|Year : 2005 | Volume
| Issue : 2 | Page : 24-30
|Comparative shear bond strength analysis of strong, intermediary strong & mild 'self etch' systems, and a hypothesis on their chemical behavior on dentin
Narasimha Bharadwaj1, KS Karthikeyhan2, VG Sukumaran1
1 Department of Conservative Dentistry & Endodontics, Sree Balaji Dental College & Hospitals, Pallikkaranai, Chennai - 601 302, India
2 Department of Conservative Dentistry & Endodontics, Meenakshi Animal Dental College & Hospitals, Alapakkam Main Road, Maduravoyal, Chennai - 600 095, India
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| Abstract|| |
The aim of the study was to evaluate the shear bond strengths of composite to dentin using strong, intermediary strong & mild 'self-etch' dentin adhesives and also identify the most probable chemistry & mechanism involved in the action of self-etching systems. The adhesives tested were AdheSE (1voclasr), i Bond (H. Kulzer). Xeno III (Dentsplv). Clearfil SE Bond (Kuraray) and Prompt-L-Pop (3M). Fifty freshly extracted noncarious, unrestored human premolars were taken & the occlusal surfaces of all teeth were ground to expose a flat surface of dentin. After polishing the dentin surface. each tooth was mounted in acrylic resin blocks of uniform dimension. The bonding area on the dentin surface was delineated by a circular outline into which the adhesives were applied following manufacturers directions. Cylinders of resin composite (Z-100) were built on the bonded dentin surfaces in two increments. Following storage in distilled water at 37"C for 24 hours, the specimens were tested for shear bond strength using an Instron Universla Testing Machine at a cross-head speed of 0.5mm / min. The results suggest that 'intermediary strong' selfetch systems demonstrated superior bond strength values compared to the rest. 'Mild' adhesives exhibited values, which were higher than 'strong' but lower compared to 'intermediary strong' systems. The 'strong' self-etch, bonding agents demonstrated the lowest bond strength values compared to all test groups.
|How to cite this article:|
Bharadwaj N, Karthikeyhan K S, Sukumaran V G. Comparative shear bond strength analysis of strong, intermediary strong & mild 'self etch' systems, and a hypothesis on their chemical behavior on dentin. J Conserv Dent 2005;8:24-30
|How to cite this URL:|
Bharadwaj N, Karthikeyhan K S, Sukumaran V G. Comparative shear bond strength analysis of strong, intermediary strong & mild 'self etch' systems, and a hypothesis on their chemical behavior on dentin. J Conserv Dent [serial online] 2005 [cited 2020 Jan 21];8:24-30. Available from: http://www.jcd.org.in/text.asp?2005/8/2/24/42601
| Introduction|| |
The current challenge in adhesive dentistry is to develop dentin adhesives that will achieve dentin bond strengths similar to those obtained between resins and asid-etched enamel  . In 1963, Buonocore  demonstrated the difference in bonding mechanisms between enamel and dentin. He stated that the formation of resin tags was the principal adhesive mechanism by which resins bonded to enamel. The idea that resin tags was the principal adhesive mechanism by which resins bonded to enamel. The idea that resin penetrates the microporosities of etched enamel resulting in a micromechanical bond is well accepted today.
As far as dentin adhesion is concerned, early attempts to bond to dentin resulted in poor bond strengths. This is not surprising given the fact that while enamel contains 4% organic content, dentin comprises 30 - 35% of organic components  . Moreover, originally it was believed that dentinal tubules are the only pores available for micromechonical retention, although these tubules contain fluid, which would be an impediment to bonding. Also, the number of tubules available for bonding varies depending on the location, with deep dentin having more tubules than superficial dentin  . Predictably, the tubular structure of dentin, high percent of organic content, intrinsic wetness and presence of smear layer coupled with other factors like type of dentin, age of teeth and direction of tubules renders dentin as a perplex substrate to bond with ,
Despite these limitations, the bonding mechanism to dentin became effective and predictable when the smear layer was removed, inter-tubular and peri-tubular dentin were dissolved, collagen fibers exposed and, after infiltration of resin monomers, a hybrid layer was formed  . This bonding mechanism became evident following the fourth generation of dentin adhesives  . Eventually, bond strength values approaching or exceeding 20 MPa have been reported in teh Fifth generation bonding agents.
