0099-2399/87/1311-0541/$02 00/0 JOURNAL OF ENDODONTICS Copynght �9 1987 by The American Association of Endodontlsts

Printed in U.S.A. VOL 13, NO. 11, NOVEMBER 1987

The Synergistic Relationship between Ultrasound and Sodium Hypochlorite: A Scanning Electron Microscope Evaluation

Jeffrey Anthony Cameron, BDS

Twenty-eight freshly extracted human teeth were hand instrumented to clinical standards. Distilled water was used as the irrigant during instrumenta- tion of all specimens in order to produce a uniform smear layer. The canals were given a final irrigation with either 4% sodium hypochlorite, ultrasonic irri- gation with different concentrations of sodium hy- pochlorite, or ultrasonic irrigation with distilled water. The results indicated that 4% sodium hypo- chlorite, or ultrasound with water, did not remove the smear layer. Four percent or 2% sodium hypo- chlorite activated by ultrasound did remove the smear layer from instrumented areas of the canal wall. It was concluded that a synergistic relationship existed between sodium hypochlorite and ultra- sound, and that the relationship was clinically sig- nificant in the presence of 2% available chlorine.

Debridement of root canals is carried out by using hand instruments along with an irrigating solution. The im- portance of the irrigation solution was recognized in 1936 by Walker (1), who recommended the judicious use of double-strength chlorinated soda because of its protein solvent and germicidal properties. Moorer and Wesselink (2) reviewed the extensive literature on the properties of sodium hypochlorite before investigating the factors affecting the protein solvent ability of this irrigant. Their studies indicated that mechanical agita- tion or fluid flow was a more important factor in the tissue dissolving ability of sodium hypochlorite than the initial percentage of available chlorine. They found ultra- sonic energy to have more influence on a hypochlorite/ tissue system than slow mechanical agitation. Richman (3) first suggested the use of ultrasound in endodontics in 1957. Although he was concerned primarily with the ultrasonic activation of hand instruments, he did sug- gest that the canal be flooded with sodium hypochlorite to dissolve organic matter and to prevent heat build up. In 1977, Takagi (4) reported on the in vivo and in vitro use of the ultrasound/sodium hypochlorite combination.

541

A scanning electron microscopic study revealed clean canal walls, and in a clinical trial fewer patient visits were needed. The effectiveness of the ultrasound-so- dium hypochlorite combination has been studied by several authors (5-10). These studies have used a variety of ultrasound generators, commercially available or prototype ultrasonic inserts, and differing strengths of sodium hypochlorite. The results have been as- sessed by low-power magnifying glass, light micros- copy, or a scanning electron microscope. The constant feature of the results was the relationship between the concentration of sodium hypochlorite activated by ultra- sound and the presence or absence of debris in the instrumented canal. Sodium hypochlorite concentra- tions of 5% (5), 3% (6), and 2.5% (7, 8) produced debris-free canals; concentrations of 1.0% (9) and 0.5% (10) were not effective in removing debris. Martin and Cunningham (11) have claimed that a synergistic rela- tionship exists between ultrasound and sodium hypo- chlorite when used in conjunction for the debridement of a root canal. The purpose of this experiment was to determine if such a synergistic relationship did exist and what effect the concentration of sodium hypochlo- rite had on the efficiency of the system. A scanning electron microscope was used to detect the presence or absence of a smear layer or surface debris on the instrumented, irrigated root canal walls.

MATERIALS AND METHODS

Twenty-eight freshly extracted single-rooted human teeth extracted for periodontal reasons were stored in water before being used in this experiment. To minimize variables between experimental groups, the teeth cho- sen all had straight roots and were approximately 21 mm in length. Mature teeth were chosen so that a final file size at the working length in the range of #40, #45, or #50 would achieve instrumentation to clinical stand- ards. A conventional access cavity was prepared in the crown of the tooth and Hedstrom files were used with a twist-pull action to enlarge the apical portion of the

