Effect of Sodium Hypochlorite Irrigation with and Without Surfactant on Maximum Operative Torque and Vertical Force of Nickel-Titanium Endodontic Instruments.

Objectives: This study aimed to evaluate the effects of root canal irrigation on maximum operative torque and vertical force of Wave One Gold Primary (WOG), Reciproc Blue R25 (RB) and Protaper Next X2 (PTN) endodontic rotary instruments during simulated root canal preparation in the presence and absence of a surfactant "benzalkonium chloride". Material and
Methods: A custom-made automated irrigation and torque/force analyzing device connected with endomotor was used for the study. Acrylic resin blocks with simulated curved root canals of 35° were randomly assigned to 6 groups for each file. The following irrigants were used during simulated root canal preparation with continuous rotation or reciprocation motion with pecking mode: 5% NaOCl and 5% NaOCl with 0.84% benzalkonium chloride. Maximum torque and vertical force values ​​of files were recorded during preparation.
Results: BAC addition to NaOCl did not significantly affect maximum torque values of instruments (p>0.05). WOG generated lowest torque value compared with PTN and RCB in both groups (p<0. 05). RB generated higher vertical force compared with WOG and PTN instruments in NaOCl+BAC group. The surface modifier decreased the maximum force value of WOG instrument (p<0.05). Discussion: Surfactan addition to NaOCl had no negative effects on maximum torque and force of the instruments.

Introduction

Endodontic rotary nickel-titanium (NiTi) file systems can be used safely for root canal preparation due to their increased flexibility, strength and ease of use compared with stainless steel instruments (). Despite the advantages, NiTi rotary instruments appear to have a higher risk of fracture (). A separation of rotary NiTi instruments occurs due to torsional or flexural fatigue (). Torsion is the most common type of failure caused by locking of the tip or another part of instrument in the root canal while the shank continues to rotate (, ). When the elastic limit of the metal is exceeded by the torque exerted by the handpiece, a fracture of the tip becomes inevitable (). For reducing the risk of fracture of the instruments, a visual inspection of the damage on the file by the physician is recommended (). Therefore, new designs, metal alloys, manufacturing techniques and movement kinetics have been introduced.

Reciprocating kinematics aims to reduce taper lock and consequently the risk of torsional failure compared with rotary motion (). WaveOne Gold (WOG; Dentsply Maillefer, Ballaigues, Switzerland) and Reciproc Blue (RB; VDW GmbH, Munich, Germany) are reciprocating systems that have different cross sections and metal treatments. WOG has a new parallelogram cross section compared with Wave One instruments. Both WOG and RB instruments are heat treated after machining, which is related to improvements in the mechanical behavior of these files ().

ProTaper Next instruments (PTN; Dentsply Maillefer, Ballaigues, Switzerland) are rotating files made of M-wire. A unique NiTi alloy manufactured by a thermal treatment process, which reportedly increased flexibility and resistance to cyclic fatigue, has higher strength and wear resistance than similar instruments made of conventional superelastic NiTi wires (, )

Sodium hypochloride (NaOCl) is the most commonly used irrigation solution for chemical preparation during root canal treatment. However, despite its excellent tissue-dissolving and antimicrobial abilities, NaOCl presents a high surface tension (). The ability of surface-active agents (surfactants) to reduce the surface tension of NaOCl solutions has been recently demonstrated (). Benzalkonium chloride (BAC) is a cationic surfactant frequently used in dentine bonding agents and orthodontic resins (). Addition of 0,084% BAC to NaOCl has been reported to reduce its contact angle and the surface energy, without affecting the free chloride content, cytotoxicity or antibacterial properties of the mixture ().

The term of “operative torque” can be defined as the quantity of torque developed during instrument progression towards the apex. It is a real time measurement of dynamic forces needed to perform the shaping of the canal.

