Shear bond strength evaluation of resin composite to resin-modified glass-ionomer cement using three different resin adhesives vs. glass-ionomer based adhesive

Introduction

Resin composites have gained considerable popularity in the restorative dentistry as a direct restorative material for both anterior and posterior teeth. However, they have several disadvantages, such as polymerization shrinkage, potential failure of adhesion leading to secondary caries, and a relatively high co-efficient of thermal expansion (1, 2). Polymerization shrinkage, mechanical and thermal stresses create contraction, internal stresses and deformation in the resin composite restoration and surrounding tooth structure. These stresses may result in de-bonding from the cavity walls or cohesive fractures of the restoration material or tooth structure (3, 4).

In contrast, glass ionomer cements (GICs), which were developed by Wilson and Kent (1971), provide long-term fluoride release, thereby decreasing the possibility of recurrent caries (5). They also have physicochemical adhesion to tooth structure, coefficient of thermal expansion similar to that of tooth structure, biocompatibility and low cytotoxicity (6,7). The reaction of GICs displays a similar behavior to dentin under thermal stimuli (8). Because of these favorable properties, it can be placed on dentin prior to the application of a resin composite. This restorative method is commonly referred to as the sandwich technique (laminate or bilayered technique) (9, 10). The rationale behind this technique is to combine the advantages of both GICs and resin composites to form one restoration in order to enhance the clinical serviceability (10-12).

The GIC considerably reduces the volume of resin composite used, subsequently minimizing the detrimental effects of polymerization shrinkage (13, 14). It also prevents the resin composite from bonding with the dentin, thereby restricting the adhesion of the resin composite to the enamel. The reduction in bonded composite surfaces decreases the configuration factor (C-factor) of the cavity (15). In addition, it acts as a stress absorbing interfacial layer between the shrinking resin composite and dentin (16). In contrast to resin bonding, the adhesion of glass-ionomer to tooth structure is less technique sensitive and its quality increases with time (8).

Two types of GICs, i.e., conventional or resin-modified GIC (RMGIC), can be used as base in the sandwich technique, although they show differences in the adhesion mechanism, setting reaction, and sensitivity to the moisture of the materials. The success of this technique depends on the bond strength of the GIC to both dentin and resin composite (17). Some studies have demonstrated that acid-etching of the cement surface enhances the bond strength due to increases in mechanical retention (18, 19); however, this has not been confirmed by other studies (20, 21).

Clinically, the etching procedure requires a two-to-seven minute waiting period for the initial setting of the GIC to be completed (18, 22). High technique sensitivity, low cohesive strength and slow setting reaction of conventional GIC, and also minimal of chemical bonding between it and resin have led to the introduction of resin-modified GIC (RMGIC) (18, 23). The RMGIC has been developed to improve tensile and fracture strengths, working time combined with a rapid set, chemical solubility, and polishing appearance (6, 12, 24, 25). The setting reaction of RMGIC follows two distinct mechanisms: resin polymerization and acid-base reaction. RMGIC also demonstrates significantly improved cohesive strength, lower modulus of elasticity and better bond strength to tooth than the conventional GIC. Nonetheless, these cements usually have somewhat inferior esthetic and abrasion resistance when compared to resin composite, thus limiting their use in high stress-bearing areas (6, 11, 25, 26).

The recent development of adhesive systems, including self-etch primers might overcome this disadvantage, as there is no need to rinse the GIC prior to application of the bonding agent (22, 27). In the last few years much attention has been focused on the development of adhesive systems. Therefore, light cured glass ionomer based adhesive (Fuji Bond LC) was introduced. It is the first bonding agent based on a dynamic glass ionomer adhesion; it is essentially a diluted version of the restorative RMGIC Fuji II LC and suitable for bonding all resin composites (28).

