO 014 diamond bur (Strauss & Co, Industrial Diamonds Ltd, Ra’anana, Israel) to assess the same area in each instrument during a micro–CT analysis.

The root canals were irrigated using 5 mL of 3% sodium hypochlorite (NaOCl; CanalPro, Coltene/Whaledent, Germany) during instrumentation. In total, 20 mL of 3% NaOCl was used for each sample. The debris on the surface of the file was wiped off with alcohol-soaked gauze after each use.

One set of new HEDM files (10/.05 and 25/.08) was used to prepare two root canals (mesiolingual and mesiobuccal) on each of the three mandibular molar teeth, totaling six root canals per set of instruments. After filing the mesial canals of each mandibular molar tooth, the files were washed with 5 mL of distilled water to remove accumulated debris. They were then cleaned in an ultrasonic bath containing detergent (acetone, ethanol, and deionized water) and sterilized at 134 °C for 4 minutes at 30 psi before being used on the next canal (17). All procedures in this experiment were conducted by a single experienced endodontist to maintain consistency in technique and outcomes.

The changes in the spirals and surfaces of the files were examined at 40X magnification under a dental operating microscope (OMS 2350, Zumax, Jiangsu, China). The files with spirals that returned to their original shape during sterilization were used to prepare the root canals of the next sample.

Micro–CT analysis

The sizes (10/.05 and 25/.08) of each new HEDM file and those used multiple times were evaluated by micro–CT (SkyScan 1274, SkyScan, Aartselaar, Belgium) four times: when intact, after filing one canal, after filing two canals and after filing three canals. Samples were scanned with a resolution of 33 µm, exposure time of 50 ms, and rotation of 0.4°. The electrical values applied to the X-ray source during scanning were 50 kV and 800 µa. Data were reconstructed with NRecon (NRecon version 1.6.9.4; Skyscan), and transverse sections were obtained. Then, the surface properties were analyzed with CTan (CTan version 1.13.5.1; Skyscan). Three-dimensional models were created using CTvol (CTvol version 1.7.4.2; Skyscan).

Statistical Analysis

Data were first analyzed using the Shapiro‒Wilk test to verify the assumption of normality; the data were not normally distributed. The volume and surface area in each file group were analyzed by applying Friedman's test. Each group comparison was assessed using the Wilcoxon signed rank test with Bonferroni correction. Statistical analyses were performed using SPSS 21.0 (IBM - SPSS Inc., Chicago, IL, USA) software, and the statistical significance level was P < 0.05.

Results

No fractures occurred in any HEDM files during root canal preparation. Figures 1 and 2 show the three-dimensional (3D) micro-CT superimposition of pre- and post-use images of the HEDM 10/.05 and HEDM 25/.08 files, respectively. These images show the surface alterations of the instruments after repeated clinical use. Color coding is used to differentiate the file surfaces: intact instruments are shown in green, while the surfaces after each usage are depicted in red (1x, 2x, and 3x).

Figure 1.

Representative three-dimensional (3D) micro–CT reconstructions of HEDM 10/.05 instrument. Color-coded instrument surface indicates a) Intact (green) and after single-use (red) b) After second-use (green) and after third-use (red) of the instrument.

Figure 2.

Representative three-dimensional (3D) micro–CT reconstructions of HEDM 25/.08 instrument. Color-coded instrument surface indicates a) Intact (green) and after single-use (red) b) After second-use (green) and after third-use (red) of the instrument.

In Figure 1, the 3D reconstruction of the HEDM 10/.05 instrument highlights the changes that occur after each use. The transition from the intact state to the post-use states visually represents the reduction in surface area. Similarly, Figure 2 presents the HEDM 25/.08 instrument in 3D, thus allowing for a direct comparison of its surface integrity before and after each use, providing a reference for understanding how the performance of the instrument may be affected by continued use.

The median and interquartile range (IQR) for the volume (V) and surface area (SA) values of the HEDM 10/.05 and HEDM 25/.08 files are presented in Table 1. Based on the obtained data, the effect size value was found to be 1.25, and from this, the obtained power value was 83%.

TABLE 1. The surface area and volume values of HyFlex EDM 10/.05 and HyFlex EDM 25/.08 files.

