| Abstract|| |
Objectives: To evaluate the effect of the eugenol exposure time of an eugenol-based provisional restorative material on the shear bond strength of two-step and one-step self-etching adhesives to dentin, at three different time intervals of 24 h, 7 days, and 14 days.
Materials and Methods: Forty extracted human posterior teeth were sectioned mesiodistally to obtain two halves and the resulting 80 halves were randomly assigned into four groups of 20 specimens each (Group-I, -II, -III, and -IV). Cavities of specified dimensions were prepared to expose dentin surface. In Group-I, temporarization was carried out with noneugenol cement (Orafil-G) for 24 h (control group). In Group-II, -III, and -IV, temporarization was carried out with eugenol cement (intermediate restorative material (IRM)) for 24 h, 7 days, and 14 days, respectively. Each group was further divided into two subgroups of 10 teeth each for two-step (Adper SE Plus) and one-step (Adper Easy One) self-etch adhesive systems, respectively. A plastic tube loaded with microhybrid composite resin (Filtek Z-350, 3M) was placed over the dentin surface and light cured. The specimens were subjected to shear stress in universal testing machine.
Results: Group-II yielded low shear bond strength values compared with Group-III, -IV, and Group-I, which was statistically significant.
Conclusions: The prior use of eugenol containing temporary restorative material reduced the bond strength of self-etch adhesive systems at 24-h period. No reduction in bond strength at 7 or 14 days exposure was observed with either two-step or one-step self-etch adhesive.
Keywords: Eugenol-based cement; self-etch adhesive; shear bond strength
|How to cite this article:|
Nasreen F, Guptha AS, Srinivasan R, Chandrappa MM, Bhandary S, Junjanna P. An in vitro evaluation of effect of eugenol exposure time on the shear bond strength of two-step and one-step self-etching adhesives to dentin. J Conserv Dent 2014;17:280-4
|How to cite this URL:|
Nasreen F, Guptha AS, Srinivasan R, Chandrappa MM, Bhandary S, Junjanna P. An in vitro evaluation of effect of eugenol exposure time on the shear bond strength of two-step and one-step self-etching adhesives to dentin. J Conserv Dent [serial online] 2014 [cited 2019 Jul 18];17:280-4. Available from: http://www.jcd.org.in/text.asp?2014/17/3/280/131802
| Introduction|| |
Self-etch adhesives are gaining popularity because of simplified bonding procedures and reduced technique sensitivity.  The self-etch approach uses acidic adhesive comonomers that simultaneously demineralize and infiltrate into the dentin. The intensity of the interaction of self-etching adhesive systems with dentin was mostly dependant on the acidity and aggressiveness of the primer used.  However, the fundamental principle of bonding to dental hard tissues remains the same, which is based on micromechanical interlocking of adhesive resin to tooth substrate. Proper bonding depends on adhesive penetration between the spaces created by acid etching and satisfactory polymerization of the bonding resin. Several factors interfere with bonding ability of adhesive, which are adhesion strategy, conditioning time, solvent removal method, thickness of adhesive layer, substrate structure, and the provisional restorative material previously used. 
In clinical practice, many times provisionalization would be required before contemplating permanent restorations for reasons like inadequate time to restore multiple carious lesions in one appointment, questionable pulpal status, inter-appointment access cavity restoration following endodontic procedures, prognostication of success following endodontic procedure, and need for time to fabricate permanent restorations from laboratories.  Among the provisional restorative materials, zinc oxide eugenol (ZOE) is widely used. , Eugenol (2-methoxi-4-allyphenol) has been shown to penetrate dentin and change its wettability and reactivity. , The effect of eugenol-containing temporary restorations on the bond strength of composite to dentin is still inconclusive.
In particular, not many studies have been undertaken to evaluate the effect of eugenol-containing materials on the bond strength of self-etching adhesives to dentin. In case of self-etching adhesive formulations, as smear layer is not completely removed with acids prior to application of self-etch adhesives, it is more likely that more remnants of eugenol molecules and residues of eugenol-containing temporary restoration will be incorporated into the hybridized complex, thus affecting the bonding performance of these new systems. 
Hence, this study was done with an objective to evaluate the effect of eugenol exposure time of an eugenol-based provisional restorative material (intermediate restorative material, IRM) on the shear bond strength of two-step and one-step self-etching adhesives to dentin, at three different time intervals of 24 h, 7 days, and 14 days.
| Materials and methods|| |
Forty sound human posterior teeth extracted for periodontal and orthodontic reasons were selected. The surfaces of the teeth were cleaned with scalers for removal of calculus and remnants of periodontal ligament and stored in normal saline.
