An Invitro Evaluation Of Shear Bond Strength Of Conventional And Resin Modified GIC To Resin Composite Using Three different adhesives.

INTRODUCTION

           As patient's demand for posterior composite restorations are on the rise, the longevity of these restorations are becoming a common concern among researchers and clinicians. The durability of this adhesive restoration seems to be closely related to the integrity of the tooth-restorativematerial interface. Failures within the bonded interface may result in undesirable effects, such aspost-operative sensitivity, marginal staining, recurrent caries, and pulp pathology. Several aspects can contribute to the poor performance of posterior restorations, including composite shrinkagestresses, thermal/ mechanical fluctuations, light attenuation, technique sensitivity and incorrectindication. In addition, posterior restorations with margins in dentin, such as Class II and Class V preparations, are more challenging, since dentin bonding is less predictable than enamel bonding.

           Resin composite restorations are technique sensitive, particularly when placed in posterior teeth where access, visibility and moisture are difficult to control.¹

           In 1977, McLean first recommended lining composite resins with glass ionomer cement. In this technique, the glass ionomer is used to replace the lost dentin as a dentin substitute. Subsequentto the placement of the liner, an adhesive material is used and the composite resin restorative is placed. McLean's suggestion was a valuable technique as glass ionomers were able to bond and seal dentin and represent a practical way of reliably restoring posterior teeth with compositeresin. It was shown that composite resin could be bonded to acid treated glass ionomer in 1985.

This technique of tooth restoration by directly overlaying bonded materials has become known as "sandwiching", "layering" or "stratification". Based on principles of "biomimesis", i.e. there placement of tissue, or a part, using materials that most closely replicate the original essence, it can be argued that the properties of glass-ionomer cements make them the best dentin replacementmaterial, popularly referred to as “man-made dentin”, and the resin-based composites are the best enamel substitute.²

    Wilson and Kent introduced conventional glass ionomer cements in 1972 and later in 1992, Resin-modified glass ionomers, also called “hybrid” ionomers, were introduced. These light curedsystems have been developed by adding polymerizable functional methacrylate groups with a photo-initiator to the formulation. Such materials undergo both an acid-base ionomer reaction as well as polymerisation by photoinitiation and self-curing mode. These materials areknown for preventing postoperative tooth sensitivity when placed under resin-based compositerestorations, protecting against bacterial ingress to dentinal tubules, fluoride ion release andantimicrobial action. Addition of the resin component within the glass-ionomer formula not onlydecreases initial hardening time and handling difficulties, but substanstially increases wearresistance and physical strengths of the cement. The photocuring polymerization allows a longerworking time, a rapid hardening on command, and early development of strength and resistanceagainst acid attack. Fracture toughness, fracture resistance, and resistance to wear are allimproved in the resin-modified glass-ionomers, in addition to the major advantages of glass ionomers such as flouride ion release, biocompatibility, favourable thermal expansion andcontraction properties and physiochemical bonding to tooth structure.³

Based upon the underlying adhesion strategy, three mechanisms are currently in use: 1) Etchand-Rinse Adhesives 2) Self-etch Adhesives 3) Glass Ionomer Cements. The search for the ideal adhesive system has led to the development of various generations of bonding systems, namely the first generation which treated enamel surface to the recent development of seventhgeneration of self-etch adhesives, in an attempt to reduce the steps involved and to furthersimplify the clinical bonding techniques.⁴

The 2 step self-etching adhesive technique is basedon the simultaneous etching and priming of enamel and dentin using an acidic primer, followedby the application of adhesive resin. It eliminates the separate etching and rinsing steps, thussimplifying the bonding procedures and reducing technique sensitivity. The seventh generationself-etching adhesives combine the etching, priming and bonding procedures into a singlesolution and single step. The self-etching approach was introduced with the purpose ofcontrolling influence of the operator,⁵ though generally associated with lower bond strengths,self-etch adhesives provided bond performances comparable to the conventional 3- steptechnique in some cases. 6 The efficiency of these simplified bonding systems is stillcontroversial.

The GIC based adhesive was first introduced in 1995 and was nothing but a diluted version of resin modified glass ionomer cement, Fuji II LC, as GIC is still considered as the only materialthat selfadheres to tooth and have documented a two-fold micromechanical and chemical bonding mechanism to tooth substrate according to Yoshida et al. Also, clinical evaluation ofGIC based adhesive system has shown promising results.⁷

The bonding mechanisms of restorative materials to tooth tissues are often explained in the literature. Yet, few studies have addressed aspects regarding the restorative materials used in sandwich techniques. Although enamel and dentin pretreatment before the application of bonding systems and restorative materials is well established in the literature, the need for GICsurface treatment before the placement of composite resin in sandwich restorations still remainsdebatable.8Several studies have evaluated the effectiveness of total-etch adhesives and selfetchadhesives to bond to enamel and dentin. However, very few studies have evaluated thetechniques of achieving improved bond strengths between the resin composite and conventionaland RMGIC, used in sandwich restorations using total-etch and self-etch adhesive systems.

