Effect of Sodium Hypochlorite Activated by Heating on the Cyclic Fatigue Resistance of OneShape Rotary Instruments

Special Article – Root Canal

J Dent App. 2017; 4(1): 372-375.

Effect of Sodium Hypochlorite Activated by Heating on the Cyclic Fatigue Resistance of OneShape Rotary Instruments

Demiral M¹, Keskin C²* and Inan U²

1Endodontics Specialist, Istanbul Çekmeköy Oral and Dental Health Center, Istanbul, Turkey

2Department of Endodontics, Faculty of Dentistry, Ondokuz Mayis University, Samsun, Turkey

*Corresponding author: Keskin C, Department of Endodontics, Faculty of Dentistry, Ondokuz Mayis University, Samsun, Turkey

Received: April 03, 2017; Accepted: April 28, 2017; Published: May 05, 2017


Introduction: The present study aims to evaluate the effect of sodium hypochlorite, which was activated by heating to different temperatures on the cyclic fatigue resistance of OneShape rotary system instruments.

Materials and Methods: This study included 30 OneShape rotary system instruments with size 25.06. The instruments were randomly divided into 3 groups as one control group and two 5.25% sodium hypochlorite groups, which were activated by increased temperature (either 37°C or 50°C) (n=10). The instruments in the irrigation solution groups were immersed into the irrigation solutions and kept rotating for 10 minutes. The instruments in the control group did not receive any pretreatment. All instruments were tested with in the artificial canal with a radius of curvature of 5mm and angle of curvature of 60°c. The dynamic cyclic fatigue testing of the instruments was performed with a fully automated device. Each instrument was tested until fracture, when the device automatically stopped at the time of it. The number of cycles per formed by each instrument was determined. The data were analyzed using one way ANOVA and Tukey tests.

Results: No significant difference between the control and the irrigation group heated to 37°C (p>0.05); whereas the specimens from the irrigation group heated to 50°C exhibited significantly higher cyclic fatigue resistance than the control and irrigation group heated to 37°C (p<0.05).

Conclusion: Activation of NaOCl by increasing its temperature to 50°C increases the cyclic fatigue resistance of OneShape 25.06 instruments.

Keywords: Instrument Fracture; Nickel Titanium; Cyclic Fatigue


The introduction of nickel-titanium (NiTi) rotary instruments to endodontic practice has provided a great advantage to clinicians in cleaning and shaping procedures, which is considered as the most important step of the root canal treatment [1]. The super elastic properties and high torsional fracture resistance of NiTi alloys have provided instruments to perform safer and more successful root canal preparation [2]. Root canal preparation complications such as canal blockage, elbow, step, apical transportation, and perforations were reported to occur less frequently in preparations performed by using NiTi rotary instruments when compared to stainless steel hand instruments [2]. However, nickel-titanium rotary instruments can still be separated without any signs or visible surface changes during clinical use [3].

The irrigation agents used during root canal treatments or sterilization processes implemented subsequently to root canal treatments may cause chemical reactions leading to the corrosion and/or formation of surface irregularities in the surfaces of NiTi rotary instruments. These irregularities and distortions can lead to instrument fracture during clinical use [4]. NiTi rotary instruments are broken due to two main reasons as torsional or cyclic fatigue failure. Torsional failure occurs when the instrument continues to rotate despite its tip is screwed in root canal, whereas cyclic fatigue failure occurs when the canal instrument is subjected to consecutive compression and tensile stresses at the level where metal fatigue is exceeded during rotation in a curved root canal [5]. Cyclic fatigue resistances of root canal instruments are tested using static and dynamic laboratory cyclic fatigue test models.

Sodium hypochlorite (NaOCl) is the most commonly used irrigation solution for the disinfection of root canal system. NiTi instruments come into contact with NaOCl in cleaning or sterilization processes during or after the root canal treatment. It has been reported that NaOCl removes nickel ions from the surface of NiTi instruments, thus forming micro-holes [6]. This corrosive effect has been reported to cause stress accumulation and cause crack propagation on the instrument. Therefore it has been stated that contact between NaOCl and instruments might negatively affect the mechanical properties of NiTi instruments [7,8]. Previous studies evaluated the effect of NaOCl on NiTi instruments manufactured from conventional, memory-controlled and heat-treated NiTi alloys and reported distortions on the surfaces of NiTi instruments as a result of NaOCl contact [9-12]. Various techniques have been suggested to activate the NaOCl including ultrasonic activation, increasing the concentration of the solution, heating, and increasing irrigation period [13,14]. It has been reported that heating the NaOCl solution enhances the tissue dissolution efficiency and antimicrobial properties of the solution [15,16].

OneShape (Micro Mega, Besancon, France) is a single instrument system used with continuous rotation and made from conventional NiTi alloy. OneShape instruments have triangular cutting tips and 2 additional cutting edges along with the working part. The design of the cross-sectional area, changing between 2 and 3 cutting edges in both apical and coronal section provides an optimal cutting efficiency [17]. Our literature research revealed that there was no study that investigated the effect of NaOCl on the cyclic fatigue resistance of OneShape instrument. The purpose of the present study is to evaluate the effect of the heated NaOCl solution on the cyclic fatigue resistance of OneShape instrument. The null hypothesis of the study is that the contact with heated NaOCl does not affect the cyclic fatigue resistance of OneShape instrument.

Materials and Methods

Specimen collection and pretreatment

OneShape NiTi rotary instruments (25.06) were collected. The surfaces of the instruments were examined under 3.5x magnification (Eyemag Pro F, Carl Zeiss, Germany) and 30 instruments without any visible surface and/or shape deformation were selected. The instruments were divided into 3 groups randomly, including the control and 2 groups of NaOCl solution heated up to 37°C and 50°C (n=10). An artificial canal made of stainless steel with an inner diameter of 1.5mm, 60° angle of curvature, and a curvature radius of 5mm, was used for cyclic fatigue testing. In a fully automatic cyclic fatigue testing device, an artificial tube made from stainless steel, which the instruments to be tested in could freely rotate without any contact was prepared (Figure 1).