|Year : 2014 | Volume
| Issue : 5 | Page : 17-22
Evaluation of thickness of hybrid layer and length of resin tags of three adhesives to the root canal dentin: An in vitro scanning electron microscopic study
Sowmya Kallepalli1, Shreemoy Dash2, Sujatha Gopal1, N.B.P. Surya Kumari1
1 Department of Conservative Dentistry and Endodontics, MNR Dental College & Hospital, Sangareddy, Andhra Pradesh, India
2 Department of Conservative Dentistry and Endodontics, St. Joseph Dental College and Hospital, Eluru, Andhra Pradesh, India
|Date of Web Publication||10-Mar-2014|
105, Rose Quratz, My Home Jewel, Madinaguda, Hyderabad - 500 050, Andhra Pradesh
Source of Support: None, Conflict of Interest: None
Objectives: This present study was done to evaluate the resin-dentin interface of three adhesive systems used for bonding the root canal under scanning electron microscopic (SEM).
Materials and Methods: Sixty single-rooted human maxillary anterior teeth were decoronated at the cementoenamel junction apical preparation done up to 60 no. K-file and obturated with cold lateral compaction of gutta-percha and resin sealer (AH Plus). The specimens were then randomly divided into four groups of 15 teeth each. Group I-"Etch and rinse" adhesive system Group II-"Self-etch" adhesive system Group III-"Resin modified glass ionomer" adhesive system Group IV-"No adhesive system" used (control). All the adhesive systems were light cured then fiber posts were luted with dual cure resin cement and the specimens were prepared for SEM examination at magnifications ×1500 and ×5000 at coronal, middle and apical thirds of root canal.
Results: Statistical analysis was performed using one-way ANOVA followed by post-hoc Tukey's honestly significant difference test. Group I showed highly statistical significant difference when compared with other groups.
Conclusion: Maximum thickness of hybrid layer and long resin tags were shown by etch and rinse adhesives followed by self-etch adhesives.
Keywords: Etch and rinse adhesives, hybrid layer, resin tags, self-etch adhesives
|How to cite this article:|
Kallepalli S, Dash S, Gopal S, Kumari NS. Evaluation of thickness of hybrid layer and length of resin tags of three adhesives to the root canal dentin: An in vitro scanning electron microscopic study. J NTR Univ Health Sci 2014;3, Suppl S1:17-22
|How to cite this URL:|
Kallepalli S, Dash S, Gopal S, Kumari NS. Evaluation of thickness of hybrid layer and length of resin tags of three adhesives to the root canal dentin: An in vitro scanning electron microscopic study. J NTR Univ Health Sci [serial online] 2014 [cited 2019 Nov 19];3, Suppl S1:17-22. Available from: http://www.jdrntruhs.org/text.asp?2014/3/5/17/128485
| Introduction|| |
The advent of resin composite restorative materials has revolutionized the field of restorative dentistry. Resin composite material, with all its advantages, also poses a challenge to the clinician to find a perfect bonding system and technique.
The concept of hybridization of dentin is the benchmark of good bonding.  Hybridized dentin reduces the risk of microleakage, the incidence of secondary caries and post-operative sensitivity that can be caused by such leakage.
Endodontically treated teeth with an extensive loss of coronal tooth structure needs to be restored with a post and core, as a foundation for the final restoration. The trend toward the use of pre-fabricated fiber posts along with the adhesive resin cements reduces the risk of root fractures as compared to custom made post and core.  The most important characteristic of the fiber posts is its modulus of elasticity which is similar to that of dentin and resin cements. 
| Materials and Methods|| |
This present in vitro study was conducted to evaluate the resin-dentin interface of three adhesive systems used for bonding the root canal dentin under scanning electron microscopic (SEM).
Selection of samples
Sixty extracted maxillary anterior teeth with relatively straight roots which were extracted for periodontal reasons were used in this study. The teeth were decoronated at the cementoenamel junction (CEJ) so that the remaining root length was 14 mm.
Working length was determined by placing a size 15 no. K-file. Apical preparation done up to 60 no. K-file. Step back preparation was done up to 80 no K-file. Periodic recapitulation was done and 2 ml of 3% sodium hypochlorite was used as an irrigant after each file and a final rinse was then performed using 5 ml of 17% ethylenediaminetetraacetic acid for 3 min and were obturated with gutta-percha (Pearl Dent) and resin sealer, AH Plus (Dentsply). Then an intraoral periapical radiograph was taken.
