For the preparation of specimens the metal die was made as suggested by WE Dilts et al., in which mold size of dimension 0.25 inch in diameter, and 0.75 inch in length was fabricated [12]. A removable metal spacer of dimension 0.25 inch in diameter and 0.11 inch in length was also fabricated to create space for luting cement. The metal spacer was placed in the metal die and all the core build-up materials i.e. Silver Amalgam (DPI Alloy Fine grain, Mumbai), Glass ionomer (GC-Gold Label, High strength posterior restorative, Tokyo/Japan), Glass Ionomer Silver Reinforced (HI DENSE, SHOFU-INC, Kyoto/Japan), Composite Resin (Filtek 2350 XT, 3M ESPE), and Resin Modified Glass Ionomer (Vitremer™ Core Buildup, 3M ESPE) were proportioned and manipulated according to manufacturer’s instructions and packed into the stainless steel metal die mold [Table/Fig-1]. A uniform load of 500gms was applied on the die until the materials were set. After the final setting of core build up materials, they were gently removed from the mold cavity and the end of the core material specimens which faced towards metal spacer was finished with 320 grit silicon carbide paper, to make it flat for bonding. The surface was then cleaned with air-water spray and dried with air for 10 seconds to simulate clinical treatment of cores. In this manner, 100 specimens were fabricated, out of these specimens, 20 specimens of each core buildup material were randomly divided into two groups (n=10) for luting with each brand of luting cement i.e. Traditional GIC (KetacTMcem Radiopaque,3M ESPE Dental Products) or Resin Modified Glass Ionomer luting cement (RelyX™ Hybrid Glass Ionomer Permanent Luting cement, 3M ESPE Dental Products). The core buildup samples were reinserted into the metal mold and the metal spacer was removed. Each brand of luting cement was mixed according to manufacturer’s instructions. The mixed cement was bonded to the core material by expressing it into the space left for luting cement in the mold. A uniform load of 500gms was applied on the die until the luting materials were set. After setting of luting cement, the bonded specimens of total dimension 0.75 inch in length and 0.25 inch in diameter of core build-up material and luting cement were carefully removed from the mold and were examined carefully and if any, faulty specimens were discarded. To stimulate the oral conditions, all the bonded specimens were stored at 37⁰C in a 0.01M lactic acid buffer solution at a pH of 4 for 7days. The preparation of 0.01 M lactic acid buffer solution of pH4, was done by adding 100ml of distilled water (H₂0) to 1ml of lactic acid and adding pellets of sodium hydroxide to maintain the pH at 4 [1]. The lactic acid buffer solution was changed daily.

After seven days of immersion the specimens were removed and thoroughly cleaned and dried. The shear bond strength test was done using Universal testing machine (Unitek 9450 PC). The specimen was positioned horizontally within the specimen holder. Shear load was applied at 90⁰ at the junction of luting agent and core material at a cross head speed of 0.5mm/min till it fractures. The values were recorded in a digital readout in Kilo Newton (KN) and were converted to MPa. The shear bond strength megapascals (Mpa) was calculated by dividing the force required to break the specimen by the core buildup-luting agent bond area (0.0492 inches²).

The obtained data was statistically analysed using Two-way ANOVA followed by HOLM-SIDAK method for pair wise comparison at a significance level of <0.05.

The Shear bond strength values of Traditional GIC luting agent with Silver Amalgam, GI, GIC Silver Reinforced, Composite Resin and RMGIC were 3.14, 3.76, 10.86, 11.59, and 15.16 Mpa with standard deviation of 0.71, 0.52, 1.83, 0.96 and 0.98 respectively. The Shear bond strength values of RMGIC luting agent with Silver amalgam, GIC, GIC Silver Reinforced, Composite Resin and RMGIC were 3.90, 8.70, 12.68, 14.04, 20.56 Mpa with standard deviation of 0.69, 0.57, 1.06, 0.67 and 2.25 respectively [Table/Fig-2].

[Table/Fig-2]:

Shear bond strength (Mpa) of core materials with Traditional GIC and RMGIC

Two-Way ANOVA showed significant difference in bond strength of luting agents (p<0.05) and core materials (p<0.05) and the interactions (p<0.05) [Table/Fig-3]. When pair wise comparison was done by HOLM-SIDAK test, then the RMGIC luting cement showed significantly higher shear bond strength values (p<0.05) than Traditional GIC luting cement for all the core buildup materials except for silver amalgam core buildup materials (p-0.9039). When core buildup materials were compared in Traditional GIC luting cement group, RMGIC core material showed highest shear bond strength followed by composite resin, GI silver reinforced, GI, and silver amalgam core materials. The difference was statistically significant (p<0.05) for all core buildup materials except between silver amalgam and GIC (p-0.9716) and GI silver reinforced and composite resin core buildup materials (p-0.9204) [Table/Fig-4]. When core buildup materials were compared in RMGIC luting cement group, RMGIC core material showed highest shear bond strength followed by composite resin, GI silver reinforced, GI, and silver amalgam core materials. The difference was statistically significant (p<0.05)) for all core buildup materials except between GI silver reinforced and Composite resin core buildup materials [Table/Fig-5].

The choice of core foundation materials is confusing and the physical and handling properties may lead the clinician to favour one material over another. Ultimately, a core build-up must be able to withstand the forces of mastication and parafunction over a period of many years [13,14]. Cementation is one of the final steps in the sequence of clinical procedure for indirect restorations. Luting agents comprise a broad category of materials used to attach and seal dental restorations and prostheses to teeth. Proper selection of a luting agent is a last important decision in a series of steps that require meticulous execution and will determine the long-term success of fixed restorations. The choice of a luting agent is dependent on the clinical situation combined with the physical, biologic, and handling properties of the luting agent. Currently, a plethora of luting agents is available and the choice of the optimal luting agent can be confusing, even for the most experienced clinician.

