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Evaluation of Shear Bond Strength of Methacrylate- and Silorane-based Composite Resin Bonded to Resin-Modified Glass-ionomer Containing Micro- and Nano-hydroxyapatite

Statement of the Problem The adhesion of resin-modified glass-ionomer (RMGI) to composite resin has a very important role in the durability of sandwich restorations. Hydroxyapatite is an excellent candidate as a filler material for improving the mechanical properties of glass ionomer cement. Purpose The aim of this study was to assess the effect of adding micro- and nano-hydroxyapatite (HA) powder to RMGI on the shear bond strength (SBS) of nanofilled and silorane-based composite resins bonded to RMGI containing micro- and nano-HA. Materials and Method Sixty cylindrical acrylic blocks containing a hole of 5.5×2.5 mm (diameter × height) were prepared and randomly divided into 6 groups as Group 1 with RMGI (Fuji II LC) plus Adper Single Bond/Z350 composite resin (5.5×3.5 mm diameter × height); Group 2 with RMGI containing 25 wt% of micro-HA plus Adper Single Bond/Z350 composite resin; Group3 with RMGI containing 25 wt% of nano-HA plus Adper Single Bond/Z350 composite resin; Group 4 with RMGI plus P90 System Adhesive/P90 Filtek composite resin (5.5×3.5 mm diameter × height); Group 5 with RMGI containing 25 wt% of micro-HA plus P90 System Adhesive/P90Filtek composite resin; and Group 6 with RMGI containing 25 wt% of nano-HA plus P90 System Adhesive/P90 Filtek composite resin. The specimens were stored in water (37° C, 1 week) and subjected to 1000 thermal cycles (5°C/55°C). SBS test was performed by using a universal testing machine at a crosshead speed of 1 mm/min. Data were analyzed by two-way ANOVA and Tukey test (p< 0.05). Results There were significant differences between groups 1 and 4 (RMGI groups, p= 0.025), and groups 3 and 6 (RMGI+ nano-HA groups, p= 0.012). However, among Z350 and P90 specimens, no statistically significant difference was detected in the SBS values (p= 0.19, p= 0.083, respectively). Conclusion RMGI containing HA can improve the bond strength to methacrylate-based in comparison to silorane-based composite resins. Meanwhile, RMGI without HA has the best bond strength to silorane-based composite resins.


