Potential Cytotoxicity of Orthodontic Aligners in the Oral Environment: A Scoping Review

Background: Clear aligner therapy (CAT) is increasingly popular due to aesthetic and functional advantages. Recent advances allow direct 3D printing of aligners, raising questions about their cytotoxicity and biocompatibility under intraoral conditions. Objectives: To review and synthesize current evidence on the cytotoxicity and biocompatibility of 3D-printed and thermoformable orthodontic aligners, and to identify factors affecting cellular responses. Eligibility criteria: Original research published from 2021 to 2026 evaluating the safety of orthodontic aligners; conference abstracts, editorials, and review papers were excluded. Sources of evidence: Medline (via PubMed), Scopus, Web of Science, and the Cochrane Library; search terms “aligner” AND “biocompatibility”; last search conducted on 31 January 2026. Charting methods: Data on authors, publication year, material type, experimental model, cytotoxicity assessment (extraction solvent, incubation period, cell line, and cell exposure), and main results were extracted independently by two reviewers. Results: Fourteen in vitro studies were included, seven on thermoformable aligners and seven on directly 3D-printed aligners; no clinical trials were identified. Material composition, post-processing, and oral environmental factors influenced cytotoxicity. Some materials exhibited acceptable biocompatibility, whereas others showed varying cytotoxic effects, indicating inconsistencies across studies. Conclusions: Directly 3D-printed and thermoformable aligners show potential for safe intraoral use, but evidence is limited to in vitro studies. Further standardized in vitro and in vivo research is needed to reliably assess cytotoxicity and ensure patient safety before widespread clinical implementation.