title: Effects of stress relaxation in beta-titanium orthodontic loops.
abstract: BACKGROUND
  This study evaluates the changes in the force system of the beta-titanium T-loop spring (TLS) caused by stress relaxation.
  
  
  METHODS
  Ninety TLSs with dimensions of 6 × 10 mm, made of 0.017 × 0.025-in beta-titanium alloy and preactivated by concentrated bends, were randomly distributed into 9 groups according to the time point of evaluation. Group 1 was tested immediately after spring preactivation and stress relief by trial activation. The other 8 groups were tested after 24, 48, and 72 hours, and 1, 2, 4, 8, and 12 weeks. By using a moment transducer coupled to a digital extensometer indicator adapted to a universal testing machine, the amounts of horizontal forces and moments and the moment-to-force ratios were recorded at every 0.5 mm of deactivation from 5 mm of the initial activation in an interbracket distance of 23 mm.
  
  
  RESULTS
  The horizontal forces and moments were higher (P <0.001) for group 1 compared with the other 8 groups, which were not different among themselves. All groups produced similar moment-to-force ratios (P = 0.600), with no influence of time.
  
  
  CONCLUSIONS
  The TLSs preactivated by concentrated bends had progressive load decreases over time, and this effect is critical in the first 24 hours.
authors.affiliation: Faculdade de Odontologia de Araraquara, Universidade Estadual Paulista, Araraquara, São Paulo, Brazil.
authors.forename: Sergei Godeiro Fernandes Rabelo
authors.initials: SG
authors.lastname: Caldas
authors.forename: Renato Parsekian
authors.initials: RP
authors.lastname: Martins
authors.forename: Rodrigo F
authors.initials: RF
authors.lastname: Viecilli
authors.forename: Marília Regalado
authors.initials: MR
authors.lastname: Galvão
authors.forename: Lídia Parsekian
authors.initials: LP
authors.lastname: Martins
mesh_terms: D000704:Analysis of Variance; D003722:Dental Alloys; D003799:Dental Stress Analysis; D008422:Materials Testing; D009969:Orthodontic Wires; D011897:Random Allocation; D018709:Statistics, Nonparametric; D013314:Stress, Mechanical; D013997:Time Factors; D014025:Titanium
chemical_list: D003722:Dental Alloys; C023988:beta titanium; D014025:Titanium