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The use of ultrasound for the removal of the smear

layer. The effect of sodium hypochlorite

concentration; SEM study

Jeffrey

A.

Cameron,

BDS

Key words:

Endodon tics, root canals, smear layer,

sodium hypochlorite, ultrasound.

Abstract

This study was carried out

to

determine the optimum

concentration of sodium hypochlorite activated by

ultrasound n eeded

to

remove the sm ear layer from

an instrumented root canal wall. The results

indicated that under the condit ions of this

experiment a

4

per cent solution of sodium hypo-

chlorite removed all of the smear layer from all 5

specimens; a 2 per cent solution was almost as

effective;

1

per c ent or 112 per cent solutions were

less effectiv e, and w ater was the least effective. A

2 per cent solution of sodium hypochlorite activated

by ultrasound would appear to be the optimum

concentration for the pro duc tion of a smear free root

canal wall und er clinical conditions.

Received

for

publication September

1986.

Accepted

April

1987.)

Sodium hypochlorite has a long history as an

endodontic irrigant. In 1936 Walker' suggested the

judicious use of double strength chlorinated soda

in addition to mechanical cleansing of the root

canal. He comm ented upon its tissue solvent and

germicidal properties, with no apparent ill effect

on living tissue. Doub le-strength chlorinated soda

is a solution of sodium hypochlorite and sodium

chloride containing at least

5

per cent available

chlorine. In 1941 Grossman and Meirnan2 used

pulp tissue from freshly extracted teeth to invest-

igate the solvent efficiency of agents such as

double-strength chlorinated soda, hydrochloric or

sulph uric acids, potassium or so dium hydroxides,

or a variety of enzymes. They found the double-

strength chlorinated soda to be the most effective

pulp tissue solvent of the chemicals tested. Since

that time many authors have used a wide variety

of tissue types to investigate the tissue solvent

properties

of

sodium hy pochlorite and the effect

of

dilution on the solvent efficiency of sodium

hypochlorite.

It is possible that th is pro tein solvent capability

could be a factor in the ability of sodium hypo-

chlorite ultrasonic irrigation to remove the smear

layer from the wall of an instru mented root canal.

Trepagnier, Madden, and Lazzari3used

0.5

per

cent, 2.5 per cent, and

5

per cent solutions of

sodium hypochlorite to dissolve the collagen

containing tissues remaining in the root canal of

instrumented human teeth. Th e

5

per cent and

2.5

per cent solutions showed no significant difference

in their solvent ability in the

5

minute treatment

time, while the 0 5 per cent solution had little

solvent action. Hand, Smith, and Harrison' used

necrotic rat epithelial and subcutaneous tissue to

investigate the effect of dilution on the solvent

action of sodium h ypochlorite. They found a 5.2

per cent solution to be more effective than a

2.5

per cen t solution which was more effective than a

1

per cent solution; a 0.5 per cent solution was

comparable with distilled water in its ability to

dissolve necrotic tissue. T he Sused necrotic tissue

from the abdominal wall of a rat to confirm the

superior solven t action of a

3

per cent solution of

sodium hypochlorite compared with a 1 per cent

solution. Gordon, D amato, and Ch ristneP exposed

vital and necrotic bovine pulp tissue to 0, 1, 3 or

5

per cent

sodium

hypochlorite. T he 3 per cent and

5 per cent solutions were equally effective in

dissolving the vital tissue. Conce ntrations of 1, 3

and 5 per cent w ere equally effective in dissolving

the necrotic tissue. Nakamura, Asai, Fujita, and

colleagues' concluded that

5

per cent and

2

per cent

solutions of sodium hypochlorite were equally

effective in dissolving bovine tendon collagen,

bovine pulp, and bovine gingiva.

Australian Dental Journal 1988;33 3):193-200.

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Senia and Marshalla used 5.25 per cent sodium

hypochlorite as an endodontic irrigation solution

in the mesial root canals of human first molar teeth.

