ENDODONTOLOGY

EFFECT OF CLEANING AND STERILIZATION PROCEDURES ON NITI ROTARY FILES – AN SEM AND EDS STUDY KUBER SOOD* B. MOHAN** L. LAKSHMINARAYANAN***

ABSTRACT Rotary nickel-titanium files have become the most widely used engine driven endodontic instruments. Whenever these instruments are re-used proper cross-infection control measures should be undertaken. After clinical use if the tenaciously bound debris is not removed from endodontic files, any method of sterlization may be in-effective. Although varying concentration of sodium hypochlorite (NaOCl) have been used to remove the organic debris, its effect on file surface with repeated cleaning and sterlization cycles have not been investigated. The purpose of this study was to assess the effect of repeated cleaning and sterilization procedures on 3 brands of Nickel-Titanium (NiTi) endodontic file using Scanning Electron Microscope (SEM) and Energy Dispersive Spectroscopy (EDS).105 New NiTi instruments from 3 different manufacturers namely ProTaper, RaCe and ENDOWAVE were subjected to 2, 5 and 10 cycles of cleaning and sterilization procedure. Cleaning procedure involved pre soaking of files in 2.5% NaOCl for 15min (Group I) and 1hour (Group II), followed by sterilization process using steam autoclave at 1210C at 15psi pressure for 15minutes. Files were observed for evidence of corrosion such as pitting or deposition of corrosion products using SEM. EDS was conducted to analyze the chemical elements in corroded & non corroded areas of the file. 2.5% NaOCl is much more corrosive when used for 1hr immersion time compared to 15 minutes. All the files showed mild to severe corrosion with 1 hour immersion time in 2.5% NaOCl – ProTaper NiTi files showed most corrosion followed by RaCe and least corrosion in ENDOWAVE rotary files. This can be attributed to various surface treatment included during manufacturing process of these files.

INTRODUCTION

care setting because of concerns about transmission

Rotary NiTi endodontic instruments are gaining

of disease via the oral cavity. Hence, the basic

popularity based on their superior preparation of

theorems of asepsis in general dentistry also apply

1

canals compared to hand instruments . Several

to NiTi files with little variance when ever the

studies have shown that these NiTi instruments can

instruments is re-used.

2

be used many times without intra canal failure .

Effective re-processing of instruments involves

Endodontic treatment may directly involve contact

cleaning to remove organic residue followed by

with saliva, blood and infected pulp tissue, carrying

sterilization4.Only a few studies have investigated

bacteria, viruses or prions from patient to patient3.

the effectiveness of cleaning methods and their effect

Cross–infection control is a major issue in dental

*Post graduate, **Reader, ***Professor and Head of Department, Department of Conservative Dentistry and Endodontics, College of Dental Sciences, Saveetha Institute of Medical and Technical Sciences, Chennai.

34

ENDODONTOLOGY

EFFECT OF CLEANING AND STERILIZATION PROCEDURES ON NITI ROTARY FILES – AN SEM AND EDS STUDY

on endodontic instruments. Studies have proved that

Sub Group B

wiping of files with gauze soaked in alcohol,

sterilization procedure

brushing files under running water or ultrasonic bath

Sub Group C

are all ineffective in cleaning the organic debris over used files

5- 7

- 5 cycles of cleaning & - 10 cycles of cleaning & sterilization procedure

. Pre soaking the endodontic files in

Each sub group consisted of 5 files each from all

NaOCl is a proven method for complete removal

three different manufacturers.

of organic debris and disinfection8-10. Although for

Control Group - No cleaning & sterlization

cleaning of instruments, the strength of NaOCl

procedure was done

solution must be balanced against potential damage

CLEANING PROCEDURE

to instruments by corrosion as it is highly corrosive

Each cleaning cycle involved scrubbing the

to metals11, 12.

files with nylon bristle brush for 20 strokes whilst

The purpose of this study was to evaluate the

files were mounted in a stand and rinsed with tap

effect of repeated cleaning and sterilization

water7. The files were then immersed into test tubes

procedures on corrosion of NiTi rotary files using

containing 2.5% NaOCl for 15 minutes for Group I

Scanning Electron Microscope (SEM) and Energy

and 1hr for Group II13. Files were then thoroughly

Dispersive Spectroscopy (EDS).

rinsed under distilled water immediately after each cleaning cycle and were allowed to dry over night

MATERIALS AND METHODS

before sterilization procedure.

