Comparison of the transformation temperatures of heat-activated Nickel-Titanium orthodontic archwires by two different techniques

• The results of the study indicate that the Bend and Free Recovery (BFR) method is suitable as a standard test method to determine the transformation temperatures of heat-activated Nickel-Titanium (Ni-Ti) orthodontic archwires.
• In this study, transformation temperature values, As and Af, are comparable between the Differential Scanning Calorimetry (DSC) and BFR test methods.
• Since BFR enables testing of unmodified, as-received wires that can be strained to clinically appropriate levels, it is a more clinically relevant test method than DSC, and it is more economical.
• In this study, the tested Ni-Ti archwires fully transitioned to austenite well before the manufacturer-listed temperatures.

ObjectivesThe purpose of this study was to investigate the suitability of the Bend and Free Recovery (BFR) method as a standard test method to determine the transformation temperatures of heat-activated Ni-Ti orthodontic archwires. This was done by determining the transformation temperatures of two brands of heat-activated Ni-Ti orthodontic archwires using the both the BFR method and the standard method of Differential Scanning Calorimetry (DSC). The values obtained from the two methods were compared with each other and to the manufacturer-listed values.MethodsForty heat-activated Ni-Ti archwires from both Rocky Mountain Orthodontics (RMO) and Opal Orthodontics (Opal) were tested using BFR and DSC. Round (0.016 inches) and rectangular (0.019 × 0.025 inches) archwires from each manufacturer were tested. The austenite start temperatures (As) and austenite finish temperatures (Af) were recorded.ResultsFor four of the eight test groups, the BFR method resulted in lower standard deviations than the DSC method, and, overall, the average standard deviation for BFR testing was slightly lower than for DSC testing. Statistically significant differences were seen between the transformation temperatures obtained from the BFR and DSC test methods. However, the Af temperatures obtained from the two methods were remarkably similar with the mean differences ranging from 0.0 to 2.1 °C: Af Opal round (BFR 26.7 °C, DSC 27.6 °C) and rectangular (BFR 27.6 °C, DSC 28.6 °C); Af RMO round (BFR 25.5 °C, DSC 25.5 °C) and rectangular (BFR 28.0 °C, DSC 25.9 °C). Significant differences were observed between the manufacturer-listed transformation temperatures and those obtained with BFR and DSC testing for both manufacturers.SignificanceThe results of this study suggest that the Bend and Free Recovery method is suitable as a standard method to evaluate the transformation temperatures of heat-activated Ni-Ti orthodontic archwires.