Evaluation of the Antimicrobial Effect and Shear Bond Strength of Orthodontic Fixed Retainers Bonded With Composite Containing Copper Oxide Nanoparticles: An in vitro Study

Abstract

Background: Following orthodontic treatment, fixed retainers help prevent teeth from returning to their original positions. However, the presence of fixed retainers in the oral cavity can hinder oral hygiene, potentially increasing plaque and calculus accumulation and contributing to caries development. Several approaches have been proposed to prevent dental caries, and one recent strategy involves incorporating nanoparticles into resin composites. Objectives: This study evaluated the effects of copper oxide nanoparticles on the antimicrobial activity and bond strength of orthodontic composites. Methods: Sixty healthy premolars were randomly assigned to three groups (n = 20 teeth/group). The teeth were mounted in pairs, yielding 10 specimens per group. Copper oxide nanoparticles (CuO NPs) were incorporated into a flowable composite at concentrations of 0% (control), 2%, and 4%. Shear bond strength (MPa) was measured using a universal testing machine. Antimicrobial activity against Streptococcus mutans ATCC 35668 was assessed using the disk agar diffusion (DAD) method. No inhibition zones were observed. These findings support only in vitro conclusions; clinical caries prevention was not evaluated. Results: The mean bond strength was highest in the control group (32.23 ± 13 MPa), followed by the 2% CuO NP group (21.06 ± 7.32 MPa), and lowest in the 4% CuO NP group (19.66 ± 7.24 MPa). Bond strength differed significantly among the groups (P = 0.013). All groups exceeded the clinically acceptable shear bond strength threshold of 6 - 8 MPa. The DAD test showed no inhibition zone. Conclusions: An inverse relationship was observed between CuO NP concentration and shear bond strength, although the values remained within clinically acceptable ranges (6 - 8 MPa). The DAD test revealed no inhibition zone under the tested conditions, suggesting limited nanoparticle diffusion in solid media; however, this finding does not conclusively demonstrate a complete absence of antimicrobial activity. Clinical caries prevention was not evaluated in this study.