5 4 2 C a m e r o n J o u r n a l o f E n d o d o n t i c s

root canal by two or three instrument sizes. Gates Glidden drills were used to remove any gross irregular- ities from the coronal portion of the root canal�9 One milliliter of distilled water was used to flush the canal between each instrument size. Although the use of water as an irrigating solution was a departure from clinical practice, it did ensure that the nature of the smear layer was constant for each group. Four instru- mented teeth were allocated on a random basis to each of the six experimental groups; four teeth received no further irrigation and acted as controls. Group 1 teeth were irrigated with 5 ml of 4% NaOCI (Zixo bleach; Ajax Chemicals, Sydney, Australia) in 1-ml increments over a 3-min period�9 A final flush of 2 ml of distilled water was used to prevent any further soft tissue solvent activity of sodium hypochlorite. The other ex- perimental groups received 3 min of ultrasonic irrigation with 5 ml of either 4% NaOCI (group 2), 2% NaOCI (group 3), 1% NaOCI (group 4), 0.5% NaOCI (group 5), or distilled water (group 6). Ultrasonic energy was intro- duced into the irrigating solution by a smooth broach (Micro Mega, Geneva Switzerland) held in an endodon- tic insert PR30 (Cavitron; Dentsply, York, PA) activated by a Cavi-Endo dental unit (Cavitron) used in the pro- phylaxis mode�9 The root canal and access cavity were filled with the test irrigant, the broach placed into the canal so that the tip was in the middle third of the root canal, and the ultrasound unit activated for 20 s. Ten seconds were allowed to remove the broach from the canal, flush the canal, and replace the broach into the canal, so that over the 3-min (6 x 30 s) period of ultrasonic irrigation, the ultrasound was activated for 2 min (6 x 20 s). All specimens received a final flush of 2 ml of distilled water�9 The root was sectioned longitudi- nally using a diamond wheel under a fine air/water mist�9 The apical half of the root was mounted onto a 10-mm diameter brass stub, air-dried, given a minimum thick- ness gold coating, and viewed in a scanning electon microscope (JEOL JSM 840; JEOL, Tokyo, Japan)�9 The image obtained was recorded on negative film (FP4; Ilford Ltd., Cheshire, United Kingdom) in a roll film back (Mamiya, Tokyo, Japan)�9 All specimens were viewed from end to end at x500 magnification before photo- micrographs were made of areas typical of the apical seat (1-mm level), the apical funnel (4- to 5-mm level), and the middle third area of the root (9-mm level)�9

To minimize the effect of operator bias, the following measures were introduced. All four control teeth were processed together�9 Six teeth, one from each of the six experimental groups, were processed and viewed to- gether. Hand instrumentation of all six teeth was com- pleted before an individual tooth was allocated a num- bered specimen container. The instrumented tooth was then given the apl~ropriate irrigation and the scanning electron microscopic root section prepared. This spec- imen was then returned to its numbered container�9 A

technical officer mounted the numbered specimen onto a coded brass stub for drying and gold coating�9 The areas selected as being typical of the 1-, 4-, and 9-mm areas were chosen by the scanning electron micro- scope director (G. W.) and a frame number was in- cluded in the scanning electron microscope data line on the photographic negative. Areas of special dental interest such as fins or lateral canals were photo- graphed by me. The negatives were printed and the photomicrographs assessed for presence or absence of smear layer�9 A field was classified "smear free" if all dentinal tubules in the field were visible and most tubules were free from a debris plug. The negative number/stub number/specimen number/experimental group number process was then reversed to identify each photomicrograph�9

Routine photographs were taken at standardized magnification of x500, x2,000 and x4,500. All photo- micrographs in this article are at x2,000 magnification and represent views typical of the retained smear layer and debris approximately 4 mm from the instrumented apical seat.

RESULTS

Group 1: 4% NaOCI

A tightly adherent smear layer was present in all instrumented areas of the root canal. The surface of the smear layer was smooth and free from soft tissue debris (Fig. 1).

Group 2: 4% NaOCI + Ultrasound

All specimens in this group were free from surface smear. Debris plugs had been removed along the entire length of the specimens (Fig. 2).

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Group 3: 2% NaOCl + Ultrasound

All specimens were free from surface smear but a debris plug was present in some dentin tubules (Fig. 3 ) .

Group 4: 1% NaOCI + Ultrasound

A surface smear was present on all instrumented surfaces�9 There was evidence of the smear layer break- ing up and lifting (Fig. 4).

Group 5: 0 . 5 % NaOCl + Ultrasound

An intact smear layer was present on all instrumented areas of the root canal (Fig. 5). All specimens receiving ultrasonic irrigation with sodium hypochlorite showed smear-free uninstrumented areas�9

Group 6: Ultrasound + Water

All specimens in this group were smeared from end to end with evidence of soft tissue-like debris on the surface (Fig. 6).

Group 7: Control

All specimens in this group were heavily smeared from end to end with soft tissue-like debris on the surface (Fig. 7).

DISCUSSION

A synergist has been described as one reagent which co-operates with another when combined with it, thus increasing the efficiency of both. It would appear from this study that when sodium hypochlorite and ultra- sound are combined, as in ultrasonic irrigation, the

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544 Cameron Journal of Endodontics

FIG 6. Ul t rasonic irr igat ion wi th w a t e r did no t remove the smear layer

(original magni f icat ion x2 ,000) .

