Efficacy of Different Irrigating Systems on Debris and Smear Layer Removal: A Scanning Electron Microscopic Study

Introduction

Mechanical instrumentation of the canals without fail generates smear layer on the walls of rootcanal.¹ This smear layer obstructs dentinal tubules, preventing medicaments, sealants and obturation materials from reaching their full potential in the lateral canals and dentinal tubules. It is a loosely adhesive structure that can harbour bacteria and prevent the obturating material from forming a strong bond with the canal wall, potentially allowing microleakage. ²

Irrigants employed while shaping the canal helps in eliminating the smear layer by penetrating the complex canal system. The irrigating solutions, along with the irrigation method and techniques used, determines the quantity of smear layer eliminated.

Improvements in irrigation techniques are required to enhance the cleaning efficiency of complex canal systems.³ The aim and objective of this research was to see how effective the Rotary canal brush, Endo Activator, Passive ultrasonic irrigation (PUI), and Continuous ultrasonic irrigation (CUI) were at eliminating smear layer and debris from the canals.

Materials and Method

The study used sixty extracted human mandibular bicuspids with single root and one canal (Figure 1). After extraction, teeth were immediately placed under running water to remove blood, followed by curettage to remove soft tissue, and then preserved at room temperature in 0.1 percent thymol solution until needed. Teeth were decoronated (Figure 2 and 3) with a diamond disk under water spray to achieve a standardized root length of 16 mm. The working length was computed by deducting 1 mm from the length at which the tip of the ISO size #10k-file was just noticeable at apical foramen. To prevent irrigants from extruding, the apical end was closed with wax. WaveOne rotary file (reciprocating system) was employed to instrument all the canals. The files were changed after using in four canals. Irrigation was done in all root canals using a #27 gauge hypodermic needle and a syringe with 6 ml of 2.5 percent sodium hypochlorite solution during instrumentation.

Division of experimental groups

Instrumented teeth were arbitrarily classified into four categories, each containing 15 specimens. All the root canals were rinsed with 2 ml of Smear Clear solution dispensed in a syringe with # 27 gauge needle, which was activated by different techniques, according to the categories (Figure 4, 5 and 6).

Group 1- Activation of irrigant by ISO #20 rotary canal brush rotated at 600 rpm attached to X-smart, for 20 seconds, 2 mm short from the working length.

Group 2- Activation of irrigant by ISO #20 EndoActivator file attached to EndoActivator handpiece, for 20 seconds, 2 mm short from the working length.

Group 3- Activation of irrigant by ISO #20 ultrasonic file (irrisafe) attached to Satelec ultrasonic handpiece, In group 1, group 2 and group 3, the activation of irrigant was repeated two more times, making a total volume of 6 ml of smear clear solution and total activation time of one minute.

Group 4- The canals were flushed with 6 ml of smear clear solution by continuous ultrasonic irrigation 2 mm short of working length, for one minute, with help of a hollow needle (#27 gauge) fixed to the ultrasonic unit via 1200 file adapter (EMS). This adapter has a hollow design which enables the flow of irrigant from VARIOS 970 ultrasonic unit (NSK) up to the tip of the ultrasonically vibrating needle.

Immediately after activation, all the canals were rinsed with 6 ml distilled water and dried using paper points. A diamond disc was used to create two longitudinal fissures on the buccal and lingual surfaces to aid in root slicing. Using a chisel, the roots were divided into two halves, with the half containing the most visible portion of the apex being preserved and coded. The specimens were dried, assembled on metallic stubs, and inspected under Scanning Electron Microscope (SEM). Photo micrographs were taken at x500 for debris and x1500 for smear layer examination from the appropriate center of coronal (10 mm to apex), middle (6mm to apex), and apical thirds of each specimen.

The photographs were evaluated using Schafer and Lohmann criteria for scoring the debris, as given below:

DEBRIS score (dentinal chips, pulp remnants, particles loosely attached to canal wall)

Score 1: Clean wall, few debris particles

Score 2: Few agglomerations

Score 3: Many agglomerations, less than 50% of canal wall covered

Score 4: More than 50% of canal wall covered with debris

Score 5: Complete covering of canal wall by debris

The photographs were also evaluated by Torabinejad et al criteria for scoring the smear layer as follows:

SMEAR LAYER (dentin particles, remnants of vital or necrotic pulp tissue, bacterial components, retained irrigants)

Score 0: No smear layer (no smear layer on surface of the root canals with all tubules clean and open)

Score 1: Moderate smear layer (no smear layer on the surface of root canals but tubules contain debris)

Score 2: Heavy smear layer (smear layer covers the root canal surface and tubules)

Results

Debris removal

The least score was noted in Group 4 (continuous ultrasonic irrigation). Hence, Group 4 displayed highest efficiency in debris removal. The results of Group 4 were different from Group 1, 2 and 3 at the coronal third which was statistically significant. At the apical and middle thirds, the difference between Group 4 and 3 was not statistically significant. The mean score for all the groups for debris at different levels of the canal is tabulated in table 1.

