Surface Activation of Zirconia Orthodontic Brackets by Multi-Gas Atmospheric Plasma: Effects on Shear Bond Strength

Zirconia orthodontic brackets exhibit favorable mechanical and esthetic properties; however, their chemically inert surfaces can limit adhesion to resin cements. This study evaluated the effect of multi-gas atmospheric plasma irradiation on the shear bond strength (SBS) of zirconia brackets. Zirconia brackets were treated with nitrogen (N₂), argon (Ar), or air plasma for 3 and 10 s and compared with untreated controls and conventional alumina-sandblasted specimens. Surface wettability was assessed by contact angle measurements, and surface chemical changes were analyzed using Fourier transform infrared (FT-IR) spectroscopy. After 10 s of plasma treatment, the water-contact angle decreased from 64.3° ± 8.4° in the untreated group to 17.9° ± 6.4°, 18.8° ± 2.7°, and 16.8° ± 5.3° with Ar, N₂, and air, respectively. For SBS testing, zirconia brackets were bonded to bovine enamel, stored in distilled water at 37 °C for 24 h, and subsequently tested (n = 12-15 per group). The mean SBS was 20.99 ± 2.87 MPa for the untreated group, 25.50 ± 2.40 MPa for the sandblasted group, and 23.63 ± 3.33, 23.99 ± 3.26, and 23.98 ± 3.58 MPa after 10-s Ar, N₂, and air plasma treatment, respectively. FT-IR revealed no new absorption peaks. Within the limitations of this study, multi-gas atmospheric plasma treatment markedly reduced the apparent water-contact angle measured on flat zirconia specimens, whereas its effect on SBS relative to the untreated group was not statistically significant.