Introduction
Currently, the most common tooth-colored restorative materials used for esthetic purposes are composite res-ins. However, recurrent caries caused by polymerization shrinkage is a major disadvantage. [1] Several restorative methods have been offered to solve or at least re-duce this problem, such as the use of liners beneath the restoration, [2] incremental placement of restorative material, [3] increasing the filler content of the composition, and lately, use of ring-opening monomers. [4] Efforts have been made to improve the clinical performance of methacrylate-based composite resins, which has led to the development of new monomers such as ring-opening silorane and new filler technology such as nano fillers. [5] Siloranes, a new class of ring-opening monomers, were produced to overcome the difficulties related to polymerization shrinkage. This new type of monomer is derived from the reaction of oxirane and silorane molecules with a volumetric shrinkage determined to be 0.99 volume%. Opening and extending of the oxirane ring during polymerization in this new system would compensate the volume reduction. [6] Filtek Z350 is a nanofilled composite resin with 65-75 wt% of silica and zirconia nanofillers, [7] which is claimed to have a low shrinkage because of its high filler content. [8] The use of a liner with lower elastic modulus or a base with fluoride release property is a clinical approach to decrease the polymerization shrinkage. [9] Lamination of dentin with glass-ionomer cement (GIC) is strongly recommended to enhance the adhesion to dentin and minimize the microleakage, particularly when the margin of the restoration is on the dentin. [10] Fluoride release, adhesion to mineralized dental tissues, and a coefficient of thermal expansion similar to that of tooth structure are some of the advantages of GIC that make it possible to be used as an alternative layer of dentin in the composite resin fillings as well as the main filling material. [10][11] However, sensitivity to desiccation and moisture and its poor mechanical properties of GIC [12] have prompted researchers to find solutions to overcome such disadvantages. [13] In one research, the use of amalgam, silver and metal powders as reinforcements in GIC powder was suggested; however, these products have inferior esthetic appearance and decrease bond strength to enamel. [14] Incorporation of a light-cured catalyst and resin into light-cured GIC can accelerate the setting reaction and improve the mechanical strength of this material. [15] Resin-reinforced GIC is extraordinarily high in flexural strength despite having lower compressive strength than the conventional GIC. [16] Mechanical strength could be improved by incorporating SiC whiskers or short fibers into GIC, [17] but very small fibers may reside in vital organs and jeopardize their health like what asbestos fibers do. [13] Hydroxyapatite (HA), a calcium phosphate, has a chemical composition and a crystal structure similar to apatite in tooth structure and in human skeletal system. It also offers excellent biological behavior and is the principal mineral component of the enamel, comprising more than 60% of dentin by weight. As GICs have been found to interact with HA via the carboxylate groups in polyacid, the incorporation of HA into GICs may not only improve the biocompatibility of GICs, but also might have the potential to improve its mechanical properties.
The bond strength may also increase regarding a composition similar to that of enamel and dentin. [18] Several studies investigated the effect of adding different amounts and sizes of apatite powder to GIC for improving the physical property of this cement. [19][20][21][22] These studies demonstrated that GICs containing hydroxyapatite exhibit better mechanical properties and higher bond strength to dentin than the conventional GICs. A study reported that hydroxyapatite-reinforced glass-ionomer, 75wt% of glass-ionomer and 25wt% of hydroxyapatite, exhibited the highest bond strength to dentin. [23] It has been demonstrated that adding nanohydroxyapatite (nano-HA) to glass-ionomer shows higher bond strength to tooth structure compared to micro-hydroxyapatite (micro-HA). The decreased size of nano-HA particles, similar to that of the minerals in tooth, leads to increased surface area and higher solubility, filling the enamel defects with higher performance ;this phenomenon occurs through releasing calcium and phosphate ions and by increasing the bond strength between the tooth and the restorative material. [24] In all the cases where the glass-ionomer is employed as a base or liner, the adhesion of this base or liner to the restorative material (particularly composite resin) has a very important role in retention, durability, and strength of the restoration. As no study was carried out on the bond strength of resin-modified glassionomers (RMGI) containing micro-HA and nano-HA to composite resin, the aim of this study was to evaluate the SBS of nanofilled and silorane-based composite resins bonded to RMGI containing micro-and nano-

Materials and Method
In this experimental study, 60 specimens were prepared.
The tested powders were prepared by mixing micro-HA (Sigma-Aldrich Inc.; USA) and nano-HA particles (Sigma-Aldrich Inc., USA) with RMGI powder (GC; Tokyo, Japan). The two powders prepared in this study included the glass-ionomer powder reinforced with 25 wt% of micro-HA powder, [23] and the glass-ionomer powder reinforced with 25 wt% of nano-HA powder.
Thus, the micro-HA and nano-HA powders were weighed carefully by using an electronic weighing machine (AND; GR+360, Japan) with a precision of 0.001.
The correct ratio was added by weighing the glass powder. In order to obtain a uniform powder in the samples, after initial mixture by hand, the mixed powders were placed in amalgam capsules in an amalgamator (Ultramat 2; SDI, Australia) for 20 seconds. Resin-modified glass-ionomer powder (Fuji II LC) with no additives was used to prepare the control samples.
Sixty acrylic (Acropars; Iran) blocks were prepared by using a metal cylinder mold measuring 25×25 mm. The resin blocks were polished smooth with 220-, 320-and 400-grit abrasive paper. A hole, 5.5 mm in diameter and 2.5 mm in height, with retentive grooves was prepared at the center of the polished surface using a #556 diamond fissure bur. [25] Samples were divided into 6 groups, each with 10 samples as follows.
In Group 1, the powder and liquid of RMGI was mixed on a wide surface of glass slabs with a powderto-liquid ratio of 3.2gr:1gr and placed in the cavity em-  Statistical analyses were performed with SPSS software (version 15) using two-way ANOVA and Tukey test. The level of significance was set at p< 0.05.

Results
The SBS values are given in Table 1   Differences in capital letters indicate statistically significant differences within columns, and differences in lowercase letters indicate statistically significant differences within rows (p< 0.05). RMGI: Resinmodified glass-ionomer, HA: Hydroxyapatite.

Discussion
Hydroxyapatite has attracted considerable attention as a biomaterial for dental applications due to its similarity in crystallography and chemical structure to that of human hard tissues, i.e. tooth and bone. A number of studies [19][20][21][22]