		HyFlex EDM 10.05						HyFlex EDM 25.08
	Surface Area			Volume			Surface Area							Volume
	Median IQR (25th -75th)			Median		IQR (25th-75th)	Median			IQR (25th-75th)				Median							IQR (25th-75th)
Intact	27.05+0.40a		26.8-27.3	2.61+0.09a	2.58-2.65		31.98+0.40a		31.6-32.08				3.53+0.00a					3.53-3.54
First Usage	25.84+0.43ab		25.4-26.2	2.53+0.07ab	2.49-2.58		30.70+0.35ab		30.5-30.8				3.46+0.06ab									3.42-3.47
Second Usage	24.42+0.48bc		24.3-24.6	2.45+0.81bc	2.41- 2.52		29.98+0.39bc		29.8-30.08				3.31+0.07bc									3.24-3.36
Third Usage	23.48+0.70c		23.3-23.7	2.39+0.07c	2.35-2.42		28.05+0.35c		27.8-28.2				3.14+0.05c									3.14-3.17
P value	0.05

*Different superscript letters indicate statistically significant differences between groups (P < 0.05) (^{a, b, c, d} for intragroup comparasion, ^x,y for intergroup comparasion)

EDM, electrical discharge machining method; IQR, interquartile range

Intact HEDM 10/.05 and HEDM 25/.08 files exhibited the highest values for both volume and surface area. With an increasing number of uses, the volume and surface area values decreased significantly (P < 0.05 or P = 0.000).

Discussion

The changes in the surface properties of files after clinical use affect their cyclic and torsional fatigue resistance and cutting efficiency (18). Surface irregularities and blunt cutting edge can affect the cutting ability of NiTi rotary files and contribute to corrosion and material fatigue (3). Additionally, exposure to chemicals during use, as well as pre-sterilization cleaning and multiple sterilization cycles, can deteriorate these instruments and increase their corrosion rate (19). Therefore, evaluating the surface characteristics of NiTi rotary files is essential to assess their susceptibility to damage. Some clinicians may reuse the same file while treating another tooth of the same patient out of convenience or for financial reasons (20). Scholars have reported different results regarding the maximum number of canals that can be treated with NiTi rotary files before they need to be replaced with new files (8). The recommended number of uses can vary depending on the complexity of the root canal system being treated and the operator's technique. This research is the first study to examine changes in surface area and volume of HEDM files after multiple uses on natural teeth, using micro-CT, thus providing valuable insights into the practical implications of reusing these instruments in clinical practice. According to the results of this study, after the use of HEDM 10/.05 and HEDM 25/.08 files in root canal preparation, a decrease in volume and surface areas was observed (P <0.05). Therefore, the null hypothesis of the study was rejected.

The HEDM instruments demonstrated greater flexibility and improved resistance to cyclic fatigue when compared to the HyFlex CM, Twisted File, and K3 files in a recent study. (21) Similarly it exhibited higher cyclic fatigue resistance than Protaper Gold and K3XF (22). The unwinding of the spirals compensate for stress during their clinical use. HEDM files return to their original shape with full shape recovery and the continuous functionality following high-temperature sterilization cycles (23). This allows for multiple uses of the same file, potentially increasing their lifespan in clinical applications. However, it is essential to acknowledge that this design feature does not completely prevent the risk of degradation over time. To use files safely during clinical applications, it may be important to understand the post-production conditions and how repeated use affects surface properties, which is critical for ensuring patient safety and effective treatment outcomes. This study addresses a significant gap in the existing literature, as there are currently no micro–CT studies investigating the changes in volume and surface area of HEDM rotary files before and after multiple uses. By understanding how changes to the surface affect the performance of these instruments, clinicians can make more informed decisions about reusing files, ultimately enhancing the quality of endodontic treatments.

Artificial or natural teeth have been used in studies to examine the changes in the mechanical properties of files after root canal preparation (13). Since extracted teeth better reflect real clinical conditions than artificial teeth, extracted teeth with inclined (20°-40°) mesial canals of mandibular molar teeth were used for root canal preparation in the present study.

Studies to assess the quantitative wear amount of used rotary files utilizing microscopic images and software analysis (24). However, no study has examined the volume and surface area changes that occur on the surfaces of files using micro–CT imaging. Micro-CT provides high-resolution, 3D images, allowing for detailed quantitative assessments of volume and surface characteristics. It provides precise volumetric data by assessing the entire 3D structure of the sample. The results of our study cannot be directly compared to those of other researchers.

In studies on surface roughness in the literature, surface topographies were reported to change after the clinical use of a rotary system file, and these changes were related to erosion, cutting surface deformation, plastic deformation and microcrack formation (25). In addition, as the surface roughness increased, the tendency of the files to break increased, and the cutting efficiency decreased (26). The files were used sequentially in this study, following the manufacturer's recommendations. To reduce the stress of HEDM 25/.08, it was suggested to use HEDM 10/.05 files as the first file for mesial root curvatures in mandibular molar teeth (27). Especially at the tip of the file, HEDM 10/.05 files exhibit openings in their spirals due to stress. This effect may be due to the small-diameter files being exposed to torsional stress in root canal preparation.

