Comparative Evaluation of Smear Layer Removal by Various Endodontic Irrigating Solutions—An In vitro Study

Abstract

Background:

Present study aimed to investigate the smear layer removal of three different endodontic irrigating solutions: Group I—17% EDTA followed by 3% NaOCl, Group II—combination of hydrogen peroxide (12.8 mL/100 mL), sodium benzoate (0.021 mL/100 mL), acetic acid (2.664 mL/100 mL), and water; Group III—combination of 4.25% citric acid, 3% doxycycline, 9% propylene glycol, and water.

Materials and Methods:

One hundred twenty-three extracted premolars were stored and decoronated. The root canals were biomechanically prepared with ProTaper rotary instruments (Dentsply) up to size F5. During instrumentation, 1 mL of 2.5% NaOCl was used as an intracanal irrigant. After instrumentation, one hundred and twenty samples were randomly divided into three experimental groups according to different irrigating solutions used. As control, three specimens were prepared in same manner as experimental groups but only irrigated with 2.5% NaOCl and distilled water not with acids or chelators. Finally, each root canal of all the experimental samples were irrigated with 2.5% NaOCl and distilled water, dried, temporarily sealed and stored and subjected to scan under SEM.

Results:

Smear layer removal efficacy of all the experimental groups were compared at all sites (coronal, middle, apical) with one another and P value was found to be <0.001, that is, highly significant.

Conclusion:

Combination of 4.25% citric acid, 3% doxycycline, 9% propylene glycol, and water showed effective smear layer removal from the coronal, middle, and apical thirds as compared to other irrigating solutions than other irrigating solutions.

INTRODUCTION

Endodontic treatment is challenging as its success lies in a variety of factors such as cleaning, shaping, and three dimensional obturation. Various irrigants are used in routine endodontic practice for smear layer removal. The most common combination for the removal of the smear layer is the alternate use of EDTA and NaOCl. NaOCl irrigation following EDTA increases the dentinal erosion effect, causing erosion of peritubular and intertubular dentin and reducing the dentin microhardness.[1,2,3] Because of these adverse effects of acidic irrigation solutions, studies are focused on new irrigation solutions. However, an ideal solution, which can remove the smear layer effectively without causing erosion on root canal dentin walls, has not been found yet.[4,5,6]

Therefore, this in vitro study was carried out to investigate the smear layer removal by three different irrigating solutions, that is, 17% EDTA followed by 3% NaOCl; combination of hydrogen peroxide (12.8 mL/100 mL), sodium benzoate (0.021 mL/100 mL), acetic acid (2.664 mL/100 mL), and water and another combination of irrigating solutions is 4.25% citric acid, 3% doxycycline, 9% propylene glycol, and water.

MATERIALS AND METHODS

One hundred and twenty-three single and straight-rooted premolars extracted for orthodontic purposes were stored in 0.1% thymol solution. The crown portion was discarded, and only the roots were kept for the study. The working length was determined with a #10 K-file. The file was inserted into the canal until it was seen at the apical foramen. Then 1mm was subtracted from this length to establish the working length. The apical end of each root was sealed with soft sticky wax to prevent the outward flow of irrigating solution through the apical foramen and to stimulate in vivo apical counter pressure during canal preparation. During the entire preparation, samples were instrumented using ProTaper rotary instruments (Dentsply) up to size F5. 0.1 mL of 2.5% NaOCl was used as an irrigant after each filling. After instrumentation, 123 samples were randomly divided into three experimental groups with 40 samples in each group and one control group with three samples:

• Group I: 17% EDTA followed by 3% NaOCl.

• Group II: Combination of hydrogen peroxide (12.8 mL/100 mL), sodium benzoate (0.021 mL/100 mL), acetic acid (2.664 mL/100 mL), and water.

• Group III: Combination of 4.25% citric acid, 3% doxycycline, 9% propylene glycol, and water.

• Group IV: Control group-2.5% NaOCl and distilled water. Then, each root canal of all the experimental samples was irrigated with 2.5% NaOCl and distilled water.

For this procedure, 27-gauge needles were attached to a 5-ml syringe and were kept 1 mm short of working length. Irrigation time for every solution was 1 min. As control, three specimens were prepared in the same manner as the experimental groups, but only irrigated with 2.5% NaOCl and distilled water, not with acids or chelators. The presence of an adequate smear layer and uneroded dentine in the control group was confirmed before final irrigation.