           Hence, the purpose of this present study is to compare the shear bond strength of conventionalglass ionomer cement and resin modified glass ionomer cement with composite resin usingtotal-etch, self-etch and GIC based adhesives.

MATERIALS & METHODS

           Hundred samples were prepared and divided into six groups of fifteen samples each and two control groups of five samples each. Samples were prepared using clear plastic straws of five mm diameter and nine mm height as shown in Figure 1and each group was prepared in the following manner. The surfaces of GIC were roughened prior to initial set, using a serrated condenser to create micromechanical retention and to mimic the clinical scenario. After the bonding procedure, composite resin (Ceram XTM Mono, DENTSPLY, DeTrey, Konstanz, Germany) is then added in increments to a height of 3mm x 5 mm diameter and each incrementis light cured for 40 secs using Ledition (IvoclarVivadent, USA) at 600mW/cm2.

FIGURE 1: SCHEMATIC FOR SAMPLE PREPARATION

Group 1: (n=15)

           Conventional glass ionomer cement (GC Fuji II Capsules, GC Corporation, Tokyo, Japan) is triturated using the Silver Mix 90 triturator for 10 secs and packed into the mould to a height of6 mm and 5 mm diameter. The specimens are stored in distilled water at 37°C for a period of24 h to simulate the oral cavity. After 24 h, GIC surface is etched using 36% o-phosphoric acidgel (Conditioner 36, DENTSPLY, DeTrey, Konstanz, Germany) for 15 secs and rinsed withwater for 20 secs. The surface is air dried using three-way syringe for 10 secs. The bondingagent, XP Bond Universal total-etch adhesive (DENTSPLY, DeTrey, Konstanz, Germany) isapplied to the etched GIC surface using a micro-brush, with vigorous scrubbing motion for 10secs and then light cured for 20 secs.

Group 2: (n-15)

           Conventional glass ionomer samples are prepared in the same way as Group1. Self-etch adhesive is applied before the initial set of glass ionomer cement (i.e. before 5 minutes), whichis verified using a sharp explorer tip. The self-etch adhesive used is Adper Easy One (3MESPE, USA) which is applied using a microbrush and rub it in for 20 secs. This is then airthinned for 5 secs and light cured for 10 secs.

Group 3: (n=15)

           Samples are prepared using resin modified glass ionomer cement (GC Fuji II LC Capsules, GCCorporation, Tokyo, Japan), which is triturated for 10 sec and packed into the mould to a heightof 6 mm. The surface is instrumented using the serrated condenser and light cured for 20 secs.

           Surface of set resin modified cement is etched using 36 % o-phosphoric acid for 15 secs and rinsed with water for 20 secs. The total-etch adhesive, XP Bond is applied using a microbrush and vigorously scrubbed for 10 secs and cured for 20 secs.

Group 4: (n=15)

           Resin modified glass ionomer cement samples are triturated, packed into moulds and cured for20 secs. Self-etch adhesive (Adper Easy One) is applied with a microbrush for 20 seconds, airthinned for 5 secs and light cured for 10 secs.

Group 5:( n= 15)

           Conventional GIC samples were triturated and condensed into moulds. The GIC based adhesive,(G-BOND, GC Corporation, Tokyo, Japan) was applied after the initial set of GIC, and left undisturbed for 10-15 secs, air thinned for 5 secs and then photo-cured for 10 secs.

Group 6: (n= 15)

           RMGIC samples were prepared in a similar manner and photo-cured. G-BOND is applied andphoto-cured according to the manufacturer's instruction. Control A: (n=5) (Conventional GIC + composite, no etch, no adhesive resin) Conventional glass ionomer (GC Fuji II Capsules) samples are prepared and composite is directly cured without etchin and bonding.

Control B: (n=5)

(RMGIC + composite, no etch, no adhesive resin) Resin modified glass ionomer samples (GC Fuji II LC Capsules) are prepared and is directly bonded to composite without etching and bonding.

           The samples were removed from the mould and mounted in a hollow plastic tube (3cm x3 cm)and filled with cold cure acrylic resin and stored in 100% humidity at room temperature for 48hours. The shear bond strength is determined using the Universal Testing Machine (LloydInstruments, LR 50K) at a crosshead speed of 1 mm/minute. A metal jig is fabricated to directthe shearing force perpendicular to the glass ionomer- composite interface as shown in Figure2.

RESULTS

           The mean shear bond strength and standard deviation was computed and analyzed by OnewayAnalysis of Variance (ANOVA) and Tukey test at a significance level of 0.05 as shown inTable 1, 2 and 3 and the graph was plotted. One-way ANOVA results showed statistically significant difference in results among the four groups and controls (p<0.001). Group 4 and Group 3 showed significantly higher bond strengths; followed by Group 6 than Group 2, Group1 and Group 5 showed the least. The shear bond strength values for resin modified glassionomer cements were significantly higher than conventional glass ionomer cement and selfetchadhesives produced significantly higher bond strength values than GIC based adhesivesand total-etch adhesives (p<0.001).

