Long-term fixed appliance treatment is prone to enamel demineralization and the formation of chalky white spots, resulting in caries with an incidence of 95% [21, 22]. The Transbond™ XT bonding system is the most extensively used bonding agent and is often used as the standard for evaluating bond strength with other bonding agents [6, 23]. Accordingly, the current study used Transbond™ XT as a control group to assess RMGIC (GC Fuji ORTHO™ LC, USA). Our results demonstrated that deproteinization of the enamel surface by using 5.25% NaClO can improve the bond strength of RMGIC to the enamel surface, meeting the clinical requirements (5.9–7.8 MPa) in the absence of acid etching . Furthermore, moisture and saliva contamination reduced the bond strength of RMGIC, but its adhesive strength remained within the clinical requirements, and fluorine was released to prevent demineralization.
RMGIC is a hybrid of GIC and composite resins containing traditional glass ionomers, monomer components, and related initiator systems . The curing mechanism of RMGIC involves the acid–base reaction of GIC and the free-radical polymerization of light-cured resins . After mixing RMGIC powder, the acid–base reaction begins. When light curing was performed, the resin component immediately initiated polymerization. We confirmed that the shear bond and tensile strength test outcomes were more significant after 24 h than performing the test directly after 30 min .
Acid etching is the standard approach for enamel bonding in which hydroxyapatite crystals on the enamel surface are dissolved. It also increases the wettability and free energy the of enamel surface and is conducive to the adhesion between enamel and resin materials . Before etching, the enamel is polished and cleaned. However, a large amount of protein remains on the enamel surface after polishing . The degree of enamel demineralization after acid etching varies because of different degrees of mineralization on the enamel surface. Phosphoric acid reportedly cannot etch the enamel surface thoroughly. Only 2% exhibits ideal acid etching [25, 26].
Furthermore, NaClO has an antibacterial effect that dissolves organic material without damaging healthy tissue or tooth structure . The enamel surface can be deproteinized with NaClO before bonding to increase the penetration of the adhesive into the enamel and further improve the bond strength. Thus, prior to acid etching, 5.25% NaClO is applied onto the enamel surface for 1 min to improve the etching property [11, 12]. The pH of NaClO is similar to that of calcium hydroxide, which reacts with OH– to form NaOH and HClO. NaOH and fatty acids react to reduce the surface tension and neutralize neutralizing the amino acids through HClO acid etching. Cl– action and cell metabolism permit OH– to bind to Ca2+ to form Ca(OH)2 . The reaction of NaClO with the soft tissue is mild and presents a chlorinated odor.
In the current study, acid-etched and non-acid-etched groups showed statistically significant differences in bonding-strength measurements. The bond strengths of the formed were significantly higher than those of the latter. This finding may be due to the fact that the bonding between the RMGIC and the treated enamel surface achieved better strength through acid etching and adequate mechanical retention between the resin materials and the micropores of the etched enamel. RMGIC without acid etching treatment of the enamel bonding relied only on the retention of the glass-ionomer chemical reaction. Results showed that in the acid-etching groups, no statistically significant difference in bond strength existed between the Transbond™ XT adhesive group and RMGIC groups. Our results were similar to those reported by Cheng et al. . They found that the bond strength of RMGIC to enamel after acid etching was greater than that of the composite resin. However, the current results were inconsistent with those of Yassaei et al. . They treated enamel surface with 37% phosphoric acid and 10% polyacrylic acid by using a composite resin and RMGIC for bonding with ceramic brackets. They found that the bond strength of the composite resin is significantly greater than that of RMGIC.
In the etching groups, deproteinization with 5.25% NaClO and acid etching of the surface under the same conditions as the experimental group with a dry surface showed greater bond strength than in the group with a wet surface. In the acid-etching groups, the moisture and non-moisture groups had statistical significance, in which the bond strength of the former was lower than that of the latter. This finding was due to the acid etching deproteinizing the enamel surface. Under moist conditions, the pores formed on the enamel surface were clogged. The bond strength of the experimental groups treated with 5.25% NaClO before acid etching did not significantly differ from that of the control group. Treatment of the enamel surface with 5.25% NaClO prior to acid etching removed organic constituents, such as proteins from the enamel surface. Consequently, the surface was uniformly etched, consistent with the findings of Ayman et al. . Previous studies have shown that the bond strength of an agent increased.