The bond strength of a luting agent to dentin or core buildup material is one of the important factors in the success of cast restoration and it should be within the range of clinical acceptability. As most of the failures of indirect restorations occur in the shear stress so in this study, the methodology used to access bond strength was the shear test as previously used by Tezvergil A et al., and Padipatvuthikul P et al., [15,16]. Sinhoreti MA et al., stated that devices like Chisel systems i.e. straight or saddle shaped chisel, stainless steel tape and piston can be used to platform the shear bond strength test [17]. In the present study ISO specified straight chisel of 5mm width was used to perform the test. In this study all the direct core materials i.e. Silver Amalgam, Glass ionomer, Glass Ionomer Silver reinforced, Composite and Tri-Polymerized Resin modified glass ionomer were selected because of their advantages like reduced chair side time, ease of manipulation and reduced cost.

Glass ionomer cements are arguably one of the most popular materials for permanent cementation [18]. But it is said that their mechanical strength is inadequate for use in stress bearing areas. Doubts have been raised regarding the cements cohesive ability to withstand the high stresses generated during contraction on setting invitro, when bonded in thin layers between two substrates. To improve the physical properties and to reduce the chances of cements cohesive failure the resin modified glass ionomer cements were developed. This innovation helped to overcome the problems associated with traditional glass ionomer cements i.e. moisture sensitivity and low physical properties. Considering this fact, traditional glass ionomer luting cement and resin modified glass ionomer luting cement were used as luting agents in this study.

Kuybulu FI et al., stated that water uptake can lead to hygroscopic expansion and dissolution at restoration margins affecting the bond of luting cements to foundation materials [6]. It is believed by some investigators that the rate of disintegration in the dilute organic acids common to the oral cavity could provide data which are more meaningful clinically than in distilled water or artificial saliva [19]. Also, when used for luting, the cement around the margins of restorations is in stagnation area and therefore subjected to pH lower than 7 through the action of bacterial plaque or sugars to produce, in particular, lactic acid [20]. According to the study done by Sandra Hewlett et al., [1] the shear bond strength of the core materials to various luting agents was significantly reduced after immersion in 0.01 M Lactic acid as compared to those stored in 100% humidity. So in this study lactic acid buffer solution of 0.01 M at pH 4 was used rather than distilled water or artificial saliva. This was done to simulate the in vivo conditions. The lactic acid of pH 4 was used based on the previous invitro studies [9,20–27].

Results of this invitro study showed that RMGIC luting cement showed more shear bond strength more than Traditional GIC luting cement. The difference in shear bond strength of the luting cements was statistically significant for all the core buildup materials except for silver amalgam core material. This may be because both the luting agents behaved in similar fashion to achieve just mechanical retention with the amalgam core. The superior shear bond strength of RMGIC luting cement may be because of its resin component which is added to enhance the mechanical and bonding properties and as the physical and mechanical properties are enhanced it reduces the chances of cohesive failure thus increasing the bond strength. The superior shear bond strength can also be attributed to the more water resistance during setting and less solubility than traditional GIC luting cements [28]. Mclean JW and Um CM stated that the main drawback of traditional GIC luting cement is susceptibility to moisture attack & subsequent solubility, if exposed to water during initial setting period [29,30]. The reduced shear bond strength may also be because of absence of resin component and reduced physical and mechanical properties. Similar results were seen in study done by Sandra Hewlett et al., [1].

These results were not in agreement with the findings of Czar Necka & Nicholson who concluded in their study that the presence of the resin phase in the resin modified glass ionomer has little or no effect on the overall interaction with aqueous medium [7]. The non significant difference in the shear bond strength for silver amalgam core material may be because both the luting agents behaved in similar fashion to achieve just mechanical retention with the amalgam core.

When the core materials were compared, irrespective of luting cements, the highest bond strength value was showed by RMGIC core material followed by Composite Resin, GI Silver Reinforced, GI and Silver amalgam. The difference in shear bond strength of Traditional GIC luting cement was statistically significant for all core buildup materials except between silver amalgam with GI and GI silver reinforced with composite resin core buildup material, and the difference in shear bond strength of RMGIC luting cement was statistically significant for all core buildup materials except between GI silver reinforced with Composite resin core buildup materials. The results of this investigation showed that RMGIC core foundations bonded to RMGIC luting cements provide the most durable combination, while silver amalgam core foundation material bonded to traditional GIC luting cement provided the least durable combination in the presence of lactic acid buffer solution. RMGIC core material showed superior shear bond strength with RMGIC luting cement because it is expected that the combination of materials of similar composition will have better bond both by mechanical retention and chemical adhesion. This result was in consistent with the findings by the study done by Sandra Hewlett [1].

Considering the result of this study it can be stated that while restoring the broken down and endodontically treated teeth, the bond strength between a core material and cement should be considered. But because clinical situations usually involve foundations composed of both restorative materials and tooth structure, the results of these studies do not endorse any core material and/or luting cements. Moreover, this invitro study evaluated the bond strength of the luting agents with different core materials by shear test, but according to some investigators that both shear and tensile bond strength are relevant for retention of indirect cement retained restorations. Further investigations are required to evaluate the bonding under more closely simulated clinical conditions.

Within the limitations of this invitro study it was concluded that, amongst both the luting cements, the shear bond strength of resin modified glass ionomer luting cement was significantly higher than traditional glass ionomer luting cement for all the core buildup materials except, for silver amalgam core buildup material. Amongst all core buildup materials, resin modified glass ionomer core material showed highest shear bond strength values followed by composite resin, glass ionomer silver reinforced, glass ionomer and silver amalgam core materials irrespective of luting cements.