In an effort to reduce the number of steps involved in bonding procedures yet retain the effectiveness of dentin adhesives a self-etch approach was identified. In these systems the procedures etching, rinsing and bonding were reduced to etching and bonding thereby eliminating one step. This 'No rinse technique' not only eliminates operator variables but also lessens clinical operating time, which is marketed as either 'single-step' or 'two-step' Self-etch dentin adhesives. Evidently, self-etching systems represent a logical step in the evolution of contemporary dentin bonding agents.
The purpose of this study was to evaluate the shear bond strengths of composite to dentin using strong, intermediary strong & mild self-etch dentin adhesives and also to identify the probable chemistry and mechanism involved in the action of self-etching systems. The systems tested include Prompt-L-Pop (3M ESPE). AdheSE (Ivoclar Vivadent), Xeno III (Dentsply), i Bond (Heraeus Kulzer) and Clearfil SE Bond (Kuraray).
| Materials and Methods|| |
Fifty non-carious unrestored human premolars, freshly extracted for orthodontic purpose were used for the study; Adequate care was taken to prevent any fracture of teeth during extraction procedures. Following extraction, the collection, storage and handling of teeth was done as per the recommendations of Occupational Safety and Health Administration (OSHA) and the Center for Disease Control and Prevention (CDC). The surfaces of all the teeth were examined under 3M Light curing visible light, and teeth with cracks and structural defects were eliminated.
The occlusal surfaces of all the teeth were ground using water-cooled slow speed diamond disc, so as to expose a flat surface of dentin with an approximate residual dentin thickness of 1.5 - 2 mm. The dentin surface was polished using 320 - 600 grit silicon carbide paper. Each tooth was mounted in 1 inch thick and 2.5 cm height blocks made of self-cure acrylic resin, such that the occlusal surfasce of the tooth is parallel to the bottom surface of the block. The bonding area on the dentin surface was delineated with a wheel diamond by marking a thin circular outline of diameter 3.5 mm into which the adhesives tested, were applied. Specimens were randomly assigned to five groups (n=10).
Group I : AdheSE (Ivoclar vivadent)
Group II : i-Bond (Hereaus Kulzer)
Group III : Xeno III (Dentsply)
Group IV : Clearfil SE Bond (Kuraray)
Group V : Prompt-L-Pop (3M ESPE)
Adhesives selected for the respective groups were applied on the prepared dentin surfaces as per the manufacturer's directions. Thereafter, cylinders of resin composite, Z 100 (3M), were built upl on the bonded dentin surfaces by following the manufacturer's instructions, using a hollow plastic matrix of diameter 3mm and height 3mm (2 increments of 1.5mm each) in all the specimens . Both increments of resin composite were individually light cured for 40 seconds with a visible light-curign unig (3M), whose light intensity was monitored with a dental radiometer. After light curing the plastic matrix was removed.
All the specimens were stored in distilled water at 37 o C for 24 hours prior to testing. The specimens were tested for shear bond strength using a Universal testing machine (Model 4411, Instron). A chisel shaped rod, traveling at a crosshead speed of 0.5 mm / min. was used to deliver the shearing force in a direction parallel to the adhesive interface between the resin composite and dentin until fracture occurred. The Shear Bond Strength values were recorded in units of Megapascal (MPa).
| Results|| |
The Shear bond strength values obtained (in MPa) for the Five test groups evaluated are given in [Table 1].
The Mean, Standard Deviation (SD) and Test of significance of values obtained between different study groups are listed in [Table 2].
