Does eugenol affect the microtensile bond strength of self-adhering composite? - An in vitro study.

Objectives: The objective of the study is to evaluate the effect of eugenol-based temporary on microtensile bond strength of self-adhering composite at 1 day and 7-day time intervals. Materials and
Methods: Occlusal enamel of 24 human molars was removed. Zinc oxide eugenol (ZnOE) and noneugenol temporary cement (ZnONE) were placed on the dentin surfaces and left for different times (1 day, 7 days). After removal of temporary cement, teeth were randomly divided into eight subgroups: subgroup GE1S (n = 3): ZnOE cement + Self-adhering composite (SAC), subgroup GE1N (n = 3): ZnOE cement + adhesive system (one coat 7 universal) + nanohybrid composite, subgroup GE7S (n = 3): ZnOE cement + SAC, subgroup GE7N (n = 3): ZnOE cement + adhesive system + nanohybrid composite, subgroup GNE1S (n = 3): ZnONE cement + SAC, subgroup GNE1N (n = 3): ZnONE cement + adhesive system + nanohybrid composite, subgroup GNE7S (n = 3): ZnONE cement + SAC, subgroup GNE7N (n = 3): ZnONE cement + adhesive system + nanohybrid composite. Four sticks per tooth were obtained, resulting in 12 sticks per group with a cross-sectional area of 0.5 mm 2. The μTBS test was performed with a crosshead speed of 0.5 mm/min. Statistical Analysis: Kruskal-Wallis and Mann-Whitney U-test were used for analysis.
Results: Highest and lowest mean value of microtensile bond strength was observed in GNE7N (12.75MPa) and GE1S (1.42MPa), respectively.
Conclusion: The presence of eugenol at early stage, i.e. 1 day, has a negative influence on microtensile bond strength of SAC. At 7 days, the negative effect of eugenol on microtensile bond strength gets nullified. Thus, a waiting period of 1 week is sufficient to overcome negative influence of eugenol-based temporaries on polymerization of SAC. Copyright:

INTRODUCTION

The world of dentistry has always been in a state of evolution where bonding strategies are concerned. The drive toward including fewer steps and higher precision regarding the various bonding strategies have recently led to many important discoveries. It is an established fact that bonding of dental composite resins to dental hard tissues gets affected by the adhesion strategy, adhesive layer thickness, structure of substrate, and type of utilized provisional restorative material.[1]

Acid etching and rinsing, which has been the pioneer of bonding in dentistry, has been slowly diminishing nowadays due to the advent of the popular self-etching adhesive systems, due to their acidic primers making them lesser technique sensitive compared to etch and rinse system. The bonding of dental composites occurs by micromechanical retention, the strength of which is a key factor for the resistance of composite restoration against the various dislodging forces in the oral environment thus preventing microleakage.

Zinc oxide eugenol (ZnOE), which is the most widely used temporary sealing material in restorative dentistry, has a radical scavenger, eugenol that inhibits polymerization of resin. Temporary cements, in general, are difficult to remove completely. In addition, it is reported that a very small bit of eugenol is released from ZnOE cement, which eventually contaminates the dentin surface by penetrating the dentinal tubules.[2] All these can affect the adhesion, wetting capability, and sealing ability of the adhesive system. However, the results of effect of eugenol on composite resin sealing are inconclusive.[3]

Self-adhering composites (SAC) released recently into the dental market have received the most attention in recent years as they can be bonded to dental tissue without application of any other adhesive system, making the procedure less time consuming and less technique sensitive.[4]

There have previously been studies regarding the effect of eugenol on the bond strength of conventional composites,[567] but the self-adhering composites have not yet been put into play regarding this context. Thus, this study aimed to evaluate the effect of eugenol on microtensile bond strength of SAC at different time interval. The null hypothesis to be tested was that there is no influence of eugenol and duration of application time on the microtensile bond strength of SAC.

MATERIALS AND METHODS

The sample size was estimated at 5% level of significance and standard deviation of 4.97. The standard deviation data (0.92–4.97) was obtained from study by Cengiz and Ünal 2019[8] that evaluated the microtensile bond strength of two self-adhesive flowable composites. The maximum SD value was used for estimation.

Noncarious, freshly extracted 24 human permanent molar teeth of similar dimensions with no cracks, decay, fracture, abrasion, previous restorations, or structural deformities (extracted for periodontal reasons) were selected. Soft tissue deposits and calculus were removed with an ultrasonic scaler (Woodpecker DTE D5, China). Teeth were checked for fracture lines under dental surgical microscope (Gippon Inc. Japan) and were selected for study. Teeth were then stored in 1% Chloramine T (HiMedia Labs., India) solution at 40°C until use. The storage period for the selected teeth was not more than 3 months.

