COMPARATIVE EVALUATION OF FRACTURE RESISTANCE OF TWO ANTIBACTERIAL COMPOSITES AGAINST A COMMERCIAL COMPOSITE-AN IN VITRO STUDY

1. Post Graduate Student, Department of Conservative Dentistry and Endodontics, Pushpagiri College of Dental Sciences, Thiruvalla. 2. Professor and Head, Department of Conservative Dentistry and Endodontics, Pushpagiri College of Dental Sciences, Thiruvalla. 3. Professor, Department of Conservative Dentistry and Endodontics, Pushpagiri College of Dental Sciences, Thiruvalla. 4. Reader, Department of Conservative Dentistry and Endodontics, Pushpagiri College of Dental Sciences, Thiruvalla. 5. Post Graduate Student, Department of Conservative Dentistry and Endodontics, SRM Dental College and Hospital, Ramapuram, Chennai. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History Received: 15 April 2020 Final Accepted: 18 May 2020 Published: June 2020

Antibacterial composites are desirable since it decreases the incidence of secondary caries. One method to impart antibacterial property is the addition of antimicrobial filler [17][18][19] . The downfall of addition of such particles is that they may hinder the mechanical properties of the parent material 17,18 . Among these properties, compressive strength has a particularly important role in the mastication process since most of the masticatory forces are of compressive nature 20 . Compressive strength after antimicrobial filler addition was investigated in only a few previous works (Hojati et al 2013, Stencel et al 2018) 18,19 . This study aimed to compare and evaluate the fracture resistance of modified composite by incorporation of 5wt% ZnO NPs & a novel antibacterial composite -Sri Chitra Thirunal Institute of Medical Science and Technology (SCTIMST) against a commercial composite. The null hypothesis was that there was no significant difference in the fracture resistance of Zinc oxide nanoparticle incorporated composite and a novel antibacterial composite (SCTIMST) against a commercial composite. Sample preparation:10 cylindrical specimens (2 mm in diameter and 4 mm in height) were prepared using mold from each study group. Polymerization was carried out at the top and at the bottom before removal of specimen at 600mW/cm 2 for 40 s each.  There was no significant difference (p >0.05) in mean compressive strength evaluation values between the groups, tested using independent sample t test. On statistical analysis, it was confirmed that addition of ZnO nanoparticles achieved higher mean compressive strength values though not statistically significant. The novel antibacterial composite containing Zn 2+ (SCTIMST) had higher mean compressive strength values though not statistically significant.

Discussion:-
Dental resin composites are at present the most popular restorative material owing to its superior aesthetics and adhesive property 1 .Despite its popularity, factors like polymerization shrinkage with subsequent micro leakage and sequelae of secondary caries, water sorption and inadequate degree of conversion have been the major hurdles in providing a clinically long-serving restoration. 14 Literature studies have shown that ZnO-NPs had been incorporated as radiopaque reinforcing fillers and to impart antibacterial properties 17,18 . In this study, the weight % of ZnO-NPs was optimized by analyzing the consistency of the mix on addition of 1 wt. % increments of ZnO-NPs to a known weight of commercial composite. On addition of ZnO-NPs beyond 5 wt. % concentration, the working characteristics of the parent composite material was found to be inferior and the material was no longer pliable. Thus, 5 wt. % ZnO-NPs was determined as the optimal concentration of ZnO-NPs to be added to the commercial composite. These modified composites were stored in completely opaque bottles until the test was performed.
The higher compressive strength values obtained for Group III, Commercial composite with 5wt% ZnO-NPs may be attributed to the increased filler content as compared to the control group. Miyazaki et al in 1991 stated that higher bond strength was obtained with the composite with the higher filler content. 21 ZnO-NPs exhibited 5-10 times more antibacterial property as compared to SiO 2 depending on the microorganism type. Therefore, fillers containing ZnO-NPs in addition to silica nanofillers could be an advantageous approach for plaque inhibition [Sevinc & Hanley 1165]. 17 ZnO-NPs are insoluble and ZnO-NPs incorporated composites should not lose their antibacterial effect over time due to lower release rates. ZnO-NPs can be used as filler particles in addition to nanosilica fillers in order to reinforce the composite [Imazato et al 2003]. 16 The higher compressive strength values obtained for Group II, novel antibacterial composite (SCTIMST) containing Zn 2+ may be attributed to the presence of inorganic-organic hybrid resins as compared to the control group. In inorganic-organic hybrid resins, organic polymer (methacrylate group) components bring good elasticity, tenacity, ductility and low density, while the inorganic components (Si, Zn 2+ )increases the hardness, stiffness and resistance to elevated temperature [Vibha C et al 2015]. 22 The present study is an in-vitro one evaluating limited properties of two novel experimental composite materials. Composite restorative materials and its clinical success depend on a multitude of factors. Further studies have to be done to evaluate various mechanical properties, curing aspects and adhesive characteristics of these materials. Clinical studies are warranted to achieve a more accurate evaluation and validation of the findings of this study.

Conclusion:-
Within the limitations of this study, it can be concluded that addition of antibacterial nanoparticles to the commercial composite and the novel antibacterial composite (SCTIMST) did not affect the strength of the composite, rather, it had a reinforcing effect as compared to the control material on evaluation of the compressive strength.

Conflicts of interest:
The authors declare that they have no competing interest.