A total of 105 new rotary NiTi files from three

STERLIZATION PROCEDURE:

different manufacturers were used for the study; 35 ProTaper (Tulsa – Dentsply, U.S.A) 35 RaCe (FKG

After cleaning procedure, the files were placed

– Dentaire, Switzerland) and 35 ENDOWAVE

in an autoclave box and autoclaved at 1210C at 15

(J.Morita, U.S.A). For cleaning and sterilization

psi pressure for 15 minutes14 followed by over night

procedure 30 files from each manufacturer were

drying before commencement of next cleaning

divided into two Group I and Group II, each group

cycle.

having 45 files each, 15 files from each

Subgroup A, B and C were subjected to 2, 5 &

manufacturer. Control group comprised of 15 new

10 cycles of cleaning & sterlization process

files, 5 from each manufacturer. Group I

respectively. Control group was neither cleaned nor autoclaved at all.

- 15 minutes soaking time in 2.5% NaOCl

Group II

EXAMINATION OF CORROSION

- 1 hour soaking time in

When the files were soaked in 2.5%NaOCl for

2.5% NaOCl

cleaning purpose, any sign for precipitated corrosion

Each group was subdivided into 3 sub groups of

products within the solution or deposited on the

15 files each Sub Group A

surface of the file, were noted, but not investigated

- 2 cycles of cleaning &

further.

sterlization procedure

After completion of cleaning and sterlization 35

ENDODONTOLOGY

EFFECT OF CLEANING AND STERILIZATION PROCEDURES ON NITI ROTARY FILES – AN SEM AND EDS STUDY

protocol, each file was dried and stored in a sterile

formation of flocculent black precipitate which

vial. Each file was provided with an identification

completely disappeared in sub group IC (10

record, used to guide the analysis by SEM. For SEM

cleaning cycle). However RaCe and ENDOWAVE

the files were mounted horizontally on stubs with

files did not show any sign of visible corrosion when

adhesive tape. The flutes of the file were assessed

immersed in 2.5% NaOCl for 15 minutes in sub

for presence of localized area of surface pitting with

group 1A, 1B and 1C respectively.

or without corrosion products precipitated on the

In Group II, with one

surface 15 and compared to surface topography of

hour soaking time in 2.5%

corresponding type of file in the control group.

NaOCl (fig.2), all ProTaper

During SEM there is subsequent emission of

files showed formation of

quantized x-ray photon along with the secondary

dense black precipitate in

electrons from the surface of the sample. These

sub group IIA (2 cleaning

quantized x-ray photons are characteristic of an

cycles), in sub group IIB (5

element. The wavelength or energy analysis of these

cleaning cycles) moderate

emitted x-rays provides qualitative elemental

amount of precipitate

identification and relative intensities of these x-rays

formation was observed

are used for quantitative analysis in EDS. With the

and

help EDS, composition of a point or defined area

precipitate formation was seen in sub group IIC (10

on the surface of the file can easily be determined

cleaning cycle).

16

very

minimal

RaCe on the other hand

to a high degree of precision (~0.1wt %) . In the

demonstrated mild flocculent brown precipitate in

present study EDS was used to identify and

sub group IIA (2 cleaning cycles) but in successive

characterize semi-quantitatively chemical elements

cleaning cycles, sub group IIB (5 cleaning cycles)

present on surface of corroded and non corroded

and sub group (10 cleaning cycles) no precipitate

files.

was observed. No sign of visible corrosion was observed in ENDOWAVE files in any cleaning cycle.

RESULTS

SEM EXAMINATION

VISUAL EXAMINATION In Group I, with 15

IN CONTROL GROUP (No cleaning and

minutes soaking time in

sterlization procedure done) fig.3

2.5% NaOCl (fig.1), All ProTaper files showed formation of flocculent black

precipitate

in

solution, in sub group IA (2 cleaning cycles), in sub group IB (5 cleaning cycles) there was reduction in 36

ENDODONTOLOGY

EFFECT OF CLEANING AND STERILIZATION PROCEDURES ON NITI ROTARY FILES – AN SEM AND EDS STUDY

ProTaper file demonstrated machining marks and sharp cutting edges with no sign of pitting or deposition of corrosion products on the surface of the file. RaCe file in SEM examination revealed smooth file surface with slight bur marks and slightly rounded cutting edges with no sign of pitting surface defects or deposition of corrosion products on the surface of the file. ENDOWAVE under SEM examination revealed ultra smooth surface with smooth rounded cutting edges and no sign of pitting, surface defects or deposition of corrosion products on the surface of the file. IN GROUP I (15 minutes soaking time in 2.5% NaOCl) In sub group IA (2 cleaning cycles) no corrosion demonstrated corrosion product deposits on the

was observed in any of the files (fig.4). Mild pitting

surface of the file (fig.6). IN GROUP II (1 Hour soaking time in 2.5%NaOCl) In subgroup IIA (2 cleaning cycles) mild pitting and deposition of corrosion products on the surface of the file was observed on the surface of ProTaper files whereas no pitting was observed for either RaCe or ENDOWAVE (fig.7). was observed in subgroup IB (5 cleaning cycles) in ProTaper and RaCe files (fig.5). However, In subgroup IC (10 cleaning cycles) moderate pitting and corrosion deposits were observed on ProTaper files surface and mild pitting & surface deposition of corrosion products was seen in RaCe whereas ENDOWAVE showed no signs of pitting but 37

