A Narrative Review of In Vitro and In Vivo Studies on Nickel Ion Release from Nickel-Containing Orthodontic Archwires

Nickel-containing archwires, especially those fabricated from nickel-titanium (NiTi) and stainless steel (SS), are essential in fixed orthodontic therapy due to their favorable mechanical characteristics. Nonetheless, concerns persist regarding nickel-induced hypersensitivity, cytotoxic effects, and the release of metal ions, particularly nickel. This narrative review examines contemporary evidence on nickel release from orthodontic archwires, extending previous systematic reviews by evaluating both laboratory (in vitro) and clinical (in vivo) investigations under diverse environmental conditions. Searches were conducted in Web of Science, Scopus, and PubMed for studies addressing the association between nickel ion release from nickel-containing archwires and environmental factors. Evidence indicates that although metal ions are released during short-term orthodontic treatment, the concentrations generally remain below harmful thresholds, with release rates influenced by factors such as pH, corrosion, duration of treatment, and environmental conditions. Long-term studies are limited and are typically confined to either in vitro or in vivo settings, rarely both. To clarify causal relationships in metal ion release, monitoring ions such as nickel in vivo is crucial, and further research is needed to evaluate their long-term effects. Additionally, collaboration among clinicians, researchers, and regulatory authorities is essential to formulate evidence-based guidelines for the selection of orthodontic materials, emphasizing patient safety and the mitigation of metal ion risks.