Effect of the regional variability of dentinal substrate and modes of application of adhesive systems on the mechanical properties of the adhesive layer.

AIM: This study assessed the effect of the dentin depth and the application mode on the hardness and elastic modulus of the adhesive layer.
MATERIALS AND METHODS: Occlusal surfaces of 48 caries-free human third molars were removed, at two levels: Superficial and deep dentin. For each type of surface, the test specimens were randomly divided into groups which underwent the application: A conventional two-step adhesive system (Adper™ Single Bond [SB]) and self-etch adhesives system (Adper™ SE Plus [SE] and AdheSE(®) [AD]). The adhesives applied were active or passive. Composite build-ups were constructed incrementally. The teeth were sectioned, embedded, and polished. The nanoindentation test was performed in the adhesive layer. The results were analyzed by ANOVA and Tukey tests.
RESULTS: In the adhesive layer, the higher hardness (0.307 ± 0.006 GPa) and elastic modulus (4.796 ± 0.165 GPa) of SE were obtained in superficial dentin in passive application. The elastic modulus of SE (4.115 ± 0.098 GPa) was lowest in active application in superficial dentin. The active application significantly increased the hardness of the SB in the deep dentin (0.011 ± 0.314 GPa) compared the superficial dentin (0.280 ± 0.010 GPa). For the AD, only the mode of application was statistically significant (P=0.0041) for the hardness, active application (0.289 ± 0.015 GPa) being higher than passive application (0.261 ± 0.013 GPa) (P=0.0042) in deep dentin.
CONCLUSION: The experimental results reveal that the mechanical properties were influenced for the application mode of adhesive systems and dentin depth.

INTRODUCTION

Several adhesive systems are manufactured and investigated with the objective of seeking an effective bond to different dentinal substrates, resulting in different substrate conditioning techniques, and application modes of these systems. Development efforts attempting to simplify the application procedure of adhesives by combining the primer and adhesive resin into a single application step may reduce hybridization effectiveness.[1] Therefore, self-etching adhesives were produced with the intention of reducing the steps in the conventional conditioning technique and mistakes during application and manipulation.

The understanding of the micromorphological and physiologically dynamic of the dentin, represents a challenging substrate for bonding.[23] As cavities become deeper and closer to the pulp, there is a variation in the number of tubules, which increases the superficial humidity of dentin and makes the action of primers critical.[4] Since the acid in the self-etching adhesive system is incorporated into the primer, it can cause an increase in permeability when it begins its demineralizing action, and consequently, make the bond to the dentin less critical.[5] The adhesives that require acid etching are apparently more sensitive to the regional differences than the self-etching adhesives.[6]

To improve the action of total-etching and self-etching adhesives, some alternatives may be used, such as the active application of the adhesive system.[78] These alternatives accelerate the evaporation of the solvents present in the adhesive systems as well as the residual water of the dentinal surface, in addition to allowing greater infiltration of the monomers into the demineralized dentin, which could contribute to the formation of polymers that are more stable and resistant to hydrolysis.[9]

The immediate and long-term bond strength at the resin-dentin interface was improved by the active application of the adhesive systems.[710] The improvement in the mechanical properties of the polymer formed within the demineralized dentin can be attributed to this discovery.[11] Therefore, this study evaluated the effects of the dentin depth and the application mode on the hardness and elastic modulus of the adhesive layer.

MATERIALS AND METHODS

Forty-eight extracted human third molars were obtained from the Tooth Bank of the “Universidade Estadual de Ponta Grossa, PR, Brazil,” and stored at 37°C in a 3% thymol solution until used. The present study was approved by the Research Ethics Committee of the UEPG, Protocol #09908/08. Three adhesive systems were evaluated: Adper™ Single Bond Plus (3M ESPE, St. Paul, MN, USA), a total-etching; Adper™ SE Plus (3M ESPE), and AdheSE® (Ivoclar Vivadent, Schaan, Liechtenstein, DDR), self-etching. The samples were divided into 12 experimental groups (n=4). The factors under study were the adhesive system, the depth of the dentinal substrate and the application modes of the adhesive systems.

