Scanning Electron Microscopic Evaluation of Surface Defects of Remover Retreatment File After Single and Multiple Uses

Podsumowanie

Here, we present a protocol for evaluating the surface characteristics of endodontic retreatment files after repeated use in retreatment procedures, utilizing scanning electron microscopy to identify and analyze potential surface defects.

Streszczenie

This study aimed to evaluate surface defects of Remover rotary Nickel-Titanium (NiTi) files after single and multiple uses in conventional endodontic retreatment procedures using scanning electron microscopy (SEM). Eighty acrylic blocks, simulating root canals with a 1.5 mm internal diameter, a 5 mm radius of curvature, and a 55° curvature, were utilized. After chemomechanical preparation and obturation, 24 new Remover files (N30, 7%, L23) were randomly assigned to three groups: single use, triple use, and six uses. The files were operated at 600 rpm with a torque of 2.5 Ncm, cleaned, and sterilized after each use.

SEM analysis at magnifications of 100x, 250x, and 500x revealed surface defects, including tip deformation, microcracks, fracture, unwinding, surface pitting, and blade disruption. Deformation was observed in 75% of the files after a single use and in 100% of the files after three and six uses. Microcracks were absent after single use but appeared in 25% and 87.5% of files after three and six uses, respectively, showing a statistically significant increase (p < 0.001). Surface pitting also significantly increased among groups (p = 0.004).

No fractures were observed in any group. The most common defects were tip deformation (91.7%) and surface pitting (70.8%). The findings suggest that repeated use of NiTi files significantly increases surface defects, elevating the risk of fatigue fractures. Thus, the results recommend limiting the reuse of Remover files to a maximum of 3x. Further research is needed to correlate defect types with anatomical factors and to assess file effectiveness in retreatment scenarios.

Wprowadzenie

Endodontic retreatment is a procedure performed when a previously treated tooth fails to heal or develops new pathologies, such as persistent infection, reinfection, or missed anatomy. The procedure involves the removal of the existing root canal filling material, thorough cleaning and disinfection of the canal system and subsequent refilling^1,2.

Nickel-titanium (NiTi) instruments are of great importance in improving and facilitating endodontic procedures due to their flexibility and high cutting efficiency^3,4. The superelasticity of NiTi instruments permits them to better adapt to canal curvature, exhibit less wear, and have a higher resistance to fracture^5,6. However, one of the major concerns with NiTi files is that they can fracture without visible deformation³.

The most common cause of fracture in NiTi rotary instruments is cyclic fatigue⁷. Cyclic fatigue occurs due to alternating tensile and compressive stresses on opposing surfaces of the instrument as it rotates continuously in a curved root canal without binding^8,9. Fracture due to cyclic fatigue results from metal exhaustion¹⁰. Several factors influence the occurrence of fracture due to cyclic fatigue, including the physical properties of the instrument^11,12, the root canal morphology¹³, repeated clinical use, and the sterilization process^14,15. Therefore, to improve the fatigue resistance of NiTi rotary files, various modifications in the manufacturing method and core diameter, as well as changes in the cutting-edge and cross-sectional designs, have been attempted¹⁶. The Remover file is a new generation file produced by thermal treatment and a special electropolishing process called C-wire. Its design features are claimed to increase fatigue resistance. The file has a 30/100 mm non-cutting (inactive) tip and a minimally invasive core diameter. It is manufactured with a variable triple helix cross section that is symmetrical for the first 3 mm and then becomes asymmetrical towards the shaft. In addition, it is designed to preserve periradicular dentin by having a 7% taper in the first 10 mm, followed by a 0% taper towards the shaft¹⁷.

Cyclic fatigue fractures in NiTi rotary files typically occur without any visible plastic deformation^18,19,20. As a result, these fractures cannot be evaluated clinically, and structural changes must be examined under high magnification using tools such as a stereomicroscope or scanning electron microscope (SEM)²¹. Due to the impracticality of performing such examinations on a routine basis, manufacturers recommend that files be used only once^22,23. However, due to the high cost of NiTi files, many clinicians choose to reuse them²⁴. Therefore, it is important to investigate the effects of clinical reuse on these files. One clinical study showed that rotary instruments can be safely reused up to 4x²⁵. However, other studies have evaluated much higher reuse rates and there is no consensus on how many times a file can be safely reused^24,26.

