Shear bond strength of a RMGIC for orthodontic bracket bonding to enamel

Orthodontic bonding materials must be biocompatible, resistant to solubility and chemical attack, and provide sufficient retention while allowing safe debonding without enamel damage. Minimum clinically acceptable adhesion is often cited as 5.9–7.8 MPa, though test conditions vary. Composite resins are widely used and provide good bonding but lack fluoride release and are susceptible to contamination issues. Glass ionomer cements (GICs), particularly resin-modified GICs (RMGICs), chemically bond to enamel, release fluoride, and tolerate moisture, potentially reducing white spot lesions and enamel damage upon debonding. However, literature on RMGIC performance in orthodontic bonding is mixed, with some studies reporting higher debonding than composites. This study investigates whether a restorative RMGIC (Riva LC HV) can compete with an orthodontic RMGIC (Fuji Ortho LC) and a primer/composite system (Transbond XT) for bonding to enamel, and evaluates which bonding protocols optimize adhesion.

Prior work indicates composites are easy to handle and yield adequate bonding but do not release fluoride and can be sensitive to moisture contamination. Conventional GICs historically showed high debonding rates, motivating development of RMGICs with improved mechanics, fluoride release, antibacterial effects, and humidity tolerance. Some clinical and in vitro studies suggest RMGICs reduce enamel damage at debonding and help prevent white spot lesions; others report higher bracket failure than composite systems. Reported acceptable SBS thresholds are 5.9–7.8 MPa. Literature shows polymerization quality (degree of conversion) affects SBS, and factors such as light unit, distance, curing time, bracket type (ceramic vs metal), and filler characteristics influence outcomes. Some studies show FUJI achieving equal or higher SBS than composites, potentially due to dual cure/acid–base reactions. There is evidence that universal adhesives with 10-MDP can enhance bonding, especially with prior phosphoric acid etching, by promoting micromechanical and chemical adhesion. Data on combining universal adhesives with RMGICs in orthodontics are limited, though restorative literature suggests improved bonding with etch-and-rinse plus universal adhesive before RMGIC application.

Design: In vitro experimental study. Materials: Three bonding materials—Transbond XT composite (TXT), Fuji Ortho LC RMGIC (FUJI), Riva LC HV RMGIC (RIVA)—used with either a one-step self-etch primer (Transbond Plus, TSEP) or a universal adhesive (Scotchbond Universal, SU). Teeth: 121 extracted human molars/premolars free of cracks, restorations, caries; stored in 1% chloramine T at 4°C and used within 3 months, per ethical approvals. Preparation: Roots partially removed; buccal/lingual enamel surfaces flattened with 800-grit to >7 mm²; specimens embedded in acrylic resin with enamel exposed; surfaces inspected at ×40 to ensure cleanliness. Grouping and bonding: Randomly assigned to 11 groups (n=11) with specific protocols: G1 FUJI; G2 OC (polyacrylic acid) + FUJI; G3 TSEP + FUJI; G4 TSEP (light-cured) + RIVA; G5 TSEP + RIVA; G6 TSEP (light-cured) + TXT; G7 H₃PO₄ + TSEP (light-cured) + RIVA; G8 SU (light-cured) + TXT; G9 H₃PO₄ + SU (light-cured) + TXT; G10 H₃PO₄ + SU (light-cured) + RIVA; G11 H₃PO₄ + RIVA. FUJI and RIVA capsules mixed 10 s in a Rotomix and injected on button bases. A 7 mm² cylindrical metal orthodontic button was bonded to each sample per protocol; excess removed. Storage: All bonded specimens stored in distilled water at 37°C for 7 days. Shear bond strength (SBS): Tested on a universal testing machine using a shear device applying load parallel to the interface at 0.5 mm/min. Maximum load divided by bonded area (7 mm²) to calculate MPa. Failure mode: After debond, inspected at ×30 to classify as AF (adhesive at enamel interface), CF-E (cohesive in enamel), MF (mixed), or CF-B (cohesive in button). Flexural modulus: Bars (25×2×2 mm) for each material (n=10) prepared per ISO 4049, light-cured 90 s total (30 s top/middle/bottom), stored 2 weeks in water at 37°C; tested in 3-point bending at 1 mm/min; Ef calculated as FL³/4BH³. Degree of conversion (DC): Cylindrical samples (6 mm × 2 mm). For FUJI and RIVA, both light-cured (20 s) and self-cured conditions were prepared (n=3 each); TXT only light-cured (n=3). Stored 1 week in water at 37°C in dark. FTIR-ATR spectra 500–4000 cm⁻¹, resolution 8 cm⁻¹, 64 scans; peak ratios at 1638 cm⁻¹ (C=C) and 1720 cm⁻¹ (C=O) measured before and after polymerization; DC = 1 − [(C=C/C=O)P / (C=C/C=O)NP]. Statistics: Normality (Shapiro–Wilk) and homoscedasticity (Levene) verified. One-way ANOVA with Tukey post hoc for SBS, Ef, DC among groups/materials; Fisher’s exact test for failure modes. XLSTAT; α=0.05.

