PubMed1982
There are two frequently used methods of aligning malposed teeth by means of an arch wire. The first method is to deflect and tie the arch wire, which has a low modulus of elasticity, into the deepest part of the bracket slot. This low-modulus arch wire is not permanently deformed, and the elastic forces in it return the arch wire (which is tied to the bracket on the tooth) to its preformed shape. The second method is to stretch an elastic material (for example, Unitek ligature threads, Unitek Alastik modules, Ormco power threads, or Ormolast ligature modules) from the bracket tie wings to a preformed stiff arch wire which has a high modulus of elasticity and the teeth move from their malposed position to the stiff arch wire form. The objective of this investigation was to stretch a thermodynamic nitinol wire from 8 percent to 12 percent of its original length and measure the shrinkage of the wire in length. If the quantity of shrinkage of a 0.010-inch nitinol ligature type of wire were great enough, the second method of moving teeth to a stiff preformed arch wire by employing the thermodynamic nitinol stretched wire as a "wire rubber band" could be used to move teeth. This could serve as a possible alternative to Alastik and elastic modules in which there is a great deal of inherent force decay and permanent deformation. When the thermodynamic nitinol wire was heated through its transition temperature range, three different magnitudes of shrinkage occurred. The first was less than 1.0 percent of the original stretched length of the wire, the second was between 6 percent and 7 percent of the original stretched length of the wire, and the third was approximately 1.0 percent.