Biocompatibility of orthodontic materials: a systematic review

AIM The biocompatibility of an orthodontic material is measured by evaluating short- and long-term effects of the material's contact with the biological components of the tissues through 1. in vitro tests 2. in vivo tests 3. tests on animal models and clinical trials. The biocompatibility of a metallic alloy is directly dependent on its corrosion effect, which seems to be minimal in nickel-free alloys and maximum in stainless steel due to oxidation of the surface layers. Biocompatibility tests would require models that reproduce the physical-chemical conditions of the oral cavity, the corrosion ability of saliva, exposure to bacterial biofilm, immune reactivity, exposure times related to the duration of orthodontic treatments. The results should also be compared with the systemic absorption of residues. To date, biocompatibility studies with heterogeneous methods and purposes have been carried out making comparison difficult. Purpose of this study was review assessing of the biocompatibility of most used intraoral dental appliances materials. METHODS PubMed was searched using various combinations of the following keywords: "orthodontic appliances" and “corrosion”, "toxicity test", "cytotoxicity", "hypersensitivity", "biocompatibility" or "safety". Inclusion criteria: Original articles published in English from 2011/06 to 2021, assessing potential effects of orthodontic appliances materials on oral tissues or systemic health. Exclusion criteria: articles not in English; articles on experimental materials and never used before in the commercial production of orthodontic appliances; articles expressing relative ratios without absolute values, articles not available online. RESULTS AND CONCLUSIONS 166 potentially relevant articles have been identified with multiple searches. All abstracts have been read. 87 were excluded for lack of consistency with the research, 32 for application of the inclusion/exclusion criteria, 9 because duplicated. The selected articles were read in full, the tests performed in each work and whether the tests were positive, or negative were considered. It was also considered whether a threshold for toxicity or reversibility of the effects was verified. 38 studies testing the following effects were selected: corrosion=12, leaching=1, cytotoxicity=23 and genotoxicity=4, sensitivity=5. 28/38 articles reported slight or moderate positivity to the tests and 6 of them verified reversibility of the effects over time. Human studies are scarce. Lowest levels of corrosion are confirmed for coated Niti alloys and lowest cytotoxicity and for nickel-free alloys. However, since in vitro studies cannot accurately reproduce the conditions of the oral cavity, any toxic effects and hypersensitivity should be investigated by implementing retrospective cohort studies or prospective observational studies.