- SBS: Highest SBS in G3 (TSEP + FUJI) 37.23 ± 3.84 MPa; lowest in G7 (H₃PO₄ + TSEP + RIVA) 17.51 ± 5.51 MPa (both p < 0.05). G2 (OC + FUJI) 27.32 ± 7.38 MPa was significantly higher than G6 (TSEP + TXT) 19.38 ± 8.92 MPa. For RIVA groups: G10 (H₃PO₄ + SU + RIVA) 25.15 ± 3.60 MPa > G11 (H₃PO₄ + RIVA) 22.09 ± 5.34 MPa ≈ G4 (TSEP + photo + RIVA) 22.03 ± 6.43 MPa > G7 (H₃PO₄ + TSEP + RIVA) 17.51 ± 5.51 MPa ≈ G5 (TSEP + RIVA) 17.54 ± 5.33 MPa. Universal adhesive with prior etch improved SBS for TXT (G9 H₃PO₄ + SU + TXT: 29.30 ± 7.54 MPa) vs TSEP + TXT (G6: 19.38 ± 8.92 MPa) and SU + TXT without etch (G8: 21.60 ± 9.31 MPa). All groups exceeded the 5.9–7.8 MPa clinical threshold. - Failure modes: Groups with SU + TXT (G8, G9) showed CF-E and MF, while other groups were predominantly AF. Table 4 counts: e.g., G8 AF=6, CF-E=2, MF=3; G9 AF=3, CF-E=3, MF=5. Differences significant (p < 0.05). - Flexural modulus: TXT 10.67 ± 0.62 GPa > FUJI 6.63 ± 1.99 GPa > RIVA 2.42 ± 0.82 GPa (all p < 0.05). - Degree of conversion (1 week): FUJI light-cured 77.69 ± 2.90%; RIVA light-cured 75.17 ± 1.90% (no significant difference); TXT light-cured 70.14 ± 1.09% (significantly lower than RMGICs). Self-cured: FUJI 65.08 ± 1.50%; RIVA 61.53 ± 2.80% (both significantly lower than their light-cured counterparts).

The study demonstrates that a restorative RMGIC (Riva LC HV) can achieve clinically acceptable and, in some protocols, competitive SBS compared to an orthodontic RMGIC (Fuji Ortho LC) and a composite system (Transbond XT). The highest SBS was obtained with FUJI when paired with a self-etch primer (TSEP), and TXT benefited significantly from phosphoric acid etching followed by a universal adhesive (SU), consistent with the chemical interaction of 10-MDP with enamel calcium. For RIVA, protocols employing a universal adhesive—especially after phosphoric acid etching—improved SBS versus TSEP-based protocols, suggesting micromechanical and chemical adhesion enhances performance. All groups surpassed Reynolds’ threshold, indicating clinical applicability; however, higher SBS correlated with increased mixed or cohesive enamel failures in SU + TXT groups, underscoring a potential trade-off between bond strength and enamel safety during debonding. Mechanical (Ef) and polymerization (DC) outcomes support these trends: TXT had the highest stiffness, while RMGICs exhibited higher DC after light curing, possibly contributing to FUJI’s high SBS. These findings inform protocol selection to balance bond reliability and enamel preservation, advocating consideration of universal adhesives to optimize adhesion, particularly with RMGICs, while being mindful of enamel fracture risk at debonding.

Under the study conditions, all three materials (FUJI, RIVA, TXT) provided SBS values well above clinical thresholds for bonding orthodontic attachments to enamel. Riva LC HV can serve as a viable alternative to Fuji Ortho LC and Transbond XT systems. Incorporating a universal adhesive, especially with prior phosphoric acid etching, can optimize SBS for both composite and RMGIC protocols, though higher SBS may increase the risk of enamel damage upon debonding. Future research should include clinical trials and in vitro aging (e.g., thermocycling, prolonged storage) to validate durability, optimize protocols for enamel safety, and assess performance under conditions mimicking brackets' opacity.

In vitro design cannot fully replicate oral conditions (saliva, masticatory and orthodontic forces). SBS was measured at 1 week, not over longer periods; thermocycling and extended aging were not performed. Degree of conversion was measured on exposed samples and may not reflect polymerization under opaque brackets/buttons. Thus, generalizability to clinical settings is limited and requires clinical validation.