|Year : 2005 | Volume
| Issue : 3 | Page : 14-22
|Evaluation of resin-dentin interfacial structures and shear bond strength of three different bonding systems - an in-vitro study
Jikky John, S Kavitha, Lakshmi L Narayanan
Department of Conservative Dentistry and Endodontics, Saveetha Dental College & Hospitals, Chennai., India
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| Abstract|| |
This in vitro study was carried out to evaluate the quality of Resin - Dentin Interface and also to determine the shear bond strength of three different bonding systems. The systems used was Gluma Comfort Bond, Adhese and Prompt. L. Pop.
The Resin - Dentin Interface was evaluated under scanning electron microscope, which showed variation in hybridization and resin tag formation between the groups. The specimens were also subjected to shear bond strength in universal testing machine. The mean bond strength of Gluma Comfort Bond (Group 1) = 18.81 MPa, Adhese (Group 2) = 23 MPa and Prompt L Pop (Group 3) = 10.83 MPa. The results were statistically analyzed using Anova and Students-t- test. 11 was found that the bond strength value of group 2 were significantly higher than group 1 and group 3.
The present study concludes that it is the uniformity and density of the hybrid layer which is more important than the thickness for the integrity of the restorative - tooth interface.
|How to cite this article:|
John J, Kavitha S, Narayanan LL. Evaluation of resin-dentin interfacial structures and shear bond strength of three different bonding systems - an in-vitro study. J Conserv Dent 2005;8:14-22
|How to cite this URL:|
John J, Kavitha S, Narayanan LL. Evaluation of resin-dentin interfacial structures and shear bond strength of three different bonding systems - an in-vitro study. J Conserv Dent [serial online] 2005 [cited 2022 Aug 10];8:14-22. Available from: https://www.jcd.org.in/text.asp?2005/8/3/14/42587
| Introduction|| |
The increasing demand of restorative esthetic treatment due to the advent of Bonding agents and composite resin materials have created a paradigm shift in treatment philosophy.
The fundamental objective of the bonding agent is to create a union of two dissimilar surfaces, by facilitating penetration of monomers and to obtain a direct contact of resin with the collagen fibers, which resulted in a mixed zone of polymerized resin and entangled collagen fibrils the hybrid layer (Nakabayashi 1992) also known as the inter diffusion zone (Van Murleek et al 1992) ,
True hybridization and resin tag formation inside the tubule is mandatory for the integrity of the tooth-restorative interface. Marginal Discoloration, post operative sensitivity and recurrent caries are the most frequent consequences of oral fluid penetration along the cavity walls towards the pulp  .
Diffusions of resin monomers into the collagen network of their inter tubular dentin has been considered important for enhancing Bond strength. Proper infiltration and retention is required because a close adaptation is always challenged by polymerization shrinkage and shrinkage stress which may result in the formation of contraction gap at the resin - dentin interface. This challenge led to the evolution of various generations of Dentin bonding systems, with complex chemical formulas with the objective of promoting chemical adhesion. But each one had their own disadvantages like low bond strength, higher polymerization shrinkage, loss of hydrophilic group and technique sensitivity, which paved way for the fifth generation bonding systems. 
The 5 th generation bonding system follow different usage protocols. First one follows a two step adhesive system where acid etching precedes the application of bonding agents. The second one is also a two step adhesive system, where the first step involves conditioning and priming of enamel and dentin simultaneously without rinsing followed by the application of bonding agent. The third is an all in one adhesive, which treats both enamel and dentin in one application. Incomplete resin impregnation of collagen network and inadequate hybridization leads to decreased bond strength. The efficacy of bonding is usually evaluated by measuring the bond strength of composite resin to dentin.
The resin - dentin interface has been examined with a variety of methods namely Microradiographs, SEM, TEM, Confocal microscopes, Argon ion etching etc. One of the main advantages of SEM is that a very high magnification can be obtained even for a relatively small regions.
The objective of the present study was to qualitatively analyze the resin - dentin interface using scanning electron microscope (SEM) and evaluate the shear bond strength of three different bonding systems.
| Materials and Methods|| |
Fifteen freshly extracted premolars were selected for the study. They were cleaned ultrasonically and stored in saline. Occulsal reduction was done and dentin exposed using a diamond disk under copious irrigation. The dentinal surface was then smoothened using 320 grit silicon carbide paper. The teeth were randomly arranged in three groups of 5 each. The materials used for this study is given in [Table 1].
