title: Effect of clinical use of nickel-titanium springs.
abstract: BACKGROUND
  Our objectives were to determine whether there are changes on the load deflection rate (L/DP) and the average force (FP) of the superelastic pseudoplateau, and whether permanent deformation is changed in nickel-titanium closed-coil springs (CCSs) after 6 months of clinical use.
  
  
  METHODS
  Twenty-two nickel-titanium CCSs (Sentalloy 100 g; Dentsply GAC, York, Pa) were subjected to tensile mechanical testing at 37°C on activations varying from 3.2 to 16.0 mm before and after 6 months of clinical use. A regression line was fitted over the most horizontal area of the unloading part of the stress-strain graph of every CCS, and its slope was used as L/DP. The FP was determined by the midpoint of the longest segment of the curve that could be fit within the regression line with a R(2) of at least 0.999, and permanent deformation was determined graphically by obtaining the strain value when the measured stress reached zero. The data were analyzed by 3 analyses of variance at 2 levels, with 5% of significance.
  
  
  RESULTS
  Time and activation significantly influenced the variables tested (P < 0.001). Time increased the L/DP and permanent deformation but decreased the FP. Activation decreased L/DP, FP, and permanent deformation. Significant interactions between time and activation were detected for FP (P = 0.013) and deformation (P < 0.001).
  
  
  CONCLUSIONS
  After 6 months of active clinical use, the analyzed springs had a significant but small increase in their L/DP; FP dropped up to 88%, and the CCSs deformed up to 1.26 mm.
authors.affiliation: Private practice, Ribeirão Preto, São Paulo, Brazil.
authors.forename: Amanda Fahning
authors.initials: AF
authors.lastname: Magno
authors.affiliation: Private practice, Ribeirão Preto, São Paulo, Brazil.
authors.forename: André da Costa
authors.initials: Ada C
authors.lastname: Monini
authors.affiliation: Professor, Faculdade de Química de Araraquara, Universidade Estadual Paulista, Araraquara, São Paulo, Brazil.
authors.forename: Marisa Veiga
authors.initials: MV
authors.lastname: Capela
authors.affiliation: Professor, Faculdade de Odontologia de Araraquara, Universidade Estadual Paulista, Araraquara, São Paulo, Brazil.
authors.forename: Lídia Parsekian
authors.initials: LP
authors.lastname: Martins
authors.affiliation: Private practice, Araraquara, São Paulo, Brazil; adjunct professor, Orthodontics Graduate Program, Faculdade de Odontologia de Araraquara, Universidade Estadual Paulista, Araraquara, São Paulo, Brazil. Electronic address: dr_renatopmartins@hotmail.com.
authors.forename: Renato Parsekian
authors.initials: RP
authors.lastname: Martins
mesh_terms: D006801:Humans; D008422:Materials Testing; D009532:Nickel; D009968:Orthodontic Appliances, Removable; D013718:Tensile Strength; D014025:Titanium
chemical_list: D009532:Nickel; D014025:Titanium