However, to date, only a few studies have evaluated the efficacy of different resin-based and glass-based adhesives on RMGIC surface before the application of the resin composite in sandwich technique and there is no consensus on the type of surface treatment modalities over RMGIC. Hence, the present study was conducted to compare the shear bond strength (SBS) of resin composite to RMGIC utilizing three different generations resin adhesives versus a GC Fuji bond LC. Moreover, this modified protocol would not only prevent moisture contamination or desiccation of the underlying GIC, but it could also save precious chair-side clinical time. The null hypotheses were that adhesive type and the acid-etching procedure have no effect on the SBS of the resin composite to RMGIC.

Materials and Methods

In this in vitro study, 21 acrylic resin blocks (Acropars, Tehran, Iran) were prepared using a cubical aluminum mould, 100×25×20 mm in dimension, which was polished with carbide polishing paper. Before applying the molding acrylic resin into aluminum mould, four water soluble tablets were located on glass slab in specified place and after the completion of the setting of acrylic resin, they were washed out.  Thus, four holes of 5-mm in diameter and 2-mm in depth were made in each block. The holes were finally filled with RMGIC (Light-Cured Universal Restorative, GC Corporation, Tokyo, Japan) by mixing it according to the manufacturer's instructions. Every effort was made to ensure that the exposed cement surface has been flushed with the surrounding specimen container. A light-emitting diode (LED) light curing unit (Demetron A.2, kerr Italia, S.p.A., Scafati, Italy) was used to polymerize the cement for 20 s at 1000 mW/cm2 light intensity. The tip of light cure unit was placed 1-mm above the surface of the cement. The RMGIC surface was not finished to a glass-smooth surface to mimic the clinical scenario.

Prior to cement surface treatment, the bonding area was demarcated with an adhesive tape with a punch hole of 3.5-mm in diameter. Thereafter, the 84 specimens were randomly divided into seven treatment groups (n=12) according to the surface treatment performed:

Group 1: No treatment was applied on the cement surface (control group.(

Group 2: The cement surface was not etched.

Group 3: The cement surface was etched with a 37.5% phosphoric acid gel (Gel Etchant, Kerr Italia S.p.A., Salerno, Italy) for 15 s.

All specimens in group II and III were rinsed thoroughly for 20s and gently air dried for 5s to remove excess moisture without desiccation of cement. Then, OptiBond Solo Plus (Kerr Italia S.p.A., Salerno, Italy) was applied to the bond area marked on the cement surfaces and cured according to manufacturer’s instructions (Table 1.(

Group 4: OptiBond XTR self-etch adhesive (Kerr Italia S.p.A., Salerno, Italy) was applied on the cement surface and cured according to the manufacturer's instructions (Table 1).

Group 5: OptiBond All-in-One self-etch adhesive (Kerr Italia S.p.A., Salerno, Italy) was treated on the cement surface and cured according to the manufacturer's instructions (Table 1).

Group 6: Fuji Bond LC adhesive (GC Corporation, Tokyo, Japan) was treated on the cement surface and light cured according to the manufacturer's instructions before adding the resin composite (Table 1)

Group 7: Same as group 6, but the Fuji Bond LC bonding agent (GC Corporation, Tokyo, Japan) was not cured over RMGIC, the GIC-based adhesive and resin composite was co-cured for 20 s. On the other hand, immediately afterward resin composite was placed on the treated cement surface, two materials light cured together for 20s.

Immediately following these procedures, a transparent plastic tube with 3-mm inner diameter and 2-mm height was filled with a microhybrid resin composite (Point 4, Kerr Italia S.p.A., Salerno, Italy, A2 Body Shade) in a one-layer increment technique and centered over the RMGIC surface in the template. Subsequently, any excess uncured resin composite was carefully removed from periphery of tubes with an explorer and cured resin composite then with sharp surgical blade. The tube was exposed to the curing light for 20s vertically and for 20s circumferentially (10s from each side) to ensure complete polymerization. For all the specimens, the tip of light cure unit was placed 1-mm away from the surface of the restoration materials. After the composite buildup, the plastic tube was carefully removed with scalpel blade, leaving the resin composite rod on the treated adhesive surface of the RMGIC block. All the experimental procedures were conducted at room temperature, and the manufacturers’ instructions were precisely followed for the all materials.