They concluded that

5.25

per cent sodium hypo-

chlorite was more effective than normal saline in

dissolving human pulp tissue, but was of

questionable value as an endodontic irrigating agent

in the apical 3 mm of narrow root canals. McCom b

and Smith9 used the scanning electron microscope

(SEM ) to study the root canal wall after hand

instrumentation in conjunction with a variety of

irrigating solutions. Six per cent sodium hypo-

chlorite was effective in removing superficial debris,

but was not capable of removing the smear layer

that was produced by standard instrumentation

techniques. Th is smear layer was thought to contain

dentine, remnants of odontoblastic processes, pulp

tissue and bacteria. L ester and Boyde'O comm ented

on the 'fuzzy' or 'smeared' appearance of the root

canal wall when 1 per cent sodium hypochlorite was

used as an irrigating solution d uring and after hand

instrumentation. Storage of these specimens in

5

per cent sodium hypochlorite for 3 days removed

part of the smear layer, but left the dentina l tubule

openings occluded with

a

plug of mineralized

debris.

Sodium hypochlor i te has been used in

conjunction with other irrigating agents in an

attempt t o achieve

a

debris free canal. Grossman

believed the effervescence produced by the alternate

use of sodium hy pochlorite and hydrogen peroxide

should remove debris from the root canal.

Experiments by M cCom b and Smith9 suggested

that the combination of

6

per cent sodium hypo-

chlorite followed by 3 per cent hydrogen peroxide

produced

a

surface similar to that produced with

water irrigation, while Svec and Harrison1zconsid-

ered the combination to be superior-to saline in the

apical third of the root canal. Rubin and colleague^'^

examined the flushing efficiency of 2.5 per cent

sodium hypochlorite alternated with 3 per cent

hydrogen peroxide and concluded that instrumen-

tation was the most important aspect of

biomechanical preparation, while Baker and co-

workers stated that the flushing action of the

solutions, and not their tissue dissolving qualities,

appeared to

be

the significant factor. Rome, Doran,

and Walker15 used G ly-Oxide in com bination with

sodium hypoch lorite (concentration not stated) and

concluded that the combination was no more

effective in preventing smear layer formation than

the use of sodium hypochlorite alone. McComb,

and Smith9 used REDTA* liberally during and

after hand instrumentation and produced a debris

laden smear layer in the apical region of the root

canal. Goldman and colleagues16 sed a perforated

needle to deliver high volum es of 5 per cent sodium

hypochlorite into the root canal. This delivery

system was able to remove the smear layer from

uninstrumented areas but not from instrumented

areas of the apical region of the root canal wall.

When the perforated needle was used to deliver

10

m L

REDTA

followed by

10

m L

5

per cent

sodium hypochlorite there was little evidence of a

smear layer in the apical third of the root canal.

Th is delivery system was not able to remove soft

tissue and de bris from a large fin adjacent to a root

canal. Berg and co-workers18used

REDTA

while

instrumenting decoronated teeth to size

70

at the

apex, with 3 mL

REDTA

as a final irrigation. T he

canal wall was clean in the coronal third bu t show ed

debris plugs in the dentinal tubules in the middle

third of the canal.

Ultrasound has been suggested as a means of

increasing the efficiency of both instrumentation

and irrigation of root canals. Takagi19 and Moo rer

and W esselinkzO ound that ultrasound markedly

improved the efficiency of sodium hypochlorite as

a tissue solvent. C ra bb zl felt it was 'possible to

render root canals extremely clean using 5 per cent

sodium hypochlorite as an irrigating solution in

combination with ultrasonic agitation and with the

minimum of instrumentation

. . .

Cameronzzwas

able to remove the sm ear layer from the apical third

of a hand instrumented root canal by the ultrasonic

activation of 3 per cent sodium h ypochlorite over

a 3 minute period. The ultrasonic energy was

delivered to the irrigant by a smooth broach held

in an endodontic insert PR30.t Cunningham and

M artinz3 used a continuous flow of

2.5

per cent

sodium hypochlorite thro ugh an endodo ntic insert

P l 0 5 t for the ultrasonic preparation and irrigation

of the root canal. Th ey found minimal smearing of

the canal wall in the apical region, with smear free

ramifications where instrumentation had not been

possible. Goodman, Reader, and Beckz4used

2.6

per cent sodium hypochlorite activated by ultra-

sound to irrigate canals instrumented by the

step-back technique. T he y found significantly less

soft tissue

1

mm from the apex after ultrasonic

irrigation, but no significant difference at the 3

mm

level. Langeland, Liao, and PasconzSwere unable

to achieve a totally clean canal when a continuous

flow of

1

per cent sodium hypochlorite was used

through the endodontic insert

P105.

Trauber and

colleaguesz6mod ified a scaler tip to activate

0.5

per

~

*Roth Drug Company, Chicago, I l l USA.

