Determination of the Transformation Temperature Ranges of Orthodontic Nickel-Titanium Archwires

Currently, manufacturers utilize no standard method for testing orthodontic Nickel-Titanium (Ni-Ti) archwires. The majority of published orthodontic studies use Differential Scanning Calorimetry (DSC) to test the Transformation Temperature Range (TTR) of Ni-Ti wires, but the Bend and Free Recovery Test (BFR) method may be more clinically applicable. Objective: To compare the Transformation Temperature Range (TTR) values, As and Af, of thermoelastic Ni-Ti archwires between: (1) Bend and Free Recovery (BFR) and Differential Scanning Calorimetry (DSC) test methods, (2) two wire sizes, (3) two wire brands, (4) two lots, and (5) tested and manufacturer-listed values. Methods: Eighty orthodontic Ni-Ti archwires were divided into 2 groups tested using BFR or DSC. Groups were further divided based on manufacturer: Rocky Mountain Orthodontics (RMO) and Opal Orthodontics (Opal) and wire size: round (0.016 in.) and rectangular (0.019x0.025 in.), resulting in 8 subgroups. Each subgroup consisted of wires from two lots. Austenite start (As) and austenite final (Af) temperatures were recorded. Data analysis was performed using Student t-tests (α≤0.05). Results: Statistically significant mean differences were observed: (1) between BFR and DSC for all As values and most Af values; (2) between wire sizes for all BFR-tested wires; (3) between brands for half of the BFR-tested groups and half of the DSC-tested groups; (4) between most lots; (5) between all tested and listed Af values by Opal and RMO. Conclusions: TTR values are comparable between DSC and BFR, but BFR is a more economical and clinically relevant method, enabling testing on unmodified, as-received wires. All wires fully transitioned to austenite well before their manufacturer-listed temperatures.