In the current literature, there is little information about the effects of irrigation solutions on torque and vertical force values during intracanal instrumentation with rotary NiTi files. In the literature, aqueous and paste type lubricants were compared during instrumentation with rotary NiTi instruments and it was reported that aqueous solutions reduced torque values of endodontic rotary files compared with paste type in simulated root canals (, ). However, there is no information about how NaOCl irrigation, with or without surfactant, changes the vertical force and torque values ​​of rotary NiTi files during intracanal instrumentation. The aim of this study was to compare maximum torque and vertical force generated by WOG, RB and PTN instruments during canal irrigation with NaOCl and NaOCl mixed with BAC. For this purpose, a customized device for automated root canal instrumentation complemented with irrigation and torque/force analysis was developed.

The hypothesis of the present study was that instrument type and surfactant addition to sodium hypochlorite does not affect the torque/force values.

Material and Methods

A customized device and software were designed and manufactured for this study. The device was used for instrumentation and irrigation of the simulated canals and determination of the torque/vertical force during instrumentation. The device consisted of a computer (Casper, Nirvana, One PC, Turkey), an endodontic motor (C-Smart-1, COXO, Guangdong, China), a moving vertical stage which the endodontic motor head is attached to using a custom-made holder, a force measuring unit, a handle on which transparent blocks were placed, and an irrigation solution pump (Figure 1 and 2). The torque sensor of the endodontic motor was modified for measuring the torque, and apically directed forces were defined.

Figure 1

The root canal instrumentation, irrigation and torque/force analyzing device used in this study

Figure 2

A: J shaped root canal model, B: Irrigation tip, C: Endodontic motor head attached to a custom-made handle, D: Force measuring unit.

The device was controlled by software on the computer. When the program was run, the instant values ​​taken from the torque and force sensors were transferred to the computer by the microcontroller.

Preparation of acrylic resin blocks

Ninety acrylic resin training blocks (V04 0245; VDW, Munich, Germany) with simulated curved root canals of 35° were used in this study. Canals and apical patencies of resin blocks were checked with a #15 stainless steel hand file. In our preliminary study, we observed that most of the instruments were broken in the canal without preshaping. Therefore the canals were shaped with Protaper Next X1 file.

Preshapings were made by only one investigator (EAA), 5 ml, %5. 25 NaOCl (Werax, İzmir, Turkey) was used for irrigation between each file. After the shaping procedure, the canals were irrigated with 5 ml, %5.25 NaOCl and dried with #20 paper points (President, Allershausen, Germany).

Preparation of irrigation solutions

5. 25% NaOCl and 0,084% BAC (Kim-pa, İstanbul, Turkey) mixed in 5.25% NaOCl were used in this study. During testing, solutions were heated at 37°C for mimicking body temperature.

Groups were designed as (n=15):

NaOCl group;

Group 1: WaveOne Gold Primary irrigated with 5. 25% NaOCl

Group 2: Reciproc Blue R25 irrigated with 5. 25% NaOCl,

Group 3: Protaper next X2, irrigated with %5. 25 NaOCl (Werax, İzmir, Turkey)

NaOCl+BAC group;

Group 4: Wave One Gold Primary irrigated with 0,084% BAC mixed in 5.25% NaOCl

Group 5: Reciproc Blue R25 irrigated with 0,084% BAC mixed in 5. 25% NaOCl

Group 6: Protaper next X2 irrigated with 0,084% BAC mixed in 5. 25% NaOCl

Endodontic motor was set to rotate in “Wave One” mode for WOG; “Reciproc All” mode for RE R25, “Protaper next” mode for PTN X2. NaOCl or NaOCl mixed with BAC were transferred to root canal orifice of the acrylic blocks during instrumentation 2 mm inward from the canal orifice with a 30 gauge needle tip attached to the solution pump. 0.2ml irrigant per second was delivered through the canal orifice during the experiment. The handpiece was programmed to move in a simulated pecking motion, that is downwards for 2 s and upwards for 1s to reach the working length (16mm) of the transparent blocks.

The software recorded and plotted the values of the torque and vertical force per millisecond. The maximum torque and vertical load values were also recorded.

Statistical analysis

The data were analyzed using the Kruskal-Wallis test with Dunn-Bonferroni correction. For all tests, the alpha-type error was set at <0.05.

Results

The median, 25th percentile, 75th percentile, minimum and maximum torque and vertical force values of each groups are presented in Table 1 and Table 2.

Table 1

Maximum torque values of tested instruments (Ncm).

