|Year : 2010 | Volume
| Issue : 4 | Page : 271-277
Efficacy of various root canal irrigants on removal of smear layer in the primary root canals after hand instrumentation: A scanning electron microscopy study
VS Hariharan1, B Nandlal2, KT Srilatha3
1 Senior Lecturer, Department of Pedodontics & Preventive Dentistry, Sree Balaji Dental College & Hospital, Chennai, India
2 Professor & HOD, Department of Pedodontics & Preventive Dentistry, JSS Dental College & Hospital, JSS University, Mysore, India
3 Associate Professor, Department of Pedodontics & Preventive Dentistry, JSS Dental College & Hospital, JSS University, Mysore, India
|Date of Web Publication||25-Jan-2011|
V S Hariharan
Sree Balaji Dental College and Hospital, Chennai
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Aim: The purpose of this in-vitro study is to determine the efficacy of various irrigants in removing the smear layer in primary teeth root canals after hand instrumentation. Materials and Methods: The present study consisted of 30 human primary incisors which were sectioned at the cementoenamel junction horizontally. The specimens were divided randomly into four experimental and one control group having six teeth each and each group was treated with the specific irrigant. 5.25% NaOCl, 5.25% NaOCl + 10% EDTA, 6% citric acid, 2% chlorhexidine, saline (control) were the irrigants evaluated for efficacy in removal of smear layer. The specimens were split along the longitudinal axis using a chisel after placing superficial grooves in cementum not extending to the root canal. The exposed surface was subjected to scanning electron microscopic analysis to reveal the efficacy of irrigants in removal of smear layer. The representative areas were evaluated twice at 15 days interval by a single evaluator. The scale for the smear layer removal by Rome et al was modified and used in the present study. Results: The pictures from the scanning electron microscopy showed that among the tested irrigants, citric acid has the best efficacy to remove the smear layer without altering the normal dentinal structures, which was supported by the lowest mean smear scores. The pictures from the 10%EDTA + 5.25% sodium hypochlorite group showed that even though it removed the smear layer, it adversely affected the dentine structure. SEM pictures of the other groups like sodium hypochlorite, chlorhexidine revealed that these irrigants does not have the capacity to remove the smear layer in primary teeth. Conclusions: The results of the present study clearly indicate the superior efficacy of 6% citric acid than the other tested irrigants on removing the smear layer in primary teeth root canals.
Keywords: Citric acid, EDTA, irrigation, smear layer, primary teeth
|How to cite this article:|
Hariharan V S, Nandlal B, Srilatha K T. Efficacy of various root canal irrigants on removal of smear layer in the primary root canals after hand instrumentation: A scanning electron microscopy study. J Indian Soc Pedod Prev Dent 2010;28:271-7
|How to cite this URL:|
Hariharan V S, Nandlal B, Srilatha K T. Efficacy of various root canal irrigants on removal of smear layer in the primary root canals after hand instrumentation: A scanning electron microscopy study. J Indian Soc Pedod Prev Dent [serial online] 2010 [cited 2019 Dec 13];28:271-7. Available from: http://www.jisppd.com/text.asp?2010/28/4/271/76157
| Introduction|| |
Preserving the integrity of primary dentition is the most important aspect of preventive dentistry. It is vital to retain the primary dentition until its normal exfoliation, as it is essential for the growth of facial-skeletal complex. One of the common reasons for the premature loss of primary teeth is dental caries. An alternative to avoid such loss would be an endodontic treatment, in which the main goal is to preserve the teeth in the same position.  A clean root canal system along with a three-dimensional seal is the clinician's path to success. The contents of root canal system are removed during the biomechanical preparation. Throughout the instrumentation, irrigation is unavoidable. It is a simple procedure, by which the loose, necrotic, contaminated materials are flushed away. If not, they are inadvertently pushed deep in to the canal, compromising the periapical tissue and permanent bud. Besides, minimizing the extrusion of pulpal and dentin residues through the apical foramen, irrigants disinfect the root canals and dentinal tubules with its antimicrobial properties.  Hence, the action of irrigants is essential.
In endodontic canal preparation, smear layer directly results from instrumentation. As the tooth structure is cut, instead of being uniformly sheared, the mineralized matrix shatters and resulting in smear layer. The scattered smear layer is found all over the dentinal surface after the instrumentation. Although there is some controversy regarding the desirability of retaining the smear layer in adhesive dentistry, in endodontics, its removal is considered to be advantageous and highly desirable. 