Smear layer removal

At the apical, middle and coronal thirds, Group 4 (continuous ultrasonic irrigation) had the least mean score. At the coronal and middle third, statistically significant difference was noted among Group 1, 2 and 3. However at the apical third, difference between Group 4 and 3 was not statistically significant. The mean score of all the groups for smear layer at different thirds of the canal is tabulated in table 2.

Discussion

In this research, the efficacy of the Rotary canal brush, EndoActivator, Passive ultrasonic irrigation system, and Continuous ultrasonic irrigation were assessed to ascertain the most efficient irrigation method for eliminating the smear layer and debris.⁴

The cleanliness of the root canal wall was assessed using a SEM since it allows assessment of the whole section of both halves of the canal by separate numeric evaluation criteria for debris and smear layer. The smear layer and debris score at the coronal, middle, and apical thirds were documented.^5,6,7 It must be emphasized that a root canal is a single entity that cannot be considered clean if any of the arbitrary levels is not clean.^3,8

Analysis of the data showed that continuous ultrasonic irrigation system (Group 4) was the best in debris and smear layer elimination, followed by EndoActivator (Group 2) and PUI (Group 3), in all the thirds of root canal. At middle and coronal thirds, findings of Group 4 were different in regard to the other categories, which was statistically significant. At the apical third, difference between Group 3 and 4 was not significant statistically. Continuous ultrasonic irrigation was used in Group 4 to ensure that the irrigant was replaced regularly. It is thought to provide an advantage over intermittent irrigation because of continuous replacement of irrigant. A flow rate of 6 ml/min was followed in this research that used a customized tip (hollow needle #27 gauge) connected to the ultrasonic unit via 1200 file adapter (EMS). Since the final apical preparation was up to 040.08 size, #27 gauge tip of the hollow needle was allowed to be placed loosely at 2 mm less than the working length. This had the added advantage of continuous replenishment of the irrigating solution and smooth surface of needle prevented gouging.⁹

A study conducted by Goel and Tewari displayed the efficiency of PUI in eliminating smear layer in the apical third, and intermittent ultrasonic activation was superior to continuous ultrasonic activation.¹⁰ Technique of continuous ultrasonic irrigation followed was different in their study. They used a ISO # 10 K-file 1 mm less than the working length and the irrigation (17% EDTA) was exchanged through a 30 gauge needle (Navi-Tip FX) at the orifice of the canal. Irrigant exchanges do not take place 1 mm beyond the needle tip. Hence, no fresh irrigant was available in the apical third of the canal where evaluation of smear layer was done. In contrast to this, in this research the irrigant was delivered through the needle tip which was 2 mm short of apex. The continuous flow and replenishment enables new active irrigant to contact canal wall and flush out the already dissolved/chelated inorganic portion of smearlayer.⁷

A study done by Adcock et al showed very minimal amount of debris in the isthmus, than in the canal. Study done by Howard et al did not show any significant difference in the cleanliness of isthmus and canal after the final rinse with Pro-ultra Piezo or the Maxiprobe. Ribeiro et al used various agitation devices to eliminate smear layer (Navi-tip FX needle, apical negative pressure irrigation, manual dynamic irrigation and continuous ultrasonic irrigation), and did not find statistical difference among them.^9,11,12

The next most successful approach for eliminating the debris and smear layer from the root canal system was passive ultrasonic irrigation (Group 3). At the coronal, middle and apical regions, Group 3 findings were statistically different when compared to Group 1 and 2. It was inefficient than Group 4 at middle and coronal area, but the results were non-significant in the apical area. The outcome of this study was consistent with those of Lee S J et al., Lui et al., Narmatha V J and Sophia Thakur, Stamos et al., and Sabins et al. ^13-17

Acoustic streaming is the best possible reason for the cleaning efficiency of ultrasonic irrigation (Ahmed et al.). It is defined as the generation of time dependent steady unidirectional circulation of fluid in the vicinity of a small vibrating object (Nyborg). The instrument should vibrate freely in the canal for efficient acoustic streaming. The highest efficiency is obtained with small file, high frequency and large displacement amplitude. Therefore, diameter of the canal is important because it can influence the amplitude displacement.¹⁸ The file should remain loose inside the canal while irrigating, without any pressure on the walls of the canal or intentional removal of dentine, characterizing the passive ultrasonic irrigation technique (Sabins et al., 2003) which helps in the reflux of irrigant, causing further debris to be pushed coronally, thus preventing irrigant extrusion into periapical tissue. Ultrasonic waves help in the energy transfer from the vibrating file to the irrigant in the canal which causes hydrodynamic turbulence, causing cavitation of the irrigant and bubble impact waves, resulting in debris removal. In this research, an ISO size #20 (irrisafe) non-cutting ultrasonic file was employed, with the canal enlargement done till WaveOne large 040.08 file.

