Do silver/hydroxyapatite and zinc oxide nano-coatings improve inflammation around titanium orthodontic mini-screws? In vitro study

Overcoming the failure percentage of orthodontic mini-screws (OMSs), which is about 30% of overall orthodontic cases, especially in malocclusion treatment that requires orthopaedic heavy forces, is a great challenge. Bacterial infections, soft tissue and bone inflammation, and weak connections between bones and the OMS surface are among the main causalities of this failure.The aim of the study is to evaluate in vitro the microbiological activities of the deposited nanomaterials (Silver/hydroxyapatite nanoparticles (Ag/HA NPs) and zinc oxide nanoparticles (ZnO NPs)) in terms of microbial inhibition. In addition, the in-vitro cytotoxicity and cytocompatibility of the synthesized nano-coatings prior to their in-vivo application in animal models were tested on four types of cells, namely, fibroblasts, osteocytes, osteoblasts, and oral epithelial cells.Ag/HA NPs and ZnO NPs were built up onto the surface of titanium OMSs by electrochemical deposition. This electrochemical deposition was performed on 50 orthodontic mini screws and the deposited materials were characterized with the aid of scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX) analysis, X-ray Diffraction (XRD) and nano-scratch test. In addition, the microbiological activities of the deposited nanomaterials were explored in vitro in terms of microbial inhibition. Furthermore, the cytotoxicity and cytocompatibility were tested on four types of cells, namely, fibroblasts, osteocytes, osteoblasts and oral epithelial cells.SEM images revealed spherical Ag NPs in the range of 40-70nm in diameter, rod-shaped HA NPs and porous scaly ZnO NPs on the surface of the OMSs. XRD analysis confirmed the crystal structures of AgNPs, HA NPs, and ZnO NPs. ZnO NPs coated OMS had the highest antimicrobial activity than Ag/HA coated OMS against Gram-positive, Gram-negative and fungal strains. Moreover, after incubation, the decrease in the number of bacterial colonies was significant with ZnO and Ag/HA nanoparticles (with the greatest decrease for the former), due to the potent antibacterial effect of nanoparticles against Escherichia coli and Enterococcus faecalis. Moreover, ZnO NPs-coated OMSs showed a better cytocompatibility with oral epithelium, bone cells, and fibroblasts compared to Ag/HA NPs.The suggested nanocoating is a promising strategy to overcome the development of an inflammatory zone around the fixed OMSs.