Comparison of the Shear bonding Strength of ION-Z GIC to a Resin Composite Using Different Adhesive Systems: RCT Study


 Background:In restorative dentistry we usually use Sandwich Technique for posterior restorations where GIC is placed below and a resin composite is placed over it. The bonding strength between these two materials are low. We are looking for the best adhesive system to put it in between. We think that the self-etching bond will give the best bonding strength between them whereas total etch will give lower bonding strength than self-etching system. Methods:ION-Z GIC was bonded to resin composite by using two different bonding agents. The thirty specimens used were prepared by using acrylic blocks with holes in each hole to retain the ION-Z GIC. The specimens were randomly divided into three groups:Group I: Control group.Group II: Total-etch adhesive was applied and cured over ION-Z GIC.Group III: Self-etch adhesive was applied and cured.The composite resin placed over the ION-Z GIC and cured. The shear bond strength was measured by shearing of the bonded specimens on Universal Testing Machine (Model 114) using speed of 0.1mm / minute. The reading was tabulated and subjected to statistical analysis using ANOVA and Tukey's test.Results:The test showed statistically significant difference between Group III and Group I and between Group III and Group II. Group III had the highest shear bonding strength.Conclusion:Self-etch adhesive agent produces have better shear bond strength to ION-Z than total-etch adhesive and to the group without any bonding agent.

Technique plays an important role in restorative dentistry, where glass ionomer cement (GIC) is placed below and a resin composite is placed over it. The superior micromechanical bond of resin composite to acid-etched enamel, the bond strength of glass Ionomer to dentin and the ability of glass Ionomer to release uoride when in contact with oral uids, combined with its low solubility, make the combination of these two materials a prudent step in improving clinical success. {2-3} This technique was developed by McLean and others in 1985; they used the dentin adhesive properties of glass ionomer cements (GICs) to seal cavities and reduce microleakage. {4} This technique bene ts from the advantages of GIC uoride release combined with esthetic resin material to enhance clinical serviceability. {5} However, the bonding strength between conventional GICs and resin composite is low because of the lack of chemical bonding between these two materials. This could be attributed to the difference in setting reactions between dental composites and conventional GICs. {5} Furthermore, failure also occurs due to sensitivity of the GIC to moisture and its progressive loss following acid etching. {6} Moisture contamination during the initial setting of GICs can cause dissolution of the weak calcium polyacrylate chains, which can degrade their physical properties. {7} In order to prevent moisture contamination during the rinsing procedure, it is mandatory to allow the initial setting of the glassionomer prior to the etching procedure. {7} Clinically, this can be achieved by waiting for two-to-seven minutes for the initial setting of the GIC to be complete before start etching and rinse procedure {7} .
Detailed clinical techniques for bonding glass ionomer to dentin, followed by etching the enamel margin and glass ionomer lining, then bonding resin composite to etched enamel and glass ionomer have been described. {8} The success of the resin composite-glass ionomer laminate restoration depends on the strength of the bond of the glass ionomer liner to dentin and the strength of the bond between the glass ionomer liner and the resin composite. {8} GIC etching time has yet to be standardized. A 30-second or less etching time has been proposed in one study, {9} {5} Due to the use of 37% phosphoric acid to etch glass ionomer, there is a need to rinse the acid before applying the bonding agent. {5} In order to prevent moisture contamination during the rinsing procedure, which may affect the integrity of the glassionomer, it is mandatory to allow the initial setting of the glass ionomer before the etching procedure. {5} This is clinically veri ed with the help of a sharp explorer {5} after waiting for 5minutes and 20 seconds (manufacturer's instructions GC ASIA Tokyo, Japan).
Since this procedure requires a waiting period, this technique is not popular among restorative dentists. {7} The recent development of adhesive systems, including self-etch primers and glass-ionomer-based adhesives, might overcome this disadvantage, as they do not require etching and rinse procedure {7} .
Self-etching systems combine the functions of primer and adhesive components and do not need an "etch and rinse" phase, which not only decreases clinical application time, but also signi cantly reduces technique sensitivity. {10} Another important advantage is that the in ltration of resin occurs simultaneously with the self-etching process, by which the risk of discrepancy between both processes is low or non-existent. {10} The self-etch effect should be ascribed to non-rinsing, polymerizable monomers to which one or more carboxylic or phosphate acid groups are grafted {7} . Hence, the present study was conducted to evaluate and compare the shear bond strength of ION-Z GIC to composite resin, using different generations of bonding systems applied on ION-Z GIC.