To obtain the different depth levels, the occlusal surfaces of the teeth were cut in a cutting machine (Isomet 1000, Buehler Ltd., Lake Bluff, IL, USA) with a diamond disk, at a speed of 350 rpm, under constant water cooling. Periapical radiographs of teeth were taken, which served as the auxiliary means of analyzing the proximity of the pulp chamber and consequently for determining the height of the occlusal cut. The cuts were made perpendicular to the long axis of the tooth, standardizing superficial dentin (SD=2.0–2.5 mm distance to pulp) and deep dentin (DD=0.5–1.0 mm to pulp).[12]

A flat dentin surface was exposed after grinding the occlusal enamel. Then, the dentin surfaces were polished on wet 600 grit silicon-carbide paper for 60 seconds to create a standardized smear layer. The adhesives were applied onto the dentin in accordance with their manufacturers’ instructions. However, before performing the active application, with the aim of improving standardization of the equivalent manual pressure that would be placed on the surface of the demineralized dentin, the operator was trained in the surface of an analytical balance (Mettler, type H6; Columbus, OH, USA).[11] In this group, the pressure was equivalent to approximately 38.9 ± 8.4 g, similar to that used in the studies of Dal Bianco et al.[7] and Higashi et al.[11] Thus, the solvent was then gently evaporated to form a slightly shiny adhesive film; it was light cured for 20 seconds using a light-curing unit. Solvent evaporation can facilitate the polymerization reaction because solvent volatilization can reduce the distance among monomers and increase the degree of conversion.[13]

After the adhesive application, three increments of 1.0 mm of composite resin Filtek™ Z350 (3M ESPE, St. Paul, MN, USA) buildups were inserted and polymerized for 30 seconds (Optilux 400, Demetron, USA). The teeth were stored in distilled water for 24 hours and the “center area” was delimited with red color in order to separate four bonded sticks of each tooth used in the nanoindentation test. After this, the teeth were adapted to an “ad hoc ” device, to obtain bonded sticks with a cross-sectional area of 0.8 ± 0.1 mm2.

The bonded sticks were fastened to “sample holders” with aid of paraffin. Initially were abraded with silicon carbide abrasive papers of decreasing abrasiveness (600, 1000, 1200, 1500, 2000, and 4000). Polishing was made with diamond suspensions (1 and 0.25 μm), in an automatic polishing device Aropol S (Arotec, Cotia, SP, Brazil) at 300 rpm. After storage in water (37°C/24 h), the specimens were submitted to the nanoindentation test.

The nanoindentation experiments were made using a Nano Indenter XP (MTS Systems Corp., Oak Ridge, TN, USA). The surface approach rate of the nanoindenter was set at 10 nm/s and the duration of the loading and unloading indentation was set at 10 seconds each, and the load was maintained constant by 4 seconds at maxim load. The tests were made at room temperatures (24°C ± 0.2°C). A Berkovich triangular pyramidal diamond indenter was employed with an applied load of 1 gf (10 mN). Twelve test indentions tests were made for each bonded stick in the adhesive layer. The hardness and elastic modulus were determined by the Oliver and Pharr[14] method.

Scanning Electron Microscopy (Shimadzu SSX-550, Kyoto, Japan) was employed to obtain indentation images. The hardness and elastic modulus (GPa) values of the adhesive layer were submitted to a three-way ANOVA test. For the contrast of the means, Tukey's multiple comparisons test was used (α=0.05).

RESULTS

The mean values and standard deviations of hardness and elastic modulus for the adhesive layer of all of the experimental groups are presented in Table 1, and it was found that:

Table 1

Means and standard deviations of hardness (GPa) and elastic modulus (GPa) from the adhesive layer for all experimental conditions

For Adper™ Single Bond Plus: In the hardness the double interaction (P=0.00056) was statistically significant. It can be noticed for the adhesive Adper™ Single Bond Plus that in SD, the passive application provided the higher hardness values and that in DD no significant difference among the means was observed (P>0.05);

For Adper™ SE Plus: The hardness demonstrated that the interaction (P=0.03699), as well as the main factors [substrate (P=0.00371), technique (P=0.00041)] were statistically significant. As regards the elastic modulus, the factor main technique (P=0.00034) and the interaction (P=0.02455) were statistically significant (P<0.05). Table 1 shows that no difference between superficial and DD was found for hardness when the adhesive was applied actively. On the other hand, when the application was passive, higher hardness values were found in the SD. A similar phenomenon occurred with the elastic modulus values of this material;

For AdheSE®: The hardness showed that only the factor technique was statistically significant (P=0.0041), the active application being [0.290 ± 0.009] statistically higher than the passive application [0.265 ± 0.013] (P=0.0042). However, as regards the elastic modulus none of the main factors [substrate (P=0.688), technique (P=0.820)], as well as the interaction (P=0.878) were statistically significant (P>0.05). As regards the hardness values, it can be observed that the active application provided the higher hardness in DD. No difference was observed among the groups as regards the elastic modulus.

