AUTHORS: DR.MIMANSHA AGARWAL, DR.MANDEEP.S.GREWAL

ABSTRACT:

The present work evaluated the influence of the artificial canal arc lengths on the  number of cycles necessary to induce fatigue fracture in engine-driven  endodontic  instruments and   the effect  of  immersion  in  different  irrigating solutions   on  the  resistance  of  the   Protaper   F2   Rotary instruments   to   cyclic  fatigue   fracture.

A total of 90 instruments were immersed in the irrigating solutions for 5 minutes. All instruments were then subjected to cyclic fatigue. The instruments were rotated at 250 rpm until fracture. The time of fracture was measured by the same operator  using stopwatch.. Summarizing the results of our study it was observed that there was no significant difference in the time taken by each instrument to fracture in all the groups according to one way ANOVA. The instruments fractured at the point of maximum curvature in all the groups. Longitudinal and fractographic SEM examinations were than performed for two instruments from each group. Changes of surface cracks, micro-cracks, and pitting was not noted for most samples under SEM.

INTRODUCTION

Chemo mechanical canal preparation during the root canal treatment involves cleaning and shaping procedures with endodontic instruments & irrigating solutions. The aim of the root canal instrumentation is to obtain a continuous tapering funnel shape, following the original canal from the coronal access to the apex (Walcott & Himel 1997)1. The function of irrigants are to act as lubricants during  the  mechanical debridement of pulpal & dentinal tissues, a medium to remove debris, a solvent to dissolve tissue, an  agent  to  promote root-canal sterility & patent  dentinal  tubules  on  the   root-canal  walls (Mueller 1983).2,3
The  fracture  of  rotary  nickel-titanium (NiTi) endodontic instrument happens  as  a  result  of   torsional  or  bending  fatigue.  A major concern when using rotary, engine-driven  NiTi    endodontic     instruments  is  the  breakage  of these instruments because of cyclic fatigue  when  used  under  low-cycle  loading.  When an endodontic instrument, within its elastic limits, rotates in the interior of a curved canal a mechanical load occurs; represented by altering tensile & compressive stresses. The cyclical repetition of these loads leads to instrument fracture through low-cycle fatigue.4 The  magnitude  of  the tensile  &  compressive    force    imposed   on  the  flexed  area  of   the instrument   depends   on   the   curvature  radius &   the diameter of the instrument. 
The   smaller   the   curvature   radius    &    the   larger   the instrument’s diameter,  the    greater    the stress induced   upon   the instrument (Helio Pereira Lopes et al 2007)4

The chemical effects of the irrigating solutions on endodontic files may also hinder their performance (Muller 1983). 2
Corrosion adversely affects the metallic surfaces by causing pitting & porosity, & decreases the cutting efficiency of endodontic files (Stokes et al 1999).5

AIMS AND OBJECTIVES

The purpose of this study is:-
1.   To evaluate the influence of the artificial canal arc length on the amount of time necessary to induce fatigue fracture in engine-driven endodontic instruments.
2.   To evaluate the effect of immersion in different irrigating solutions on the resistance of the ProTaper F2 Rotary instruments to cyclic fatigue fracture.

MATERIAL AND METHODS

A total of 90 ProTaper F2 Rotary instruments (Dentsply Maillefer, Ballaigues, Switzerland), all from the same production lot were assigned to six different groups of 15 each.

Instruments in all the groups were immersed in the irrigating solutions for 5 minutes, as it is assumed that the instrument comes in contact with the irrigant for approximately 5 minutes during the chemo mechanical preparation.

1. Group 1 = Distilled Water
2. Group 2 = 3% NaOCl at room temperature
3. Group 3 = 3% NaOCl at 50°C
4. Group 4 = Smear Clear
5. Group 5 = 17% EDTA
6. Group 6 = MTAD

All instruments in all the groups were then subjected to tests of resistance to fracture because of cyclic fatigue using an apparatus specially designed for the purpose.

An artificial canal was made out of stainless steel with a width of   1.1 mm and a depth of 3 mm, a total length of 21 mm, and arc on the tip with a curvature radius of 6.0 mm. The arc measured 9.4 mm (90º curvature) and the straight part 11.6 mm .4


During the tests the stainless steel canal was lubricated with glycerin to reduce the instruments friction against the canal wall & heat release 4.
The instruments were rotated at a nominal speed of 250 rpm, 2 N cm torque until fracture, using an electric micro-motor with right hand rotation. The time of fracture was measured by the same operator using stopwatch. The instant of fracture was based on the visual and tactile observation of the fracture occurring in the instrument.

All the fractured instruments were than secured with their tips in separate containers.
Longitudinal and fractographic SEM examinations were than performed for two instruments from each group, using ZEISS EVO Series Scanning Electron Microscope Model EVO 50.

Samples were mounted on a circular metallic sample holder. The samples were fixed onto the sample holder rigidly enough, using sticky carbon tape, so that they do not fall off easily while handling. Since an electron beam is incident on the samples for SEM analysis it is essential that the samples are electrically conducting, this was achieved by coating the samples with 20 - 50 nm thick gold or silver. Bio-Rad Polaran sputter coater was used for this purpose.

