Shear bond strength of composite resin to resin-modified glass ionomer cement using 2-hydroxyethyl methacrylate-based and 2-hydroxyethyl methacrylate-free adhesive system.

BACKGROUND: Sandwich technique combines the properties of both composite resin and glass ionomer cement to make the most of the physical and esthetic properties of each material. Bonding agents improve the wettability of GIC surface and improves the bond between composite resin and GIC. The aim of this study was to evaluate and compare shear bond strength of composite resin to resin modified glass ionomer cement using HEMA-based and HEMA-free adhesive systems. An in vitro study.
MATERIALS AND METHODS: Total 30 disc-shaped samples were prepared with resin modified glass ionomer cement (RMGIC). Samples were divided into three groups, each group containing 10 samples. Group I (n=10): Nano-hybrid composite resin (NHCR) was bonded to RMGIC without any adhesive system. Group II (n=10): NHCR was bonded to RMGIC using hydroxyethylmethacrylate (HEMA)-based adhesive system. Group III (n=10): NHCR was bonded to RMGIC using HEMA-free adhesive system. The shear bond strength was tested using Universal testing Machine and the results were calculated using one way ANOVA and Post-Hoc test.
RESULTS: Maximum shear bond strength was recorded in group III where HEMA-free adhesive used with a mean value of 6.13±1.859 MPa followed by group II where HEMA-based adhesive used with mean value of 4.38±1.533 MPa. The control group showed least shear bond strength.
CONCLUSION: Application of HEMA-free adhesive (OptiBond All-In-One) resulted in greater shear bond strength between RMGIC and composite resin than HEMA-based adhesive (Single bond Universal Adhesive).

INTRODUCTION

Glass ionomer cement (GIC) was introduced in the 1970s by Wilson and Kent. One of the important qualities of GIC is its property to release fluoride that provides anticariogenic effect. Other than this, it provides chemical adhesion to the tooth structure, biocompatibility, and low cytotoxicity.[1] To improve the physical and chemical properties of conventional GICs, resin-modified GIC (RMGIC) was introduced.[23]

Composite resins are used in both anterior and posterior restoration due to its esthetic properties. However, there are some disadvantages of composite resins such as polymerization shrinkage, microleakage, and pulpal irritation.[1] To reduce microleakage at restoration margins, GIC is used as a base beneath composite resin restoration. This is also known as “laminate restoration” or “sandwich technique.” This technique was developed by McLean in 1985 to seal cavities and reduce microleakage.[12] This technique combines the properties of both composite resin and GIC to provide dentin adhesion, fluoride release, as well as esthetics and polishability.[34] The rationale behind the technique is to make the most of the physical and esthetic properties of each material, as it combines the dentin adhesion and fluoride release of glass ionomer as well as the esthetics and polishability of resin.[56]

Bonding agents improve the wettability of GIC surface and improve the bond between composite resin and GIC or its modified types.[2] Adhesive agents have undergone various modifications such as changes in viscosity, modification of primers, and addition of nanofillers to improve the bond strength between the tooth and composite resin.[7]2-hydroxyethyl methacrylate (HEMA) is a water-soluble methacrylate monomer frequently present in dental adhesives. It enhances the wetting properties of dental adhesives, and thereby, positively influences bond strength to dentin.[8] However, studies have shown that restorations bonded with HEMA-containing adhesives are more susceptible to water absorption and hydrolytic degradation.[9] Some studies have shown that removal of HEMA from self-etch adhesives would minimize water sorption, while others have observed that HEMA would be beneficial for the adhesive system.[10] HEMA is considered one of the most potent allergens among the monomers.[11] Fast penetration of noncured monomers through the skin and gloves can cause contact dermatitis in dental personnel.[12] The disadvantages of HEMA have led to the introduction of HEMA-free less hydrophilic adhesives which may show reduced water sorption, higher stability of mechanical properties, stability of interfacial bond, improvement in bonding durability, and reduced allergenic potential.[13]

During the restoration of deep Class II cavities, GIC, and resin interface may communicate with the oral environment. This may lead to bond failure. Different studies[135613] have been carried out to measure shear bond strength between GIC and composite. However, literature has failed to reveal the comparison of the shear bond strength after use of HEMA-based and HEMA-free adhesives. On this background, the aim of this in vitro study was to evaluate the shear bond strength of composite resin to RMGIC using HEMA-based and HEMA-free adhesive system.