Groups		Median	25th Percentile	75thPercentile	Min.	Max.
	WOG	0.19 a,b	0.17	0.20	0.15	0.20
	RCB	0.41 a	0.39	0.44	0.38	0.45
	PTN	0.88 b	0.74	1.10	0.64	1.16
	WOG	0.18 c	0.16	0.22	0.14	0.23
	RCB	0.37 d	0.33	0.39	0.30	0.41
	PTN	1.13 c,d	1.02	1.37	0.90	1.71

Same superscript letters in the same column indicate statically differences between groups by the Bonferroni test for pair-wise comparison (p<0.05). (Min: minimum, Max: Maximum)

Table 2

Maximum vertical force values of tested instruments (gram force).

Groups		Median	25th Percentile	75thPercentile	Min.	Max.
	WOG	250.38 x	208.04	346.24	160.66	382.29
	RCB	260.54	196.54	350.46	147.99	373.17
	PTN	160.79	151.88	182.62	129.57	194.95
	WOG	149.10 x,y	110.45	182.72	75.85	265.85
	RCB	401.39 y,z	353.01	430.05	289.91	506.66
	PTN	130.83 z	119.45	184.30	107.54	215.81

Same superscript letters in the same column indicate statically differences between groups by the Bonferroni test for pair-wise comparison (p<0.05). (Min: minimum, Max: Maximum)

In NaOCl group, WOG instruments generated lower torque than RB and PTN instruments (p<0.05). In NaOCl+BAC group, PTN generated higher torque than WOG and RCB (p<0.05). BAC addition to NaOCl did not significantly change the torque values of each instrument (p<0.05).

No significant difference was found between instruments in NaOCl group in terms of maximum vertical force (p<0.05). Nevertheless, in NaOCl+BAC group, RB showed higher force than WOG and PTN (p<0.05). The vertical force generation of WOG instruments decreased in NaOCl+BAC group compared to NaOCl group (p<0.05).

Discussion

In this study, we aimed to evaluate the effects of NaOCl, with or without surface modifiers, on the maximum operative torque and maximum vertical force values of PTN F2, WOG Primary and RB R25 files during intracanal instrumentation using acrylic resin blocks. Peters et. al. reported that acrylic blocks with simulated canals had similar behavior as mandibular incisors when measuring the torque of rotary instruments (). Therefore, in the study, we preferred to use acrylic resin blocks to provide standardization among samples.

Studies evaluating the torsional resistance of rotary NiTi files are generally carried out by burying the tip of 3-5 mm of the files in acrylic or composite resin, operating the file in rotation motion and recording the maximum torque value immediately before the breakage occurs. These studies are carried out according to ISO 3630-1 standards. Nevertheless, this method doesn’t provide sufficient information about the torque values generated by the file and canal walls in the dynamic clinical conditions in curved canals. Furthermore, the reciprocation files are not designed to continuously rotate in one direction until fracture. In other words, there is a gap between clinical use and experimental methods. So far, different techniques have been designed and introduced for determining the torque and vertical load similar to the clinical conditions to overcome this gap (-).

Studies comparing aqueous and gel-type irrigants on torque and apically directed force showed that aqueous irrigants decreases torque and force compared with paste type lubricants and dry controls (, ). This can be explained by the fact that the use of irrigants can reduce the torsional fracture risk by decreasing the torque values.

Using the files with pecking motion has been shown to decrease the risk of fractures by reducing flexural resistance and screw-in forces (, ). Hence, specially designed software enabled the files to work in pecking 2 mm forward and 1 mm backward motion.