ENDODONTOLOGY

EFFECT OF CLEANING AND STERILIZATION PROCEDURES ON NITI ROTARY FILES – AN SEM AND EDS STUDY

In sub group IIB (5 cleaning cycles); ProTaper

file surface by the

file surface demonstrated moderate pitting along

manufacturer.

with mild surface deposits. RaCe demonstrated mild

In Group I (15 minutes

pitting on the surface but ENDOWAVE showed no

soaking time in 2.5%

sign of pitting except some surface roughness in form

NaOCl)

of surface deposits was noted (fig.8).

Decrease levels of Ni & Ti were observed in ProTaper in sub group IB and IC corresponding to 5 & 10 th cleaning cycles respectively. However EDS profile of RaCe and ENDOWAVE remained unchanged in sub groups IA, IB and IC. In Group II (1hr soaking time in 2.5% NaOCl) ProTaper files in subgroup IIA (2 cleaning

In subgroup IIC (10

cycles) depict depletion of both Ni & Ti from the

cleaning cycles) all three brands

of

surface of the file, which continues with successive

files

cleaning cycles in sub group IIB and IIC. The

demonstrated severe to moderate

pitting.

ProTaper

showing

corrosion

formed on the surface of ProTaper file reveals

maximum pitting and

complexes of Nickel &

ENDOWAVE showed

Copper. This Copper

moderate pitting & surface

deposits

may have come from

roughness

Gold-Copper coatings

(fig.9).

on handled of ProTaper

EDS ANALYSIS

files and presence of Cu

In 2 Dimensional elemental mapping of control

may

files (fig.10);

enhance

corrosion

the

process

(fig.11).


ProTaper file and ENDOWAVE shows only Ni & Ti peaks which depicts that there is no

RaCe files in sub group IIA (2 cleaning cycles)

surface coating over these files.

showed no depletion of Ni & Ti levels from the


RaCe file shows peaks of Ni & Ti along with

surface of the file, indicative of no corrosion. But a

peaks of Ag & N which points towards silver

significant decrease in Both Ni & Ti was seen in

Nitride coating procedure done to harden the 38

ENDODONTOLOGY

EFFECT OF CLEANING AND STERILIZATION PROCEDURES ON NITI ROTARY FILES – AN SEM AND EDS STUDY

sub group IIB (5 cleaning

studies17 which suggested Nickel is selectively

cycles) & sub group IIC

removed from the surface layer producing

(10cleaning

cycles)

micro-pitting during corrosion of NiTi. This

which indicate corrosion

study proves that even some amount of

taking place on the

titanium is also lost during corrosion.

surface of the file (fig.12)

DISCUSSION

ENDOWAVE files

Rotary NiTi files have become an important

did not depict depletion

armamentarium in endodontic practice while re-

of Ni & Ti from the

using these instruments cross – infection control is

surface of the file in sub

a major issue in the dental care. Organic debris

group IIA (2 cleaning

adheres tenaciously to these rotary instruments after

cycles) and IIB (5 cleaning cycles). However, there

clinical use. Hence to remove the organic matter

was mild depletion of Ni & Ti levels seen in sub

and debris completely from the files, effective

group IIC (10 cleaning cycles) depicting minimal

cleaning procedures must be carried out before

corrosion on the surface of ENDOWAVE files

proceeding to the sterlization 48 . Till date no

(fig.13), although it is important to understand that

satisfactory protocol for cleaning of instrument before sterilization has been formulated 7. The efficacy of NaOCl as tissue – dissolving and disinfecting agent depends on its concentration and time of exposure18. But NaOCl was also been proven to be corrosion to many metals19, 20 and selectively removes Nickel from NiTi alloy17. Hence in current study NaOCl was used as it removes the organic debris completely during cleaning procedure. But the strength of NaOCl solution and duration for which the instrument is in

correlation between peak amplitudes and

contact with NaOCl must be balanced against

concentration of element on the file surface is only

potential damage to instrument by corrosion.

semi – quantitative.