The geometry of the indentation and the capacity of positioning accuracy of the nanoindentation technique in the adhesive layer (CA) are shown in Figure 1a–b.

Figure 1

SEM images of nanoindentations in the adhesive layer (CA) area. (a) Indentations made straight, the center of the CA, at ×700 magnification. (b) Indentation in CA, at ×1000 magnification

DISCUSSION

The heterogeneous structure of dentin makes bonding to dentin more difficult, generating conflicting results regarding the quality of this procedure, when applied to superficial or deep dentin.[15] Results showed higher mechanical properties, closer to the DEJ.[16-18] Thus, the mechanism of bonding to deep dentin, which has a larger number of dentinal tubules with larger tubule diameter, is considered more critical than bonding to superficial dentin.[19]

In most of the in vitro studies, the methodology is to obtain flat dentinal surfaces by wear with abrasive paper or cuts with diamond disks, perpendicular to the long axis of teeth, providing different levels of depth, which are measured directly on the specimens with the aid of a digital pachymeter, often disregarding the morphological variations of each tooth.[1117] The lack of uniformity of the substrate, which differs according to location, morphology, distribution of the structural elements, permeability, and intrinsic humidity results in variations in hardness being observed on the surface available for adhesion, and hence affecting the bond strength.[2021]

In the present study, with the aim of standardizing the superficial and deep dentin, periapical radiographs of teeth were taken, which served as the auxiliary means of analyzing the proximity of the pulp chamber and consequently for determining the height of the occlusal cut. Thus, the variability of the dentin according to the anatomy of the dentin-pulp complex was seen, with superficial dentin obtained from perpendicular cuts corresponding to the same type of dentin.

Regarding the active application of adhesive systems in dentinal substrate has been an easy alternative for clinical application, resulting in better performance of these systems.[7] In this study the active application provided the higher hardness in both superficial and deep dentin for the adhesive system AD, and it was used in a standardized manner, similar to that used with conventional adhesive systems in the studies of Dal-Bianco et al.[7] and Higashi et al.[11] In addition, the mechanical pressure applied to the microbrush actively promotes the greatest potential to dissolve the smear layer, as well as an increase in the agitation of the molecules, allowing greater evaporation of the solvent and therefore better diffusion of monomers into dentin, thus leading to an increase in the percentage of polymer formation and improving the mechanical properties of the adhesive layer.[22]

The previous results in the literature demonstrated that the degree of polymerization is negatively correlated with the amount of solvents present in the adhesive,[2324] meaning that the higher the amount of water/solvent, the lower the degree of polymerization.[11] Both the behavior of SB and AD with regard to the elastic modulus was unchanged, meaning that irrespective of the application mode or dentinal substrate, this property is not affected.

In each adhesive system, there is a known solvent concentration, in which maximum monomer conversion is reached, which appears to be related to viscosity, and could influence its mechanical properties. Although not measured in this study, it was visually evident that SE is more viscous than SB. Thus, the rewetting of the substrate during the adhesive technique might have been beneficial to SE, decreasing the viscosity of the adhesive and enabling greater mobility of the components in the reaction during polymerization. However, this hypothesis should be the subject of future studies for confirmation.

The polymerization shrinkage of composite resins can be minimized when adhesive systems with a low elastic modulus are applied in a thinner layer, whereas systems with a higher elastic modulus should be applied in a thicker layer to achieve this purpose.[25] Thus, a sufficiently flexible layer of intermediate resin could withstand the polymerization shrinkage stress of the composite resin as well as improve the distribution of stresses induced by thermal changes, absorption of water and occlusal force at the interface.[17] However, more studies can be performed by relating the mechanical properties of the adhesive layer and the thickness of the adhesive systems, because the polymerization shrinkage can be minimized when adhesive systems are applied in appropriate thick layers, from the analysis of elastic modulus.

CONCLUSION

Within the limits of this study, the experimental results reveal that the mechanical properties were influenced for the dentin depth and application mode of the different adhesive systems.