Adhesive Properties of Bonded Orthodontic Retainers to Enamel: Stainless Steel Wire vs Fiber-reinforced Composites

Purpose: The objectives of this study were to compare the bond strength of a stainless steel orthodontic wire vs various fiber-reinforced composites (FRC) used as orthodontic retainers on enamel, analyze the failure types after debonding, and investigate the influence of different application procedures of stainless steel wires on bond strength. Materials and Methods: Caries-free, intact human mandibular incisors (N = 80, n = 10 per group) were selected and randomly distributed into 8 groups. After etching with 37% H3PO4 for 30 s, rinsing and drying, bonding agent (Stick Resin) was applied and light polymerized. Then one of the following FRC materials was applied on the flowable composite (Stick Flow) using standard molds: group 1: Angelus Fibrex Ribbon; group 2: DentaPreg Splint; group 3: ever- Stick Ortho; group 4: Ribbond. In group 5, Quad Cat Wire was applied in the same manner as in FRC groups. In group 6, after applying bonding agent (Stick Resin), Quad Cat Wire was placed directly on the tooth surface and covered with Stick Flow composite. In group 7, after bonding agent (Heliobond) was applied, Quad Cat Wire was placed directly on the tooth surface and covered with Tetric Flow composite. In group 8, after applying bonding agent (Heliobond) and polymerization, Tetric Flow composite was applied, not polymerized, and Quad Cat Wire was placed and covered with Tetric Flow again. Specimens were thermocycled for 6000 cycles between 5°C and 55°C and loaded in a universal testing machine under shear stress (crosshead speed: 1 mm/min) until debonding occurred. The failure sites were examined under an optical light microscope. Data were analyzed using one-way ANOVA and the Tukey-Kramer adjustment test (α = 0.05). Results: Significant differences were found between the groups (p = 0.0011) (ANOVA). Bond strength results did not significantly differ either between the FRC groups (groups 1 to 4) (6.1 ± 2.5 to 8.4 ± 3.7 MPa) (p > 0.05) or the wire groups (groups 5 to 8) (10.6 ± 3.8 to 14 ± 6.7 MPa) (p > 0.05). Failure types varied within the FRC groups, but mainly composite was found left adhered on the enamel surface at varying degrees. In the stainless steel wire groups, when the retainer was applied onto the bonding agent and then covered with flowable resin, partially attached composite on the enamel was often found after debonding. When the wires were embedded in the flowable composite, the Heliobond group (group 8) showed more adhesive failures between the enamel and the composite compared to group 5, where the bonding agent was Stick Resin. Conclusion: Regardless of their application mode, stainless steel orthodontic bonded retainers delivered higher bond strengths than those of fiber retainers. The differences were statistically significant compared to those of Angelus Fibrex Ribbon and DentaPreg Splint. Keywords: bond strength, fiber-reinforced composite, lingual retainer, orthodontics, relapse, stainless steel wire