INTRODUCTION

Resin-modified glass ionomers (RMGIs) were introduced as a hybrid between conventional resin composites and glass ionomers.¹ Because these materials do not have the strength or wear resistance of conventional composites,² their use as posterior occlusal load bearing restoratives is questionable.^3-6 Resin-modified glass ionomers have poor retention as pit and fissure sealants⁷ but excellent longevity as cervical restorations⁸ and as liners and bases.⁹ In these applications, it is critical for the RMGI to develop an effective bond to tooth structure.

Traditional glass ionomers bond to dentin by an ionic bond with hydroxyapatite, and conventional composite materials bond to dentin through micromechanical interlocking with collagen fibrils and dentinal tubules. RMGIs contain components of glass ionomers (fluoro-aluminosilicate glasses and polyacrylic acid) as well as resin composites (photo or chemical initiators and methacrylate monomers).¹⁰ Due to their hybrid nature, RMGIs bond to dentin through both an ionic bond between polyacrylic acid and hydroxyapatite and mechanical interlocking with collagen and the resin monomer.¹¹ The initiation of this bond can be attributed to the various methods of polymerization of the RMGIs.

These materials polymerize by up to three mechanisms: 1) an acid-base reaction between the polyacrylic acid and the fluoro-aluminosilicate glass, 2) a photo-initiated free-radical reaction between methacrylate monomers, and 3) a chemically-initiated reaction between methacrylate monomers remaining after photo-initiation.¹² Recent studies have shown the acid-base and photo-initiated free-radical reactions inhibit each other, with photopolymerization of RMGIs reducing the acid-base reaction.^10,13 Another recent study concluded that the acid-base induced chemical interaction between RMGI and dentin is the main mechanism of bonding with RMGI.¹⁴ Therefore, it is hypothesized that RMGIs allowed to polymerize without light activation will develop bond strength through the ionic bonding between RMGIs and dentin and should therefore have an equivalent or higher bond strength than light-cured RMGIs.

To test the hypothesis, the bond strength of RMGI to dentin with and without light polymerization was compared. It was assumed that bond strength to dentin with uncured RMGI would be attributed to an acid-base reaction. The null hypothesis was that there would be no difference between the light-cured and uncured groups.

MATERIALS AND METHODS

Sixty freshly extracted human molars were collected from the University of Alabama at Birmingham School of Dentistry. The occlusal surfaces of the teeth were ground to expose midcoronal dentin with a grinding wheel (Model 108; Wehmer Co, Addison, IL, USA). Specimens were polished with 600-grit SiC paper under water. The teeth were evenly and randomly divided into six groups: 1) Ketac Nano (KN), 2) KN without light cure (woLC), 3) Fuji Filling LC (FF), 4) FF woLC, 5) Fuji II LC (FII), and 6) FII woLC.

Immediately following polishing, all groups (n=10) were conditioned and primed according to the manufacturers' instructions (Table 1). A transparent rubber tube (1.54 mm inner diameter and 4 mm length) was filled to a depth of 2 mm with the corresponding RMGI and pressed to the prepared surface of each sample. Groups 1, 3, and 5 were then light polymerized with an Elipar S10 LED curing light (813 mW/cm²_, 3M ESPE, Seefeld, Germany) from above and both sides for 20 seconds and placed in a damp, dark box. Groups 2, 4, and 6 were placed in the damp dark box within one minute after affixing the rubber tube without light curing. The box was then placed in a 37°C incubator (Queue; ThermoElectron, Waltham, MA, USA) for 24 hours.

Table 1:  Products Used in This Study

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The specimens were removed from storage and dried with absorbent wipes. The rubber tube was cut away to reveal a 1.54-mm diameter RMGI cylinder bonded to each specimen. The specimens were loaded into a custom fixture in a universal testing device (Instron, model 4411, NVLAP, Canton, MA, USA) that secured the teeth with the bonded RMGI from three sides. A blade attached to the upper member of the Instron was used to fracture the specimens at a crosshead speed of 1 mm/min. The maximum load required to separate the RMGI cylinder from dentin was recorded. Shear bond strength (SBS) was determined: maximum load/surface area of composite post. The fracture surfaces of the specimens were examined under light microscopy at 100× magnification (VHX-600, Keyence Co, Osaka, Japan) to determine the mode of failure.

The normal quantile plot of shear bond strength by treatment was examined to determine acceptable normality for the data. Differences between treatment groups were analyzed statistically by analysis of variance (ANOVA) (α=0.05). Groups were compared to the mean using a Tukey post-hoc test (α=0.05).

RESULTS

Table 2 shows the mean SBS and standard deviations of all groups tested. Two KN, all KN woLC, and seven FII woLC specimens debonded before testing. For these groups, the specimens with premature failures were given a value of zero and included in the statistical analyses. A one-way ANOVA revealed significant differences among testing groups (p<0.05). A Tukey/Kramer analysis revealed no significant differences in bond strength between the three light-cured RMGIs (Ketac Nano, Fuji II LC, and Fuji Filling). KN was assumed to be greater than KN woLC because the bond with KN woLC was too weak to survive testing. FII was significantly greater than FII woLC (p<0.05), but no significant difference was observed between FF and FF woLC. Fracture modes of failure are presented in Table 3. Uncured specimens most frequently fractured by adhesive failure, while some light-cured specimens failed cohesively. The ratios of adhesive:mixed:cohesive fractures for cured KN and FF, respectively, were 4:5:1 and 4:3:3, and uncured KN and FF were 8:2:0 for both. The same ratio for cured and uncured FII samples were 7:3:0 and 8:2:0, respectively.