Specimen preparation for μTBS

Occlusal enamel was removed using a trimmer with water irrigation. The exposed dentin was polished with 600-grit SiC paper under water irrigation for 30s, to obtain a uniform smear layer.[9] The exposed area did not exceed 1.5 cm in width and 2.0 cm in length. The specimens were then ultrasonically cleaned in distilled water for 5 min to remove any remaining silicon carbide dust particles.

Specimens were randomly divided into two groups, as follows:

Group GE (n = 12) in which all the exposed dentin surface of all the samples was covered with ZnOE cement (DPI, Mumbai, Maharashtra, India) of approximately 2 mm. Three scoops of ZnO powder were mixed with 4 drops of eugenol.

Group GNE (n = 12) in which all the exposed dentin surfaces of all the samples was covered with noneugenol temporary cement (MD-Temp Plus, META BIOMED Corporation Limited, Korea) of approximately 2 mm thickness. Each group was divided into two subgroups (n = 6) on the basis of time period for which temporary cement was left. In subgroup GE1 and GNE1, temporary cement was left for 1 day and then removed. In subgroup GE7 and GNE7, temporary cement was left for 7 days and then removed. Temporary cement was ultrasonically removed using periodontal scaler tip after desired storage period in incubator at 37°C and 100% humidity followed by thorough rinsing with 5 ml of distilled water.

Each subgroup was further divided into two groups (n = 3) on the basis of type of adhesives used for bonding.

Subgroup GE1S (n = 3): ZnOE cement + self-adhering flowable composite (Dyad Flow, Kerr Corporation, Orange, USA).

Subgroup GE1N (n = 3): ZnOE cement + adhesive system (one coat 7 universal, Coltene Whaledent, Switzerland) + nanohybrid Composite (Herculite Precis, Kerr corporation Orange, USA).

Subgroup GE7S (n = 3): ZnOE cement + self-adhering flowable composite.

Subgroup GE7N (n = 3): ZnOE cement + adhesive system + nanohybrid composite.

Subgroup GNE1S (n = 3): noneugenol cement (ZnONE) + self-adhering flowable composite.

Subgroup GNE1N (n = 3): ZnONE cement + adhesive system + nanohybrid composite.

Subgroup GNE7S (n = 3): ZnONE cement + self-adhering flowable composite.

Subgroup GNE7N (n = 3): ZnONE cement + adhesive system + nanohybrid composite.

Composite build-up of 5 mm was done following incremental technique, using precut standardized plastic tubes which had a diameter of 6 mm and height 5 mm on all specimens of subgroup GE1S, GE1N, GE7S, GE7N, GNE1S, GNE1N, GNE7S, GNE7N. Each increment of 2 mm composite was polymerized with LED light-curing unit (Woodpecker Lux V, Guilin Woodpecker, China) with a intensity of 1200 mW/cm2 for a period of 20 s. Specimens in each subgroup (GE1S, GE1N, GE7S, GE7N, GNE1S, GNE1N, GNE7S, GNE7N.) were sectioned perpendicularly to the adhesive interface using a low-speed diamond saw underwater irrigation (Isomet, Buehler Ltd, USA). Four sticks per specimens were produced resulting in twelve specimens per subgroup. Each stick being 1 mm × 1 mm thick and 10 mm long. The sticks were then attached to a custom-made jig using screws and cyanoacrylate glue and this jig was attached to the Instron universal testing machine (Apex Assessment lab, Ghaziabad, Uttar Pradesh, India) and finally stressed until failure with tensile load at crosshead speed of 0.5 mm/min. The force at which failure occurred was recorded in Newton, and cross-sectional area of specimens was measured with a digital calliper in mm sq. to compute the bond strength in megapascal (Mpa). Results obtained were subjected to statistical analysis. Data were collected and Kruskal–Wallis and Mann–Whitney U-test were used with statistical significance set at 5%. The data analysis was done with IBM Statistical Package for the Social Sciences version 19 (IBM Corp, Armonk, N.Y., USA).

RESULTS

Highest mean value of microtensile bond strength was observed in GNE7N (12.75MPa) while lowest mean value of microtensile bond strength was observed in GE1S (1.42MPa). The mean microtensile bond strength values (in MPa), standard deviation, and standard error of different groups were calculated [Table 1]. Mean values of microtensile bond strength (MPa) for different groups can be arranged asGE1S

Table 1

Means and standard deviations for microtensile bond strength (MPa) of different groups using Kruskal-Wallis test

Group	Mean±SD	SE
GE1S	1.42±0.515	0.148
GE1N	7.00±1.537	0.443
GE7S	1.83±0.389	0.112
GE7N	10.17±3.298	0.952
GNE1S	1.83±0.389	0.112
GNE1N	11.50±3.398	0.980
GNE7S	2.00±0.707	0.217
GNE7N	12.75±2.927	0.844

SD: Standard deviation, SE: Standard error

On multiple comparison of microtensile bond strength (In MPa) of different groups at various time intervals [Table 2], statistically insignificant difference was observed in the mean rank microtensile bond strength values of GE1S and GNE1S; GE7S and GNE1S; GE7N and GNE1N; GE7N and GNE7N; GNE1N and GNE7N (P > 0.05). However, rest all comparisons in different groups were statistically significant.