The root canal walls of each sample were enlarged with the low speed reamers of sizes 3 or 4 depending on size and shape of the root. The depth of post space preparation was 9 mm.
The samples were randomly allocated to four groups of 15 samples each.
Group I-"Etch and rinse"-"One bottle" adhesive system: Etching with 35% phosphoric acid (Ivoclar Vivadent) for 15 s, washed and dried. The primer/adhesive was applied and light cured (LC) for 60 s.
Group II-"Self-etch" adhesive system: It was applied for 15 s and brushed for another 15 s, so that the total reaction time was not less than 30 s. The self-etching adhesive was applied and cured for 60 s.
Group III-"Glass ionomer" adhesive system: 20% poly acrylic acid conditioner was applied for 10 s, rinsed and dried. The powder and liquid were manipulated according to manufacturer's directions and applied in a thin layer. It was LC for 60 s.
Group IV-"No adhesive system used": Etching with 35% phosphoric acid (Ivoclar Vivadent) for 15 s, washed and dried. No adhesive was applied. This acts as a control group.
Fiber posts (Bioloren) were then cemented with dual cure resin cement (Calibra). After complete setting of the cement, samples were stored in water at room temperature.
After 1 week, the root samples were sectioned through the center of the long axis of the post using a diamond saw wheel at a slow speed under water. One segment from each split specimen was selected, based on how well the splitting was done on them. Three locating notches for standardized examination of the interfaces were made at 2 mm (coronal 3 rd ), 5 mm (middle 3 rd ) and 8 mm (apical 3 rd ) apically to the CEJ and were prepared for SEM examination. Specimens of all the groups were evaluated for uniformity and thickness of hybrid layer at the standardized magnification of ×1500 and extent of penetration (resin tag length) at a magnification of ×5000 [Figure 1] and [Figure 2] using etch and rinse, self-etch and glass ionomer based adhesive systems on root canal dentin at coronal, middle and apical third of the root canal.
The tabulated observations were then statistically analyzed using one-way ANOVA at a significant level of (P < 0.05) at each third of the root canal. Post-hoc Tukey's honestly significant difference test was also performed to compare the variation among the groups. The software and version used for analysis was SPSS 16 (Statistical Product and Service Solutions Inc., Chicago, USA).
| Observations and Results|| |
The mean and standard deviation were calculated for each group [Table 1] and [Table 2]. It was found that the Group I, Group II and Group III showed maximum hybrid layer thickness and long resin tags in the coronal third with statistically significant difference (P < 0.05) among the three groups. There was no hybrid layer and resin tag formation in Group IV acting as a control.
|Table 1: mean, SD and test of significance of mean hybrid layer thickness among different sites for different study groups|
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|Table 2: Mean, SD and test of significance of mean depth of penetration of resin tag amoung different sites for different study groups|
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Group I showed statistically significant difference when compared to Group II, Group III and Group IV [Table 3] and [Table 4].
|Table 3: Mean, SD and test of significance of mean hybrid layer thickness among different study groups for different sites|
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|Table 4: Mean, SD and test of significance of mean depth of penetration of resin tag among different study groups for different sites|
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| Discussion|| |
For an endodontically treated tooth, a post is mandatory when an inadequate amount of tooth structure is remaining to support the core material. The bonding of the post with the dentin reduces the concentration of stresses in the remaining root structure and distributes the forces more equally over the entire bonded interface. ,
The use of adhesive materials in combination with resin cements and fiber posts reduces the stress created by post-cementation. This led to an increased interest in studying about bonding of adhesive systems to root canal dentin. ,
Although a number of factors like interpenetration, micro mechanical interlocking and chemical bonding have been postulated to contribute for effective bond between resin and tooth structure,  hybrid layer and resin tag formation have been reported to play a major role in resin retention. ,,
The overall bond strength of resin composite to dentin can be regarded as a summation of the individual bond strengths provided by intra-tubular penetration (resin tag formation) and penetration into partially demineralized dentin (hybrid layer formation). 
Hybrid layer is the resin-infiltrated enamel, dentin or cementum. It is neither resin nor tooth but a hybrid of two. Resin tag is the extension of adhesive resin into open dentinal tubules. 
A new classification of adhesives was introduced at a symposium in Philadelphia in 2000, based on the number of working steps and treatment of smear layer. ,,
- Etch and rinse adhesives.
- Self-etch adhesives.
- Resin modified glass ionomer adhesives.
In etch and rinse technique, the biggest challenge for the hybrid layer and resin tag formation comes in the form of smear layer. It is necessary to acid etch the dentin. Acid etching removes the smear layer, opens the dentinal tubules, increases dentinal permeability and decalcifies the inter-tubular and peritubular dentin. 
