| Abstract|| |
Background and objective : True adhesion has been the 'holy grail' of restorative dentistry. Iftrue adhesion of restorative materials to tooth structure is to be achieved, optimal retention must be obtained and microleakage must be prevented. This study was conducted to evaluate the correlation between microtensile bond strength and microleakage using two different adhesives systems, which differed in their mode of adhesion.
Method : Box type Class I occlusal cavities were prepared on twenty molar teeth. The teeth were assigned to two groups Group I (Gluma Comfort Bond and Charisma), Group 11 (i-Bond and Charisma), each comprising of ten teeth. Restored teeth were subjected to thermocycling, followed by dye immersion and sectioning. Microleakage evaluation for the extreme end slices was done using dye immersion technique and then viewed under stereomicroscope. Microtensile bond strength testing of the middle two slices was done using an Universal Testing Machine (Instron).
Results : Statistically, Group I showed higher microtensile bond strength and lower microleakage value when compared to Group 11. A highly significant negative correlation was found between microtensile bond strength and microleakage.
Interpretation and conclusion
· Smear layer removing adhesive system exhibited higher bond strength and less leakage than smear layer dissolving adhesive system.
An inverse relationship exits between microtensile bond strength and microleakage.
Keywords: Microtensile bond strength, microleakage, Class I cavity, relationship, smear layer removal and dissolution.
|How to cite this article:|
Elizabeth SK, Manjunath M K. Association between microtensile bond strength and microleakage at the resin based composite/dentin interface - an in vitro study. J Conserv Dent 2007;10:134-40
|How to cite this URL:|
Elizabeth SK, Manjunath M K. Association between microtensile bond strength and microleakage at the resin based composite/dentin interface - an in vitro study. J Conserv Dent [serial online] 2007 [cited 2013 May 20];10:134-40. Available from: http://www.jcd.org.in/text.asp?2007/10/4/134/43036
| Introduction|| |
With the evolution of aesthetic dentistry and with the advancement in dentin adhesives, the clinical application of resin based composites have been extended to encompass posteriorrestorations  .
Microleakage developed due to penetration of bacteria, enzymes, bacterial byproducts, various acids and bases and even water at the tooth-restoration interface and has been regarded as detrimental to the longevity of the restorations. It occurs when the materials are poorly adapted to the cavity; when they pull away from a cavity wall during polymerization; when liners or smear layers dissolve or when recurrent caries occur at the tooth-restoration interfacial gap , . The ability of a material to prevent this penetration is evaluated using a number of microleakage assessment techniques  .
Numerous studies are evident as regards to shear bond strength and microleakage, which being the two characteristics of adhesives that are frequently evaluated in in-vitro studies of dentin bonding systems. An "ideal" dental adhesive would provide high bond strengths and would eliminate microleakage. However, the relationship between shear bond strength and microleakage performance is not clearly understood and is still conflicting  .
To date, most leakage studies and bond tests have been performed in different specimens because the specimens needed to be sectioned in order to measure leakage. Since, the recently developed microtensile bond tests by Sano et al in 1994, permits the creation of 8-10 specimens per bonded tooth, it has become possible to evaluate bond strengths and leakage studies using the same specimen  .
With the microtensile bond test, measurement of bond strength to small surface areas (0.5-1mm 2 ) has become possible. It permits the testing of irregular surfaces and cavities that are more clinically relevant than flat surfaces, which is still not possible with conventional shear and tensile bond tests  . But studies regarding the microtensile bond strength and microleakage and their co-relation are few and inconclusive. Hence this study was undertaken.
| Materials and Methods|| |
Standard box type Class I cavities were prepared on the occlusal surface using a high speed airotor handpiece and diamond point (SF12, ISO I11/014) under constant water spray. After preparing cavities on ten teeth, a new diamond point was used. Standard cavities were prepared with 3mm width bucco-lingually and 3mm depth as measured from the mesial and distal marginal ridges. The width and depth were verified using a Williams periodontal probe. 1 mm each on both mesial and distal marginal ridges were left intact.
After preparing the cavities, the teeth were stored in distilled water at 37°C. Before restorative procedures, the teeth were randomly assigned to the two treatment groups often teeth each.
n=10; Gluma Comfort Bond and Charisma
n= 10; i - Bond and Charisma
All the restorative procedures were carried out according to the manufacturer's instructions. Restorative material Charisma was placed in small increments and each increment was light cured for 20-40 seconds. After the restorative procedures the specimens were stored in distilled water at 37±2°C for48 hours. The specimens of the two groups were subjected to 500 cycles of thermocycling in a thermocycler where they were immersed in two baths, at 5°±2°C (cold cycle) and 55°±2°C (hot cycle) with a dwell time of 30 seconds each and a transfer time of 10 seconds between each bath (dry cycle).