Busslinger it al 199810 found 5.25% NaOCl to

INFERENCE

produce corrosion in light speed NiTi files when


ProTaper NiTi files showed more corrosion

used for 30 and 60 minutes. However, Haikel et al5

followed by RaCe and least by ENDOWAVE

found no significant effect on NiTi files after


In all the groups more corrosion was seen with

immersion in 2.5% NaOCl for 12 to 48hrs. Various

1hr immersion time in 2.5% NaOCl.

studies have proven 1% NaOCl solution alone is in-effective in organic debris removal 7, 8 .In a


EDS was suggestive of that unlike previous

comprehensive study by Moorer et al 21 he 39

ENDODONTOLOGY

EFFECT OF CLEANING AND STERILIZATION PROCEDURES ON NITI ROTARY FILES – AN SEM AND EDS STUDY

concluded 2.5% NaOCl completely dissolves

silver Nitride coating which increases hardness of

organic debris. Hence, in the present study 2.5%

the file and makes it more corrosion resistant as this

NaOCl was used for the study for two different

coating did not corrode from the surface of the file

soaking periods.

in many areas. ENDOWAVE showed the best result out of 3 brands of files, this can be due to ultra

Various studies22, 23 have stated that autoclaving

smooth surface of ENDOWAVE files as seen in SEM.

upto 10 cycles does not affect torque, hardness or

Manufacturer

micro structure of NiTi files. Hence, for the present

claims

that

each

file

is

electrochemically polished after mechanical

study the files were autoclaved for 2, 5 & 10 cycles.

polishing which makes the surface ultra smooth

To evaluate corrosion SEM was used because

reducing friction and may be responsible for its less

corrosion is a surface phenomenon and it provides

corrosion. Also few SEM pictures showed deposition

correct topographic analysis of file surface. EDS was

of chloride crystals on the surface of file to avoid

use to determine the components of endodontic file

that file should be wiped with alcohol to remove

alloy in corroded and non corroded areas although

crystal deposition after cleaning the file with

it is important to observe that correlation between

NaOCl25.

peak amplitudes on EDS and concentration of

In the present study it was observed that in

chemical elements in the analyzed area is only semi

group I (15 minutes soaking time in 2.5%NaOCl)

– quantitative16. Stokes et al (1999)

all the brands of files showed minimal amount 15

speculated that

corrosion on the file surface till the fifth cleaning

manufacturing factors affect the corrosion of NiTi

and sterlization cycle. A soaking time of one hour

endodontic instruments. This was also observed in

in group II can be detrimental to the NiTi instrument

the present study as ProTaper demonstrated

with repeated cleaning and sterlization procedures.

maximum corrosion among 3 files this can be

Hence, it can be concluded that 2.5% NaOCl can

attributed to (1) machining marks on the file surface

be effectively be used to clean NiTi instruments

which acts as crevices and initiate corrosion (2)

without causing any potential damage to the

Other factor can be gold – copper platting on the

instrument.

handle of ProTaper files which when comes in

Although no direct studies have correlated

contact with NaOCl corrodes leaching Free Cu++

corrosion of endodontic files with instrument

ions which forms corrosion product with Nickel and

fracture due to fatigue. But it is likely in clinical

promote further corrosion. This finding was

scenario pitting corrosion acts as localized area of

confirmed by EDS.

stress concentration and crack propogation26, which

RaCe files showed relatively less corrosion

leads to instrument fracture. Further studies have to

when compared ProTaper. This can be speculated

be done to investigate the effect of corrosion on the

to due to grinding & mechanical polishing done

torsional strength and cyclic fatigue life of the NiTi

during manufacturing process to remove machining

rotary instruments.

marks. Also EDS reveals that the file is coated with 40

ENDODONTOLOGY

EFFECT OF CLEANING AND STERILIZATION PROCEDURES ON NITI ROTARY FILES – AN SEM AND EDS STUDY

CONCLUSION

11. Edie JW, Andreasen GF, Zaytoun MP (1981). Surface corrosion of nitinol and stainless steel under clinical conditions, The Angels orthodontist: 51,319-324.

Cleaning and sterilization is mandatory to prevent cross – infection, so under the condition of

12. Aten JC (1993). The stimulation of clinical corrosion of endodontic files, Master off sciences thesis. Northern western university.

this study it can be concluded that
2.5% NaOCl for 15minutes can be effectively

13. M.Darabara, L.bourethis, S.Zinelis (2004). Susceptibility to localized corrosion of stainless steal and NiTi endodontic instrumentsin irrigating solutions, International endodontic journal: 37, 705-710.

used for cleaning.
ProTaper files were found to be more prone

14. Bruce.R, Hilt, Charles J.cunningham, Chiayi Shen(2000). Torsional properties of stainless steel and nickel-titanium after multiple autoclave sterlization, Journal of endodontics: 26, 76-82.

for corrosion than RaCe followed by ENDOWAVE

15. Stokes, Upjohn Pruett (1999). Effect of disinfecting solution on endodontic instrument. International endodontic journal: 34, 512-517.

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