Pirani et al. (8) examined the surface properties of HEDM files by SEM after the preparation of 10 molar teeth, with each file having a curvature of 50°–70°. The authors reported that none of the HEDM 25/.12 and 25/.08 files were deformed, except for slight deformations on the surfaces of the HEDM 10/.05 files. Scholars have reported that the irregularities on the surfaces of new HEDM files do not pose a danger after use. Although the authors reported that both files had oxide layers on their surfaces, they also reported that EDM may be more durable than CM, which consists of B2-austenite and B19-martensite phases, because it contains a large amount of R phase (8). While these mechanical properties are closely related to the safety and efficacy of HEDM files during clinical use, small EDM files were reported to be permanently deformed due to their higher susceptibility to torsional failure than larger instruments. Caution was recommended regarding reuse of small HyFlex EDM rotary instruments (7).

Uslu et al. (13) evaluated the changes in the surface properties of HyFlex CM 25.08 and HEDM 25/.08 files before and after use with 3D optical profilometry. Although both files tested in this study were made of CM alloy, changes occurred in the surfaces of the files after use, and the preservation of surface roughness values was observed with EDM technology.

Irrigants were found to reduce the risk of torsional fractures by reducing the torque values of Ni-Ti rotary instruments. (28) However, these instruments showed a variable increase in surface roughness after being exposed to irrigants. (29) NaOCl can cause micropitting by removing nickel from the instrument surface. This can cause corrosive areas on the NiTi instrument affecting the surface roughness. (30) Therefore, NaOCl irrigation with a concentration of 3% was chosen in the present study to reduce the amount of available chlorine that would attack the alloy. In a study, immersion in NaOCl reduced the cyclic fatigue resistance of ProTaper Next, Hyflex CM, and HEDM instruments, with a greater effect observed on those made from CM wire (31).

Changes in the surface structures of EDM files might occur due to sterilisation periods (18). Repeated sterilization contributes to surface roughness and changes in mechanical characteristics. Hyflex EDM and WaveOne Gold files were reported to show similar surface changes when subjected to multiple usage and autoclaving cycles (32). In a study in which the changes on the surfaces of HEDM files during autoclave sterilization were examined with atomic force microscopy, the surface roughness of the new brand HEDM file was reported to change at a low level after the first sterilization, and the surface roughness increased significantly after the fifth sterilization (18). After being applied as instruments, heating and severe deformation in the canal, a decrease in hardness and an increase in the elastic moduli of the EDM files were observed. These results were explained by the re-dissolution of the Ni₄Ti₃ phase and the stress-induced martensitic transformation of some austenite into an additional R-phase, which is consistent with the phase changes observed in the EDM files used (23). In this study, the files were sterilized by autoclave after each use to mirror the conditions of actual clinical use as much as possible. The decreases in the surface area and volume values of the files and the effects of sterilization on the deformations on the surface can be explained in this manner.

Considering the limitations of controlled laboratory conditions, the setting may not fully replicate the complex biological and mechanical interactions present in vivo, thus making it difficult to directly correlate the findings to the clinical performance of the instruments. Potential biases from operator variables and the anatomical canal configuration of mandibular molars may limit the applicability of findings to other types of teeth with different canal anatomies (33). Further research considering a diverse range of operators and anatomical canal variations is needed to enhance the generalizability of these results.

Although the results of the study show that the volume and surface area changes could be analyzed using micro–CT, further studies are needed to examine how the micro-CT findings correlate with the cyclic and torsional fatigue of files in clinical use and their relationships with the incidence of fracture. SEM and mechanical testing can be further combined with micro-CT to analyze surface details, revealing micro-cracks or assessing mechanical integrity.

Conclusion

Within the limitations of the study, micro-CT analysis revealed that the surface area and volume of HEDM files decreased progressively with each repeated use, along with observable deformations on the surfaces after multiple uses. Micro-CT analysis can be utilized to assess the volume and surface area changes of rotary instruments. Clinicians should monitor the integrity of files, consider the effects of sterilization, and the risks associated with reusing files to enhance endodontic treatment outcomes, particularly in complex canal anatomies where the risk of instrument failure is higher.

Availability of data and materials

The data in this study are available upon request from the corresponding author.