Comparisons of the thermodynamic properties of three nickel-titanium orthodontic archwires

The unique memory property of thermodynamic wire is only partially understood. It is believed to result from the wire's inherent capability to markedly alter its atomic bonding forces as a function of temperature. This shape recovery phenomenon may be the result of a transition in crystal structure that occurs by deformation and cooling. When the transition is reversed, by heating, the structure reverts to its original form and abrupt property changes occur. The purpose of this study was to determine the transition temperature ranges (TTR) of three commercially available thermodynamic archwires and to determine the rate of recovery of the wires when bent to a uniform shape. A jig was constructed to hold the wires and was suspended in a water bath within a plexiglass box. The temperature of the water bath was gradually increased. A program was written to acquire a single video frame from a running video tape and then allow the operator to graphically overlay the position of each wire specimen. The results indicate that the TTRs for the three commercially available thermodynamic wires are of similar magnitudes (x = 6.7 degrees C, 6.2 degrees C and 6.7 degrees C). The greatest differences were in the standard deviations (1.3 degrees C, 2.2 degrees C and 3.7 degrees C) which may be a function of manufacturing during alloying of the wire and/or its heat treatment.