After thermocycling all the teeth were embedded into rectangular aluminium blocks (lined with modelling wax on the inner surface), of 1 x 1" x 0.75" and were stabilized with the help of self curing epoxy resin, exposing only the crown portion of the teeth. As the self cure resin starts setting, the epoxy resin block is removed from the aluminium blocks. All the specimens of the two groups were colour-coded for reference.
The entire surface of the crown of each tooth was coated with two layers of nail varnish except for a 1mm width around the cavosurface margin. The resin blocks were then immersed in a 2% Methylene Blue buffered solution for 4 hours, after which they were thoroughly rinsed under tap water for 10 minutes and left to dry for 24 hours before sectioning.
Each specimen was then fitted onto the chuck of the sample holding arm of the Isomet saw and were sectioned longitudinally in a bucco-lingual direction into four rectangular slices. The two extreme end slices i.e., the mesial and distal end slices were subjected to measure microleakage using stereomicroscope at 10x magnification [Figure 1].
The scores for microleakage were calculated along both the enamel and dentinal walls and in the dentin towards the pulp.
The following criteria was used to score the extent of microleakage:
0 - No leakage
No dye penetration or leakage [Figure 5].
1 - Mild leakage
Leakage extending upto the dentino-enamel junction [Figure 6].
2 - Moderate leakage
Leakage extending beyond the dentinoenamel junction excluding the pulpal floor [Figure 7].
3 - Severe leakage
Leakage extending onto the full dentinal wall including the pulpal floor [Figure 8],[Figure 9],[Figure 10].
Microtensile Bond Strength Testing
The middle two slices were of 1 mm thickness each. To obtain a cross-sectional surface area of 1 mm 2 for the bond strength measurement, the adhesive dentin interface was reduced from the mesial and distal aspects to an average width of 1 mm with an airotor and a fine diamond point (SF 41,10109/010) which has a bur head diameter of l mm under constant water spray, resulting in hour-glass shaped specimens [Figure 2] and [Figure 3].
Each specimen was then consecutively bonded to the custom made aluminium jig using cyanoacrylate based adhesive. It was bonded in such a way that the adhesive dentin interface will correspond with the rectangular slot on the jig. This jig was then held vertically in the upper and lower jaws of the universal testing machine [Figure 4]. The specimens were tested for microtensile bond strength using a 100N load at a cross-head speed of lmm/min in the universal tester that was operated by a computer using Instron Software. The maximum load to debond the specimens was recorded in Newtons. Microtensile bond strength was calculated in Megapascals(MPa) by the ratio of maximum load in Newton to the cross-sectional area of the bonded interface in square millimeters.
Statistical Methods applied
Following statistical methods were applied in the present study.
- Pearson's product moment correlation
- Independent samples 't' test
All the statistical operations were done through SPSS (Statistical Presentation System Software) for Windows Evaluation version 14, 2006) SPSS Inc. NewYork.
| Results|| |
Between Group I and Group II, a highly significant difference was observed in their mean microtensile bond strength values as the obtained 't' value of 21.93 was found to be significant at .000 level. From the mean values it is clear that Group I had significantly higher microtensile bond strength value than Group II. The mean difference was 15.42 [Table 3].
Between Group I and Group II, a significant difference was observed in their mean microleakage values as the obtained 't' value of 2.64 was found to be significant at.012 level. From the mean values, it is clear that Group 11 had significantly higher microleakage than Group I. The mean difference was -.90 [Table 4].
When Group I and Group II, were combined, a highly significant negative correlation was obtained between microtensile bond strength and microleakage, where correlation coefficient of - .577 was found to be significant at 0.000 level. In other words, as the microtensile bond strength increased, microleakage decreased linearly and significantly. Thus, overall we find an inverse relationship between microtensile bond strength and microleakage [Table 5].
| Discussion|| |
Since the evolution of enamel and dentin bonding agents, sealing ability and bond strength evaluation are the two main types of laboratory tests used to evaluate the performance of restorative systems in vitro. The feasibility of using these tests to evaluate restorations placed in cavity preparations permits the evaluation of a specific material in a condition that is clinically relevant  . This study was undertaken to evaluate the correlation between microtensile bond strength and microleakage in the same Class I cavities using two different adhesive systems.
Dye penetration was chosen as a measure of microleakage detection for the present study because it is simple and fast. It was advantageous in that it can be detected in dilute concentrations, is non-toxic and the results can be evaluated quantitatively.
In this study 2% Methylene Blue was preferred because Methylene Blue can serve as an adequate indicator for passage of microorganisms, large size endotoxins and also toxic agents of low molecular weight , .