In previous studies that have evaluated the reuse of NiTi files, the primary focus has been on the effects of root canal widening and shaping on the fracture resistance of the files. A review of the literature, therefore, reveals that there is only one study that specifically evaluates the repeated use of retreatment file systems²⁷. The aim of this study is to evaluate the impact of repeated use on the surface characteristics of the Remover file using scanning electron microscopy (SEM). It is hypothesized that increased clinical use will result in an increase in surface defects, thereby elevating the risk of fatigue fractures. The specific objective is to analyze the changes in surface defects of the Remover file after single and multiple uses, and to discuss the implications of these changes for clinical practice.

Protokół

1. Sample procurement

1. Procure 80 acrylic blocks with an internal diameter of 1.5 mm, a radius of curvature of 5 mm, a 55° curvature, and a working length of 16 mm.

2. Cleaning and shaping procedure

1. Set the endomotor to a torque of 2.0 Ncm and a speed of 300 rpm.
   1. Attach a 10/.04 taper file to the motor and use it in a back-and-forth motion until the working length (16 mm) is reached, ensuring that it does not bind.
   2. Irrigate the canals with 5.25% NaOCl.
   3. Attach a 15/.04 tapered file to the motor and use it in a back-and-forth motion until the working length (16 mm) is reached, ensuring that it does not bind.
   4. Repeat steps 2.1.2 and 2.1.3 with 20/.04, 25/.04, 30/.04, and 35/.04 taper files, used sequentially at the working length (16 mm).
   5. Dry the canals with paper points.

3. Obturation

1. Check the fit of a guttapercha cone to the canal.
2. Inject the bioceramic canal sealer into the canal and fill it with bioceramic sealer.
3. Insert the appropriate gutta-percha cone into the sealer-filled canal. Cut the gutta-percha 2 mm below the canal orifice using a heat tool.
4. Take a periapical radiograph to verify the canal fillings (see Figure 1).
5. Store the specimens in an incubator at 37 °C and 100% humidity for 2 weeks.

4. Retreatment procedure

NOTE: A total of 24 new Remover files (23 mm) were used in the present study. The files were randomized into three groups of eight samples each. In determining the number of samples and files used in this research, the quota sampling method was used, considering the budget and the sample sizes of other reports in the literature²⁷.

1. Operate the files at 600 rpm and 2.5 Ncm torque according to the manufacturer's instructions. Use the files with a back-and-forth motion without applying apical pressure until they are 3 mm short of the working length.
2. Remove the file from the canal when resistance is felt and irrigate with 5.25% NaOCl solution.
3. Repeat steps 4.1 and 4.2 until the desired length is reached.
4. Clean and sterilize the instruments in an autoclave for 18 min at 134 °C before shaping the specimen.
   NOTE: The files in the first group were used for retreatment in eight curved canals. The files in the second group were used for retreatment 3x each, and the files in the third group were used for retreatment 6x each. The procedures were repeated in group 2 and group 3 according to the number of uses.

