Assessment of metal ion accumulation in oral mucosa cells of patients with fixed orthodontic treatment and cellular DNA damage: a systematic review

Intraoral fixed appliances remain in the potentially corrosive environment of the mouth for an average of two years. Over time, corrosion causes the release of metal ions, such as nickel and chromium. These metals can become allergenic and cytotoxic, causing different conditions in the human body. The aim of this study therefore is to carry out a systematic review of the available scientific evidence on the accumulation of metal ions, and the genotoxic and cytotoxic effects in oral mucosa cells deriving from short- and long-term exposure to them. The systematic review is reported in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement. The primary outcome (quantification of metal ion deposits and assessment of their genotoxic and/or cytotoxic effects) and secondary outcome (complementary analysis of cytotoxic and genotoxic effects) were examined. The Cochrane Collaboration tool and Toxicological data Reliability Assessment Tool (ToxRTool) were used for quality assessment. Once the search was performed, a total of seven articles met the inclusion criteria and were included in this study. Two main techniques were used to assess genotoxic effects: alkaline comet assay (6/7) and micronucleus method (1/7). Cytotoxicity was evaluated (4/7) using the trypan blue dye test. Accumulations of nickel (7/7), chromium (5/7), and other metals (zinc, cobalt, iron, manganese, molybdenum, titanium) were also quantified. The results allowed us to conclude that release of metal ions and acute cell and DNA damage in oral mucosa cells takes place in the early stages of treatment. However, more long-term studies are needed to evaluate chronic exposure to metals and DNA damage, as well as cellular capacity to recover DNA integrity.