RESULTS

GRAPH 1 : MEAN AND STANDARD DEVIATION OF  LENGTH (MM) AT WHICH THE INSTRUMENTS FRACTURED  IN DIFFERENT SOLUTIONS

ANOVA OF BROKEN AT LENGTH (mm)

	Sum of Squares	df	Mean Square	F	Sig.
Between Groups	3.300	5	.660	1.905	.102
Within Groups	29.100	84	.346
Total	32.400	89

GRAPH 2 : MEAN AND SANDARD DEVIATION OF TIME(MINUTES) AT WHICH THE INSTRUMENTS FRACTURED IN DIFFERENT SOLUTIONS

ANOVA OF TIME (Minutes)

	Sum of Squares	df	Mean Square	F	Sig.
Between Groups	.926	5	.185	1.867	.109
Within Groups	8.330	84	.099
Total	9.256	89

SEM PHOTOGRAPHS OF GROUP 1 (MAGNIFICATION 1000x)	SEM PHOTOGRAPHS OF GROUP 2 (MAGNIFICATION 1000x)	SEM PHOTOGRAPHS OF GROUP 3 (MAGNIFICATION 1000x)
SEM PHOTOGRAPHS OF GROUP 4 (MAGNIFICATION 1000x)	SEM PHOTOGRAPHS OF GROUP 5 (MAGNIFICATION 1000x)	SEM PHOTOGRAPHS OF GROUP 6 (MAGNIFICATION 1000x)

DISCUSSION

Instrumentation has a key role in the cascade of treatment procedures to eradicate microbes in the root canal system. Moreover instrument shapes the root canal in such a way that effective irrigation becomes possible.6 The use of irrigating solutions is an important part of endodontic treatment. The irrigants facilitate removal  of necrotic tissue, microorganisms and dentin chips from the root canal by a flushing action. In 1960, introduction of Nickel Titanium alloy by W.F Buehler at the Naval Ordnance Laboratory has revolutionized metallurgy. Ni-Ti alloy is found to have unique properties like super elasticity and shape memory, which preserves the canal geometry during cleaning and shaping (Walia et al 1988) .6 The super elasticity of Ni-Ti allows deformation of as much as 8% strain to be fully recoverable compared to a maximum of less than 1% with stainless steel (Thompson S.A. 2000). 7,8 The Ni-Ti mechanically driven instrument when subjected to continuous rotation , undergoes unidirectional torque inducing constant stress and strain to the files (Berutti et al 2003).This subjects the instrument to a constant and variable strain depending on the canal curvature and the hardness of the dentin to be removed. This metal fatigue eventually leads to cumulative micro-structural changes that ultimately lead to failure of the instrument (Svec et al 2000). Failure of the endodontic rotary instruments occur under two circumstances: torsional fracture and bending or flexural fracture (Sattapan et al  2000). Torsional fracture occurs when a part of the instrument is locked in a canal while the shaft continues to rotate.8 When an instrument is rotating around the curve, it is compressed on the inner side of the curve and stretched on the outer side of the curve. With every 180 degrees of rotation, the instrument flexes and stretches over and over again, resulting in cyclic fatigue and, eventually, fractures. The larger sized or greater taper file sustains more compressive and tensile forces due to increased metal mass (Camps et al 1995).This study used the ProTaper F2 because it has been shown to possess lower resistance to fracture because of cyclic fatigue than the other instruments in the ProTaper series (Fife et al 2004).9   It is known that NaOCl is corrosive to metals. The corrosion patterns, involving selective removal of nickel from the surface, can create micro-pitting ( Sarkar et al 1983).10  It is supposed that these micro-structural defects can lead to areas of stress collection and crack formation, weakening the structure of the instrument (Oshida et al 1992).11 NaOCl is considered to be more effective at 50°C hence its heated to check its effect on the instruments at the raised temperature (Berutti et al 2006).12 The study shows no significant difference in the no. of cycles required to fracture the instrument on exposure to these irrigants. In this study, the instruments dipped in 3% NaOCl at 50⁰C took the least amount of time to fracture, though there was no significant difference in the time taken by each instrument to fracture in all the groups according to one way ANOVA. 17% EDTA is found to have lowest corrosion resistance. Due to current paucity of data on the effect of MTAD and Smear Clear, on the instruments, during irrigation, we decided to check the torsional fatigue of ProTaper instruments after subjecting them to these different irrigating solutions.

In this study, the instruments dipped in 17%EDTA took the maximum amount of time to fracture, those dipped in Smear Clear fractured earlier than 17% EDTA.  Instruments dipped in 3% NaOCl at room temperature fractured earlier than those dipped in MTAD, followed by distilled water. The instruments dipped in 3% NaOCl at 50⁰C took the least amount of time to fracture, though there was no significant difference in the time taken by each instrument to fracture in all the groups according to one way ANOVA.

The instruments fractured at the point of maximum curvature in all the groups. The fractured instruments when viewed under SEM presented ductile morphological characteristics. Longitudinal changes of surface cracks, micro-cracks, and pitting was not noted for most samples under SEM. Thus, the cyclic fatigue of rotary Nickel Titanium endodontic instruments remains unaffected with the irrigating solution used during chemo-mechanical preparation of the root canals.

Apart from the canal curvature and the irrigating solution used, other factors such as the no. of times the instrument is used, manufacturing defects, static or dynamic motion of instrumentation13, torque14 and speed of rotation15 also play an important role and hence have to be looked into by the clinician while determining the cyclic fatigue of the instrument.

CONCLUSION

The following conclusions were drawn from this study:

1. The mean length at which the instruments fractured was 15.7 mm. The results show that the fractured surface of the tested instruments occurred close to the middle point of the canal arc length.
2. The instruments dipped in 17% EDTA took the maximum amount of time to fracture and those dipped in 3% NaOCl at 50⁰C took the least amount of time to fracture, though there was no significant difference in the time taken by each instrument to fracture in all the groups according to one way ANOVA.
3. SEM evaluation demonstrated that the fractured surfaces had ductile morphological characteristics. The presence of dimples with varied forms was identified in these surfaces. Longitudinal changes of surface cracks, micro-cracks, and pitting were not noted for most of the samples under SEM, indicating that there was no surface change on the instrument due to the irrigating solutions.

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