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Abstract This research aimed at investigating the antibacterial effect of Octenidine dihydrochloride when incorporated in an experimental dental flowable resin composite material. Different concentrations were used and tested in order to increase the antibacterial effect without compromising the mechanical properties of the resin composite. A commercially available flowable resin composite material (Herculite Ultra Flowable by Kerr Corporation) was used as a control group in this study. The following properties were tested: diametral tensile strength, compressive strength, flexural strength, water sorption, solubility, degree of conversion, dentin bond strength, and the antibacterial activity against Streptococcus mutans and Lactobacillus casei at different time intervals. Eight similar specimens were made for every test in each group, and all specimens were tested for all the previous properties. Diametral tensile strength test was conducted by subjecting cylindrical specimens to compressive loading using the universal testing machine till the point of fracture. The load was applied vertically on the lateral portion of the cylindrical specimen, producing tensile stresses perpendicular to the vertical plane passing through the center of the specimen. The fracture load was then recorded and the ultimate diametral tensile strength was calculated according to a mathematical equation. As for the compressive strength test, it was done by subjecting cylindrical specimens to compressive loading using the universal testing machine till fracture occurred. where the compressive strength was automatically recorded and calculated using a computer software program. Moreover, the flexural strength was obtained by subjecting bar shaped specimens to a 3-point loading using a universal testing machine. Summary and conclusions 135 Water sorption and solubility measures were recorded after one day and repeated after 7 days immersion in distilled water. The different masses and volumes of each disc were documented after each test, and the water sorption and solubility values were obtained and represented from these numbers using mathematical equations. The degree of conversion was measured by chemical analysis using FTIR (Fourier Transform Infra-Red Spectroscopy). Degree of conversion was performed after 24 hours of photo curing. Dentin bond strength test was assessed using shear bond strength. Specimens were stored in an incubator at 37°C in 100% humidity for 24 hours, then subjected individually to shear bond stress at cross head speed of 1 mm/min until fracture occurred. Shear bond strength was calculated by dividing the maximum load by the cross-sectional area of the bonded interface. Fracture mode analysis was observed using a stereomicroscope with a magnification of 40X to determine the failure mode in each specimen. The antibacterial activity of the Octenidine dihydrochloride was assessed using the agar disc diffusion test. The antibacterial activity was assessed at different time intervals against two different bacterial strains namely Streptococcus mutans and Lactobacillus casei. On investigating the diametral tensile, compressive, and flexural strength results, group 1(control group) showed the highest mean values among all the tested groups, followed by groups 2,3, and 4 respectively. The mechanical properties of the formulated flowable resin composite were significantly lower than that of the commercially available control group. Adding to that, the incorporation of the antibacterial agent Octenidine dihydrochloride significantly lowered the mechanical properties among the tested groups. Summary and conclusions 136 Regarding the water sorption and solubility values, group 1 (control group) showed the lowest values in both tests, followed by groups 2,3, and 4 respectively. Incorporation of Octenidine dihydrochloride significantly increased the water sorption and solubility values, however values were still within the accepted ranges for clinical use. Results of the degree of conversion measured at 24 hours of curing revealed a significant increase in the degree of conversion of the commercially available control group (group 1) in comparison to all the experimental resin composite in this study. Also, group 2 showed a significant increase in the degree of conversion when compared to the other two groups 3 and 4 which contain the antibacterial agent Octenidine dihydrochloride. On the contrary, incorporation of the antibacterial agent OCT showed a significant decrease in the degree of conversion when compared to groups free of the OCT. Concerning the dentin bond strength test the values shown in all groups followed the same pattern of results shown in all other mechanical tests mentioned previously, where group 1 showed the highest mean values followed by groups 2 then groups 3 and 4 respectively. However only with group 4 the values obtained were below that accepted for clinical use. As for the antibacterial effect against Streptococcus mutans, both groups 1 and 2 contained no Octenidine dihydrochloride thus they exhibited no antibacterial effect. However, groups 3 and 4 had a significant antibacterial effect that decreased over time for the first 3 months, then significantly increased after 6 months’ time. Regarding the antibacterial effect against Lactobacillus casei, results were similar regarding groups 1 and 2, however the antibacterial effect significantly increased over time in both groups 3 and 4. Within the limitations of this study, the developed experimental resin composite may be clinically attractive. Additional future evaluation of biocompatibility, together with other mechanical and physical properties of the experimental resin composite is recommended. Summary and conclusions 137 Conclusions Based on the results of this study as regards the effect of incorporating different percentages of Octenidine dihydrochloride into experimental flowable composite, it can be concluded that: 1. Octenidine dihydrochloride positively influenced the antibacterial activity of the flowable resin composite against Streptococcus mutans and Lactobacillus casei. 2. Adding Octenidine dihydrochloride to the experimentally formulated flowable resin composite met the ADA specifications regarding the diametral tensile strength, compressive strength, flexural strength, water sorption, solubility, and degree of conversion. 3. Octenidine dihydrochloride had a negative effect on the bonding of the experimentally formulated flowable resin composite to the dentin. Recommendations for further research: 1. In-depth exploration of the effect of using Octenidine dihydrochloride together with different filler types, sizes and shapes while taking into consideration the different patterns of filler distribution would be very helpful. 2. More methodological work is needed on how to incorporate different weight percentages of the antibacterial agent. 3. Further research might include chemical analysis and characterization of the antibacterial agent, and possibly exploring the effects of combining another antibacterial agent with the Octenidine dihydrochloride and assessing the long-term effect on the material’s properties. 4. Although methodologically challenging, it would be very useful to conduct longer term studies, and re-asses the material’s properties at different time intervals |