Hobson confirmed that three-forth of the radicular dentin of necrotic primary teeth is infected. This fact confirms the importance of instrumentation and irrigation in endodontic to eliminate the root canal infection, to increase the root dentin permeability and to maintain the asepsis of the canals.  Keeping this in mind, the present study is aimed to assess the capacity of various irrigants in removing the smear layer in primary teeth root canals after hand instrumentation.
| Materials and Methods|| |
The present study is aimed to reveal the efficacy on smear layer removal of the following root canal irrigants after hand instrumentation in primary tooth root canals.
- 5.25% Sodium hypochlorite.
- 10% EDTA.+ 5.25% Sodium hypochlorite.
- 6% Citric acid.
- 2% Chlorhexidine digluconate.
Thirty primary anterior teeth were collected from various hospitals in and around Mysore for various other reasons like trauma, over retained teeth. Primary incisors with at least two third of root intact were included in the study. Radiographs were taken in Department of Pedodontics and Preventive dentistry, JSS Dental College and Hospital to rule out abnormal pulpal pathology, gross deviation from the normal anatomy, internal resorption, obstruction within the canal system.
Selected teeth were stored in 0.9% saline at temperature 4˚C till further use in the refrigerator in Department of Pedodontics and Preventive dentistry, JSS Dental College and Hospital. Superficial grooves were put mesiodistally along the longitudinal axis in cementum not extending to the root canal using diamond disk to facilitate smooth split in the latter stages. This was done under constant irrigation with saline to avoid dehydration and heat generation.
A no.10 K-file was placed in the canal until it was just visible at the apical foramen. The working length was established by reducing 1 mm from this point. Instrumentation was done with separate set of 'K' files for each group. In the present study, for irrigation, 25 gauge needles were used and the depth of the irrigation needle was calculated by reducing 2 mm from the working length. All these procedures were done in constant atmospheric temperature to avoid the variation due to temperature and in sterile condition. Chemo mechanical preparation was done according to the conventional step back preparation using K files. The canals were enlarged with K files till 45 sizes. During each instrumentation, canal was irrigated with 3 ml saline. Following this, the samples were divided randomly into five groups as follows:
- Saline group
- Sodium hypochlorite group
- Sodium hypochlorite +EDTA group
- Citric acid group
- Chlorhexidine group
During each instrumentation, irrigation was done with saline. Canals were then irrigated with 10 ml of 0.9% saline as a final irrigation.
Sodium hypochlorite group
During each instrumentation, irrigation was done with saline. Canals were then irrigated with 10 ml 5.25% sodium hypochlorite as a final irrigation.
Sodium hypochlorite + EDTA group
During each instrumentation, irrigation was done with saline. After instrumentation, irrigation was done with 10 ml 5.25% sodium hypochlorite then 10 ml of 10% EDTA for 2 min and followed by sodium hypochlorite as final irrigation.
Citric acid group
Canals were irrigated with saline during each instrumentation. Final irrigation was done with 10 ml of 6% citric acid for 1 min.
During instrumentation, canals were irrigated with saline and final irrigation was done with 10 ml of 2% chlorhexidine gluconate.
All the samples were finally irrigated with 10 ml of distilled water to avoid sedimentation of crystals within the canals.
Splitting of samples
The specimens were split along the longitudinal axis using a chisel through the previously placed grooves. One half of sample was used. The other half was discarded. Then the samples were transferred to the testing lab in a sterilized plastic container without any contamination.
Scanning electron microscope
The exposed surfaces were subjected to scanning electron microscopic analysis to reveal the efficacy of smear layer removal. The surfaces were sputter coated with gold-palladium particles of 35 nm size. Then the surfaces were evaluated under SEM. The sample was divided into cervical, middle, apical. Representative photographs from each root third were taken for all the samples. The samples were assessed blindly by the single examiner, between 15 days interval to avoid the visual memory. The scores were given to the images according to the scoring criteria which is a modification of scoring method of Rome et al.
0 - No smear layer, all dentinal tubules open and no erosion of tubules.
1 - No smear layer, all dentinal tubules open and erosion of tubules.
2 - Minimum smear layer > 50% dentinal tubules visible.
3 - Moderate smear layer; < 50% of dentinal tubules open
4 - Heavy smear layer; outline of dentinal tubules obliterated.
Scores given by the single examiner were evaluated with Kruskal-Wallis H test for intra-examiner variability. There was no significant difference (P<0.05) between the scores at each root third (cervical, middle, apical) for all groups. So it was decided to use the scores obtained during first time for the further studies.
