Nickel-based orthodontic archwires—particularly those composed of nickel-titanium (NiTi) and stainless steel (SS)—are essential components in fixed orthodontic systems due to their favorable mechanical performance. Nonetheless, apprehensions persist regarding nickel-related hypersensitivity, cytotoxicity, and ion emission. This narrative review investigates current evidence concerning nickel ion release from orthodontic materials, expanding on earlier systematic reviews by assessing both in vitro and in vivo experiments under diverse environmental scenarios. Searches were conducted in PubMed, Scopus, and Web of Science databases to identify studies evaluating the correlation between environmental factors and nickel ion release from nickel-containing archwires. Findings demonstrate that metal ion emission occurs in the early phases of treatment but typically remains below toxic limits, with factors like pH, corrosion rate, treatment duration, and environmental exposure influencing the extent of release. However, long-term investigations remain limited and are generally confined to either in vitro or in vivo contexts rather than combining both. To clarify causal associations regarding metal ion emission, in vivo tracking of nickel and other ions is essential, along with further exploration of chronic effects. Additionally, collaboration among clinicians, scientists, and regulatory agencies is crucial for formulating evidence-based standards in orthodontic material selection, ensuring patient safety while minimizing ion exposure risks.