The teeth were sectioned mesiodistally to obtain two halves. Each half was then embedded in autopolymerizing acrylic resin to facilitate handling and keeping the buccal and lingual surfaces exposed. Cavities measuring 4 mm mesiodistally, 4 mm occlusogingivally, and 2 mm in depth were prepared with a straight fissure (ISO#10, Horico, Germany) diamond bur in high speed hand piece under air water spray to expose the dentin surface. The resulting 80 halves were randomly divided into four main groups containing 20 specimens each. In Group-I, temporarization was carried out with noneugenol cement (Orafil-G, Prevest Denpro Limited, India) for 24 h and served as a "control." In Group-II, -III, and -IV, temporarization was carried out with eugenol cement (IRM, Dentsply, USA) for 24 h, 7 days, and 14 days, respectively, before bonding. The specimens were stored in 100% humidity at 37°C till the bonding procedure was carried out. After the planned observation period was over, the temporary restorative material was removed with scaler and cavity was cleaned with pumice-water slurry and air dried. Each main group was then divided into two subgroups of 10 specimens each according to the bonding agents used, two-step self-etch (Adper SE Plus,3M ESPE, USA) and one-step self-etch (Adper Easy One, 3M ESPE, USA) adhesives, respectively. Bonding agents were applied according to manufacturer's instructions. A transparent plastic cylindrical tube measuring 3 mm in height and 2 mm in internal diameter was loaded with microhybrid composite resin (Filtek Z-350, 3M ESPE, USA) and placed over the dentin surface and was cured for 40 s using light curing unit (LEDition, Ivoclar Vivadent, Lichenstein).
Prepared specimens were subjected to shear stress estimation in a universal testing machine (Lloyd, LR 50K, UK) with a crosshead speed of 1 mm/min until failure. Peak failure load was converted to shear bond strength by dividing failure load with bonding area and values were expressed in MPa.
Data were analyzed using Student's 't'-test, one-way ANOVA (analysis of variance), and post hoc test.
| Results|| |
One-way ANOVA test [Table 1] revealed that when dentin-bonding agent's two-step and one-step self-etch adhesive systems were used according to their manufacturer's instructions, two-step self-etch adhesive system showed higher shear bond strength than one-step self-etch adhesive system in all groups and the difference was statistically significant (P < 0.001).
|Table 1: One-way ANOVA test performed to compare the mean shear bond strength values between control and experimental groups in two-step (Adper SE Plus) and one-step (Adper Easy One) self-etch adhesive systems|
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Student's 't'-test [Table 2] revealed that in Group-I, -III, and -IV two-step self-etch adhesive system produced higher shear bond strength than one-step self-etch adhesive system and the difference was statistically significant (P < 0.001). In Group-II, no difference in shear bond strength was observed between two-step and one-step self-etch adhesive system (P = 0.095).
|Table 2: Student's 't'-test performed to compare the mean shear bond strength values between two-step (Adper SE Plus) and one-step (Adper Easy One) self-etch adhesive systems within the control and experimental groups|
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Post hoc test [Table 3] revealed that when two-step and one-step self-etch adhesive systems were used, Group-I showed higher shear bond strength than Group-II and the difference was statistically significant (P < 0.001). No statistically significant difference in shear bond strength was observed between Group-I, -III, and -IV (P > 0.05). Group-II showed the least shear bond strength than Group-I, -III, and -IV and the difference was statistically significant (P < 0.001).
|Table 3: Post hoc test performed to find out mean differences between two-step (Adper SE Plus) self-etch adhesive subgroups and also mean differences between one-step (Adper Easy One) self-etch adhesive subgroups|
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Overall deductions of statistical analyses
Shear bond strength values from either bonding agents at 24 h with prior eugenol exposure produced the least values in contrast to other three groups. Values at 7 and 14 days with prior eugenol exposure groups produce comparable shear bond strength values to noneugenol group. Two-step self-etch produced better values in comparison with one-step system.
| Discussion|| |
The primary aim of provisional restorations is to protect the dentin-pulp complex from noxious stimuli prior to definitive restoration.  Among the provisional restorative materials, ZOE is widely used. Reasons for its popularity include excellent cavity seal against leakage, obtundant effect, inexpensiveness, and ease of removal. ,, During routine treatment, the use of eugenol-containing provisional cements prior to definitive restorations with composite resins has to be considered critical because eugenol disturbs the polymerization of resin materials. ,,
A chelation reaction occurs when zinc oxide is mixed with eugenol. The set cement consists of unreacted zinc oxide particles embedded in a zinc eugenolate matrix. ,, Nevertheless, the reaction is reversible; when the set cement contacts water, the eugenolate at the surface hydrolyzes to liberate eugenol.  Hume et al., in 1984 reported that eugenol released from ZOE mixtures can penetrate dentin and interact with resin-based restorative materials.  Excess water breaks the coordinate bond, resulting in the formation of free eugenol and insoluble zinc hydroxide. 
