An evaluation of the shape-memory phenomenon of nickel-titanium orthodontic wires

This investigation was undertaken to quantitatively analyze the shape-memory phenomenon of wires made of seven commercially available nickel-titanium alloys. The shape memory was determined by calculation of the percent shape recovery that occurred when the wire of each alloy was first plastically deformed below its TTR and then heated to a temperature above its TTR. The findings indicate that: 1. The mean percent recovery ranged from 89% to 94% for Ni-Ti, nitinol, Orthonol, Titanol, Sentinol Light, and Sentinol Medium alloys. The Sentinol Heavy alloy showed a mean recovery of 41.3%, which was significantly less than that of the other alloys. 2. It appears that Sentinol Heavy wire showed relatively less percent recovery because its TTR was close to room temperature. This resulted in minimal plastic deformation, because the alloy recovered its original length almost immediately, and this also indicates the importance of a proper TTR. It was concluded that the TTR should be reasonably higher than the oral temperature for clinical application of the shape-memory phenomenon of nickel-titanium alloys. 3. The heat-recovery temperature was kept to 300 degrees F in this study for maximum possible recovery. Although the results indicate that the recovery was around 90% for most alloys tested, future studies are required to determine the shape recovery at or just above oral temperature.