Table 2:  Mean Shear Bond Strength (MPa) and Standard Deviation of the Resin-modified Glass Ionomers Tested (N=10)

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Table 3:  Mode of Failure of Each Group

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DISCUSSION

Light-cured RMGIs develop bond strength through the fast-acting light-initiated reaction of resin monomers as well as an acid-base reaction and a chemically-initiated free-radical reaction. Non–light-cured RMGIs polymerize through only the acid-base and chemical-initiated free-radical reactions. Our study reports low bond strength of non–light-cured RMGIs (excluding FF) and relatively high bond strength of cured RMGIs. Therefore, our results suggest that most RMGIs do not develop adequate bond strength without light-activated resin polymerization, rejecting the null hypothesis.

The non–light-polymerized Fuji Filling LC was the only non–light-cured material to produce the same SBS as the light-cured specimens. The manufacturer of this material indicated that a chemical initiator is included in FF but not FII, and it is possible that the chemical initiator activated the free radical resin polymerization in FF. The manufacturer of KN indicated that a chemical initiator is not included in their material. Therefore, the KN woLC and FII woLC groups polymerized only through an acid-base reaction.

The contribution of the acid-base and free radical reactions on the mechanism of bonding of RMGIs has not been completely elucidated, and both mechanisms of bonding have been reported in dental literature. Mitra and others¹⁵ and Coutinho and others¹⁶ analyzed the reaction between methacrylated copolyalkenoic acid (a key component of RMGIs) and pure hydroxyapatite (HAP) crystals. Fourier transform infra-red spectroscopy and X-ray photoelectron spectroscopy analyses indicated an ionic bond between the methacrylated polyalkenoic acid and HAP, suggesting an acid-base reaction. On the other hand, scanning electron microscopy analysis of interfacial microstructure between RMGIs and dentin, performed by Coutinho and others,¹⁶ showed a submicron hybrid layer of RMGI and an analysis by Mitra and others¹⁵ showed tag-like structures of RMGI penetrating dentin. Carvalho and others¹⁷ describe a similar phenomenon terming it a demineralized, resin-infiltrated zone—a characteristic of free-radical polymerization based bonding.

A study by Cardoso and others¹⁴ measured the bond strength of RMGIs bonded to fractured dentin with and without the use of a conditioner. Micrographs revealed the presence of a hybrid layer in the conditioned dentin specimens but not in the unconditioned specimens. However, no difference in bond strength was measured between the conditioned and unconditioned dentin groups. Based on these observations, the authors suggested that the mechanical interlocking provided by a hybrid layer is not the primary mechanism of bonding with RMGIs. The authors later acknowledged the possibility of a nanometer-scaled hybrid layer created by the mild self-etching properties of the polyalkenoic acid present in the RMGI. In summary, the effect of either bonding mechanism towards the adhesion of RMGIs to dentin is still unclear.

Traits of both ionic (acid-base) and resin infiltration (free-radical) bonding are reported in current literature; however, this study suggests resin infiltration creates the strength of the bond of RMGI to dentin. In vitro testing has shown that the SBS of glass ionomer materials is weaker than that of resin composites. Additionally, previous studies have reported that RMGIs have SBS values greater than glass ionomer materials but less than composites.^18,19 Therefore, it would be expected that conditions that enable a RMGI to behave like a resin composite material, such as light polymerization, would favor higher SBS values.

The failure analysis revealed that more of the light-cured specimens of FII and KN failed by cohesive and mixed failures. This result further indicates that a stronger bond was formed between light-cured RMGI and dentin than uncured RMGI and dentin. The failure modes of cured and uncured FF were nearly identical, most likely due to the chemical-initiated free radical polymerization of the uncured FF.

All RMGI groups in this study were applied preceding a coat of conditioner or primer as indicated by the manufacturer. FF and FII both use a conditioner. Conditioners can remove the smear layer and partially demineralize dentin,²⁰ and the use of a conditioner can significantly improve bond strength.^21,22 KN uses a light-cured primer. The requirement to light polymerize this primer indicates that it contains a photo-initiated resin component—similar to an adhesive. Pereira and others²³ concluded that the SBS of RMGI to dentin improved with the use of an adhesive, a technique that facilitates resin bonding. The layer of adhesive, however, decreases the fluoride release from these materials.^24-25 The results of Pereira and others reinforce the finding that light-activated resin polymerization provides the strength of the bond between RMGI and dentin.

A limitation of this study is that only three RMGIs were examined. Varying chemistries of other RMGIs may produce different results. Future studies could compare the SBS of additional RMGIs.

CONCLUSIONS

KN and FII have increased bond strength after light polymerization. The results suggest that light activation is needed to obtain optimal bond strength to dentin with some RMGIs. Clinically, KN and FII should be light cured after placement to achieve optimum bond strength.