The null hypothesis of this study is that there is no difference in the shear bond strength of composite resin to RMGIC using HEMA-based and HEMA-free adhesive system.

MATERIALS AND METHODS

RMGIC (Fuji II LC, GC America, Alsip, IL, USA) bonded to the nanohybrid composite (Filtek Z-250 XT 3M ESPE, St. Paul USA) using two different bonding agents, HEMA-based adhesive (Adper Single Bond 2-3M ESPE, St. Paul USA) and HEMA-free adhesive system (OptiBond All–In-One; Kerr Co, Orange, CA, USA.) Ethical approval for this study was obtained from the Institutional Ethical Committee.

Preparation of specimens

A total of 30 disc-shaped samples of light-cured RMGIC (Fuji II LC, GC America, Alsip, IL, USA) were prepared using stainless steel molds (10-mm diameter and 2-mm thickness). The RMGIC was manipulated according to the manufacturer's instructions and filled in the molds. The surface of the filled mold was pressed with a glass slide and light cured for 30 s using a light cure unit (LED Light Curing system, Kerr Corp, Orange, CA, USA). Specimens were randomly divided into three groups of ten specimens each, the groups were as follows:

Group I (n = 10): Control group. RMGIC ( Fuji II LC, GC America, Alsip, IL, USA) and composite resin (Filtek Z-250 XT 3M ESPE, St. Paul USA) were bonded without using any adhesive system

Group II (n = 10): RMGIC (Fuji II LC, GC America, Alsip, IL, USA) and composite resin (Filtek Z-250 XT 3M ESPE, St. Paul USA) were bonded with HEMA-based adhesive (Adper Single Bond 2-3M ESPE, St. Paul USA). The adhesive agent was applied using applicator tip and air-dried with air syringe followed by light-curing for 30s

Group III (n = 10): RMGIC (Fuji II LC, GC America, Alsip, IL, USA) and composite resin (Filtek Z-250 XT 3M ESPE, St. Paul USA) bonded with HEMA-free adhesive (OptiBond All-In-One; Kerr Co, Orange, CA, USA). The adhesive agent was applied using the applicator tip and air with air syringe followed by light curing for 30s.

After this procedure, a transparent plastic ring, 6 mm height and 5 mm internal diameter, was centered over RMGIC. The composite resin was condensed into a transparent plastic ring using incremental curing technique, with each increment of 2 mm following curing the plastic ring was removed. All the procedures were carried out at the room temperature.

The samples were stored in distilled water at room temperature for 24 h prior to shear bond strength testing.

The samples were placed in between the jigs of universal testing machine (Model UNITEST-10, Instron Corp, USA), and a pointed shearing rod was placed on composite resin and RMGIC sample interface. It was subjected to static loading at a rate of 0.5 mm/min until fracture occurred. Data were analyzed with one-way analysis of variance and post hoc test.

RESULTS

The null hypothesis for this study was rejected as the difference between the experimental groups was found to be statistically significant.

One-way analysis of variance test

Mean shear bond strength of the tested groups are shown in Table 1.

Table 1

Mean and standard deviation of shear bond strength values (MPa) in the groups under the study of the resin-modified glass ionomer cement bond to the composite using different adhesive systems

Shear bond strength (MPa)	n	Mean±SD	F	df	P
Group I (no adhesive)	10	1.75±0.479	24.2	2	<0.001 (highly significant)
Group II (HEMA-based adhesive)	10	4.38±1.533
Group III (HEMA-free adhesive)	10	6.13±1.859

HEMA: 2-hydroxyethyl methacrylate, SD: Standard deviation

The maximum shear bond strength values were recorded in Group III, showing a mean value of 6.13 ± 1.859 MPa compared to Group II with mean shear bond strength of 4.38 ± 1.533 MPa. Control group, where no adhesive was used, showed the least mean shear bond strength of 1.75 ± 0.479 MPa for restoration, as shown in Table 1. Pair-wise comparison between these groups revealed that the difference of shear bond strength was statistically highly significant as shown in Table 2.

