Effect of different surface treatments on shear bond strength between zirconia and metal orthodontic brackets

Objective: The chemical inertness and uniform texture of zirconia make it difficult to establish a strong bond between orthodontic brackets and zirconia surfaces. This study evaluated the effect of different mechanical surface treatments on the shear bond strength (SBS) between orthodontic brackets and zirconia.Material and Methods: Sixty zirconia specimens (8 × 8 × 2 mm) were divided into six groups according to surface treatments: No surface treatment (C), Sandblast with 50 μm Al₂O₃ (SB), Diamond bur grinding with extra fine 15 μm (DE), Fine 40 μm (DF), Medium 80 μm (DM), Coarse 120 μm (DC)A contact profilometer measured surface roughness (Ra). All specimens were treated with Clearfil Ceramic Primer Plus, and Transbond XT adhesive was used to bond orthodontic brackets. SBS testing was performed following thermocycling. Stereomicroscopy was used to investigate failure modes, and the adhesive residue was measured using the Adhesive Remnant Index (ARI).Statistical Analysis: The results of surface roughness and shear bond strength of all groups were analyzed using One-way ANOVA was used to compare groups, with a significance level set at p < 0.05. Mode of failure were analyzed using the Chi-square test to evaluate the scores among the adhesive subgroups. Pearson’s correlation analysis was conducted to assess the relationship between surface roughness (Ra) and shear bond strength (SBS).Results: Group DE (15 μm) demonstrated the highest SBS (14.56 ± 4.02 MPa), significantly higher than Groups DM and C (p < 0.05). All groups showed SBS values exceeding the minimum clinical threshold (5.89–7.85 MPa), indicating clinical applicability. Most specimens had an ARI score of 1, suggesting that more than half of the adhesive remained on the bracket base.Conclusions: Mechanical treatment with extra-fine diamond bur grinding, followed by chemical conditioning with a 10-MDP-containing primer, significantly improves bracket bonding to zirconia. This protocol may provide a reliable and effective method for orthodontic bracket adhesion in patients with zirconia restorations