The effects of aging process and preactivation on mechanical properties of nickel-titanium closed coil springs

Abstract

Background: The most favorable feature of nickel-titanium (Ni-Ti) alloys in orthodontics is
producing constant forces in a wide deformation range. Ni-Ti closed coils produce compressive
strength and can be used for several purposes such as space closure. The aim of this study was
to evaluate the effects of the aging process (temperature changes and prolonged strain) on Ni-Ti
closed coils and to assess the effects of preactivation on forces generated by these coils.
Materials and Methods: A total of 60 Ni-Ti closed coils (G&H Wire Co., Indiana, USA) were
divided into four groups (n = 15). Two groups were kept in room temperature and two were
incubated in 37°C in normal saline for 45 days. All the samples were extended to 30% of their
original length. One group of the incubated coils and one group of room temperature coils were
preactivated the same amount of 30%. The incubated groups also received 1000 thermocycles on
days 22 and 45. The unloading forces were measured by a universal testing machine on days 0, 22
and 45. The data were analyzed using SPSS. Signifi cance was set at 0.05.
Results: The unloading forces of the Ni-Ti closed coils was not affected by prolonged heat and
moisture either in preactivated groups (P = 0.8) or the nonpreactivated groups (P = 0.6). Furthermore,
preactivation had no signifi cant effect on the unloading forces of Ni-Ti closed coils (P = 0.7).
Conclusion: Within the limitations of this study, both aging process and preactivation do not affect
on mechanical properties of nickel-titanium closed coil springs. Preactivation had no effect on the
forces generated by Ni-Ti closed coils.
Key Words: Closed coil springs, mechanical properties, nickel-titanium