The etched dentin should not to be dried before application of the bonding agent.  One should achieve a moist but not desiccated dentin a gentle air dry or blotting the surface. Here we need a system that can compete with moisture, replace it and take the monomer along with it and then evaporate. 
In order to overcome the phenomenon of over wetting and over drying Nakabayashi and Pashley  hypothesized that if the bonding step with acidic conditioner to remove the smear layer was eliminated, then the bonding procedure would become simpler.
This led to the development of self-etching/self-priming system, where the monomer could penetrate into underlying intact dentin to form both hybridized smear layer and hybridized dentin. 
Glass ionomer cements, with their advantage of chemical adhesion, were modified by the incorporation of resin component to act as a glass ionomer adhesive for composite (Fuji Bond LC).  Although the predominant mode of bonding of resin based glass ionomer adhesive with dentin is of chemical adhesion, the presence of resin renders it possible for the hybridization to occur.
Hence, in the present study, it has been included to evaluate its bonding ability by observing the hybrid layer and resin tag formation.
In this study, Group I showed the maximum values for both hybrid layer thickness and resin tags length when compared to Group II, Group III Group IV, which were statistically significant.
Possible reasons could be that in case of Group I, the specimens were etched with phosphoric acid and the reaction products were rinsed off. This results in complete removal of smear layer and smear plugs from the dentin, so that the dentin permeability increases and there is better infiltration of resin and hence aids in the better penetration of the resin tags and also thick hybrid layer. 
The hybrid layer of Group II was thinner when compared to Group I as self-etching adhesive materials eliminate the need of rinsing the tooth structure, the by-products of dentin yielded by the low pH of the adhesive system may lead to a limited demineralizing action, restricting penetration of the adhesive system to the most superficial dentin layers.  Moreover, the mineral components from the smear layer may neutralize the acidity of these self-etching systems. 
Another reason could be that etch and rinse adhesives are applied using a Wet Bonding technique in which water is retained in collagen fibrils and acetone was able to diffuse and dislocate water from the moist collagen network and thus promoting the infiltration of resin monomers through the nano spaces of the collagen web and resulting in a dense hybrid layer and also enhancing the bond strength. 
Although current approach for bonding appears to depend on its effective penetration of partially demineralized dentin (hybrid layer formation) and intra-tubular dentin (resin tag formation), chemical interaction between the bonding agent and the substrate have also been reported as one of the contributing factor. 
Compared with coronal and middle third, both the groups showed poor performance in the apical third. The probable reasons could be the inefficiency of light transmitting post on curing of the adhesive, , lack of aggressiveness/pressure during the application of adhesive due to the narrowness of the canal in the apical third  and also, failure to rinse off the conditioner resulted in an incomplete resin penetration of the demineralized dentin and minimal hybrid layer formation. 
Fuji Bond LC is a diluted version of the restorative resin-modified glass ionomer cement.  Glass ionomer adhesives have been documented to produce a hybrid layer with a thickness of about 0.5-1 μm.  And this was attributed to the partial demineralization through the use of 20% polyacrylic acid conditioner, which removes the smear layer, without removing the smear plugs.  It was demonstrated that the micromechanical interlocking obtained with glass ionomer based adhesive is similar to mild self-etch approach, where demineralization can occur only to a depth of 1 μm. ,
Compared to coronal third and middle third, Fuji bond LC showed poor performance in the apical third. The inadequate curing of adhesive could have led to poor performance of Fuji bond LC in the apical third. 
Fuji bond LC showed no resin tag formation in the coronal, middle and apical third, where only resin plugs are visible which was due to the ability of the polyacrylic acid conditioner to completely remove the smear plugs. 
Group IV that is the control group where only etching is done with no Adhesive system applied resulted in no hybrid layer formation and resin tag formation in the coronal, middle and apical thirds of the root canal. The resin cement undergoes maximum polymerization shrinkage in the root canals during light curing that result in de-bonding of the resin cement from the dentin walls thus creating interfacial gaps. ,
| Conclusion|| |
Within the limitations of our study, etch and rinse adhesives performed better than self-etch and resin based glass ionomer based adhesives, but more studies are needed to evaluate the clinical performance of self-etching and glass ionomer adhesives.
| Acknowledgments|| |
We would like to thank Defense Metallurgical Research Laboratory, for providing me permission and technical guidance in performing the study.
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[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3], [Table 4]