Adhesive strength of metal brackets on existing composite, amalgam and restoration-enamel complex following air-abrasion protocols

Bracket adhesion on restored tooth surfaces is occasionally necessary in clinical orthodontic practice. The objective of this study was to compare the effects of two air-abrasion methods on adhesion of metal brackets to enamel, resin composite, amalgam and composite/amalgam-enamel complexes. Cavities in standard dimensions (12.56 mm2) were filled with resin composite (Anterior Shine, Cavex) and amalgam (non-gamma 2, Cavex) on bovine incisors (N=40), which were then embedded in acrylic resin. Metal brackets were bonded on the following surfaces (n=10 per group): (1) enamel, (2) enamel-composite, (3) enamel-amalgam, (4) composite, and (5) amalgam. All restorative materials were either silica (SiO2)-coated (CoJet, 30 μm) and silanized (ESPE-Sil) or air-abraded with alumina (Korox, 50 μm, Al2O3) and silanized (Monobond Plus). Enamel was etched with H3PO4 for 30 s in Groups 1-3. Metal brackets were bonded onto the conditioned substrates. Specimens were stored in distilled water (24 h, 37 °C) following bonding. The brackets were then debonded using a Universal Testing Machine (1 mm/min). Shear bond strength (SBS) data were recorded and failure types were categorized. Data (MPa) were analyzed using 1-way and 2-way ANOVA, Tukey׳s post hoc test and 2-parameter Weibull distribution. While substrate type significantly affected the SBS (p<0.001), surface conditioning did not show a significant effect (p=0.256). Interaction terms were not significant (p=0.159). Mean SBS was significantly higher (p<0.001) on enamel (26.72 MPa), composite (29.97-31.37 MPa) and enamel+silica-coated composite complex (25.89 MPa) than those of other groups (10.96-20.64 MPa). The presence of amalgam resulted in the lowest SBS regardless of the conditioning method (10.96-12.41). Air-abrasion with Al2O3 followed by Monobond Plus and silica-coating and silanization did not show significant difference (p>0.05). Weibull distribution presented lower shape for restoration-enamel complexes (2.20-6.31) compared to single component surfaces (10.14-12.15). SBS on composite was similar to that of enamel but it presented predominantly cohesive failures. Failure types were frequently cohesive in composite alone or composite-enamel complex.