Influence of Temperature on the Cyclic Fatigue Resistance of Reciproc Blue Instruments

INTRODUCTION

Nickel–titanium (Ni-Ti) instruments have been introduced to facilitate the instrumentation of curved root canals.¹ These instruments have undeniable qualities such as superelasticity and shape memory.² Nevertheless, many variables can contribute to the unexpected fracture of clinically used instruments.² Cyclic fatigue has been indicated as one of the main causes³ of repeated compressive and tensile stresses in the curved area of the root canal.¹

Nickel–titanium alloys possess two crystalline phases, namely, austenite and martensite, which are influenced by tensile stress and temperature.⁴ A high percentage of the martensitic phase promotes an increase in cycle fatigue resistance as a result of the larger number of interfaces that impair the propagation of fissures.^5,6 The characteristics and relative proportions of each microstructural phase determine the mechanical properties of the metal.⁷

In an attempt to optimize the structure of Ni-Ti wires for automated instruments, a series of procedures has been developed to produce Ni-Ti alloys with a martensitic phase that is substantially more stable under clinical conditions.² Within this context, heat treatments produce a better arrangement of the crystal structure.⁶

Temperature is one of the variables that considerably influence cyclic fatigue resistance.^8–10 The in vivo intracanal temperature ranges from 31°C to 35°C,¹¹ i.e., at human body temperature (37°C), all instruments are in a mixed austenitic–martensitic phase.¹² As the instruments are used inside the canal, surrounded by periodontium at body temperature, special attention should be paid to this circumstance in mechanical tests of endodontic instruments.^8,11

Reciprocating mechanical instruments were used from 2008, with the introduction of instruments of the Reciproc® series.¹³ Looking to improve the mechanical properties of the original version and to ensure greater safety of the procedures, manufacturers introduced “Blue” heat treatment in the new generation of these instruments. The Reciproc blue (RB) instruments (VDW GmbH, Munich, Germany) are fabricated with a Ni-Ti M wire and have inactive tip and S-shaped cross section that reduces the metal core.¹⁴ Therefore, the aim of this study was to evaluate the cyclic fatigue resistance of RB 40/0.06 instruments tested at room temperature (20° ± 0.5°C) and at body temperature (37° ± 0.5°C) in a simulated stainless steel canal with a curvature angle of 60°.

MATERIALS AND METHODS

The study was conducted in the University of Pernambuco Research and Biomaterials Center (Camaragibe, PE, Brazil). Twenty-four new RB 40/0.06 instruments (length: 25 mm) were randomly divided into two groups (n = 12) according to the temperature used (20°C or 37°C). The instruments were checked under a stereomicroscope (SteREO Discovery V12; Zeiss, Germany) before use to confirm the absence of defects or deformations such as distortion or burrs in the cutting blades.

Dynamic fatigue testing was performed using an artificial stainless steel canal with a 60° curvature angle and a 5-mm radius of curvature.¹⁵ The tests were carried out by a single operator using a 6:1 reduction handpiece (Sirona Dental Systems GmbH, Bensheim, Germany) coupled to a Silver Reciproc motor (VDW, Munich, Germany). A custom-made support was used to maintain the handpiece and stainless steel canal static during use, only allowing free rotation of the instrument. The instrument to be tested was mounted in the handpiece and inserted 17 mm into the artificial canal parallel to its vertical part, following the direction of the instrument’s tip to the base. The device was completely submersed in distilled water. The temperature was controlled throughout the experiment at 20°C or 37°C with an underwater thermometer and a thermostat. An immersion water heater was used for heating and the water was cooled with ice (see Fig. 1).

The endodontic instruments were operated according to the manufacturer’s recommendations. The instrument was held in the static position and rotated freely in the axial direction until the occurrence of fracture. The exact time was measured by video recording, which was started when the instrument was triggered and stopped when the fracture was visually detected. The time to fracture is given in seconds. After fracture, the length of the fragment was measured with a digital caliper to the nearest 0.01 mm.

For descriptive analysis of the data, the mean and standard deviation were calculated using the IBM SPSS 23.0 program. The Shapiro–Wilk test and Student t test with Tukey’s posttest were applied, with p < 0.05 indicating statistical significance.

RESULTS

Table 1 shows the mean and standard deviation of the time to fracture of the instruments at 20°C and 37°C.

At 20°C, the RB instruments fractured at an average of 18.06 minutes. These results were approximately three times higher than the same instruments when tested at 37°C (an average of 6.73 minutes until fracture), considering p value less than 0.001.

The time to fracture of the RB instruments differed significantly between the two temperatures (p < 0.001). No significant differences were observed in fragment size (p %3E; 0.05).

