Orthodontic bonding to 3D-printed and milled zirconia surface-conditioned by different protocols: a comparative analysis of shear bond strength, surface topography and surface restorability

Abstract Shear bond strength (SBS) of orthodontic brackets to conventional and milled zirconia surfaces has been widely studied; however, data on 3D-printed zirconia are lacking. This study compared the effects of three surface conditioning protocols on the SBS of orthodontic brackets bonded to 3D-printed and milled zirconia, focusing on surface topography and restorability. Thirty-six rectangular specimens (20 × 15 mm ² , 1.5 mm thick) were fabricated from each material and assigned to three groups ( n = 10): bur roughening (BR), grit-blasting (GB), and tribochemical silica coating (TS). Metal orthodontic brackets were bonded to conditioned zirconia surfaces using orthodontic adhesive. Surface topography was assessed by profilometer [Ra] and scanning electron microscopy at different intervals. After 5000 thermocycles, SBS values were measured in a universal testing machine, followed by failure mode analysis. Data were analyzed with SPSS at a significance level set at p < 0.05. Notably, surface conditioning, material type, and their interaction had a significant effect on Ra and SBS ( p < 0.05). For both materials, the TS group exhibited the highest SBS (8.06 ± 0.53 MPa for 3D-printed and 9.41 ± 0.54 MPa for milled zirconia), which significantly exceeded the GB and BR groups ( p < 0.05). Both GB and TS coating achieved clinically acceptable bond strengths of 6–8 MPa. In terms of surface roughness, all surface conditioning protocols significantly increased Ra, but TS demonstrated the highest restorability for both 3D-printed (79.57 ± 7.57%) and milled (72.20 ± 14.29%) materials compared to the GB ( p > 0.05) and BR ( p < 0.05) groups. GB and TS coating demonstrated optimal SBS and surface restorability in orthodontic bonding to 3D-printed and milled zirconia. Overall, TS coating showed superior outcomes.