Methods:
ION-Z GIC was bonded to a resin composite by using two different bonding agents, a Total-etch adhesive and a Self-Etch adhesive.
Preparation of the specimens: The thirty specimens used in this investigation were prepared by using acrylic blocks. A total number of 30 acrylic blocks was prepared using a cuboidal plastic mold. In each block, four wells of 6 mm diameter and 2 mm depth were prepared by drilling holes in each block, to retain the ION-Z GIC.
With the aid of disposable micro applicators, one drop of the liquid on the holes walls was applied during 10 seconds, then the holes were washed (according to manufacturer's instructions).
The holes were then lled with self-cure ION-Z GIC by mixing it according to the manufacturer's instructions (one scoop of powder mixed with one drop of liquid) and covering the holes with glass plates to produce a smooth surface. The glass plate was carefully removed to ensure that the glass ionomer surface was smooth and not pitted. The specimens were randomly divided into three groups of 10 specimens each. The groups were: Group I: Acts as a control group, no adhesive agent was applied between the ION-Z GIC and resin composite.
Group II: ION-Z GIC with a thin layer of total-etch adhesive was applied according to the manufacturer's instructions and cured, and then a cylinder of composite resin was added and cured over the specimen.
Group III: Same as group II, but a thin layer of self-etch adhesive was applied and cured over ION-Z GIC.
Immediately following this procedure, a transparent plastic ring, 4 mm in height, with a 5 mm internal diameter, was centered over the ION-Z GIC. The composite resin was condensed into a transparent plastic ring, using an incremental curing technique, above the ION-Z GIC substrate, and all sides of the specimen were cured to ensure complete curing of the material. Following the curing the plastic ring was removed. All the procedures were conducted at room temperature.00 Permission was taken from faculty of Dentistry to use the shearing measure machine at Faculty of Mechanical and Electrical Engineering. The shear bond strength was measured by shearing of the bonded specimens on Universal Testing Machine (Model 114) which was located in strength materials lab, using speed of 0.1mm / minute. The shearing apparatus was constructed to grip the acrylic block, and a wedge blade system was designed to apply a shear force on the adhesive interface.
The reading was tabulated and subjected to statistical analysis using ANOVA and Tukey's test. Mean and standard deviation were calculated for each group by using the ANOVA test and intergroup comparison was done by the multiple comparison test-Tukey's test, which revealed a statistical signi cance among the groups.

Results:
The mean shear bond strengths were calculated for each group. (Table 1) The maximum shear bond strength values were recorded for Group III, where the self-etch adhesive was used, with the mean value of 9.151 MPa and a standard deviation of 1.26. On the other hand, group I recorded minimum shear bond strength, with a mean of 6.158 MPa and standard deviation of 0.912.
Group II with total-etch adhesive recorded a mean shear bond strength of 6.966Mpa and a standard deviation of 0.976. ( Table 2).
The shear bond strengths (MPa) for the groups: (Table 1)    The test showed statistically signi cant difference between Group III and Group I.
On the other hand, there is no difference between Group II and Group I.