One-way ANOVA was used to calculate the p - value and Multiple Range Test by Tukey - HSD procedure was employed to identify the significant groups at 5% level. The mean shear bond strength values in Group 1 (14.69 ± 0.93) and Group III (14.55 ± 1.00) are significantly higher than the mean shear bond strengths of Group 11 (11.13 ± 0.64), Group IV (12.39 +0.71) and Group V(10.80 +0.98). Also, the mean shear bond strength of Group IV (12.39 ± 0.71) is significantly higher than the mean shear bond strengths of Group 11 (11. 13 ± 0.64) and Group V (10.80 ± 0.98) ; Graph 1.
Although many theories & hypothesis have been put forth as regards to the action of self-etch adhesives, the exact chemistry & mechanism of action of these systems is still obscure. As self-etch systems eliminate the step of rinsing, there were several queries as to the fate of residual acid in dentin, the termination of the etching action, the depth of demineralization & the mechanism which self-limits its action. Most of the currently available literature establishes dentin as an excellent buffer to the residual acid, however studies are still inconclusive. The rationale behind the use of phosphate esters in the composition of these sytems by the manufacturer has offered us a clue to identify the most logical explanation of their behavior in dentin.
An ester is nothing but a combination of acid and an alcohol. According to fundamentals of organic chemistry  ."An ester is an inert or neutral compound which is susceptible to hydrolysis upon contact with an acidic or alkaline medium resulting in the free release of its constituent acid". All of our self-etch systems supplied contain a phosphate ester and Resin in its composition.
The Chemistry behind the evolution of self-etch systems could probably be the use of organic esters in conjunction with resins (Phosphate ester and methacrylate resin), which upon hydrolysis yields highly ionizing phosphoric acid (Ka = 7.5 x 10-3)5. The free phosphoric acid released is thought to bring about the etching action. Subsequent polymerization of the methacrylate resin results in solidification of the material that does not support further chemical reactions to occur, as on polymerization the reacting components enter into a solid phase. This solid phase transformation may be construed as one of the reasons that may 'self-limit' the action of these adhesives.
The sequence of events is illustrated as below:
Self-etch Adhesive contains -4 Phosphate Ester & Methacrylate Resin Adhesive (Phosphate ester & methacrylate resin) + Tooth Structure -> Phosphoric acid + Polyacrlyic acid
(As the dissociation constant of phosphoric acid (Ka = 7.5 x 10 -3 ) is higher than polyacrlyic acid (8.9 x 10 -5 ), the release of phosphoric acid suppresses the release of polyacrylic acid)
Phosphoric acid released -) Etching action of the adhesive. Subsequent polymerization (Light curing the adhesive) -+ Solidification of the adhesive ester.
[According to fundamentals of organic chemistry. "when a material solidifies it does not support further chemical reactions to proceed, as on solidification by polymerization the reacting components enter into a solid phase"].
Solidification of the adhesive - Self-limiting action (Stop the etching process)
| Discussion|| |
Van Meerbeek  classified the self-etch adhesives based on the number of components by which they are supplied, the number of steps of application and the pH of the adhesive. Accordingly, the five self-etching adhesive systems selected for the study were categorized based on each criteria as follows:-
Prompt-L-Pop recorded significantly low shear bond strength values (mean = 10.80 ± 0.98) compared to all other test groups excepting i Bond with which there was no statistical variation. According to Van Meerheek  , `strong' self-etch adhesives (Prompt-L-Pop) usually have a pH of 1 or below. Acids that are too `strong' may raise a few concerns as regarding their clinical behavior. The first concern is that the high acidity exhibited by `strong' self-etch adhesives results in rather deep demineralization defects thereby exposing the collagen so deeply that resins may not penetrate completely. This results in an uninfiltrated weak collagenous layer of dentin that is susceptible to longterm degradation  . The next akpect is the possibility of denaturation of collagen by too `strong' an acid treatment, wherein it is unlikely to expect bonding to occur with the altered or denatured collagen , .