Group I specimens were treated with 35% phosphoric acid for 15 seconds, rinsed for 1 minute and blot dried. The adhesive was then applied and polymerized using Astralis light curing unit for 20 seconds.
In group 2 primer was brushed on to the surface of the dentin for 30 sec and the excess amount of primer was removed with a strong stream of air. Bonding agent was applied for 10 seconds and polymerized using Astralis light curing unit for 20 seconds.
In group 3 adhesive was rubbed on to the tooth surface using the micro brush for 15 seconds and light cured.
In all specimens, composite resin (Solitaire, Heraeus Kulzer) was placed incrementally on the tooth surface to a thickness of 2mm and then light cured for 40 seconds.
SPECIMEN PREPARATION FOR SEM EVALUATION
Vertical sectioning of the tooth was done mesiodistally using a diamond disk and smoothened using a 320 grit silicon carbide paper. The specimens were decalcified using 37% phosphoric acid for 60 seconds, and deprotenization done by immersing in 2% NaOCI for 120 seconds. The samples were rinsed with water and dried. They were then treated with ascending concentrations of ethanol (50%, 75%, 90%) for desiccation of the specimens. These specimens were mounted on stubs and evaluated under Scanning Electron Microscope.
SPECIMEN PREPARATION FOR SHEAR BOND TESTING
Thirty freshly extracted pre-molars were taken, ultrasonically cleaned and stored in saline. Occulusal reduction was done using diamond disk under copious water irrigation with slow speed hand piece. The teeth were randomly divided into three groups of 10 each.
GROUP -1 (Smear Layer Removing)
The SEM photographs showed [Figure 1],[Figure 2]
A cylinder was fabricated using cellophane matrix, (2mm in diameter and 4mm in length) and were bonded to the tooth surface using the respective bonding agents, according to the manufacturer instructions. Composite resin was packed in I mm increments and light cured for 40 sec, using Astralis curing unit. The teeth were embedded into acrylic resin blocks and then immersed in distilled water at 37°C for 24 hrs before testing.
During testing, the specimen was positioned horizontally in a Universal testing machine (Instron) and a shear force was applied at a cross head speed of 1 mm/minute, using a knife-edge shearing instrument. The specimens were continuously loaded until fracture occurred. The shear bond strength was calculated and recorded in MPa.
The data was tabulated and statistically analyzing using ANOVA and students t-test.
| Results|| |
The SEM photographs of the resin-dentin interface in the various groups are given in [Figure 1],[Figure 2],[Figure 3],[Figure 4],[Figure 5],[Figure 6] with respective sub headings.
- Non-continuous non-homogenous hybrid layer with a thickness of 5-6 p.m
- Resin impregnation inside the peritubular and intertubular dentin resulting in uniform resin tag formation with lateral branches.
- There was no evidence of smear layer
GROUP 2 (Smear Layer modifying)
The SEM photographs showed [Figure 3],[Figure 4]
- Continuous, homogenous hybrid layer without voids ranging in thickness between 4-5 µm.
- Uniform resin tag formations with lateral branching.
- Smear layer was infiltrated by adhesive and was seen scattered on the dentin surface.
GROUP -3 (Smear layer dissolving)
The SEM photographs showed [Figure 5],[Figure 6]
- Few short resin tags without lateral branches and tail-like extensions was seen penetrating the dentinal tubules.
- No evidence of smear layer.
The data and statistical analysis are summarized in [Table 2],[Table 3] and [Table 4].
[Table 2] shows shear bond values for the three bonding systems. The mean shear bond strength for Group I was 18.81 MPa, Group 1123 MPa and Group 111 10.85 MPa respectively.
[Table 3] shows the data of Analysis of variance (ANOVA) for shear bond strength of three different bonding systems. The results were analyzed and found to be statistically significant at 1 % level (P<0.01). Fishers test was done to evaluate the variation between the groups and within the groups. Parametric test was calculated between all the groups and the groups showed significant value at 5% level.
In [Table 4], Coefficient of variation was listed and Group 11 was found to be more consistent than Group I and Group III.
| Discussion|| |
The trend towards adhesive dentistry started in the mid 1960's with the advent of first commercial restorative resin composites, and in early 1970, with the introduction of the acid etch technique. 
Since then, there has been continuous progress in developing more refined and diversified restorative composites, along with steady improvement in bonding agents. Effective adhesion to enamel was achieved with relative ease and has repeatedly proven to be a durable and reliable clinical procedure.  Adhesion to dentin surface has always been a difficult and challenging task because of its heterogeneous structure.
