THE EFFECT OF TEMPERATURE ON THE ELASTIC RESPONSES TO LONGITUDINAL TORSION OF RECTANGULAR NICKEL-TITANIUM ARCHWIRES

To investigate responses to longitudinal torsion and the effect of tem perature on the torsional stiffness of superelastic nickel titanium ar chwires, eight batches of rectangular wires were tested at 18, 27, 37, and 40 degrees C ambient temperature. The wires were twisted 25 degre es and studied in deactivation. The resulting torque-twist diagrams sh ow that only half the wires had discernible deactivation plateaus at b ody temperature. The plateaus were generally narrow (1 to 3 degrees) a nd started at 20 to 23 degrees of torsional twist. The clinical signif icance of these deactivation plateaus is debatable. Only one wire had a well-defined plateau that was fairly wide (6 degrees) and started at a lower level of twist (17 degrees). The wires without plateaus when deactivated from 25 degrees of twist were retested at body temperature . All exhibited deactivation plateaus subsequent to activating twists of 45 and 60 degrees, and the plateaus became more distinct as the deg ree of prior activation increased. This indicates that the stress impa rted on the alloys by 25 degrees of activating twist is insufficient t o induce martensitic transformation at body temperature. As prescripti ons advocate bracket pretorque of less than 25 degrees for a maxillary central incisor, the clinical relevance of alloys requiring large act ivations before they demonstrate deactivation plateaus is questionable . Half the wires tested were markedly influenced by ambient temperatur e changes; the other half were relatively insensitive to temperature. Responsiveness to thermal stimuli seemed closely related to superelast ic tendency.