Scanning electron microscopy examination

The roots were split into two halves, and each half was prepared for SEM examination. Scanning electron microscope photographs for each sample were taken from randomly selected areas of each third (coronal, middle, and apical) at a magnification of ×1000. These photographs were evaluated in a blind manner. The average of the scores for all three experimental groups and each section (coronal, middle, and apical) of the experimental groups was used for the statistical analysis.

The samples were scored against the following five-point scale described by AI Kilani et al. in 2003: Score (1)-clean root canal wall, only few small debris particles; Score (2)-light coverage of debris, <25% tubules covered; Score (3)-moderate coverage of debris, >25% but <50% tubules covered; Score (4)-heavy coverage of debris, >50% but <75% tubules covered; and Score (5)-complete or nearly complete root canal wall covered by debris. The results obtained were statistically analyzed using the Kruskal-Wallis test.

RESULTS

Data was tabulated and statistically analyzed for all three experimental groups and at different levels using the Kruskal-Wallis test.

Results of the study are summarized in Tables 1-4.

Table 1.

Statistical assessment of smear layer removal score by different experimental groups at the coronal site

Group		Coronal (mean)		Kruskal-Wallis test
				K (observed value)		K (critical value)		Degree of Freedom		alpha		P (two-tailed)
I		3.1		61.141		5.991		2		0.05		<0.0001
II		3.725
III		1.7

Table 2.

Statistical assessment of smear layer removal score by different experimental groups at middle sites

Group		Middle (mean)		Kruskal-Wallis test
				K (observed value)		K (critical value)		Degree of freedom		Alpha		P (two-tailed)
I		2.8		61.762		5.991		2		0.05		<0.0001
II		3.7
III		1.575

Table 3.

Statistical assessment of smear layer removal score by different experimental groups at the apical site

Group		Apical (mean)		Kruskal-Wallis test
				K (observed value)		K (critical value)		Degree of freedom		Alpha		P (two-tailed)
I		2.675		71.463		5.991		2		0.05		<0.0001
II		3.975
III		1.525

Table 4.

Comparison of the mean overall score of smear layer removal by different irrigating solutions at coronal, middle, and apical sites

Group		Mean (C + M +A)		Kruskal-Wallis test
				K (observed value)		K (critical value)		Degree of freedom		Alpha		P (two-tailed)
I		2.85		90.516		5.991		2		0.05		<0.0001
II		3.8
III		1.6

Table 1 shows the comparison of different experimental groups at the coronal site. P value for coronal is <0.0001. Mean of different experimental groups at coronal site: 3.1, 3.725, and 1.7, respectively, which shows that combination of 4.25% citric acid, 3% doxycycline, 9% propylene glycol, and water has best smear layer removal efficacy and combination of hydrogen peroxide (12.8 mL/100 mL), sodium benzoate (0.021 mL/100 mL), acetic acid (2.664 mL/100 mL), and water has least at coronal site.

Table 2 shows the comparison of different experimental groups at middle site. P value for middle is <0.0001. Mean of different experimental groups at middle site: 2.8, 3.7, and 1.575 respectively which shows that combination of 4.25% citric acid, 3% doxycycline, 9% propylene glycol, and water has best smear layer removal efficacy and combination of hydrogen peroxide (12.8 mL/100 mL), sodium benzoate (0.021 mL/100 mL), acetic acid (2.664 mL/100 mL), and water has least at middle site.

Table 3 shows the comparison of different experimental groups at the apical site. P value for apical is <0.0001. Mean of different experimental groups at apical site: 2.675, 3.975, and 1.525, respectively, which shows that combination of 4.25% citric acid, 3% doxycycline, 9% propylene glycol, and water has best smear layer removal efficacy and combination of hydrogen peroxide (12.8 mL/100 mL), sodium benzoate (0.021 mL/100 mL), acetic acid (2.664 mL/100 mL), and water has least at apical site.

Table 4 depicts that overall mean of different experimental groups for smear layer removal at coronal, middle, and apical sites is 2.85, 3.8, and 1.6, which signifies that combination of 4.25% citric acid, 3% doxycycline, 9% propylene glycol, and water has best smear layer removal efficacy and combination of hydrogen peroxide (12.8 mL/100 mL), sodium benzoate (0.021 mL/100 mL), acetic acid (2.664 mL/100 mL), and water has least at all the three sites.