DISCUSSION

The prevalence of fracture of mechanical Ni-Ti instruments ranges from 0.4% to 5%.¹⁶ Aiming to reduce the probability of occurrence of this phenomenon, manufacturers made improvements in the cross-section designs and fabrication methods, including Blue heat treatment.^2,14,17 Studies suggest that heat-treated instruments possess greater cyclic fatigue resistance than conventional Ni-Ti instruments.^6,16,18–24 Considering the evidence indicating a change in fatigue resistance when instruments are tested at different temperatures, studies highlight the importance of reporting this change in cyclic fatigue resistance tests.^8,9,21–27

The results of this study show a greater cyclic fatigue resistance of RB 40/0.06 instruments tested at 20°C (an average of 18.06 minutes) when compared to body temperature (37°C—average of 6.73 minutes), with an approximately 2.68-fold higher mean time to fracture at 37°C. Comparison of the results is difficult since there are no studies on cyclic fatigue resistance using RB instruments with a tip diameter of 40 and conicity of 0.06 in artificial canals with a 60° curvature angle and a 5-mm radius. In addition, no standardization for cycling fatigue testing exists.¹⁶

Inan et al.²³ evaluated the cyclic fatigue resistance of Reciproc and RB instruments at 37°C and concluded that the RB instruments exhibited greater fatigue resistance than their predecessors. However, that study used 25/0.08 instruments in a canal with severe curvature (90°) and a 2-mm radius. The mean time to fracture of the RB 25/0.08 instruments was 3.51 minutes [based on the number of cycles to failure (NCF) and considering 300 rotations per minute in the “Reciproc All” mode], while the present study found a mean time of 6.73 minutes at the same temperature. This difference can be explained by the more marked curvature and smaller radius of the simulated canal used by the cited authors, in agreement with the study of Pruett et al.,¹ which showed that the cyclic fatigue resistance of an instrument decreases with increasing curvature angle and decreasing radius.

Klymus et al.²² evaluated the impact of temperature (20°C and 37°C) on the cyclic fatigue resistance of X1 Blue (25/0.06), RB (25/0.08), and WaveOne Gold (25/0.07) instruments. Similar to the present study, in addition to the temperatures tested, the authors used a simulated stainless steel canal with a 60° curvature angle and a 5-mm radius. The results showed greater cyclic fatigue resistance for the instruments submitted to Blue heat treatment. A significant reduction (p %3C; 0.05) in time to fracture and NCF was observed for all instruments at 37°C. The authors concluded that cyclic fatigue resistance of all instruments tested substantially decreases at this temperature. The mean time to fracture of the RB 25/0.08 instruments was 11.57 minutes at 20°C, which decreased to 5.06 minutes at 37°C. Our findings showed a mean time of 18.06 minutes (20°C) and 6.73 minutes (37°C), but we tested RB 40/0.06. This difference in tip diameter and conicity may explain the differences in the mean times to fracture.

Plotino et al.²¹ investigated the influence of temperature (0, 20, 35, and 39°C) on the cyclic fatigue resistance of Reciproc and RB (25/0.08) instruments in stainless steel canals with a 60° curvature angle and a 5-mm radius. The authors observed that the RB instruments were significantly (p < 0.05) more resistant to cyclic fatigue than the Reciproc instruments at all temperatures tested and that fatigue resistance proportionally increased with decreasing temperature (0°C %3E; 20°C > 35°C > 39°C). Thus, the authors concluded that Blue heat treatment increases significantly the cyclic fatigue resistance and that the temperature affects significantly the life span of Ni-Ti instruments; when the room temperature increases, the fatigue resistance of the instruments decreases. These data agree with the present study in which the mean time to fracture also decreased with increasing temperature.

The present results support previous studies showing a reduction in the fatigue time with increasing temperature for heat-treated instruments.^{9,10,21,22,25–28} Caution is necessary when extrapolating laboratory studies to clinical practice because of their limitations; however, literature data highlight the need to identify cooling alternatives of instruments applied in vivo to root canals, increasing the safety and the time to fracture of the instrument used.

CONCLUSION

An increase in temperature reduces the cyclic fatigue resistance of RB 40/0.06 instruments. The results of the study suggest that search for an intracanal cooling system may favor the fracture resistance of the tested instruments.

CLINICAL SIGNIFICANCE

A cooling system of the root canal system is important in endodontic clinic as it favors the cyclic fatigue resistance of Ni-Ti instruments.

Reciproc® is an instrument widely used in the practices of endodontic clinic. The Blue treatment has brought the proposal of high flexibility and resistance to cyclic fatigue, allowing greater security in the endodontic treatment of curved root canals, which was demonstrated in the present study.

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