DISCUSSION

The capsulated form of conventional (GC Fuji II Capsules) and RMGIC (GC Fuji II LC) weretriturated using Silver Mix 90 triturator as automatically mixed encapsulated cements ensureeasy handling, standardized powder liquid ratio, and a homogenous consistency to the cementpaste.⁹ Mount in 1994 recommended capsulation as the ideal method for dispensing GIC.¹⁰

           Studies have shown that variations in glass and liquid composition, powder/liquid ratio, pretreatmentand size of glass particles, mixing by hand or in vibrational or rotational devices havean additional influence on the final mechanical properties of GIC.¹¹

The present study used XP BondTM as the onebottle total-etch adhesive system. Previous studies have shown that XP Bond adhesive system based on ter-butyl alcohol as solvent, reported higher micro-tensile bond strengths on both moist and dry dentin and significantly reduces sensitivity to the technique.^12,13

A study previously done by Taggart and Pearson, proposed that etching of conventional GIC after 24 h of maturation showed improvement in bond strengths.¹⁴ In Group 1, where etching and bonding was performed after 24h, did not show higher values when compared to other groups. The etching and bonding procedures were performed according to the manufacturer's instruction, after 24 h of placement of triturated mass in the mould. The mean shear bond strengths obtained for Group 1 (2.36 MPa) was the lowest obtained amongst all groups. This could be attributed to minimal chemical bonding of conventional GIC to resin composite due todifference in setting reactions.¹⁵ The nature of bonding here is solely depended on mechanicalretention and the micromechanical retention seen with etching enamel and dentin could not beachieved in conventional GIC. The result is lower than previous studies which have shownvalues of shear bond strength of self-cure GIC bonded to resin composite ranging between 3.6 -8.0 MPa after a 30 second etching time with 37 per cent phosphoric acid.¹⁶ Another reason forlow bond strengths could be due to the moisture contamination occurring during storage, whichcan interfere with the penetration and resin tag formation during the bonding procedure.¹⁷

           On the contrary, higher bond strength values of 6.19 MPa were obtained in Group 3, where RMGIC was bonded to composite resin using XP Bond. The inclusion of resin component intothe conventional GIC allows a rapid development of strength and more resistance to earlymoisture contamination. RMGIC have the additional advantage of directly bonding to resincomposite by producing a catalyst rich air- inhibited layer which can polymerize with thecomposite and produce chemical bonding.^18,19The result of the current study shows that shearbond strength values obtained for RMGIC were statistically higher than that for  conventionalGIC.

           The self etch adhesive, Adper Easy OneTM which is a single bottle adhesive that combines etchant, primer and adhesive into one solution, is a recently introduced seventh generation adhesive system. The pH of this elf-etch adhesive is greater than 2, hence considered as a mildself-etch adhesive group.²⁰ It contains HEMA (2-hydroxyl ethyl methacrylate), a hydrophilicmonomer. The hydrophilicity of HEMA helps in enhances its wetting property and improvesbond strength.²¹ Also, it contains ethanol as a co-solvent which is highly polar and results information of hydrogen bonds with the solutes and allows for greater resin infiltration²² Thehighest shear bond strength values were obtained for Group 4 (8.12 MPa), in which RMGICwas bonded to composite using Adper Easy One, could be explained on this basis. Higher bondstrengths were found for the restorative RMGIC than for conventional GIC which wasstatistically significant (p<0.001). This is probably due to its higher cohesive strength, ²³ sincecompositions of both materials are qualitatively similar. This result is in contrast to the resultsof previous studies which reported lower bond strengths with self-etch adhesives. Accordingto the results of the present study, shear bond values of self-etch adhesive is statistically higherthan that of total-etch adhesives.

In groups where G-BOND was used, higher values (5.66 MPa) were seen when RMGIC was bonded to composite than conventional GIC (2.09 MPa). This could be attributed to the similarity in composition of GIC based adhesive with the restorative materials used.⁷

The Group 2, in which conventional GIC is bonded to composite using Adper Easy One, priorto initial set (i.e before 5 mins), showed low bond strength values of 2.56 MPa. This is incontrast to the result obtained from a previous study by Gopikrishna et al, who reported bondstrength values of 4.5 MPa.²⁵ The reason for this poor bond strength might be due to the mildpH of self-etch adhesive, which results in weaker etching effect and lesser thickness of hybridlayer.²⁶

Hence, within the limitations of this study, it can be concluded that:

1. In a clinical Class II posterior restoration, RMGIC is a better substitute for dentin than Conventional GIC.
2. Self-etch adhesive, Adper Easy OneTM showed better bond strengths than GIC based adhesive, G-BONDTM and total etch adhesive, XP BondTM in bonding GIC to composite in a sandwich restoration.