In NaOCl group, the lowest torque value was observed in Wave One Gold Primary files compared with Reciproc Blue R25 and Protaper Next X2 (p<0,05). Although it was not statistically significant, the highest maximum torque value was observed in the PTN group. Designs, surface treatments, production processes and working kinetics of all these files are different from each other. These properties may affect the stress generation between the file and root canal. PTN instruments work with continuous rotation movement while WOG and RCP work with the reciprocation movement. The reciprocation movement includes clockwise and reverse cycles. It is assumed that counterclockwise rotation of reciprocating motion diminishes the torsional stress exerted on the file during the active canal shaping procedure compared with continuous rotation (). Similarly, in a study, it was shown that reciprocating motion improves the cyclic fatigue compared to continuous motion systems (). Although both RCB and WOG are reciprocating instruments, lower torsional stress of WOG may be related to its structural properties. In a study comparing cross-section area of WOG Primary and RCB R25 at D5 level, RCB was reported to have larger core area than WOG (). Although, some previous studies reported that an increase in the file's core diameter would enhance its resistance to the torsional stress (, ), the torque values ​​formed between dentine wall and the instrument may be affected by many factors such as design features, the presence of irrigation solution or the physician’s manipulation in clinical or experimental conditions.

Boessler et. al. evaluated aqueous ve gel-type lubricants in simulated root canals using Profile 30.06. They showed that aqueous type lubricants decrease the maximum torque, full torsional load and maximum force compared with dry controls (). Mazzoni et. al. compared saline, NaOCl, EDTA and gel-type EDTA using Edge Taper F2 files (). They showed that irrigants decrease the mean operative torque compared with saline. The highest decrease was observed in aqueous EDTA group in their study. Since dentin discs are used in the study, EDTA may soften the dentin and reduce the torsional resistance against the file. However, in our study, we aimed to achieve standardization by eliminating the effects of the solution on dentin by using acrylic blocks.

BAC addition to NaOCl did not significantly affect maximum torque values of instruments compared with NaOCl group (p<0.05). Bukiet et. al. suggested that percentage of 008 BAC additions decreased the contact angle and surface energy without affecting antibacterial properties, free chloride content and cytotoxicity of NaOCl (). Baron et. al. showed that BAC addition increased the antibacterial properties of NaOCl (). These studies showed that adding BAC to NaOCl affects the properties of solution positively. Similar to these positive results, this study showed that addition of BAC to NaOCl had no effects on the maximum torque generated by the instruments used in the study.

There was no statistically significant difference between files in the NaOCl group in terms of maximum force (p<0.05). The addition of BAC to NaOCl caused a decrease in the maximum force value of WOG files (p<0.05). A disadvantage of NiTi rotary instruments is the tendency to bind into the root canal. Due to the spiral configuration of the instrument, blades are bound to the root canal dentin and cause a "screw-in effect". This effect generates a force to the apical direction (, ). Reciprocating movements may reduce the screw-in effect because a momentary clockwise rotation may relieve the stress when the instrument is trapped in dentin during counterclockwise rotation. Although RB files also work with reciprocation motion, their s-shaped cross-section and having 8% taper 3 mm from the tip may have caused more force compared with WOG instrument’s parallelogram-shaped cross section and %7 taper.

The addition of BAC to NaOCl only caused a decrease of the force for WOG instruments, but did not cause a statistically significant change for other files. The BAC solution may have facilitated the movement of the WOG files, reduced the screwing effect, or had a lubricating effect between the canal wall and file. Some more detailed studies are needed on this subject.

The instruments working with continuous rotation movement were expected to cause more force generation by screwing in to the root canal. However, reciprocating motion exhibited higher forces than continuous rotation in most studies because reciprocating instruments can be pushed into the canal during the counterclockwise rotation (). The results of this study showed that, PTN did not cause a higher vertical force generation. Another possible reason could be that 6% taper of Protaper Next may have reduced the apical pressure required to reach the working length in previously expanded canals.

In the study, it was not possible to clean the resin chips formed during the preparation of the canals due to the design of the device. This was one of the limitations of the study.

As a result of the study, BAC addition to NaOCl has no negative effect on the maximum torque and maximum apical force of the Wave One Gold Primary, Reciproc Blue R25 and PTN X2 instruments. Vertical force generated by WOG decreased with BAC addition to NaOCl.

Conclusions

The results of this study demonstrated that the instruments working with reciprocation motion had lower operative torque compared with instruments working with continuous rotation motion in the presence and absence of surfactant. Surfactant addition to NaOCl did not cause any change in the torque values ​​of the instruments. While there was no significant difference in the vertical force generated by the instruments in NaOCl group, surfactant addition reduced the vertical force generated only by WOG instruments.