These may be the reasons for the PUI system’s effectiveness in eliminating smear layer and debris in this research.¹³ Various studies done by Khaord P et al., Khalap ND et al., concluded that sonic activation of irrigant by EndoActivator was more efficient in eliminating smear layer and debris than passive ultrasonic irrigation system. According to them, the reason behind this result was the unwanted dampening effect of amplitude of its characteristic nodes and antinodes pattern, in passive ultrasonic irrigation especially when instrument touched the lateral walls of a shaped canal.¹⁹ However in this research, an ISO size #20 (Irrisafe) non-cutting ultrasonic file was employed, with the canal enlargement with (040.08) large WaveOne file. This would allow free oscillation of small file, thus enhancing the influence of the technique as file would not be stressed under contact with lateral canal wall. Another similar study by Blank-Goncalves et al compared conventional irrigation versus sonic and ultrasonic irrigation system. According to their results, sonic and ultrasonic irrigation showed better removal of smear layer than conventional needle irrigation in the canal; statistically the difference between sonic and ultrasonic irrigation system was not significant.¹ Endo Activator (Group 2) showed the next best effective irrigation method in removal of debris from the canal. The Group 2 results were statistically significant than Group 1 (canal brush), in coronal, middle, and apical regions. In comparison to Group 1 in coronal, middle, and apical regions, the difference in smear layer removal was not statistically significant. But Group 2 was less efficient in removing debris and smear layer from coronal, middle, and apical regions. Group 2 showed better debris removal than Group 1. Frequency of 1 to 10 KHz was used for sonic activation, which is lower as compared to ultrasonic (25-30 KHz). Consequently, the velocity of the streaming irrigant is lower. Moreover, the pattern of oscillation of sonic instruments are different. They have one node near attachment of the file and one antinode at the file tip as compared to the ultrasonically vibrating file, which has several nodes and antinodes along the length of the file.⁴ The synergistic relations between velocity of acoustic streaming and frequency may be the reason for superior effect of the ultrasonic systems.²⁰

In 1983, Chow defined three parameters for successful root canal debridement. For an irrigating solution to be mechanically effective in removing all the debris particles, it has to: (a) reach apex; (b) create a current; (c) carry particles away. The non- significant difference of Group 2 in removing smear layer in comparison with Group 1 can be explained as follows. Endo Activator sonic agitation expedites elimination of smear layer by creating eddy currents, sonic disruption of the smear layer fragments occurs without a mechanism to carry the particles away by continuous irrigant flow, probably resulted in entrapment and settling of loose debris or fragments around the demineralized dentine collagen network along the walls of the canal. As this 3-dimensional collagen network is highly porous, it would be difficult for small particles to be dislodged again once they are trapped by this fish like matrix. According to Al-Jadaa, the sonic systems wavelength is very long to produce adequate irrigant streaming, and the energy is very low for the activation of the irrigant.²¹

The results of this research correlates with various studies by Mancini M et al., Rodig et al., Capar ID et al., Rodig T et al., Narayan GS et al., Caron et al., Niu et al., Keir DM et al., which showed EndoActivator was better in clearing smear layer and debris than canal brush.^14,22 This might be because of the positive effect of sonically induced acoustic micro-streaming and heat. However, studies by Singh N et al., showed that canal brush charred the smear layer from the coronal, and apical regions more efficiently than EndoActivator.³ Torres DU compared EndoActivator with conventional Max-I-probe irrigation for smear layer elimination.²³

The findings showed that compared to Max-I-probe irrigation, the EndoActivator system did not improve smear layer elimination. In comparison to the others in this research, rotary canal brush assisted irrigation (Group 1) was the least efficient irrigating system for eliminating the debris and smear layer from the canal. The high mean scores of Group 1 were due to the friction created between the brush bristles and the canal irregularities that results in the dislodgment of the radiolucent bristles inside the root canal. The push and pull motion of the brush which mechanically displaces the tissues, the friction generated between the brush and canal wall prevents the solution to flow inside. In this research, the canal brush was employed with a circumferential and 1- 2 mm up and down motion for 60 seconds, in a slow hand piece. Different result would have been obtained if the brush was employed for a longer time. Modifications to the brush may also increase its efficiency to clean the canal walls.²⁴ The observations of this research are in contrast to those of Al-Hadlaq SM et al., Salman MI et al., Garip Y et al., Weise et al., and Keir et al., who found that irrigation along with the canal brush improved root canal cleanliness by removing debris and smearlayer.^24,3,25,5,21

Regardless of irrigation procedure, the middle and coronal regions of the root canal walls were significantly cleaner than the apical third owing to the fact that higher amount of circulating irrigant and greater diameter of the coronal and middle thirds than the apical third. The apical thirds failed to remove the smear layer when the apical foramen was occluded.

Conclusion

The analysis of this research showed that:

• No irrigation technique achieved complete root canal cleanliness.

• The most efficient approach for eliminating debris and smear layer from root canal walls was Continous ultrasonic irrigation, followed by PUI, EndoActivator, and Rotary canal brush.

• The middle and coronal thirds of the root were cleaner than the apical third, regardless of irrigation systems.

Conflict of Interest

None.