We appreciate being given the opportunity to discuss the details of our study 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, 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 effects ; 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. 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.
Hafez et al 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.