Discussion:
Glass Ionomer Cement has been used as a liner restoration in the posterior teeth due to its chemical bonding with the teeth. Nevertheless, because of its weak resistance toward the occlusal force, resin composite was used as a nal restoration because of its cosmetical and physical properties.
On the other hand, we still have to nd a way to get the best bonding strength between those two materials. Thus, we did this RCT study.
Thirty specimens were prepared with ION-Z and resin composite which was bonded by using two different kinds of bonding agents and the bond strength was measured.
The result of the study has concluded that a self-etch adhesive agent produces better shear bond strength to ION-Z, which is highly signi cant as compared to a total-etch adhesive and to the group without any bonding agent.
Proper bond strength between resin composite and ION-Z is necessary for the success of sandwich technique. Applying self-etch adhesive over ION-Z creates a stronger bond of composite resin to ION-Z compared with total-etch adhesive. The total-etch system needs two separate phases of rinsing and drying and has also a higher technique sensitivity. (17,18) Moreover, the GIC surface may become weak during the drying phase. (19) Owing to the acidic monomer in its composition, the self-etch adhesive does not need the etching phase, rinsing and drying. (20) Self-etch adhesives are categorized into groups of mild, intermediate and strong based on their pH level and etching potential. (21) Thus, their ability in creating a bond between the composite and ION-Z may be different. (21) Our study showed that self-etch adhesive has a higher shear bond strength between the ION-Z and composite. Similar results were achieved by Kandaswamy et al. who reported that the mild self-etch bonding provided higher shear bond strength. (22) This might be due to the lower acidity of the mild selfetch adhesive compared with the strong and intermediate self-etch adhesives. According to organic chemistry, when a weak acid invades something, it induces a minimum excitation in the ions, and hence the salt crumps formation will be minimal (23) . Cations such as Ca2+ and Na+ that are not excited and are present in large amounts for effective interaction, especially in a conductive reaction medium like GIC, instigate strong ionic reaction with the bonding agents. (24,25) It seems that the lower acidity of mild selfetch adhesive leads to the higher shear bond strength.
Additionally, in our research, the strong total-etch adhesive system in group II had a lower shear bond strength compared with the other group of self-etch bonding (group III). Some previous studies showed that using self-etch adhesive with a lower pH (1-0.8) created lower shear bond strength. (22,26,27) Stronger acid neutralizes more cations, resulting in salt crumps formation. Therefore, the structure of the GIC is weakened and fragile, thus consequently the bond will be weakened (22,24) . This research also found that self-etch adhesives improved the bond between the composite and ION-Z GIC compared with the total-etch adhesive.
Arora et al. reported that the self-etch adhesive caused a stronger shear bond between the composite and RMGIC. (28) Similar result was achieved in a study conducted by Chandak et al. on the same issue. (29) Another study also showed that using self-etch adhesive on the surface of RMGIC had the potential of creating a better bond strength with the resin composite. (30) This might be due to the acidic pH of selfetch adhesive. The acidic characteristic of self-etch adhesive causes super cial dissolution of GIC and consequently improves the bond between composite resin and GIC. (31) Etching the surface of GIC with 37% phosphoric acid leads to dissolution of the lower layers of GIC matrix and therefore, would decrease the cohesive strength of the GIC which subsequently can affect the bond strength of the composite and GIC adversely. (30,32) The porosity created on the GIC surface due to the phosphoric acid is different from that caused by self-etch adhesive. Super cial destruction by means of acid-etching leads to an undesirable surface bond with the composite. It seems that application of an acid with a similar acidity of the self-etch adhesive helps creating better shear bond strength.
On the other hand, self-etch adhesive has a lower viscosity compared with the total-etch adhesive. (33) In a research, Mount found that bonding with a lower viscosity caused low contact angle on the surface; thus, it improved the wettability and strengthened the bonding of resin composite and GIC (34) It seems that low viscosity of self-etch adhesive has more potential of wettability compared with the total-etch adhesive; so, it provides greater shear bond strength between the composite and GIC.
Previous studies on self-etch adhesives showed that this system bonds with the calcium in the structure of the teeth; (35) therefore, it can possibly bond with the calcium in the structure of GIC and create a higher shear bond strength compared with the total-etch adhesive. (7) From the results of the present study it can be concluded that the application of self-etch bond agents improves the wettability of ION-Z to adhere to composite resin, thus promoting a strong shear bond between ION-Z and the resin composite.

Declarations:
Funding: No funding was received. The authors con rm the independence of this research completely from any governmental or non-governmental authorities or local /international organizations and the research is self-funded by Sana Lala.