Then the statistical evaluation was done [Graph 1].[Additional file 1]
| Results|| |
The pictures from the scanning electron microscopy showed that among the tested irrigants, citric acid had the best efficacy to remove the smear layer without altering the normal dentinal structures which was supported by the lowest mean smear scores (P>0.001) than the other tested irrigants. The pictures from the 10% EDTA + 5.25% sodium hypochlorite group showed that even though it removed the smear layer, it adversely affected the dentine structure by means of conjugation of dentinal tubules, erosion of peritubular dentin, and break down in the intertubular dentin. SEM pictures of the other groups like sodium hypochlorite, chlorhexidine revealed that these irrigants do not have the capacity to remove the smear layer in primary teeth.
The intragroup comparison of efficacy of root canal irrigants within the cervical, middle, apical thirds showed that there was no significant difference (P>0.05), even though apical third was less than the other root thirds.
| Discussion|| |
Smear layer is a microscopic layer of residues coming from the scoured dentin due to the use of rotary instruments and endodontic files during endodontic treatment. Under the scanning electron microscopy (SEM), the smear layer is viewed as a uniform, dense layer of an amorphous structure that completely obliterates the entrance to the dentin tubules and drastically reduces the permeability of the dentin. 
McComb and Smith were the first to report the presence of smear layer on permanent root canal walls. The smear layer is an amorphous structure composed of an organic portion, that is coagulated proteins, necrotic and normal pulpal tissue, saliva, microorganisms, etc and an inorganic portion consisting minerals from the dentinal structure. 
Need for removal of smear layer
To enhance the antibacterial action of the irrigants
Authors have concluded that Streptococci multiplied and invaded in the radicular dentinal tubules deep inside (up to 792 μm)  and the extent of bacterial invasion was time-dependent.  In a similar study, Streptococcus sanguis (S. sanguis), Actinomyces naeslundii, Prevotella intermedia and S. sanguis were observed at a depth of 792 μm inside the dentinal tubules.  Smear layer will plug the orifice of the dentinal tubules, thus preventing medicaments and filling materials from directly contacting the dentin.  If the smear layer is removed, root canal irrigants can enter the dentinal tubules, travel up to the distance where microorganisms are present and eliminate them. Studies have shown that sodium hypochlorite can not destroy bacteria within tubules closed by a smear layer covering. 
To reduce the microleakage
In order to create a fluid tight seal, it is imperative that the endodontic filling material closely adapts or bonds to the tooth structure. However, it is impaired by the presence of endodontic smear layer, which invariably forms after instrumentation.  Endodontic sealers act as a glue to ensure a good adaptation of root canal filling material to the canal walls. If the smear layer is not removed, the filling material may occasionally be glued to the dentin in the smear layer as well as to exposed parts of the canal walls. Not being firmly attached to the dentin, the smear layer may laminate off the canal wall and create a false seal, thus forming voids in the filling and eventually leading to expected environment for the microleakage. Studies have suggested that the removal of smear layer significantly improves the tightness of the sealing in the coronal area.  Kennedy stated that an absence of smear layer will reduce the apical microleakage of the root canal treated canals than the canals with the smear layer intact. 
For better adaptation of paste to the dentinal walls
The root canal system of the primary teeth should be cleaned, decontaminated, shaped, and enlarged, since the canal has to be filled with the nonsetting pastes. These pastes should penetrate the tubules in order to minimize bacterial contamination and not allow reinfection of the root canal system. Findings suggest that smear layer plays an important role in sealer penetration into the dentinal tubules, as well as in the potential clinical implications.  There are more number of studies explaining the smear layer removal in permanent root canals and conclusive guidelines are there for the smear layer removal in permanent teeth. But we found less number of studies available regarding smear layer removal of primary teeth. Thus the present study intended to find out efficacy of the various root canal irrigants on removal of smear layer in root canals of primary teeth.
Scanning electron microscopic pictures of saline group revealed the presence of extensive sludge layer made up of residual debris organic debris and smear layer. In electron microscopic view (×2000), the presence of heavy smear layer was clearly noticed at all three root thirds, invariably in all samples [Table 1],[Figure 1].
|Table 1 :Comparison of efficacy on smear layer removal in primary root canals by various irrigants |
Click here to view
Results of the present study indicated that the saline group does not have influence on the removal of smear layer which has proved by the highest score. This is supported by the earlier studies by McComb et al,  Baumgartner et al,  in permanent teeth and Salama et al,  in primary teeth.