Table 2

Intergroup comparison and its statistical significance

Groups (I)	Groups (J)	Mean difference (I-J)	SE	P
Group I	Group II	−2.63*	0.634	0.001
Group I	Group III	−4.38*	0.634	<0.001
Group II	Group III	−1.75*	0.634	0.03

*The mean difference is significant at the 0.05 level. SE: Standard error, HEMA: 2-hydroxyethyl methacrylate

DISCUSSION

The sandwich technique has been advocated to restore deep proximal cavity where isolation is questionable.[14] In this study to simulate sandwich restoration, composite resin was bonded over RMGIC disc. Some studies have suggested that application of RMGIC instead of conventional GIC provide better mechanical properties, more resistance to moisture, reduction in microleakage, and higher bond strength to composite resin.[15] The inclusion of resin in RMGIC resulted in the formation of resin tags in dentinal tubules and allowed the material to polymerize upon light activation, that supplemented chemical bond that GIC achieves with the tooth structure by bonding micromechanically. This double adhesion mechanism is the main determined of retention and provide better marginal sealing and reduce the microleakage.[16] RMGIC used in sandwich technique reduces the bulk of overlined composite material, thereby reducing polymerization shrinkage and enhance the marginal adaptation.[14]

Composite restorations in the oral cavity are routinely exposed to thermal changes. The material should be tested after 24 h of storage in water or after thermocycling process.[17] Thermocycling is a questionable method in in vitro studies because of variation in thermocycling temperature and lack of standardization, but in most of the studies, the temperature was set at 5°C–55°C ± 2°C and dwell time was 15 s.[18] The disadvantage of this method is that, the temperature used to stress restorations may not be the real temperatures of cold and hot food/beverage tolerated intraorally, and the increase in dwell time exceeds real clinical condition which leads to fatigue of composite material which may adversely affect the mechanical properties.[18] On this background, all the samples were kept in water for 24 h prior to testing instead of thermocycling.

Dental adhesive provides retention to composite resin and withstand mechanical forces. Furthermore, a good adhesive should be able to prevent leakage at the margins of restoration. Clinically, failure of restorations occurs more often due to inadequate sealing.[19] The total-etch adhesive systems need etching before applying. The etching and rinsing process may remove calcium and aluminum from glass ionomer and reduce the cohesive strength of it.[20]

Dental adhesives improve bond between GIC and composite by producing a rough surface in which glass particles stand out above the matrix, thus the resin is able to penetrate into micropores between the particles and providing mechanical interlocking.[21]

The self-etch approach either two- or one-step adhesives are more user-friendly due to time-saving and simplified procedure.[3] HEMA is a hydrophilic monomer present in the adhesive system, and it acts as a cosolvent and minimizes phase separation, thus increasing miscibility of hydrophobic and hydrophilic component in the solution.[9] HEMA-containing adhesives are more susceptible to water contamination, as HEMA in the uncured adhesive may absorb water which can lead to dilution of monomers to the extent that polymerization is inhibited.[20]

Presence of acidic monomer into the primer of self-etch adhesive system causes penetration of monomer; and hence, it does not require etching as a separate step. Acidic monomers increase wetness of the dentin, so the presence of HEMA in these adhesives would not be necessary.[9] After polymerization, HEMA will exhibit hydrophilic properties and will lead to water uptake and consequent swelling and discoloration, as it is fixed in polymer chain for longer time.[22] It also adversely influences the mechanical strength. Furthermore, high amounts of HEMA will result in flexible polymers with inferior quality.[23]

Both HEMA-free adhesive (OptiBond All-In-One) (pH = 2.5) and HEMA-based adhesive (Adper single Bond) (pH = 2.7) are acidic in nature, thus it improves the bond between RMGIC and composite resin, as it dissolves superficial surface of GIC.[5] HEMA-free adhesive has acetone and ethanol that balance solvent evaporation and maintain the wetness of the substrate.[9] Another explanation for good performance of HEMA-free adhesive is that it contains glycerol phosphate dimethacrylate monomer, a surfactant monomer that facilitates the penetration of hydrophobic component and reducing phase separation.[24]

Limitation of study, in this in vitro study, shear bond strength of composite over RMGIC has been evaluated. Although in clinical situations, composite and RMGIC will be bonded to cavity surfaces, which will further affect the shear bond strength clinically. Hence, further studies are necessary to simulate such an intraoral condition.

CONCLUSION

Within the limits of the above study, it was concluded that:

Application of bonding system results in an increase in shear bond strength between RMGIC and composite resin

Application of HEMA-free adhesive (OptiBond All-In-One) resulted in greater shear bond strength between RMGIC and composite resin compared with HEMA-based adhesive (Single bond Universal Adhesive).

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.