For Clearfil SE Bond, the shear bond strength value (mean = 12.39 ± 0.71) was found to be statistically higher than Prompt-L-Pop and i Bond but lower compared to Adhe SE and Xeno 111. The pH of Clearfil SE Bond is about 2, which is categorized as `mild' by Van Meerbeek. He suggested that the advocation of `mild' self-etch systems results in superficial demineralization that occurs only partially, keeping residual hydroxyapatite still attached to collagen , . Nevertheless, sufficient surface-porosity is created to obtain micromechanical interlocking through hybridization  . The thickness of the hybrid layer is, however, much smaller than those produced by the `strong' self-etch or etch and rinse approach but has been proven to be of less importance with regards to actual bonding effectivenss , . The preservation of hydroxyapatite within the submicron hybrid layer may serve as a receptor for additional chemical bonding , and are expected to be particularly advantageous. Keeping hydroxyapatite around collagen may also protect the collagen against hydrolysis and, thus, early degradation of the bond ,, . which could probably be attributed to its superior performance compared to Prompt-L Pop and i Bond.
AdheSE and Xeno III recorded significantly superior shear bond strength values (mean = 14.69 ± 0.93 and 14.55 ± 1.00 respectively) compared to all other test groups. However, there exists no statisticasl variation between AdheSE and Xeno Ill. Both these adhesives can be categorized as `intermediary strong' based on their pH, which is about 1.5  . Most typical of these adhesives is the two-fold build-up of the dentinal hybrid layer with a completely demineralized top layer and a partially demineralized base. Following an `etch and rinse' or `strong self-etch' approach, the transition of the exposed collagen fibril network to the underlying unaffected dentin is quite abrupt whereas following an `intermediary strong' self-etch approach, the deepest region of the hybrid layer upto a maximum of I µm still contains hydroxyapatite, by whi ch the transtion of the hybrid layer to the underlying unaffected dentin is more gradual. At the same time, these adhesives are relatively more acidic compared to `mild' self-etch adhesives, by which better micromechanical interlocking is achieved  . The only difference that existed between AdheSE and Xeno III lies in its mode of application. The former is a two-step self etch adhesive whereas the latter is a single-step system. Although being a single step adhesive, Xeno III is supplied in a two-component form which is mixed just prior to application. Hence, the number of steps in bond application probably does not seem have a deleterious effect on the bond strength as exhibited in the present study.
i Bond recorded significantly low shear bond values (mean = 11.13 ± 0.64) compared to the remaining test groups excepting Prompt-L-Pop with which there was no statistical difference. Although they are classified as 'intermediary strong' self-etch adhesives, they are supplied as a single-component system in a 'pre-mixed form. The exact cause for its poor performance is unknown. However, it may be hypothesized that pre-mixing by the manufacturer may result in an adverse interaction between the primer and the resin, which can probably interfere with the bonding ability of the adhesive, thereby hindering its performance. This could probably be attributed to the low bond strength values obtained by the adhesive i Bond in this study.
| Conclusion|| |
Within the confines of the present study the following can be concluded with :
- 'Imtermediary strong' self-etch adhesives (AdheSE and Xeno III) exhibited significantly superior shear bond strength values compared to both `strong' (Prompt-L-Pop) and `weak' (Clearfil SE Bond) self-etch systems, which implies that pH of the adhesive has a significant effect on the bond strength.
- Separate application of the two components, primer and bonding agent (AdheSE) or mixing of the components just prior to application (Xeno III) does not alter the bond strength, indicating that the number of steps in adhesive application probably does not seem to have a deleterious effect on the performance of selfetch systems.
- Pre-mixing of primer and bonding agent by the manufacturer, supplied in the form of `single component' system (i Bond) seems to have a deleterious effect on the bonding ability of the adhesive.
- An original hypothesis on the chemistry and probable mechanism involved in the action of self-etch adhesives illustrated by a logical explanation of their behavior in dentin.
| Acknowledgements|| |
Dr. K. Palanivelu: Manager, Central Institute of Plastic Engineering & Technology (CIPET)
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Department of Conservative Dentistry & Endodontics, Sree Balaji Dental College & Hospitals, Pallikkaranai, Chennai - 601 302
Source of Support: None, Conflict of Interest: None
[Table 1], [Table 2]
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