Notably, the difference was not statistically significant after 5.25% NaClO enamel-surface treatment . However, Justus et al.  showed that the bond strength between RMGIC and the composite resin significantly increased after treating the enamel surface with 5.25% NaClO. This finding may be due to the use of 10% acid as a bonding agent in the former, whereas the latter used 37% phosphoric acid. Polyacrylic acid (10%) did not affect the deep areas and inflicted less damage to the enamel surface, so the enamel bond strength was low.
In the non-etching group, bond strength was significantly reduced, but the results were within the required clinical bond strength of 5.9–7.8 MPa. Moreover, the bond strength of the 5.25% NaClO-treated experimental group was significantly higher than that of the experimental group without 5.25% NaClO. This result indicated that 5.25% NaClO can deproteinize and expose the entire enamel surface. RMGIC can perform the acid–base reaction inside the glass ion and fully bond with the enamel surface under the unetched condition to achieve the clinically required bond strength. No significant difference in bond strength existed between the moisture and non-moisture groups, indicating that the surface treatment with NaClO was unaffected even in a humid environment.
The bonding failure of the enamel-treated surface with acid etching occurred primarily at the adhesive–bracket interface. Surface fracture of the enamel without acid etching occurred primarily on the enamel–adhesive surface. The enamel surface remained a high-bond-strength agent at the adhesive–bracket interface after the brackets fell off. This phenomenon may contribute to the formation of sufficient bond strength between the adhesive and enamel surface, which remained intact. ARI is one of the standard auxiliary indices used to evaluate the bonding properties of adhesives and can better respond to the failure position of the bonding agent. Our study showed that enamel thickness was reduced by the process of polishing, etching, bonding, and removal of residual bonding agents during orthodontic treatment at approximately 125 µm . Therefore, for orthodontic brackets, selecting the appropriate bonding agent system was vital to keep the enamel surface intact after bracket removal.
In the current study, most of the interface failures of acid-etching groups were at the bonding agent–bracket interface. Conversely, Yassaei et al.  showed that the bonding failure of RMGIC occurs at the enamel–adhesive interface. The interface failure of non-acid-etching groups primarily occurs at the enamel–adhesive interface, indicating that the RMGIC relies mostly on chemical retention by the acid–alkali reaction inside the glass ions on enamel surfaces that are not acid etched. Other studies have suggested that enamel may fracture when subjected to SBS, particularly when applying a 5 MPa load . Herein, the enamel surface of the non-acid-etching group was treated with 5.25% NaClO. The bond strength measured in the non-moisture group was 13.44 ± 2.14 MPa, and that measured in the moisture group was 12.67 ± 2.80 MPa. These results met the clinical requirements of bond strength and did not inflict damage to the enamel surface. Recent studies have also shown that resin-modified glass ionomer adhesives can inhibit the growth of Streptococcus mutans and reduce demineralization [32, 33].
SEM images showed that the enamel surface treated with 35% phosphoric acid underwent enamel-center demineralization and phosphoacrylate dissolution, but the interstitial and peripheral enamel parts were intact. Using 5.25% NaClO and 35% phosphoric acid, demineralized enamel-glazed parts were observed. Phosphoacrylate was also used to dilute the peripheral area of the glazed part. The methods used for retention purposes were based on previously described techniques. The porous area provided adequate retention over a wider surface area, depending on the size and depth of the pores. Acid etching did not result in a clear deep porous morphology and absence of micromechanical retention . To observe the protein coverage of the enamel surface, the three groups were polished with 5.25% NaClO without any further treatments. Results showed that 5.25% NaClO had an acceptable deproteinization effect. The removal of the proteins by chemical means had no major effect on the elastic modulus and hardness of the dental enamel , as well as the laser surface treatments on the SBS between zirconia and veneering ceramic [36, 37]. In recent years, studies have been conducted on the application of CO2 laser pretreatment of the enamel surface to prevent enamel demineralization owing to acid erosion. However, these methods are complex and unstable [38, 39]. Therefore, enamel-surface treatment with 5.25% NaClO before bonding the bracket to the enamel by using RMGIC may provide good bond strength and inhibit enamel demineralization.
Recently, patients have requested the use of ceramic brackets because of their demand for maintaining an aesthetic smile. Ceramic brackets are hard, and their bond strength is extremely high. However, despite the hard properties of ceramic materials, they exhibit brittleness. Thus, changes in the ceramic bracket size of approximately 1% may affect their fracture and bond strength . Bracket fractures reportedly lead to a large amount of bracket material being retained on the tooth surface, leading to a high risk of enamel damage during grinding [23, 38]. Hence, ceramic brackets may require moderate bond strength with the enamel surface rather than strong bonding to minimize the risk of enamel damage.