After the preparation, each of the specimens were kept moist to avoid any dehydration changes and cracking during the laboratory procedures which might have affected the bond strength. Subsequently, all the specimens were stored in an incubator with 100% humidity at 37 ± 1°C for one month and then thermocycled (Vafaei Industrial Factory, Tehran, Iran) 1500 cycles between 5°C to 55°C to simulate  clinical situation with a dwell time of 1-minute in each bath and transfer time 5s.

The shear bond test was performed using a Universal Testing Machine (Zwick GmbH & Co, Ulm, Germany) at a cross-head speed of 1 mm/min until fracture occurred. The shear load was applied as close as possible to the adhesive interface between the resin composite and RMGIC utilizing a blunt knife-edged apparatus and the maximum load required to de-bonding the two materials was recorded for each specimen (Fig 1). The SBS was calculated in Mega Pascal (MPa), which is derived by dividing the maximum load force (N) at the time of fracture by the bond area (πr²).

After mechanical failure, the fracture modes in all the specimens were evaluated by one observer (MS) under an optical microscope (Olympus Optical Co., Tokyo, Japan) at 20x magnification. Accordingly, the specimens were classified into three groups: Fractures were called “adhesive failure” when the resin composite was removed from the glass-ionomer surfaces without residual debris, “cohesive failure” when fracture occurred inside the restorative materials, and “mixed failure” when combination of both cohesive and adhesive failures were observed.

Data were analyzed using SPSS-18 software (SPSS Inc, Chicago, IL, U.S.A.). Descriptive statistics including the mean, standard deviation, maximum and minimum of the SBS were accounted for each group. Normal distribution of the data and homogeneity of variances were confirmed using Kolmogorov-Smirnov and Levene's tests, respectively (p>0.05). Therefore, one-way analysis of variances (ANOVA) was used for the comparison among all groups and post-hoc Tukey test for pairwise comparisons of the groups. The failure mode frequencies were analyzed using the fisher’s exact test. P value < 0.05 was considered statistically significant.

Table 1. Various restorative materials used in the study and mode of their applications according to the manufacturers' instructions.

Fig 1: Schematic diagram of the specimen for shear bond strength test

                               Results

The descriptive statistics on the mean SBS (MPa) of resin composite to RMGIC utilizing different adhesives at the fracture for each group are tabulated in Table 2. The one-way ANOVA revealed that the SBS was significantly different among the experimental groups (P<0.001). The maximum SBS mean values (24.62±3.70 MPa) were recorded for Group 4, where OptiBond XTR self-etch adhesive (Kerr Italia S.p.A.) was applied on the RMGIC surface; on the other hand, the control group which had not receive any pretreatment displayed minimum SBS values (18.15±3.38 MPa(.

Comparisons of the groups using post-hoc Tukey test indicated that the differences in SBS between groups 3 (OptiBond Solo Plus without acid-etching) and 6 (Curing Fuji Bond LC before applying resin composite) with the control group was not significant (p>0.05); however, in the other groups (II, IV, V and VII) had a significantly higher SBS compared to the control group (p< 0.05). Based on the results of the present study no significant differences were found between groups that bonded with different adhesives (p>0.05) (Table 3).

Microscopic examination of interfacial debonding revealed that the majority of failure modes were adhesive failure followed by mixed failure, except for the group III that cement surface was etched with phosphoric acid that showed 58.3% mixed failure (Table 4). The fisher’s exact test showed that significant differences were not observed in frequency of the failure mode across the test groups (p=0.207).

Table 2. Mean values, standard deviation (SD), minimum and maximum of the SBS (MPa) of resin composite bonded to the RMGIC by using different adhesives (n=12).

Table 3. Pairwise comparison between the groups and its statistical significance using post-hoc Tukey test.