Table 2

Multiple comparison of microtensile bond strength (MPa) of different groups at various time intervals using Mann-Whitney U test

Comparison	Group	Mean rank	P
GE1S versus GE1N	GE1S	6.50	0.000
	GE1N	18.50
GE1S versus GE7S	GE1S	10.00	0.039
	GE7S	15.00
GE1S versus GNE1S	GE1S	10.00	0.039
	GNE1S	15.00
GE1N versus GE7N	GE1N	8.83	0.011
	GE7N	16.17
GE1N versus GNE1N	GE1N	7.63	0.001
	GNE1N	17.38
GE7S versus GE7N	GE7S	6.50	0.000
	GE7N	18.50
GE7S versus GNE1S	GE7S	12.50	1.00
	GNE1S	12.50
GE7S versus GNE7S	GE7S	11.67	0.446
	GNE7S	13.33
GE7N versus GNE1N	GE7N	11.38	0.432
	GNE1N	13.63
GE7N versus GNE7N	GE7N	9.92	0.072
	GNE7N	15.08
GNE1S versus GNE1N	GNE1S	6.50	0.000
	GNE1N	18.50
GNE1S versus GNE7S	GNE1S	11.67	0.446
	GNE7S	13.33
GNE1N versus GNE7N	GNE1N	11.04	0.309
	GNE7N	13.96
GNE7S versus GNE7N	GNE7S	6.50	0.000
	GNE7N	18.50

DISCUSSION

For bond strength evaluation, many mechanical testing methods such as shear, tensile, and microtensile tests have been suggested. For accurately measuring the bond strength between an adhesive and a substrate, the bonding interface should be the most stressed region. However, many studies reported that some bond strength tests like shear bond strength test and macrotensile bond strength test do not appropriately stress the interfacial zone.[1011] In contrary, μTBS test has several advantages over other bond strength testing methods, allowing appropriate alignment of the samples, thus, a more homogeneous distribution of stress. In addition, it provides better economic use of samples, better control of regional differences and gives the ability to test irregular surfaces making it the most sensitive method used for evaluating and comparing of bond strengths.[11]

In the present study, the μTBS bond strength of self-adhesive composite was significantly lower (P = 0.005) than the bond strength of nanohybrid composite when restoration was delayed for 1 day and 7 days, when eugenol was used. In addition, similar findings were observed even when noneugenol temporary cement was used for 1 day and 7 days. In general, μTBS of SACs was found inferior to self-etch adhesive system with nanohybrid composite. Poitevin et al.[12] and Fu et al.[13] had similar observation where in their studies they found that the μTBS to dentin of the SACs was significantly lower than that of other adhesive system combined with composites. Based on the pH 1.9 declared from the manufacturer, it is possible to speculate that self-adhering composite interacts with dentin in the similar manner to a mild self-etch adhesive. Moreover, proper wettability of an adhesive material onto a substrate promotes a close adhesive substrate interaction. Dyad Flow being a viscous material lacks the ability to wet self-etched collagen fibrils.[13] Despite the description of manufacturer (Kerr), no chemical analytic data on the bonding potential of GPDM, an acidic monomer, are available. It is indicated that the GPDM “etches” rather than “bonds” to hydroxyapatite.[14] To achieve self-adhesiveness, it is speculated that a relatively viscous (flowable) composite should contain a functional monomer that rather possesses an effective chemical bonding potential, as it cannot penetrate deeply. On the contrary, self-etch adhesive (one coat 7 universal) which although can be categorized as mild and ultra-mild self-adhesives (pH = 2–2.5), according to the classification given by De Munck et al.[15] in light of bond durability, has some unique properties since not all hydroxyapatite is removed from the interaction zone, much calcium is available for additional chemical interaction with specific adhesive functional monomers.[16] In addition, this adhesive contains the functional monomer 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP), which enables an intensive and stable chemical bond with hydroxyapatite.[17]