194

tPR30/P105:

avitron. Dentsply,

York

PA, USA.

Australian

Dental

Journal 1988;33:3.

7/26/2019 Cameron 1988

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cent sodium hypochlorite but were unable to

remove debris from the apical third of an

instrumented root canal.

Th is review of the literature would indicate that

the volume o f irrigation solution, the concentration

of available chlo rine and rate of fluid flow all have

an effect on the efficiency of sodium hypochlorite

as an endodontic irrigant. This experiment was

designed to standardize

(1)

the volum e of irrigating

solution, (2) the source of ultrasonic energy, and

(3) he design of insert and probe u sed for the ultra-

sonic activation of the irrigant. T he sm ear layer was

also standardized as far as possible by using w ater

as the irrigating liquid during hand instrumen-

tation. T he effect of varying the concentration of

available chlorine was to be assessed by studying

the apical third of the root canal with

a

scanning

electron microscope with pa rticular reference to the

presence of a smear layer.

Materials and methods

Twenty-five recently extracted, single-rooted

human teeth were stored in water prior to

instrumentation. T he apical funnel was enlarged by

two sizes of instrument and th e canal slightly flared

using H files throug h a conventional access cavity;

one 2.2 mL cartridge of anaesthetic solution was

used to flush debris from the canal during

instrumentation. These teeth were allocated to 5

groups each containing 5 teeth on a random basis.

A stock solution of

4

per cent sodium hypochlo ritet

was diluted as necessary with distilled water to

provide test solutions containing either 4 per cent

(Group l),

2

per cent (Group 2), 1 per cent (Grou p

3),

or

1/2

per cent (Group 4 available chlorine.

Diluted test solutions were prepared daily as

required. A cartridge of anaesthetic solution was

used to provide a test so lution of

0

per cent available

chlorine (Group 5). Each group received 3minutes

of ultrasonic irrigation using one of the concen-

trations of sodium hypochlorite as the irrigating

solution. T he technique o f ultrasonic irrigation was

described in the first part of this study.zz

A

smooth

broach w ithout a spiral handleg was placed into an

endodontic insert PR30 so that approximately

25

mm of the broach extended from the h ub of the

insert. Ultrasonic energy was provided by

a

dental

unit I used at thre equ arte r power in the prophylaxis

mode. T h e root canal an d access cavity were filled

with the test irrigant, the broach placed into the

root canal so that it did not touch the canal wall

and the tip of the broach was in the middle-third

of the root canal. T he ultrasound unit was activated

for 20 seconds. Ten seconds was allowed to remove

the broach from the canal, replenish the irrigant,

and replace the broach into the canal. Thi s process

was repeated

so

that every canal received 2 minutes

of ultrasound during a

3

minute test period. Any

further action of the sodium hypochlorite was

halted with a final irrigation of

2.2

mL of

anaesthetic solution. T he apical half of the root was

removed and sectioned longitudinally with

a

diamond wheel under a fine air water spray. The

specimens were mounted on

a

10 mm diameter

brass stub, air dried, given a minimum thickness

of gold coating and viewed in a scanning electron

microscope.1 Th e image so obtained was recorded

on negative film** in

a

roll film back.?? All

specimens were viewed from e nd to en d at x 500

magnification prior to photographing areas typical

of the apical seat, the apical funnel

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Fig.

4.

Half per cent sodium hypo chlorit e irrigation removed most soft tissue debris but appeared to have no effect on the superficial

smear layer. Orig. x

2000.

Fig. 5.-Ultrasonic irrigation with water did not remove the superficial smear layer

or

retained

soft

tissue debris. Orig.

~ 2 0 0 0 .

smear layer lifting to reveal the dentinal tubules

below (Fig. 3). Th ere w as little variation from area

to area

or

from specimen to specimen.

the smear layer (Fig.

5).

The small cracks in the

smear layer were thought to have occurred during

preparation of the specimens.

Group

4: 1/2

per cent

so ium

hypochlorite

All specimens in this group presented an intact

smear layer on all instrumented surfaces. Some

cracks were present in the smear layer, but it was

felt that these were as a result of shrinkage during

specimen preparation (Fig. 4 .

Group 5: water

A heavy, tightly adheren t smear layer was present

on the surface of every specimen. There was

evidence of soft tissue remnants

on

the surface of

Discussion

In the literatu re reviewed, the efficiency of a root

canal irrigation technique was evaluated by the light

microscope or by the scann ing electron microscope

(SEM). The light microscope was used to study

stained, serial, horizontal sections of the root canal.