DISCUSSION

In present study, Tables 1 and 2 show the statistical assessment of smear layer removal score by different experimental groups at coronal and middle site, respectively. Results depicted that group III (4.25% citric acid, 3% doxycycline, 9% propylene glycol and water) showed better results at coronal and middle third as compared to group I (17% EDTA followed by 3% NaOCl) and group II {hydrogen peroxide (12.8 mL/100 mL), sodium benzoate (0.021 mL/100 mL), acetic acid (2.664 mL/100 mL), and water}. Table 3 shows the comparison of smear layer removal by all the three groups at apical third with group III samples (4.25% citric acid, 3% doxycycline, 9% propylene glycol, and water), showing better removal of smear layer as compared to other two groups. This could be explained due to the inadequate penetration of 17% EDTA in the narrow portion of the canal. Takeda et al. in 1999, Torabinejad et al.[7] in 2003, reporting EDTA to be less effective due to its poor diffusibility in the narrow portions of the root canals. Whereas lower surface tension and increased diffusibility of MTAD were documented by Torabinejad et al.[7] who concluded that BioPure MTAD (citric acid based irrigant) was more effective in the apical third of the root canal to that of 5.25% NaOCl and 17% EDTA. Gupta et al.[8] concluded that MTAD (citric acid based irrigant) flush was the most effective debridement regimen in all the three thirds of the canal showing its ability to reach the apex with no conjugation and erosion of dentinal tubules.

Table 4 depicted better results of group I (17% EDTA followed by 3% NaOCl) samples as compared to group II {combination of hydrogen peroxide (12.8 mL/100 mL), sodium benzoate (0.021 mL/100 mL), acetic acid (2.664 mL/100 mL), and water} at all the three sites.

Effectiveness of Group I endodontic irrigant used in removing smear layer could be because of the chelating action of EDTA which causes demineralization of inorganic portion of the smear layer and root dentin surface along with exposure of collagen fibers. NaOCl dissolves organic tissue of the smear layer and exposed collagen fibers of the root dentin, leaving the entrances to the dentinal tubules more open. The dual-irrigation regime of NaOCl and EDTA has been used for removing the debris and smear layer, resulting in successful debridement and aided in enlarging narrow or obstructed root canals.[9] Goldman et al. (1982) and Yamada et al. (1983) found that the use of a high volume final flush with 17% EDTA followed by NaOCl effectively removed the smear layer.[10] Balaji et al.[11] found that the dual-irrigation regime of NaOCl and EDTA removes the debris and smear layer, resulting in successful debridement and aided in enlarging narrow or obstructed root canals. The overall significance of the present study laid in the observation that no single endodontic irrigant can disinfect the canal and able to completely debride the smear layer and necrotic debris from the entire root canal system. Combination of different solutions is necessary to achieve the desired outcomes. Group III (combination of 4.25% citric acid, 3% doxycycline, 9% propylene glycol, and water) performed significantly better in removing smear layer at coronal, middle, and apical site as compared to other combination of irrigating solutions, that is, Group I (17% EDTA followed by 3% NaOCl) and Group II, that is, combination of hydrogen peroxide (12.8 mL/100 mL), sodium benzoate (0.021 mL/100 mL), acetic acid (2.664 mL/100 mL), and water.

CONCLUSION

Being known to the complex anatomical structure of the root canal, it is important to use an appropriate combination of irrigating solutions while cleaning and shaping the root canal. This will enable us to flush out loose debris, lubricate the dentinal walls, and dissolve the organic matter in the canal. Within the limitations of the study, the following conclusions have been drawn.

All three combinations of irrigating solutions removed the smear layer from different levels of the root canal (coronal, middle, apical).

• The (Group III) combination of 4.25% citric acid, 3% doxycycline, 9% propylene glycol, and water showed effective smear layer removal from the coronal, middle, and apical thirds as compared to other irrigating solutions.

• Combination of hydrogen peroxide (12.8 mL/100 mL), sodium benzoate (0.021 mL/100 mL), acetic acid (2.664 mL/100 mL), and water (Group II) showed least smear layer removal at all the three sites as compared to Group I (17% EDTA followed by 3% NaOCl and Group III (combination of 4.25% citric acid, 3% doxycycline, 9% propylene glycol, and water).

Further research is needed to develop an appropriate combination of irrigating solutions that will give 100% results in cleaning the root canal system.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.