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efficiency of both components is increased�9 When 5% sodium hypochlorite is used during root canal instru- mentation and irrigation, the canal surface has a smeared appearance. Lester and Boyde (12) rendered instrumented, irrigated teeth anorganic by immersion in 5% sodium hypochlorite for 3 days. Although this tech- nique removed the superficial smear layer, the dentinal tubule openings were still occluded by a mineralized plug. In the present experiment, 3 min of ultrasonic irrigation with 2% sodium hypochlorite was able to achieve a cleaner canal wall than did immersion for 3 days in a 5% solution. The fluid flow created by ultra- sound, together with replacement of the irrigating so- lution within the canal, resulted in a dramatic increase in the efficiency of sodium hypochlorite as an endodon- tic irrigant. Whe~ ultrasonic energy is applied to a liquid, shock waves travel through that liquid to create violent movement within the liquid and a scrubbing effect on

the walls of the container�9 In endodontics, the ultrasonic energy passes through the irrigating solution and exerts its scrubbing effect on the root canal wall. In the present experiment, the scrubbing effect of ultrasonic irrigation with water was not capable of removing the smear layer from an instrumented root canal wall within a 3- min period�9 This would indicate that the importance of the scrubbing action of ultrasound could have been overestimated and that the fluid flow within the sodium hypochlorite could have been more important than the mechanical effect of the shock waves on the smear layer�9 However, it is possible that these shock waves could remove loosely attached material from a fin or from an uninstrumented area.

These results are in agreement with those obtained by previous workers (5, 6, 8, 12). When sodium hypo- chlorite is used as an irrigant, a smear layer is produced on an instrumented canal wall. If sodium hypochlorite with more than 2% available chlorine is used for ultra- sonic irrigation, the smear layer is removed within 3 min (6). When solutions with less than 2% available chlorine are used, the smear layer is not removed. These results could explain the apparently contradictory results of different workers who have investigated ultrasonic in- strumentation or ultrasonic irrigation and who have limited their experiments to a single concentration of sodium hypochlorite (5-11)�9

The results would also confirm the claim that a syn- ergistic relationship exists between sodium hypochlo- rite and ultrasound when they are combined during ultrasonic irrigation�9 While either component used by itself is unable to remove the smear layer, the combi- nation of ultrasound with sodium hypochlorite produces a debris-free canal wall. The synergistic relationship becomes clinically significant with hypochlorite solu- tions containing more than 2% available chlorine.

CONCLUSIONS

It was calculated that, under the conditions of this experiment:

1. Solutions of sodium hypochlorite with up to 4% available chlorine will not remove a smear layer from an instrumented canal wall.

2. Ultrasonic shock waves traveling through water will not mechanically remove a smear layer.

3. Two percent sodium hypochlorite activated by ultrasound will remove a smear layer within 3 min.

4�9 A synergistic relationship does exist between ul- trasound and sodium hypochlorite, and this relationship is clinically significant when solutions containing 2% available chlorine are used.

This research was funded in part by a grant from the Australian Dental Research Trust.

I wish to thank Gary Weber of the scanning electron microscope unit, Newcastle University, for his technical assistance.

Dr. Cameron is a suburban general practitioner in Newcastle, Australia.

Vol. 13, No. 11, November 1987

References

1. Walker A. A definite and dependable therapy for pulpless teeth. J Am Dent Assoc 1936;23:1418-25.

2. Moorer WR, Wesselink PR. Factors promoting the tissue dissolving capability of sodium hypochlonte. Int Ended J 1982;15:187-96.

3. Richman MJ. The use of ultrasonics m root canal therapy and root resection. J Dent Med 1957;12:12-6.

4. Takagi K. Bastc chn~cal studies of root canal irrigation by ultrasound. Aichi Gakuin J Dent Sci 1977;14:341-62.

5. Crabb HSM. The cleansing of root canals. Int Ended J 1982;15:62-6. 6. Cameron JA. The use of ultrasonics ~n the removal of the smear layer: a

scanning electron microscope study. J Endodon 1983;9:289-92.

Ultrasound and Sodium Hypochlorite 545

7. Goodman A, Reader A, Beck M, Melfi R, Meyers W. An in vitro compar- ison of the efficacy of the step-beck technique versus a step-back/ultrasonic technique in human mandibular molars. J Endodon 1985;11:249-56.

8. Cunningham WT, Martin H. A SEM evaluation of root canal debridement with the endosonic ultrasonic synergistic system. Oral Surg 1982;53:527-31-

9. Langeland K, Liao K, Pascon EA. Work-saving devices in endodontics: efficacy of sonic and ultrasonic techniques. J Endodon 1985;11:499-510.

10. Tauber R, Morse DR, Sina~ IA, Furst ML. A magnifiying lens comparative evaluation of conventional and ultrasonically energized filing. J Endodon 1983;9:269-73.

11. Martin H, Cunningham W. Endosconic endodontics: the ultrasonic synergistic system. Int Dent J 1984;34:198-203.

12 Lester KS, Boyde A. Scanning electron microscopy of instrumented, irrigated and filled root canals, Br Dent J 1977;143:359-67.