Th is technique demonstrated soft tissue within the

main canal and the contents of any uninstrumented

fin

or

cul-de-sac. T he sca nning e lectron microscope

was used to study longitudinal sections of the root

canal. It could dem onstrate gross debris within the

Australian Dental Journal 1988;33:3. 197

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canal, the presence of a smear layer on the canal

wall

or

debris in the mouth of a dentinal tubule.

Because of the proven efficiency of ultrasonic

irrigation using a higher concentration of sodium

hypochloriteyzZt was felt that the finer detail

offered by the SEM would be needed to demon-

strate differences in the experim ental groups. It had

not been planned to carry out a statistical analysis

of the results, but rather to present photomicro-

graphs of the least smeared and most smeared

regions in each group. However, the appearance of

the instrumented areas in

4

of the 5 experimental

groups was

so

consistent that one photograph could

be used to represent a particular group. Only the

2 per cent sodium hypochlorite group showed a

range of smeared and smear-free instrumented

areas. In this group the one specimen with some

retained smear layer was a fine upper lateral incisor

that had been enlarged by two in strum ent sizes to

file size

35

at the apex.

It

was possible that the

narrow diameter of the root canal was

a

factor in

the retention of some of the smear layer. T he smear

layer produced on the root canal wall during hand

instrumentation will contain organic tissue in the

form of pulpal soft tissue, predentine and the

organic com ponen t of dentine. Lester and Boyde'O

suggested that 'it is compo sed o f translocated

dentine deformed under high pressure'. In the early

stages of instrumentation the smear layer could have

a relatively high organic pulpal content. As

instrumentation progressed less pulpal tissue would

remain to be included in the smear layer,

so

the

protein solvent capability of sodium hypochlorite

would be less relevant. At the completion of

instrumentation, when the apical seat had been

formed and the canal wall had been flared, the

smear layer could have an inorganic content

approaching the 65 per cent inorganic content of

intact dentine. T hi s 'mineralized' sm ear layer could

react with EDTA more readily than with sodium

hypochlorite. This hypothesis is in agreement with

the results obtained by Berg and colleagues'8 who

used ED TA to remove the smear layer from a canal

instrumented to file size 70 at the apex. It is also

possible that part of the success of their exp eriment

could be attributed to th e large diameter of the root

canal.

All

of

the specimens receiving ultrasonic sodium

hypochlor i te i r r iga t ion showed smear-free

uninstrumented areas surrounded by instrumented

areas, yet the ultrasonic water specimens showed

an intact, com pletely smeared canal wall. As all of

the specimens were enlarged to the same degree,

one would anticipate uninstrumented areas to be

present in these Group

5

specimens. It was

concluded that the uninstrumented areas were

obscured by a smear layer on the surface of the

predentine. It is possible that this area of smear

layer was a debris slurry plastered on to the surface

of the predentine, and was similar in appearance

to the smear layer formed by the pressure of

instrumentation on the dentine surface. This

concept of smear layer formation in uninstrum ented

regions would h elp explain why som e areas of smear

layer adhere to the instrumented root canal wall

while in oth er areas the smear layer lifts off quite

readily. After

3

minutes of ultrasonic irrigation with

1/2

per cent sodium hypochlorite the smear layer

and predentine had been removed from uninstru-

mented areas to reveal clean calcospherite structures

on the canal wall. It is possible that the most

efficient way of obtainin g a clean, smear-free canal

would be to keep instrumentation to a minimum,

so

that smear layer formation was minimized; the

organic debris would be dissolved by a protein

solvent rather th an mechanically removed by hand

instrument

at

ion.

In order to keep the experimental variables to a

minim um it was decided to use water as the irrigant

during instrumentation rather than the concent-

ration of sodium hypochlorite appropriate for each

experimental group. While this was a departure

from clinical practice for hand instrumentation, it

did ensure that the significance of hypochlorite

concentration during ultrasonic irrigation was

emphasized. Another factor influencing this

decision was the development

of

ultrasonic and

sonic devices that utilize water as the irrigant during

root canal preparation. T h e manufacturers of some

of these ultrasonic devices have suggested the use

of ultrasound with either sodium hypochlorite or

EDTA as the final irrigation of the root canal.

