The Effect of Strain on Transformation Temperature Range of Orthodontic Nickel-Titanium Archwires

Since its introduction to the orthodontic field in the 1970s, Nickel-Titanium (Ni-Ti) has become the predominant material for the initial leveling and aligning phase of orthodontic treatment. The alloy’s ability to reversibly transform between two lattice structures determines the material’s properties, and thus, its clinical practicality. This transformation occurs as a result of a change in ambient temperature or in the area of a locally applied stress. The range of temperatures over which Ni-Ti transforms from one lattice structure to the other is called its transformation temperature range (TTR). Consequently, TTR is largely responsible for the clinical application of Ni-Ti. The aim of this study was to examine how varying levels of strain affected the transformation temperature of two brands of Ni-Ti archwires using the bend and free recovery (BFR) method. Strain was altered in this study by two different techniques. The first technique was by varying the diameter of the mandrel over which the Ni-Ti wire was bent, where a smaller diameter mandrel results in a higher strain placed on the archwire. The second technique of varying wire strain was by bending two different dimensions of wires over the same size mandrel, where a larger wire results in a higher strain placed on the archwire. Additionally, the manufacturer-reported transformation temperatures of the wires were compared to the transformation temperatures obtained in this study. The results demonstrated that TTR values are affected by strain. Increased strain resulted in increased transformation temperature values and decreased range. While this was the trend for increased strain whether by decreasing mandrel diameter or increasing wire size, one company did not have similar results when strain was increased by increasing wire dimension. This may have been a result of only testing two production lots. Additionally, there was a statistically significant difference between the obtained values in this study and those reported by the manufacturers.