ABSTRACT
Introduction: This study was designed to investigate the antimicrobial and physical properties of orthodontic composite resin modified by the addition of an antimicrobial agent. Materials and Methods: Transbond XT (3M Unitek), a photo‑activated, light‑cured composite resin, modified by the addition of chlorhexidine, in concentrations 2.5 wt% was formed into a uniform disc and also used for direct bonding of the brackets on the freshly extracted premolars for testing. The tablets of each group and a control group were subjected to the agar plate diffusion test to measure the zone of bacterial inhibition. In addition, they were incubated for 24 h in brain heart infusion medium inoculated with Streptococcus mutans and examined for antimicrobial action. A total of 80 extracted premolars were collected and divided into two sets of 40 teeth each. Stainless steel preadjusted edgewise appliance (PEA) brackets were bonded by using control and experimental composites. A universal testing machine was used to determine the shear bond strength. The first set of teeth was tested after 24 h and the second set after 25 days of storage in distilled water. Time‑dependent release of antimicrobial agents from the modified composites was also monitored spectrophotometrically. Results: The findings indicated that (1) addition of chlorhexidine to the orthodontic composite resin enhanced its antimicrobial properties, (2) there was no significant difference between the bond strengths of the control and the experimental resins tested after 24 h and 25 days and (3) maximum release of chlorhexidine from the modified resin was much higher than the minimum inhibitory concentration level.