It has been shown that fluid flow improved the

protein solvent activity of sodium hypochloritez0.2'

and that ultrasound was an effective method of

producing fluid flow. Wh en assessing the efficiency

of any ultrasonic irrigation technique one would

have to consider the nominal power of the ultra-

sound generator, the power of the insert (if

applicable), an d the efficiency of the p robe in trans-

mitting th e energy from th e insert to the irrigation

liquid. Most dental ultrasound prophylaxis units

have enough power to activate an endod ontic insert.

Of the two endodontic inserts presently available

for magnetostrictive units the older design

PR30

is

the more powerful and is more suited to ultra-

sonic irrigation; th e less pow er hl

P105

with a flow

through irrigation system seems better suited to

canal instrumentation. A piezo electronic unit with

a well designed endodontic instrument holder

198

Australian Dental Journal 1988;33:3

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should be able to transmit adequate pow er for both

ultrasonic irrigation and instrumentation. The

author has had no success in creating an efficient

endodontic insert by m odifying a periodontal insert.

Endodontic hand instruments welded onto a perio-

dontal insert tended to break, an d a spring loaded

clip welded to the insert was not efficient in trans-

mitting ultrasonic energy to the retained instru-

ment. Conventional hand instruments such as

reamers, files, smooth broaches or

hand pluggers

have been used in the endodontic insert in pilot

studies by the au thor. T h e most efficient instrument

was a smooth broach without a spiral handle; one

brand4 of smooth broach seemed to be very efficient

in transmitting ultrasonic energy and did not

fracture as easily as any other brand tested.

In this experim ent ultrasonic water irrigation had

no apparent effect on the smear layer,

so

it would

appear that ultrasound

per s

does not mechanically

remove the smear layer. McC om b and Smith' found

that

6

per cent sodium hypochlorite did not remove

the smear layer, so the efficiency of ultrasonic

irrigation with either

4

per cent

or

2 per cent

solutions must have been as a result of the fluid flow

within the system. In this study the tip of the ultra-

sonic probe did not extend beyond the middle-third

of the root canal, yet in g rou ps 1 and 2 th e effects

of

the fluid flow extended right to t he apical seat.

Th is ability to clean a canal wall by the use of ultra-

sonic irrigation has benefits beyond t he removal of

the smear layer. Hand instrumentation does not

prepare all surfaces on the canal wall equally,I4 nor

do all operators use hand instruments with th e same

e f f i c i e n c ~ . ~ ~ith ultrasonic irrigation th e operator

holds the tip of the ultrasonic probe within the

irrigating liquid, and the ultrasound forces the

liquid against the canal wall. This minimizes the

effects of operator technique, and permits the

cleansing of irregularities in the canal wall. Ultra-

sonic irrigation has the ability to exert its cleansing

ability beyond th e main root canal into an adjacent

fin or into the is thmus in

a

lower molar

Because of this ability to clean beyond the main

canal, ultrasonic irrigation must be considered

superior to EDTA

or

the EDTA/sodium hypo-

chlorite combination, which tends to leave debris

in a fin.

Summary and conclusions

Twen ty-five recently extracted hum an teeth were

enlarged by two ins trum ent sizes using Hed strom

files with water as the irrigant. Each gro up of five

teeth received

3

minutes of ultrasonic irrigation

with either

4

per cent, 2 per cent,

1

per cent, 1/2

per cent

or 0

per cent sodium hypochlorite as the

irrigant. The scanning electron microscope was

used to determine the presence or absence of a

smear layer in the apical third of each specimen.

It was concluded that ultrasonic irrigation with 4

per cent or 2 per cent sodium hyp ochlorite was an

efficient method of removing the sm ear layer from

instrumented areas of the root canal; ultrasonic

irrigation with 1/2 per cent sodium hypochlorite

was capable of removing the smear layer from

uninstrumented areas of the canal wall. A 2 per cent

solution of sodium hypochlorite activated by an

efficient ultrasound delivery system is recom-

mended for th e final cleansing of instrum ented root

canals.

Acknowledgements

T he author wishes to thank the Australian Dental

Research Fu nd for its financial assistance;

Mr

Gary

Weber of the Electron Microscope Unit, University

of Newcastle, for his technical assistance, and Miss

Robyn Westbury for processing the manuscript.

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Address

or

correspondencdreprints:

P

Box 101,

Charlestown, New South Wales, 2290.

200

Australian

Dental

Journal 1988;33:3.