Scanning electron microscopic pictures of sodium hypochlorite group demonstrated the absence of superficial debris but presence of smear layer. Under high magnification (×2000), the presence of smear layer was noticed at all three root thirds (cervical, middle, apical) [Table 1],[Figure 2]. Even though some samples showed the removal of smear layer partially, it was clearly understood that the ability to remove the smear layer was incomplete.
The scores of the sodium hypochlorite group proved that its efficacy was clearly better than that of the saline group. But still smear layer removal was incomplete. According to various authors like Mc Comb et al,  Baumgartner et al,  sodium hypochlorite did not have the efficacy to remove the smear layer in permanent teeth. But the present study is done in the primary teeth root canals. Similar study which was conducted in primary teeth by Salama et al showed that 5.25% sodium hypochlorite had the efficacy in removal of smear layer.  It can be concluded that even though sodium hypochlorite partially removes the smear layer, it is incomplete and not acceptable.
In EDTA + sodium hypochlorite
In EDTA + sodium hypochlorite group, the SEM pictures revealed that there was no smear layer in all three root third levels (cervical, middle, and apical) almost in all samples except in apical region of two samples. Even though EDTA + sodium hypochlorite group showed better results in smear layer removal, damage to the dentinal tubules was found [Table 1],[Figure 3]. Under the electron microscope view, it was undoubtedly noticed that there was conjugation of dentinal tubules, erosion of peritubular dentin, and break down in the intertubular dentin almost in all three root thirds (cervical, middle, apical).
The similar type of damage was earlier reported in permanent teeth studies, but with 17% EDTA.  The work by Marshall showed that there was a substantial difference in the microstructure of primary dentin as compared to permanent dentin, substantial differences with location, and the relatively common occurrence of microcanals.  This could have been the reason for the occurrence of erosion even in the low concentration of EDTA in our study.
The erosion does consider being the most harmful for the use in primary teeth, whose root canal dentin thickness is diminished, which makes it damage-prone. This can be added to the fact that the potential effects of the damage to the dentin structure are not totally known.
Scanning electron microscopic pictures of citric acid group showed the absence of superficial debris, smear layer in all three root thirds (cervical, middle, apical). Findings were similar for all the samples except apical region of one sample.
Under high magnification (×2000), opening of the dentinal tubules was evidently noticed. There were no signs of erosion of dentinal structures. In the citric acid group, least scores were obtained which meant there was almost complete removal of the smear layer. Citric acid at 6.0% may be considered as an option for smear layer removal in primary teeth, since it demonstrated better results than EDTA + sodium hypochlorite group in the present study [Table 1],[Figure 4].
On comparing with all other tested irrigants, there was highly significant difference (P<0.001) found, in which citric acid had better efficiency on smear layer removal than others.
In chlorhexidine group, the smear layer removal was least among the tested irrigants which is comparable to the control group (saline). Scanning electron microscopic pictures of chlorhexidine showed the presence of heavy smear layer similar to saline group. Studies on permanent teeth by Yamashita J C et al,  also showed the similar results [Table 1],[Figure 5].
| Conclusion|| |
- The present study is done to evaluate the efficacy of various root canal irrigants on removal of smear layer in primary teeth root canals using scanning electron microscopy after hand instrumentation. It was found that 6% citric acid has the best smear layer removal efficacy without affecting the normal structure of dentinal tubules in primary root canals.
- 10% EDTA + 5% sodium hypochlorite combination causes unwanted damage to the dentinal tubules, though it removes the smear layer.
- 5.25% sodium hypochlorite, saline and 2% chlorhexidine do not have the potential to remove the smear layer and among these sodium hypochlorite is comparatively better than the other two.
- Based on the results, the final suggestions drawn for the primary teeth root canal irrigation are as follows:- During instrumentation, the canals can be irrigated with saline. The final irrigation should be done with 6% citric acid to remove the smear layer followed by 2% chlorhexidine digluconate to potentiate the antimicrobial action and substantivity.
- Still further evaluation should be done in the areas like biocompatibility and antimicrobial action before finalizing the above suggestions.
| Acknowledgement|| |
To Dr. SVSG Nirmala MDS, Former Professor, JSS Dental College, Mysore, Mr.Valluru Ravi, Department of Pharmaceutics, JSS College of Pharmacy College, Mysore, for their contributions in this study.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
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