title: 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.
  
  
  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. Significance 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 significant effect on the unloading forces of Ni-Ti closed coils (P = 0.7).
  
  
  CONCLUSIONS
  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.
authors.affiliation: Dental Materials Research Center and Department of Orthodontics, School of Dentistry, Isfahan University of Medical Sciences, Isfahan, Iran.
authors.forename: Shiva
authors.initials: S
authors.lastname: Alavi
authors.affiliation: Department of Orthodontics and Dental Students Research Center, School of Dentistry, Isfahan University of Medical Sciences, Isfahan, Iran.
authors.forename: Alireza
authors.initials: A
authors.lastname: Haerian
keywords: Closed coil springs; mechanical properties; nickel-titanium