Bond strength was reported as the initial mechanical load to fracture divided by the defined cross-sectional area of the bond. Almost all bond strengths were categorized as tensile or shear bond strength until recently Sano et al developed the microtensile bond strength testing methodology. The new method carries several advantages over conventional shear and tensile load strength methods because it permits the use of only one tooth to fabricate several bonded dentin-resin slabs, allows for testing substrates of clinical significance such as carious dentin, cervical sclerotic dentin and enamel, results in fewer defects occurring in the small area (0.5-1.5mm 2 ) specimens, as reflected in higher bond strengths and allows for the testing of regional differences in bond strengths within the same tooth  .
So, in the present study, the new methodology was employed which permits to evaluate bond strength and microleakage using the same specimen.
In this study, correlation between microtensile bond strength and microleakage was drawn using two different adhesive systems. The two dentin bonding systems, Gluma Comfort Bond and i-Bond which were chosen for the present study differed in their mode of adhesion. Gluma Comfort Bond is a 5 th generation bonding agent wherein the primer and adhesive are mixed together and applied as a single system. It is categorized as 2 step smear layer removing system. It utilizes an adhesion approach that results in complete removal of the smear layer with acidic conditioners that are simultaneously applied to enamel and dentin with a total-etch technique.
Their mechanism is principally based on the combined effect of hybridization and formation of resin tags. The conditioners or acid etchants are used with the objective not only to completely remove the smear layer but also simultaneously expose a microporous scaffolding of collagen fibrils by superficial demineralization. The primer contains monomers with hydrophilic properties that have an affinity for the exposed collagen fibril arrangement and hydrophobic properties for co-polymerization with the adhesive resin. The major role of the adhesive resin is to stabilize the formed hybrid layer and to form resin tags in the unplugged dentinal tubules.
In the present study, specimens bonded with Gluma Comfort Bond showed higher mean bond strength value and lesser mean microleakage scores. This is because the adhesive monomers have completely occupied the interfibrillar spaces around exposed collagen fibrils creating a proper hybrid layer. Also, Gluma Comfort Bond is an ethanol based adhesive. Numerous studies have shown that ethanol based adhesives perform better than acetone based adhesives as ethanol is less volatile and wont leave any residual water on the surface upon evaporation. So less number of blisters or porosities are created , .
The second group of dentin bonding system used was i-Bond which is a 7 th generation bonding agent wherein the conditioner, primer and adhesive are combined in one bottle and is known as 'All-in-One" adhesives. They are categorized as smear layer dissolving adhesive. The non-rinsing all-in-one adhesives contain aqueous mixtures of high concentration of acidic monomers generally phosphoric acid or carboxylic acid esters often mixed with water to make the adhesive systems sufficiently acidic to cross the smear layer and to form a bond with the underlying dentin. The acidic monomers partially demineralize the smear layer and the underlying dentin surface without removing the dissolved smear layer remnants and unplugging the tubule orifices. They incorporate the smear layer in the hybrid layer. They vary in their acidity by virtue of the composition and concentration of polymerisable acids and acidic resin monomers in these systems. Water is an essential component as it provides hydrogen ions that are indispensable for demineralization of the smear layer and hard tissue.
Apart from simplification the rationale behind these systems is to superficially demineralize dentin and simultaneously penetrate it with monomers which can be polymerized in situ. A continuum from the unaltered dentin to the adhesive resin is created without the formation of unpolymerized hydrophilic monomers at the base of the demineralized dentin layer which could be highly sensitive to hydrolysis.
But in the present study, the specimens bonded with i-Bond showed lower mean bond strength value and higher mean microleakage scores. Several mechanisms may account for this poor performance. When the acidic monomer starts etching the surface, the acidic monomer is converted to a negatively charged monomer molecule. The negatively charged chains may in turn bond to positively charged sites present in their vicinity, preventing the acidic molecules from diffusing deeper into the tooth structure.
The significant negative correlations between microtensile bond strength and microleakage indicate that high bond strengths are associated with low microleakage and vice versa.
| Conclusion|| |
Within the limitations of this in vitro study:
- The mean microtensile bond strength of Gluma Comfort Bond was much higher than the mean microtensile bond strength of i-Bond. A highly significant difference was observed.
- The mean microleakage value of Gluma Comfort Bond was lower than the mean microleakage value of-Bond. A significant difference was observed.
- The smear layer removing group of dentin bonding system (Gluma Comfort Bond) performed better than the smear layer dissolving group (i-Bond) of dentin bonding system.
- In general, an inverse relationship exists between the microtensile bond strength and microleakage at the resin dentin interface.[Table 1],[Table 2]
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Shilpa K Elizabeth
Department of Conservative Dentistry and Endodontics, J.S.S Dental College and Hospital, S.S Nagar, Mysore- 570015
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]