Moist bonding technique was used because when the dentin is exposed to a dehydrated condition, it results in collapsed collagen fibers, which prevents formation of hybrid layer, whereas in over wet dentin the collagen network swells up increasing their diameter and length, reducing the width of peritubular spaces and in turn reducing the diffusion of monomers which interferes with the bonding of resin to dentin. ,
The samples were decalcified and deprotinized to demineralize and solubalize minerals with in the hybrid layer that was not protected by resin infiltration. This procedure exposes the resin tags to their full length and demarcates the bottom of the hybrid layer.
Gluma comfort bond (Group 1) is a fifth generation bonding agent were, etching was followed by the application of bonding agents. This group showed impregnation of the adhesive inside the peritubular dentin resulting in uniform resin tag information with lateral branches and non homogenous, less continuous hybrid layer with thickness varying from 5-6µm. The shear bond strength was 18.81MPa.
The reason behind non-homogenous hybrid layer can be due to the presence of glutaradehyde in adhesive, which could have reduced the dentin permeability by occluding the dentinal tubules.  Also studies have showed that placement of primers and adhesive together in single bottle, will interfere with the penetration of the primer at times and creates a weaker layer at the bottom of the hybrid layer and consequently the bonding is compromised. 
The absence of smear layer could be due to the separate etching procedure of dentin that has removed the mineral phase and increased the porosity of these tissues enormously.  The exposed collagen may provide reactive groups that can chemically interact with bonding primers. The amino groups may act catalytically to facilitate polymerization reaction and also the exposed collagen serves as a frame work for the creation of resin demineralized dentin-layer. 
Adhese (Group 2) is a self etching primer. Selfetching primers were introduced in 1992 to avoid acid etching and rinsing. These molecules penetrate through the smear layer into the underlying intact dentin to form the hybridized smear layer and hybridized dentin  . The primer is generally an aqueous solution, of 20wt Methacryloxy ethyl phenyl phosphoric acid (Phenyl-P) and 30wt % 2-Hydroxyethy methacrylate (HEMA). This phenyl-P / HEMA primer has a dual function. It demineralizes the dentin surface to remove smear layer and primes the dehydroxyapatite substrate to further improve monomer permeability. 
The hybrid layer formed was continuous, homogenous without voids ranging in thickness of 4-5µm. Uniform resin tag formations with lateral branches were observed. The shear bond strength was 23Mpa.
According to the manufacturer, the bis-acrylamide contains an amide and acrylic group. The amide group binds with the collagen and the acrylic with the monomers. The phosphoric acid in the self etching primer creates channels through the smear layer and aids in the penetration of the primer which coats the surface and binds the monomer. In group 2 the smear layer was seen scattered on the dentin surface, which was due to the relatively high pH.
Effective wetting and infiltration of monomers into the smear layer reinforce the bonding of the smear layer to the underlying dentin surface and formation of a micro mechanical or perhaps a chemical bond to the underlying dentin which can be the reasons for the highest bond strength of group 2. ,
Smear layer dissolving/All in one adhesive, Prompt L Pop (Group III) has the etchant, primer and the adhesive in one solution. The SEM did not reveal the hybrid layer formation, instead short few resin tags with tail-like projections were seen penetrating the dentinal tubules and there was no evidence of smear layer. This system contains a mixture of more acidic monoester, which could be the reason for low pH, which resulted in complete dissolution of smear layer. ,
Group III exhibited the least bond strength of 10.85 Mpa when compared to the other two groups which may be due to the mixture of primer and adhesive as a single component that compromised on the penetration of the adhesive.
The results of this qualitative and quantitative study do not represent a hybridization standard for the adhesive system used, since other factors are also involved namely, dentinal substrate (moist / dry bonding), solvents in the adhesive used (acetone, ethanol, water), depth of dentin (superficial / deep) and relative humidity of the environment.
| Conclusion|| |
To conclude, it is the quality (the uniformity and density) of the hybrid layer and not the thickness that matters for good bond strength which contributes to the integrity of the tooth restorative interface. The depth of penetration of the resin tags into the dentinal tubules may not play a major role in achieving good shear bond strengths.
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Department of Conservative Dentistry and Endodontics, Saveetha Dental College & Hospitals, Chennai.
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
[Table 1], [Table 2], [Table 3], [Table 4]
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