The μTBS of SACs after 7 days of eugenol-based temporary cement retention was significantly more than when restoration was done after 1 day. The same type of results was also observed for self-etch adhesives (one coat 7 universal), with nanohybrid composite. The reason for the low bond strength after 1 day may be due to the high release of eugenol, which occurs in the 1st h of contact with moisture, which may be responsible for the reduction of μTBS of both SACs and self-etch adhesives.[7] The presence of water from both the oral medium and dentinal tubules may favor a reversible reaction capable of releasing eugenol from the zinc eugenolate matrix, which is incorporated into the subjacent dentin. These characteristics favor the accumulation of free nonreacted eugenol in the smear layer and dentinal tubules, which would bond to the free radical, postponing resin polymerization, and compromising the bond strength of adhesive systems.[18] Result of the current study is similar to those of Carvalho et al.[19] in which, irrespective of the type of adhesive (etch-and-rise or self-etch), bond strength was significantly affected after 1 day of contact with ZnOE. After maintaining ZnOE restoration for 7 days, the μTBS values for both SACs and self-etch adhesive systems were re-established. Silva et al.[1] using ZnOE provisional restoration for 7 days also found no adverse effect of eugenol on the bond strength of adhesive to dentin. The probable explanation for this may be that the effect of eugenol gets neutralized with time, and thus, the negative effect of presence of eugenol on polymerization of resin is eliminated so, it can be speculated that in case, if eugenol-based temporary is being used and SACs are selected as restorative materials, a waiting period of 7 days should be done. However, in contrary, there is also a study done by Pinto et al.[7] which found that the lower μTBS of a self-etch adhesive to dentin when ZnOE had been previously placed over the substrate for 1 day and 7 days. The reason for contrary results in the current study may be due to difference in methodology from that of Pinto et al.[7] In this study, ZnOE temporary cement removal was performed with a scaler, followed by cleaning with pumice water slurry using a slow-speed handpiece. Both approaches have been proven effective in removing the remaining provisional material as compared to removal only done manually with stainless steel spatula and pumice water slurry using a slow handpiece. Thus, it is expected that the combination of the two approaches ultrasonic and rotary would have resulted better cleaning thus leading to less interference with bond strength. In the present study, the μTBS testing of restoration done after 1 day of eugenol-based temporary cement retention of SACs was compared with that of SACs done in noneugenol group at the same period, there was a significant decrease in μTBS in eugenol group reflecting the negative effect of eugenol on bonding. Similar results were also observed for nanohybrid composite resin. These results are in accordance to Silva et al.[1] and Carvalho et al.[19] who used ZnOE provisional restoration for 1 day and found similarly negative effect of eugenol on the μTBS of adhesives to dentin irrespective of the type of adhesive (etch-and-rise or self-etch) used as compared to μTBS of adhesive to dentin in noneugenol group. When the μTBS of SACs done after 7 days of eugenol-based temporary cement retention was compared with the μTBS of restoration done by SACs after 1 day and 7 days of noneugenol-based temporary retention, the difference in the μTBS was statistically insignificant (P = 1.000 and 0.446), respectively, which leads to prove the fact that a waiting period of 7 days neutralizes the effect of eugenol, eliminating its negative effect on the polymerization of resin. Thus, the results obtained will be the same as if the restoration is done after temporization with a noneugenol-based cement. Hence, it can be speculated that in case, if eugenol-based temporary is being used, a waiting period of 7 days is beneficial when using SAC.

In the present study, the μTBS of the restoration done after 7 days of retention of eugenol-based temporary cement was compared with the restoration done after 1 day and 7 days of noneugenol-based temporary cement in self-etch adhesives, the values were found to be statistically insignificant (P = 0.432 and 0.072). The reason for this may be again the neutralization of eugenol, which must have left the substrate as suitable for bonding as when no eugenol was used, and it was inferred that the negative effect of eugenol on self-etch bonding system vanished after 7 days. Thus, the μTBS appeared the same as compared to noneugenol temporary-based cement.

The μTBS of SACs after 7 days of noneugenol-based temporary cement retention was compared with when restoration was done after 1 day, the μTBS value was statistically insignificant (P = 0.446). The inference of this is that the absence of eugenol results in consistent bond strength irrespective of time period of restoration whether 1 day or 7 days.

The various time durations of eugenol retention were investigated in the present study to determine its effect on microtensile bond strength of self-adhesive composite and nanohybrid composite, so that a proper protocol can be followed while usage of eugenol-based temporary cements in relation to dental composites.

CONCLUSION

Within the limitations of the present study, it is possible to conclude that:

The μTBS of self-etch adhesives is better than SACs

Presence of eugenol, at early stage, i.e., 1 day has a negative influence on bonding of composites

At 7 days, the negative effect of eugenol on μTBS of composites got nullified. Thus, on the basis of observations in the present study, it can be recommended that if ZnOE-based temporary is used, a minimum of waiting period for 7 days is required for permanent restoration for nullifying the negative effect of eugenol on μTBS of composites.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.