5. SEM analysis

1. Sample preparation and loading
   NOTE: Take the necessary precautions to avoid contamination when handling the sample (e.g., wear gloves). Do not place the sample in a gold sputtering system as the surface is nickel-titanium.
   1. Mount the sample on an SEM stub using conductive double-sided carbon tape.
   2. Attach the stub to the stage and tighten the side screw (see Figure 2).
2. SEM operation
   1. Open the SEM sample chamber and remove the stage.
   2. Place the sample stub on the stage and secure it in place.
   3. Insert the sample stage into the sample chamber and close the chamber.
   4. Switch on the pumps and wait for the system notification of the vacuum.
   5. Open the SEM software and select the required operating voltage between 1 kV and 30 kV.
3. Image analysis
   1. Have a trained investigator take images of the 4 mm distal end, which is the active part (area of interest), at standard magnifications of 100x, 250x, and 500x. Use an unused Remover file as a reference to evaluate the surface characteristics of the samples (see Figure 3).
   2. To commence the Auto Focus function, select the key icon within the SEM software. The resulting focused image of the sample is the desired endpoint.
   3. Set the magnification to the minimum zoom level of 50x.
   4. Enable the fast scan mode for efficient image acquisition.
   5. Adjust the focus using the coarse focus mode until a preliminary focus is achieved.
   6. Gradually increase the magnification to observe the desired feature. Use the coarse focus knob to achieve a rough focus, followed by the fine focus knob for precise focusing. Repeat this step for each magnification increase.
   7. Increase the magnification until the desired feature is observed. Adjust the coarse focus knob to roughly focus the image at this magnification. Then, use the fine focusing knob to improve the focus to obtain a focused image at the desired magnification. Repeat this step each time the magnification level is increased.
   8. Once the desired magnification is reached, refine the focus using the fine focus knob for optimal clarity.
   9. For enhanced image clarity, further increase the magnification to a near-maximum level and adjust the focus using the fine focus knob. If clarity is still not sufficient, adjust the stigmation in both the x and y axes. Continue fine-tuning the focus and stigmation until the clearest image is obtained at the higher magnification.
   10. After achieving a high-quality image of the sample, return to the desired magnification level. Capture the image by pressing the photo button. Choose either slow photo mode for higher quality and resolution, or fast photo mode for quicker capture.
   11. Repeat these steps for each sample.
   12. Download the images to the computer.
   13. Have two calibrated examiners analyze all SEM images by reviewing the images on a computer screen and recording the presence and type of deformations that occur in the files. The deformations include tip deformation, microcracks, fracture, unwinding, surface pitting, and blade disruption (Figure 4, Figure 5, Figure 6, Figure 7, and Figure 8).
   14. Have the same examiners analyze the collected data twice at 1 week intervals.
       NOTE: Differences of opinion in the interpretation of SEM images of the samples between the observers are to be discussed until a consensus is reached.

6. Statistical analysis

1. Present descriptive statistics as counts and percentages.
2. Perform analyses using statistical analysis software. Evaluate the differences between groups using the Fisher-Freeman-Halton Exact test. Set a type 1 error rate of 0.05 (two-tailed), and consider p < 0.005 statistically significant.

Wyniki

Deformation was observed in 75% of files after single use and in 100% of files after three and six uses, but the differences between the groups were not statistically significant (Table 1). The evaluation of deformation types among groups is shown in Table 2. When the types of deformation were evaluated separately, no microcracks were observed after a single use, while microcracks were observed in 25% of the files after three uses and in 87.5% of the files after six uses; this difference...

Dyskusje

This study evaluated the presence and types of microscopic defects on the external surfaces of Remover files after single, triple, and six-times use in acrylic blocks simulating curved canals. Ideally, human teeth are recommended for use in studies evaluating the fracture resistance of files to better simulate clinical use²⁸. In their study, Peters and Barbakow²⁹ found an increase in fracture initiation and propagation rates in instruments used in blocks compared to extract...

Materiały

Name	Company	Catalog Number	Comments
Acrylic block	ArdaDent Medical,Ankara,Turkey		for obturation
DiaRoot Biosealer	DiaDent, South Korea	BS23101161	for obturation
DualMove Endomotor	MicroMega, Coltene, France	52002023	for preparation
EndoArt  Smart Gold	EndoArt, Inci Dental, Turkey	SGK10114	for initial preparation
Gutta Percha	EndoArt, Inci Dental, Turkey	GD23080701	for obturation
Quattro ESEM	Thermo Fisher Scientific, USA		SEM analysis
Paper Points	Dentsply Maillefer, Ballaigues, Switzerland	1I0305	for dry to root canal
Remover File	MicroMega, Besançon, France	891144/873757/	for retreatment procedure
Sodium Hypochlorite	Saba Chemical&Medical, Turkey	3010225	for irrigation
SPSS v29	IBM SPSS Corp, Armonk, New York, USA		Statistical analysis