Authors' response.

Abstract

We appreciate being given the opportunity to discuss the details of our study1Farhadian N. Usefi Mashoof R. Khanizadeh S. Ghaderi E. Farhadian M. Miresmaeili A. Streptococcus mutans counts in patients wearing removable retainers with silver nanoparticles vs those wearing conventional retainers: a randomized clinical trial.Am J Orthod Dentofacial Orthop. 2016; 149: 155-160Abstract Full Text Full Text PDF Scopus (41) Google Scholar and hope the following explanation will be helpful. In our study, silver nanoparticles were synthetized by a physical method to incorporate them into polymer particles, with hot helium gas as a carrier. No increased risk by storage time has been reported for this type of nanoparticles. The coating method was more easily available in our research center. As reported in the article, the surfaces of the retainers were polished as well as the conventional retainers, and there was no observable difference between them regarding surface smoothness. Our study protocol was for about 2 months of clinical study of antibacterial properties of the product, and as mentioned in the Discussion, investigating the mechanical properties of retainers was not the aim. Laboratory studies have produced variable results regarding the mechanical properties of polymethyl methacrylate. In our previous preliminary study,2Miresmaeili A. Atai M. Mansouri K. Farhadian N. Effect of nanosilver incorporation on antibacterial properties and bracket bond strength of composite resin.Iranian J Orthod. 2012; 7: 14-19Google Scholar even the lowest concentration of 500 ppm/weight of silver nanoparticles 10 to 20 nm in diameter incorporated into composite resin had a significant effect on the inhibition of S mutans growth. Determination of minimum inhibitory concentration could be an absolute necessity for future studies to optimize the size vs the concentration of silver nanoparticles. Metallic silver (physically synthetized) appears to pose a minimal risk to health, whereas soluble silver compounds are more readily absorbed and have more potential to produce adverse effects3Wijnhoven S.W.P. Peijnenburg W.J.G.M. Herberts C.A. Hagens W.I. Oomen A.G. Heugens E.H.W. et al.Nano-silver a review of available data and knowledge gaps in human and environmental risk assessment.Nanotoxicology. 2009; 3: 109-138Crossref Scopus (1013) Google Scholar; this was the main reason that we did not use smaller colorless silver nanoparticles (usually synthetized chemically in colloidal form). The type of Streptococcus in our preliminary study was ATCC35668. Atomic absorption tests were done to ensure that no particles entered the digestive or respiratory tracts. Cytotoxicity could significantly increase when silver nanoparticles of 20 nm in diameter or less are used.4Park M.V. Neigh A.M. Vermeulen J.P. de la Fonteyne L.J. Verharen H.W. Briedé J.J. et al.The effect of particle size on the cytotoxicity, inflammation, developmental toxicity and genotoxicity of silver nanoparticles.Biomaterials. 2011; 32: 9810-9817Crossref PubMed Scopus (761) Google Scholar Silver nanoparticles have been used in wound dressing, catheters, and medical devices, and there is no consensus on their toxicity to humans. Long-term vs short-term studies or in-vitro vs in-vivo studies have shown different results.5Ge L. Li Q. Wang M. Ouyang J. Li X. Xing M.M. Nanosilver particles in medical applications: synthesis, performance, and toxicity.Int J Nanomedicine. 2014; 9: 2399-2407PubMed Google Scholar Hafez et al6Hafez H.S. Selim E.M. Kamel Eid F.H. Tawfik W.A. Al-Ashkar E.A. Mostafa Y.A. Cytotoxicity, genotoxicity, and metal release in patients with fixed orthodontic appliances: a longitudinal in-vivo study.Am J Orthod Dentofacial Orthop. 2011; 140: 298-308Abstract Full Text Full Text PDF PubMed Scopus (102) Google Scholar reported some damage to mucosal cells even by chromium or nickel in routine fixed orthodontic appliances. Damage to cells in laboratory tests could not be the reason for absolute negation. Antibacterial properties of retainers with silver nanoparticlesAmerican Journal of Orthodontics and Dentofacial OrthopedicsVol. 150Issue 2PreviewWe read with interest the article entitled “Streptococcus mutans counts in patients wearing removable retainers with silver nanoparticles vs those wearing conventional retainers: A randomized clinical trial” in the February issue of the Journal.1 Since we are also working in this area, we had a few questions and would like some explanations, if possible. Full-Text PDF