Laboratory analysis of superelastic NiTi compression springs

The force/compression characteristics of 32 commercially available nickel titanium (NiTi) compression springs from seven distributors were investigated in vitro in order to support the orthodontist in deciding to select an appropriate spring for a given treatment. The geometrical properties of the coil springs, i.e., inner spring diameter, winding diameter and winding configuration, differed significantly, as well as the alloy composition and the thermo-mechanical treatment of the springs. All springs were mounted on a guiding rod made of an orthodontic steel wire (dimension: 0.016" x 0.022"), were compressed to a maximum extent and then relieved. Force/compression characteristics were measured at ambient temperatures of 27 degrees C, 37 degrees C and 47 degrees C. Three specimens were taken from each individual compression spring of a certain manufacturer and batch to check for constant material behaviour. A possible influence of sterilization on the mechanical properties of the compression springs was studied by autoclaving one spring of each manufacturer five times (6 minutes at 134 degrees C) and subsequently performing a force/deflection measurement. The NiTi compression springs were classified into three groups and covered a broad range of orthodontic forces between 0.5 N and 3.5 N. The width of the superelastic plateaus of the different NiTi coil springs reached from 0% to 66% of relative compression. An increase in the application temperature from 27 degrees C to 47 degrees C caused a rise in the height and a shortening of the width of the superelastic plateau. The resultant change in plateau force was as high as 0.4 N to 0.9 N, depending on the spring type investigated, the width of the plateaus was shortened by 4.0% to 15% of relative compression. All compression springs investigated displayed constant material behaviour within a certain batch. However, differences from one batch to another had a high level of significance. Forces on the plateau varied from one batch to the other by about +/-18%. An influence of sterilization on the force/compression behaviour could not be proved. Consequently, springs made of superelastic NiTi alloys cover a broad field of application with predefined and nearly constant force levels.