The polymerization of composite resins and resin-bonding systems takes place by free radical addition polymerization, which can be inhibited by any material that reacts with a free radical.  Eugenol is a radical scavenger and inhibits the polymerization of resin materials. ,, Rotberg and Deshazer in 1966 reported that eugenol causes release of calcium from dentin due to its complexing properties. This may have softening effect on dentin.  Inadequate polymerization coupled with softening of dentin leads to decreased shear bond strength and increased microleakage, resulting in clinical complications such as fractured restoration, hypersensitivity, secondary caries, and surface discoloration.
This study was performed to evaluate whether residual eugenol affected the bond strength of self-etching adhesives to dentin from previous provisional restorations and the effect of exposure time on the shear bond strength. Three different time intervals (24 h, 7 th day, and 14 th day) were evaluated in this study as the amount of residual eugenol left behind may vary at a given point of time thus influencing the bond strength values of composite. 
The results showed that exposure of dentin surfaces to eugenol-containing cement for 24 h had a negative effect on the shear bond strength of self-etch adhesive systems. However, when bonding was delayed for 7 or 14 days, there was no reduction in bond strength of self-etch adhesive systems.
This can be explained by the fact that the diffusion rate of eugenol released from ZOE cement increased to a peak up to 24 h (about 0.3 nmol/min) and then decreased slowly thereafter. After 14 days the diffusion through dentin diminished to 0.08 nmol/min.  Therefore, it is expected that the eugenol concentration in dentin after 1 week or 14 days will not significantly affect bond strength when compared with 24 h as eugenol concentration is decreased to non-inhibitory levels. 
Hume in 1988 found the concentration of eugenol in aqueous phase to be in the order of 10−2 M just beneath the ZOE cement and 10−4 M adjacent to the pulp.  This means that the concentration of eugenol is higher at the surface dentin adjacent ZOE cement and decreases toward the pulp. The residual cement and eugenol could penetrate dentin to change the wettability and reactivity of dentin.
Removal of temporary restorative material with scaler, followed by cleaning with pumice-water slurry has proved effective in removing the remaining temporary material (Yap et al., 2001).  However, in the present study values at 24 h showed the ineffectiveness of this method in eliminating eugenol residues. Mechanical removal of temporary cements is not totally effective and cement remnants were observed microscopically on surfaces that appeared macroscopically clean. ,, This may be a reason why eugenol cements affect bond strength as suggested by Baier in 1992 and Terata in 1993. 
The hydroxyl group of the eugenol molecule tends to protonize the free radicals formed during polymerization of resin-based materials, thereby blocking their reactivity and reducing the degree of conversion of these materials.  The inhibited polymerization leads to increased surface roughness, reduced microhardness, and color stability of resin composites cured in contact with cavities previously restored with ZOE cement. 
The results showed that Group-I, -III, and -IV exhibited greater shear bond strengths for two-step self-etch adhesive (Adper SE Plus) than one-step self-etch adhesive (Adper Easy One). This is in accordance with several previous studies. ,
In contrast to our study, previous studies showed that Adper SE Plus, which is a strong self-etch adhesive (pH <1) often been documented with low bond strength values, especially to dentine due to their initial high acidity that causes deep demineralization. Adper Easy One, which is a mild self-etching adhesive (pH >2) demineralizes dentin only to a depth of 1 μm, keeping residual hydroxyapatite still attached to collagen. The preservation of hydroxyapatite within the submicron hybrid layer may serve as a receptor for additional chemical bonding. Such mild self-etching adhesives are found to have higher bond strength. ,
The adaptation of self-etch adhesives to the resin-dentin interface was good without voids or separation of phases; showing a thin continuous hybrid layer. It was concluded that such an adaptation of resin with dentin tissue was required for better treatment outcome to reduce postoperative sensitivity. ,
Contrary to etch and rinse adhesives, the self-etch adhesive systems are applied directly over the contaminated smear layer. This can explain the pronounced reduction in bond strength for self-etch bonding approach following pretreatment with eugenol-containing temporary restoration. Studies have reported that a thin film (0.9-3.0 μm) of smear layer covers the dentin surface on grinding.  Therefore, when eugenol-containing temporary restoration is placed over the smear layer and left for 24 h, eugenol would probably leach into smear layer and pass through it to reach the dentinal tubules, thereby contaminating the surface dentin.
Based on our results, it is logical to use ZOE provisional restorations for a week or longer. Clinically, 7 days is a reasonable period for provisional restorations in most situations. Nonetheless, additional studies are necessary to evaluate other periods of exposure between 24 h and one week.
| Conclusions|| |
Within the limitations of the study, it can be concluded that
- The previous use of provisional restorations containing eugenol for one or two weeks did not interfere with bond strength.
- In contrast, the exposure to eugenol-based material for 24 h reduced the bond strength of self-etching adhesives to dentin.
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Anila Bandlapalli Sreenivasa Guptha
LS Nursing Home, Somaguddu Road, Near TB, Challakere, Chitradurga - 577 522, Karnataka
Source of Support: None, Conflict of Interest: None
Clinical trial registration JCD_439_13
[Table 1], [Table 2], [Table 3]