Deformation of orthodontic archwires over time.

Most previous studies of archwire deformation over time (hereafter referred to as "time-dependent deformation of orthodontic wires") have been conducted at a constant room temperature. In the clinical situation however, arch wires are exposed to 37 degrees C as well as to periods of temperature increase when hot foods or fluids are ingested. The effects of the latter on time-dependent behaviour are largely unknown. Since the introduction of direct electric resistance heat treatment to superelastic nickel titanium wires, there have been no reports on its effect on time-dependent deformation. This study investigated the effects of repeated temperature increases (70 degrees C) on stainless steel, nickel titanium and beta titanium wires. The wires were deflected by approximately 3 and 5 mm on two jigs for periods of 1 minute, 1, 7, 14 and 28 days. Permanent deformation was measured optically with a measuring microscope and the amount of time-dependent deformation was calculated. Beta titanium wires demonstrated the greatest amount of time-dependent deformation; followed by non-superelastic nickel titanium, stainless steel, and superelastic nickel titanium. Exposure to repeated temperature (70 degrees C) increases and direct electric resistance heat treatment of superelastic nickel titanium did not affect time-dependent behaviour.